ebay u-blox LEA-6T GPS module teardown and initial test

[Gyro]

Prompted by voltsandjolts's post in this thread:

https://www.eevblog.com/forum/projects/it's-feb-2016-which-ebay-gpsdo/msg875683/#msg875683

I decided to buy on of the u-blox LEA-6T GPS navigation modules currently available on ebay at under £20. It has duly arrived in 10 days flat to the UK from HK, complete with some mounting hardware and a micro-USB adapter board. This is the Timing version of the module, although for some reason being sold for drone use. Being the LEA variant, there are two programmable timing pulse outputs available.

The first thing to do was hook it up to the adapter board and try it with the u-blox u-center s/w available from their website. It works fine and appears very sensitive, getting a 3D lock downstairs in a 2 story house.

Time to pull it apart and work out the connector pinouts as I couldn't find anything on the web (there is documentation for 'Pixhawk' interfaces but these doesn't appear to match). The unit is held together with 4 screws with a further one locating the PCB in the bottom of the case to ease assembly. The top surface on the PCB is occupied by the 25mm passive patch antenna only:



On the bottom of the PCB is a u-blox LEA-6T-0-000, together with a compass IC, backup battery and other parts. This version of the LEA-6T is hard-coded rather than Flash based, but the board does include an ST 32k bit EEPROM for long term settings storage. The memory backup battery is rechargeable. There's an on-board 3V3 LDO regulator (taking the 5V supply from USB), also included are PTC and reverse polarity diode protection. There are also two LEDs, a red one on the 3V3 supply, and a green one, transistor buffered from the Timepulse signal.

The interface connector on the board has a pinout as follows:

Pin   Wire Color   Signal
1   Yellow      RXD1
2   Green      TXD1
3   Blue      Compass I2C
4   White      Compass I2C
5   Black      GND
6   Red      +5V
7   Orange      USB D-
8   Brown      USB D+

Note that there are no Timepulse outputs on the connector, not helpful for timing use. As I am going to use the module as a timing reference I decided to remove the Compass IC and re-use pins 3 and 4. A little wasteful I know, but compass modules are really cheap these days. The mod was easy, a wire from pin 3 to the 'pps' pad on the PCB. The Timepulse2 pin on the LEA-6T module (pin 9) is very close to the connector so easy to bridge a wire across to pin 4 on the connector. As the outputs are capable of running up the 10MHz so ideally you would us coax,  but I found that the signal at the end of the short supplied cable still looks pretty good, so I have left it at that for now. The photo shows the modified PCB:



I haven't tested the serial RXD and TXD signals. The LEA-6T module uses the VDDusb pin to decide whether USB is connected. This pin is hard-wired to 3V3 so this might override the serial port, I don't know. Not an issue for me as I would only be connecting them to a USB-serial module anyway. If serial-only operation is needed, the track to the VDDusb pin and associated decoupling cap are easily accessible so it would be simple to hardwire it to ground instead.

Now on to the USB adapter board. The Connecting cable is split out into a 5 pin and 4 pin connector, strangely the USB signals are split between the two connectors. The adapter board matches these to a microUSB connector. NOTE that there are NO protection components - even the 22R USB series resistors are missing, so this will only be a temporary measure. In addition to the connectors, the board also has footprints for two 0.1" headers... one is a 2 pin with USB D+ and D- (not much use) but the other is a 4 pin with +5V, GND, and 2 unconnected holes. It was very easy to add a couple of wire links to connect the new Timepulse and Timepulse2 signals to these unused pins (see photo). Usefully, once configured the unit will run standalone (no USB) by supplying 5V to the header.



I've also drawn a basic schematic of the other bits on the GPS PCB (sorry it's a bit fuzzy):



So how does it perform? I followed the advice given in the u-blox timing app-note, disabling SBAS, setting stationary mode etc. The app note also shows how to configure the Timepulse signals to only output when GPS is locked. I've set Timepulse to be a slow pulse signal (10s) as it has the led attached, and Timepulse2 as a high frequency output. I tried it on 8MHz and 10MHz, as expected there is significant jitter at 10MHz as it isn't an integer  divide of the 32MHz TCXO. At both frequencies there is sawtooth jitter visible (as expected).

I've done some initial testing with my Philips PM6622 Timer/counter. Even when Timepulse2 set to the more jittery 10MHz, when the counter is set to 10sec gate (0.1Hz resolution) the frequency display is stable at the least significant digit (1e-8). In timer mode, with Timepulse set to 10secs, again the display was stable within 1 digit (100ns), again stable to 1e-8. As you can expect 1 digit variation due to timing jitter I increased the Timepulse time to the maximum 60 secs. This still showed only 100ns jitter, so stability is better than 2e-9. I've run the module though a fair number of powers cycles and re-acquisitons and it always displays the same readings so I've got good confidence that the output it is also accurate to this level. When the OCXO output of the PM6622 and Timepulse2 are both displayed on a scope you can see the slow rolling of one signal versus the other even with the jitter, so it is easy to adjust the OCXO frequency by eye (good old analog method).

My intention is to use the module to discipline a Datum 10MHz OCXO that I've got, but it is good to see that the module on it's own is good enough to calibrate any normal timer/counter to a practical level of accuracy standalone.

[aroby]

Scullcomm has a good video series on using other modules from Ublox to build a reference standard.  As yet, it's not disciplining anything. 

[splin]

He seems to be a bit confused about the EEPROM losing its contents when the Supercap voltage drops too far  - whereas it will be the Ublox module losing data from its internal battery backed up RAM.

I don't think the Ublox module even interfaces to the EEPROM, it being provided for use by an external microcontroller.

Providing a battery for backup is still a good idea though, if only to keep the Ublox's RTC going to speed up acquisition on power up, but not for maintaining configuration. It would be better for the micro to configure the Ublox at every startup rather than relying on the Supercap/battery to avoid having to connect it to a PC for reconfiguring whenever the battery fails.

[uncle_bob]

Hi

If you ignore the sawtooth correction output on the serial port, you will have about +/- 10 ns of peak jitter on any of the timing outputs. In a rapidly changing temperature environment (out in the drafts on the bench) this may average out fairly well. If you mount it in a nice box and enclose everything, it may not average out quite as well.

Bob

[Theboel]

Hi Gyro,

I believe You have run the uCenter if You dont mind I like to see the deviaton map chart and satellite level just for comparison to my 6M

[bingo600]

Hi

If you ignore the sawtooth correction output on the serial port, you will have about +/- 10 ns of peak jitter on any of the timing outputs. In a rapidly changing temperature environment (out in the drafts on the bench) this may average out fairly well. If you mount it in a nice box and enclose everything, it may not average out quite as well.

Bob

Shouldn't the LEA-6T be able tp apply sawtooth correction internaly for the 1-PPS , or did i miss something.

I'm actually considering to skip my M12-T+ & Dallas delayline proj. , in favor of a LEA-6T.

I'm going to dicipline a PRS-10

/Bingo

[Gyro]

He seems to be a bit confused about the EEPROM losing its contents when the Supercap voltage drops too far  - whereas it will be the Ublox module losing data from its internal battery backed up RAM.

I don't think the Ublox module even interfaces to the EEPROM, it being provided for use by an external microcontroller.

Providing a battery for backup is still a good idea though, if only to keep the Ublox's RTC going to speed up acquisition on power up, but not for maintaining configuration. It would be better for the micro to configure the Ublox at every startup rather than relying on the Supercap/battery to avoid having to connect it to a PC for reconfiguring whenever the battery fails.

Yes, good video but there do seem to be a few factual errors relating to the 6T, such as not being able to do 10MHz. Yes the module can interface to an EEPROM directly as long as it's an I2C 32k bit. There's a setting in u-center configuration to write EEPROM as well as BB RAM.

I'd definitely go for the LEA over the NEO for the extra Timepulse output, even though it costs a bit more.

One strange thing I've noticed on the maximum timepulse, the datasheet says 0.25Hz, ie 4 seconds - I've had it up to 60 seconds without any problem (as my original post). The maximum time the input field on u-center will accept is 90s (90000000us) but it didn't give any timepulses out at that. Still once per minute is potentially a useful value anyway.

[Gyro]

Hi

If you ignore the sawtooth correction output on the serial port, you will have about +/- 10 ns of peak jitter on any of the timing outputs. In a rapidly changing temperature environment (out in the drafts on the bench) this may average out fairly well. If you mount it in a nice box and enclose everything, it may not average out quite as well.

Bob

Hi Bob,

Yes, I suspect that the biggest source of error is still the inevitable 1 digit uncertainty on the timer gate (100ns). I don't have the resolution to get down to the true jitter level of the module itself... but that's what you want for a calibration source isn't it. I was just expecting to have to build a disciplined oscillator to get to that level.

I'll probably build the disciplined oscillator anyway as I have an Adret Synthesized sig gen with a slightly drifty OCXO and a lock input. Once locked I should have a nice accurate 300Hz (20Hz with relaxed spec) to 180MHz  with 1Hz resolution.

The first thing to do will be a 'head unit' with the GPS module mounted on top (it's self contained so no need to enclose in the box) with buffering for the two timepulse signals, a local psu reg. and protection for the USB. I can then mount it high up (at the moment it's sitting on the bench below shelves full of equipment  which isn't great - but still good enough to get the results I posted). I can then place the disciplined OCXO and output buffers/connectors more conveniently.

Chris

[Gyro]

Hi Gyro,

I believe You have run the uCenter if You dont mind I like to see the deviaton map chart and satellite level just for comparison to my 6M

Yes, sure. I'll try re-positioning it a bit first, as I just mentioned, it's currently sitting on the bench right under shelves full of equipment so that I can get to the counter and scope, so I'm seeing max deviations of up to 10m!

[@rt]

I thought this was a teardown
Here’s the guts of LEA-5T, a bit distorted under magnification.

I got a bunch for free, but was only confident with one of them after desoldering from rejected boards,
then hand assembled from there

[Gyro]

Haha, you win! 

Seriously though, it's hard to visualize just how tiny that construction is until you actually see the module in real life.... and then to have customers putting it through reflow again on their boards!

[uncle_bob]

Hi

If you ignore the sawtooth correction output on the serial port, you will have about +/- 10 ns of peak jitter on any of the timing outputs. In a rapidly changing temperature environment (out in the drafts on the bench) this may average out fairly well. If you mount it in a nice box and enclose everything, it may not average out quite as well.

Bob

Shouldn't the LEA-6T be able tp apply sawtooth correction internaly for the 1-PPS , or did i miss something.

I'm actually considering to skip my M12-T+ & Dallas delayline proj. , in favor of a LEA-6T.

I'm going to dicipline a PRS-10

/Bingo

Hi

This is a very basic issue with all GPS modules.

The PPS edge comes off of a free running clock that is internal to the module. It may be a TCXO, it may be a PLL clock locked to the TCXO. The module simply uses that clock and works from it. The GPS data comes in and a "time solution" is formed from a bunch of math. The module decides which TCXO edge is closest to the solution and uses that to trigger the PPS output. It then reports the residual error via software.

Bottom line, there is no getting around it. You *have* to do something with the sawtooth information out of the module. Either you do a delay line and a bunch of code or you put the information into your control loop.

Bob

[uncle_bob]

Hi

If you ignore the sawtooth correction output on the serial port, you will have about +/- 10 ns of peak jitter on any of the timing outputs. In a rapidly changing temperature environment (out in the drafts on the bench) this may average out fairly well. If you mount it in a nice box and enclose everything, it may not average out quite as well.

Bob

Hi Bob,

Yes, I suspect that the biggest source of error is still the inevitable 1 digit uncertainty on the timer gate (100ns). I don't have the resolution to get down to the true jitter level of the module itself... but that's what you want for a calibration source isn't it. I was just expecting to have to build a disciplined oscillator to get to that level.

I'll probably build the disciplined oscillator anyway as I have an Adret Synthesized sig gen with a slightly drifty OCXO and a lock input. Once locked I should have a nice accurate 300Hz (20Hz with relaxed spec) to 180MHz  with 1Hz resolution.

The first thing to do will be a 'head unit' with the GPS module mounted on top (it's self contained so no need to enclose in the box) with buffering for the two timepulse signals, a local psu reg. and protection for the USB. I can then mount it high up (at the moment it's sitting on the bench below shelves full of equipment  which isn't great - but still good enough to get the results I posted). I can then place the disciplined OCXO and output buffers/connectors more conveniently.

Chris

Hi

One much easier solution is to simply calibrate your local standard directly against the output of the GPS. Divide the standard to 1 Hz and compare them on a scope or with a counter. Do a "every once and a while" adjust on the standard until they stop moving relative to each other. You can get down into the 1x10^-11 range pretty easily doing this. Been there / done that a lot of times. No control loops, no nasty added noise, nothing more to break, no two to five years debugging firmware for corner cases.

Some math:

+/- 10 ns on the GPS module
One hour is 3,600 seconds
1x10^-8 at 1 second goes to 1/3600 x10^-8 at 1 hour
You now are at 3x10^-12 resolution.

Bob

[splin]

He seems to be a bit confused about the EEPROM losing its contents when the Supercap voltage drops too far  - whereas it will be the Ublox module losing data from its internal battery backed up RAM.

I don't think the Ublox module even interfaces to the EEPROM, it being provided for use by an external microcontroller.

Providing a battery for backup is still a good idea though, if only to keep the Ublox's RTC going to speed up acquisition on power up, but not for maintaining configuration. It would be better for the micro to configure the Ublox at every startup rather than relying on the Supercap/battery to avoid having to connect it to a PC for reconfiguring whenever the battery fails.

Yes, good video but there do seem to be a few factual errors relating to the 6T, such as not being able to do 10MHz. Yes the module can interface to an EEPROM directly as long as it's an I2C 32k bit. There's a setting in u-center configuration to write EEPROM as well as BB RAM.

Whilst the NEO-6M supports external EEPROM, the NEO-7M which he is using, does not. It appears that the manufacturers are using the same board for NE-6, 7 and 8 variants but haven't bothered removing the EEPROM when the latter modules are used.

I'd definitely go for the LEA over the NEO for the extra Timepulse output, even though it costs a bit more.

Another advantage of the LEA that wasn't pointed out is that it can be told its actual location and thus provide GPS timing with only one satellite visible (but with reduced accuracy) as it doesn't require the usual 3+ to triangulate its position.

[uncle_bob]

Another advantage of the LEA that wasn't pointed out is that it can be told its actual location and thus provide GPS timing with only one satellite visible (but with reduced accuracy) as it doesn't require the usual 3+ to triangulate its position.

Hi

The qualification on that is:

Static location
Full survey compete

Bob

[@rt]

Too right! They are quite a “one shot deal”. Those nasty connectors, I don’t know what you call them.
After heaps of work trying to desolder, the one I saved was by simply turning the PCA upside down and
hitting the back of the board with a blow torch until gravity took the Ublox module off.
This one has been in service and reliable for a year!

QA told me this would never work and the sad part is there’s no way to tell until it’s reused.

Haha, you win! 

Seriously though, it's hard to visualize just how tiny that construction is until you actually see the module in real life.... and then to have customers putting it through reflow again on their boards!

[uncle_bob]

Hi

You can get the modules off a board. The problem is indeed that the BGA can easily get mucked up. Without an X-Ray there is no easy way to see if you have nuked it. There are a couple of other fine pitch parts on the board that you *might* be able to visually inspect.

Simple answer - a "new" LEA6T on eBay isn't all that expensive. The LEA-8T should start showing up on eBay soon.

Bob

[Gyro]

Hi

One much easier solution is to simply calibrate your local standard directly against the output of the GPS. Divide the standard to 1 Hz and compare them on a scope or with a counter. Do a "every once and a while" adjust on the standard until they stop moving relative to each other. You can get down into the 1x10^-11 range pretty easily doing this. Been there / done that a lot of times. No control loops, no nasty added noise, nothing more to break, no two to five years debugging firmware for corner cases.

Some math:

+/- 10 ns on the GPS module
One hour is 3,600 seconds
1x10^-8 at 1 second goes to 1/3600 x10^-8 at 1 hour
You now are at 3x10^-12 resolution.

Bob

Good point Bob, It was certainly very easy to ignore the fuzz and see the underlying roll even when comparing the two 10MHz signals (with the non integer divide jitter). Thanks for doing the maths, I hadn't realised it was so easy to get to that sort of resolution. Maybe manual is the way to go after all, certainly easier than optimising the PLL loop filter.

Chris

[Gyro]

Whilst the NEO-6M supports external EEPROM, the NEO-7M which he is using, does not. It appears that the manufacturers are using the same board for NE-6, 7 and 8 variants but haven't bothered removing the EEPROM when the latter modules are used.

Ah, I see. That seems like a bit of an oversight! I wonder if it was an introduced 'feature'. Glad I've got the 6 in that case, it will probably get used infrequently enough for the battery to go flat on a regular basis.

Another advantage of the LEA that wasn't pointed out is that it can be told its actual location and thus provide GPS timing with only one satellite visible (but with reduced accuracy) as it doesn't require the usual 3+ to triangulate its position.

I didn't realize that was only available on the LEA modules. I've been waiting for the deviation graph to give me a nice stable averaged location to try. I've only tried Survey-in mode so far.

[uncle_bob]

Hi

There are differences between the T and "not T" modules in terms of what is or is not supported. They are (as noted above) quite similar in terms of hardware. The firmware loads are what turns a LEA-6 into a LEA-6T. In some cases the docs's are abundantly un-clear about just what you get on this or that part. By far the best approach is to write some code and try it and see.

Bob

[Gyro]

Hi Gyro,

I believe You have run the uCenter if You dont mind I like to see the deviaton map chart and satellite level just for comparison to my 6M

Here you go, I couldn't find a way to capture the whole screen with the deviation window open so you've got them as two separate captures. The unit is still sitting on the bench (upstairs, but under a loft filled with *****).

I thought Trimble studio was good, but u-center is much more fun!

Coordinates removed for privacy reasons.

[lars]

Thanks Gyro,

for this thread and especially the schematic. It revealed for me why the PPS is only 2Volts out and not 3.3V. This gave me problem when I tried to connect directly to my Arduino GPSDO controller. I had to add an HCT08. Today I added a base resistor for the transistor that drives the green LED. First I tried with 10kohm between the PPS and base. Now I got almost 3.3V out but the green LED still is very bright. On the base I have about 1.5V. Maybe i should have changed the 33ohm resistor. But on module two I added 100kohm instead and now the brightness is about same as red LED. Voltage on base is about 0.9V.

On one module I have also taken away the amplifier and bias resistor and added a coax with SMA (was a uFL-SMA cable) so I can have an external antenna. With internal antenna I have about 30dB C/N and not so many satellites. With the module in an eastern window I have about 40dB C/N. With an external small antenna I have about 50dB C/N and it most of the time tracks 10-12 satellites. During a test of one day the lowest was 7 satellites and I am in Sweden. I haven´t yet tested the ADEV and MDEV for the module with the external antenna. But for the internal antenna with about 30dB C/N I got MDEV 3E-12 at tau 1000s in fixed mode. Not so bad to be in house. Without fixed mode it was about three times higher. This is about the same as I get with my M12 GPS module with external antenna!

I have had no problem to save the surveyed result to the EEPROM. Just marked all options in config.

I have also used the rxd and txd together with a USB-TTL adapter without problem. With the supplied USB board I had problem with external 5V supplied via a diode to the small board. Sometimes the PPS stopped working if I disconnected the USB. With the serial I haven´t had that problem. My intention is to use the TP error (saw tooth corr) into the Arduino controller via serial (let´s see when it happens as I am not a good programmer). As the Arduino controller TIC has 1nS resolution it would be interesting to see if the result get´s better with saw tooth correction.

Lars

[Gyro]

Hi Lars,

Glad it's been of some use. I was a bit puzzled by the LED drive too, I was expecting a standard common emitter with base resistor but nothing buzzed out until I realized what it actually was. I'll probably do the same, or increase the 33R to reduce the LED current.

Thanks for the information on connecting the external antenna (rather more of an achievement), that's quite a signal level improvement! I had been wondering myself as I have an active one available, but always nice to see someone else try it first. The LEA has all the  antenna supply and current monitoring functions so I may give it a try. A remote antenna would certainly be more convenient to use.

Nice to have confirmation that the serial connections do work - I was concerned that the USBvdd (USB present) signal would override them, I didn't want to have to make it switchable. I'll be very interested in how your sawtooth correction experiment works out. 

Chris.

Edit: The LEA/NEO-6 Hardware integration manual and GPS Antennas App Note have lots of useful Antenna data just in case you haven't already read them (you probably have):

https://www.u-blox.com/sites/default/files/products/documents/LEA-NEO-MAX-6_HIM_%28UBX-14054794%29.pdf

https://www.u-blox.com/sites/default/files/products/documents/GPS-Antenna_AppNote_%28GPS-X-08014%29.pdf

[@rt]

At the time there was no way that I could find to buy a bare module off the roll.
You could only buy a prebuilt module with optional ceramic antenna.
I recall Ublox ignoring me when requesting a sample of the bare module,
and some old fashioned integrity prevented me from stealing from my employer.

My goal to be consistent with the larger project was to construct everything down to the lowest reasonable component.

Some DC short circuits coupled with a capacitor make the best antennas!

Hi

You can get the modules off a board. The problem is indeed that the BGA can easily get mucked up. Without an X-Ray there is no easy way to see if you have nuked it. There are a couple of other fine pitch parts on the board that you *might* be able to visually inspect.

Simple answer - a "new" LEA6T on eBay isn't all that expensive. The LEA-8T should start showing up on eBay soon.

Bob

[Theboel]

@ Lars, I agreed with You about "internal" or patch antenna I have place the antenna on my 28m tower the result is not spectacular so the better antenna is must in my case I use a trimble bullet antenna I found in the flea market.

@ Gyro, Thank You Sir for the picture I can confirm Your deviation map is almost same with mine when I use "internal antenna" even when I placed it on 28m above ground. 

@ @rt have You try furuno ? they more cooperative and  the price is not to high compare to T model from Ublox here the http://www.furuno.com/en/products/gnss-module/GT-87
I believe You can buy in EU or USA.

@ Uncle bob, sorry Sir I got lost with the Your explanation about sawtooth correction,
when we use PLL with time constant 30 to 40 second are this correction still needed ?   

[Gyro]

At the time there was no way that I could find to buy a bare module off the roll.
You could only buy a prebuilt module with optional ceramic antenna.
I recall Ublox ignoring me when requesting a sample of the bare module,
and some old fashioned integrity prevented me from stealing from my employer.

My goal to be consistent with the larger project was to construct everything down to the lowest reasonable component.

Some DC short circuits coupled with a capacitor make the best antennas!

Wow @RT, that's an awesome construction, I feel quite embarrassed now about having chickened out and bought an ebay module (I always use Manhattan style breadboarding if I can possibly get away with it), though as you say bare modules are almost impossible to get.

That helical antenna is a thing of beauty!   

[@rt]

The Garmin 60csx handheld had QFH antenna. Comparative drift tests with later models that have ceramic antennas,
clearly favoured the earlier 60csx so far as actual GPS hardware goes.
I’d always go with it for GPS where possible now... obviously not for aircraft!

Don’t be embarrassed, these modules weren’t designed for protoboard. I'm much more proud of the antenna.
I even made the optional EEPROM board to fit that box, but opted out of using it. Just so surprised how fast it was

[Gyro]

An Interesting development (well maybe for just a few anyway!).

I've been tracing out the RF section of the board and I think I've found an error. The 6 pin package near the RF pins of the module is a MMIC LNA, the board has amplification. I'm not sure of the exact part number, the package is marked 'LY' (but it's not a TOSHIBA dual AF transistor!). The pinout matches the AVAGO MGA-61562, and I think there's a Chinese version with the same part number, but no hits for 'LY'.

The photo shows what I found on the board (top diagram) versus what seems to be the standar application circuit (bottom). On the board the RF supply choke to the output pin is taken from the Bias Pin rather than decoupled VCC. That doesn't look right to me, I'm not sure how  much performance impact it might have though. It might just possibly account for performance being better with the circuit removed and an external passive patch antenna fitted (as Lars has done).



The other thing is that I'm not sure what value the bias resistor should actually be. The one fitted is 10R, but that coincides with the resistor recommended by u-blox for antenna overcurrent sensing, the Avago datasheet shows 680R for the bias resistor.

Section 2.2.2.7 of the LEA/NEO 6 Hardware integration manual Fig 22 shows the internal Ant power switching and how it should be connected, The RF input pin is tolerant of Antenna supply, but u-box don't recommend applying the supply directly to that pin (because it bypasses the internal switching and current sensing). I'm not sure if it's worth messing around with that though as the LNA is always powered.

I'm thinking of diving in and modifying / correcting the LNA circuit, any thoughts on whether this is deliberate (maybe required by whatever part it actually is) or just a design error ... and possible performance impact if so?

Chris

[uncle_bob]

An Interesting development (well maybe for just a few anyway!).

I've been tracing out the RF section of the board and I think I've found an error. The 6 pin package near the RF pins of the module is a MMIC LNA, the board has amplification. I'm not sure of the exact part number, the package is marked 'LY' (but it's not a TOSHIBA dual AF transistor!). The pinout matches the AVAGO MGA-61562, and I think there's a Chinese version with the same part number, but no hits for 'LY'.

........

Chris

Hi

If it really is the Avago part and if the resistor is really 10 ohms, the part should be pulling about 80 ma. That should be pretty easy to figure out. Is it blazing hot when you put your finger on it? At 80 ma, its running at max gain, reasonably low noise figure, and pretty good IP3. A quarter watt is not a good thing to dump into a normal battery powered design, the data sheet focuses on more rational power vs performance regions of use.

One of the most basic points about GPS is fairly simple:

As received, the signal is buried deep in the noise. Noise figure is more important than gain with these modules. The only real reason to have a gain block is to overcome losses between the antenna and the module. If you have no gain and your coax has 3 db of loss, your noise figure is 1 + 3 = 4 db. With a (hopefully) low loss pc board trace as your only connection to the antenna, gain may not be needed at all. To the extend that gain can overwhelm the AGC on the device, it can actually hurt performance.

Bob

[Gyro]

Hi Bob,

The same thoughts had occurred to me, no it isn't burning hot, in fact the whole unit is pulling well within the u-blox LEA current spec. That's why I'm wondering what sort of 'look-alike' part it might be - surely they wouldn't have fitted an Avago one (actually the device marking doesn't match). The absolute max current for the bias pin on the Avago part is 10mA anyway so it would be long departed by now - maybe it is dead and just not getting in the way of the signal too much.

Sure, the only time I've seen LNAs is at the head end (LNBs etc) to compensate cable loss, the u-blox manual warns about the dire consequences (edit: in performance that is) of overload, so why go to the cost and hassle of putting one on the board? 

Now I'm wondering whether I can bridge out the parts well enough to look anything like a controlled impedance stripline still.

Chris

[uncle_bob]

Hi

I'd bet it's some sort of LNA block that just happens to pinout like the one you found. It's not real clear what happens with their part from their data sheet at 10 ohms. Smoke and crater in the board is a good guess. It might work ... who knows.

The part is small enough that you probably can take it off the board and put in a tiny wire jumper. It should be small enough to not matter much.

Bob

[Gyro]

Yes probably. 

I'll try at bypassing it tomorrow after a bit more searching, the trouble is that it will be a distinctly one-way journey - there's no way I'll get it back on!

Chris

[uncle_bob]

Yes probably. 

I'll try at bypassing it tomorrow after a bit more searching, the trouble is that it will be a distinctly one-way journey - there's no way I'll get it back on!

Chris

Hi

Jumpering around it is going to be futile. You will destroy it trying to do that. It's either "pull it off" or "don't do anything".

The rated noise figure on the LEA-6T is only 3.2 db. That's not really great. Something in the 1 db range is pretty typical for a front end spec on GPS these days. With a good sat view that would be the difference between a 52 db c/n and a 50 db c/n. The real issue is how it impacts acquisition.  Unless you run two devices side by side, you will never see the 2 db delta after things lock up. There are simply to many other variables.

One thing you *will* see is better jamming (crud) immunity. If you live in the real world (where everything seems to emit RFI) that is more likely to be a limiting factor than noise figure.

Bob

[Gyro]

Well that was an interesting day...

In order to try and get some sort of qualitative assessment of the modifications I ran multiple runs in each configuration using cold-restarts each time. Each time it was positioned on the same spot on the bench.

1) I started with the original (slightly strange looking circuit as I documented in reply #27). Average time to 3D lock from cold start approx 1':30", HDOP 2.5. Number of satellites (green) 6.

2) I then moved the LNA output supply inductor sideways so that it was feeding directly off the supply decoupling cap rather than via the 10R. Result 3d lock a little quicker at around 1':20", HDOP around 1.8, No of satellites 6-7. OK, improving.

3) Removed the 10R resistor feeding the LNA (subsequently found to be feeding pins 4 and 5). Big improvement. 3D lock in 45-55 secs, HDOP around 1.3 (lowest I saw was 0.9), No of sats 8+ (10 max).

4) Fingers crossed. Time to remove the LNA, bridged pins 3-6 on the footprint with fine wire. Also removed the output supply inductor (obviously). 3D lock in ~2 mins (sometimes need to hold it above my head to get it to lock). HDOP 2.0, No of sats around 6. Oops 

In original configuration (1) It would occasionally lose lock, especially when I was leaning over it on the bench. In configuration 3 it was pretty much impossible to make it lose lock, even cupping my hands over the antenna. In configuration 4 it loses lock fairly easily when lean over it or cover it.

Conclusion... One modification too far! The LNA was obviously doing something useful, I tried to reflow it back on, but even on my Leister Hotjet S's lowest flow rate it kept blowing away. On the second attempt it pinged out of my tweezers and into oblivion. 

The LNA's identity is still unknown, The Maxim MAX2659 has a /shutdown on pin 5, but it also has VCC on pin 4 and doesn't need the supply fed to the output via an inductor, so that doesn't tie up. There are others that have a shutdown/ bypass mode. I suspect that it's probably a Chinese part, badly integrated onto the board.

My best bet at this point seems to be to implement u-blox's LNA supply voltage circuit properly and try it with my active external antenna.

I hope this will be of some help to others tinkering with the RF performance.

[PA0PBZ]

Did you measure the voltage at pin 4 in the original configuration? I don't think there will be a significant drop over 10R and it would not make any difference to put the coil directly on the rail.
Also, satellites move  so you can't really compare the performance like you did.

Anyway, I feel your (lna) loss 

The LNA's identity is still unknown

This looks close:

[jpb]

I've not been following this thread very closely, but I thought this module based on open-source hardware may be of interest since you can get the schematics and layout of a working timing system:

http://shop.sysmocom.de/products/osmo-lea6t-gps

(There is a link on the page.)

[Gyro]

Thanks a lot guys, both for the links and the sympathy!

Yes, it was that google image that first put me onto the 61563, I couldn't get access to pin 5 under the package so I assumed that it was tied low. Being high rules that one out and opens up the ones with /shutdown-bypass on that pin. No there wasn't a significant drop across the 10R resistor, however in RF terms anything hanging off it is a long way from the supply decoupling cap. I still can't make sense of Pin 4 possibly being a supply when power is being supplied to the output pin via the inductor - actually a step I didn't document, removing the inductor caused no-lock, so power must have been passing to the LNA that way.

Yes, good point on the satellite positions, and I guess density with time, I was relying mainly on cold-reset lock time which was significantly faster in config 3 every time.

Thanks again,

Chris

[PA0PBZ]

I wonder if the choke from pin 6 to rail is a red herring, if you google " gps lna marking ly" you find a few links to aliexpress showing a MAX2659 with the text "Marking LY". I don't think the choke would hurt a MAX2659.

[Gyro]

Hey PA0PBZ, well spotted, my first confirmed link between the two!  Actually I'd never have found it from here (I did a lot of searching for those terms yesterday), a search through google UK doesn't bring up the relevant hits until you add the magic word aliexpress - curious.

The good thing is that RS components sell them at £5 for a pack of five. Surely I must be able to get one of them down successfully, even if dead bug style.

I think you're right about the choke (although it did seem to make a difference), the Maxim schematic shows an internal series capacitor as part of the output matching circuit - maybe it's different on Chinese clones.

Thanks very much. 

[uncle_bob]

Hi

The interesting thing about HDOP / PDOP / TDOP and all the other  DOP's:

On most receivers, this is purely a geometric calculation. If the sat's are at the following angles, plug it in and out comes a number. It does not mater what the signal to noise is as long as the sat is being tracked. Please do *not* ask how many hours of staring at data and how many receivers it took to confirm this.

Time to first fix (cold start) is also a bit wonky. The GPS makes guesses about where it will find a sat. The more modern modules have a lot more horsepower (correlators). They make way more guesses so the likely first fix is at the first integrator dump. Signal to noise *does* degrade the time to first fix. Unfortunately, having a set of sats in a "bad geometry" also does the same thing.

Yes, it's painful. The only real answers are a common antenna and a group of devices *or* a fairly expensive signal generator. (which breaks and costs $10K or so to fix ...).

Bob

[Vgkid]

This is like my module.

[Gyro]

Another update for those who can still stand it.

Firstly an apology for the rather downbeat post yesterday. It was rather depressing (not) to see the then unidentified 1.5x1mm LNA disappear over my shoulder into the gloom together with apparent performance.

@Bob: Many thanks for the additional background, I am not a GPS expert I'm afraid (in fact my RF experience is limited to HF band military transceivers, consumer satellite/terrestrial STB design and far too many EMC tests over the years!). Agreed my measurements were not at all definitive, although over multiple restarts in each configuration there was definitely a 'feel' of better/worse. Not being in the position to do side by side comparison or  any relevant GPS test gear it will have to stand as the best I can do.

@Vgkid: Thanks, it hadn't even occurred to me to check out the u-blox eval kit schematics. They seem to have had several 'goes' at each one, from basic direct connector to LNA + SAW. I note that u-blox say that performance improves with an external LNA, but their eval design always pairs it with a SAW filter, which makes sense because additional gain would always be needed to overcome the filter loss.

Today's progress then... As I would need to order some MAX2659s, this was the best opportunity to try the external active antenna (the LNA would need to be removed or isolated by track cut anyway to allow clean access to the LEA-6T's RF input).

The first step was to properly implement the LEA-6T's LNA supply by adding the 10R overcurrent sense resistor between the VCC_RF and V_ANT. I fitted the resistor and then found a short between the two pins, after removing the resistor I found that the pins were shorted together by an inaccessible pcb track (these guys really can't follow a datasheet!). With some solder braid I was able to clean and then remove one of the PCB pads (sigh) and refit the resistor directly to the LEA module. On trying to enable RF power using u-center (actually the default setting)  I found no voltage at the RF input pin, 3V3 was present at the resistor and no drop across it. Ho hum, I'm not sure it this was due to thermal damage from the above or because the original configuration had 3V3 applied directly to the RF input via the choke on the LNA output. Not the end of the world, I could do the same again.

I've fitted the RF coax from my cheap ebay active magnetic patch antenna to the board (I'd previously inserted an in-line RF bulkhead connector in this for mounting in a box (with the Trimble Resolution-T) so it can be unplugged as needed.  To supply power in place of the LEA's faulty internal supply I've bridged a low vaue RF inductor from the adjacent VCC decoupling cap to the RF input (see photo).



The result is WAY better than the on-board antenna in any of yesterday's configurations! Using the same unscientific (sorry Bob) metrics on a series of 10 Cold restarts over several hours I found: 3D lock in <50 secs (often under 40), HDOP 0.8-1.1, Number of active satellites 8-11 (after 15 mins). Unscientific but clearly much better! The testing was done with the Antenna mounted on a small steel plate, on top of the shelves, just below the ceiling but beneath a rather a lot of storage in the loft. Its also been wet today so the roof tiles were wet. Cable length is rather long at 3-4m.

This is the ideal configuration for me (and I guess what I would have hoped for all along). It allows me to encase the module and have the clock signals accessible (rather than having to buffer them from a 'head unit'), better for USB integrity too. For the additional cost of the active antenna it seems a good solution.

I've attached a photo of what is probably the final configuration. I've removed the patch antenna from the 'back' of the board so that I could drill holes to add strain relief for the antenna coax.

Chris.

[uncle_bob]

Hi

The uBlox modules do not have an internal antenna supply. They simply route the voltage on the "antenna DC in" pin to the antenna pin via a choke. If you put +5 on the pin, you get +5 on the antenna. Same goes for +3.3V on the pin.

Bob

[Gyro]

Thanks Bob, but according to the Hardware Integration Manual the module puts out an LC filtered VCC on VCC_RF (pin18). Linking this to V_ANT (pin 19) via a 10R resistor is supposed to provide overcurrent protection/detection. Yes, for a +5V Antenna you would feed the 5V directly into V_ANT. I was definitely seeing 3V3 on V_ANT, it just wasn't making it through the internal switch to the RF_IN pin when commanded.

I've attached the block diagram.

Chris

[uncle_bob]

Thanks Bob, but according to the Hardware Integration Manual the module puts out an LC filtered VCC on VCC_RF (pin18). Linking this to V_ANT (pin 19) via a 10R resistor is supposed to provide overcurrent protection/detection. Yes, for a +5V Antenna you would feed the 5V directly into V_ANT. I was definitely seeing 3V3 on V_ANT, it just wasn't making it through the internal switch to the RF_IN pin when commanded.

I've attached the block diagram.

Chris

Hi

In about 99% of the places you see the module used, Vcc RF goes to nothing and there is a seperate supply going to Vcc Ant. The 10 ohm resistor is pretty much useless as over current protection.

Some math:

P = E^2/ R

If E = 5 and R = 10 you need a resistor that will handle 2.5W

I = E/R

Your short circuit current will be 1/2 A. That pretty much takes out the choke they use to put the DC on the antenna. It *may* take out the ferrite bead that does the Vcc RF.

The next layer to the problem is that there is no ESD protection (or lightning protection) on the antenna input. With a 10 ohm resistor, you may / may not get a "spike" back into your main system.

It is ...errr ... possible ... that ... somebody... might ship a few of these into the field before working that all out and fixing the others.

Bob

[Gyro]

Ah, I didn't say it was a GOOD system, just what is in there  It should have at least given me volts on the Antenna feed if there wasn't an internal fault.

The VCC_RF looks like a bit of a waste of a pin to be honest, but I suppose it might save a couple of external passives in very space constrained designs (mind you, those would probably be using the NEO anyway).

Agreed on the resistor dissipation, a little better at 3V3 but not by nearly enough. I think the intention is probably that something notices (don't know if the internal  f/w has this capability) and turns it off before cooking becomes a problem, not exactly failsafe though. I note that the datasheet max V_ANT current is 100mA.

My current solution seems fine anyway, I have no need to switch the antenna supply and the external inductor is just taking the place of the internal one. The 3V3 regulator current limit and thermal shutdown should provide a worst case blunt short protection.

Chris

[uncle_bob]

Ah, I didn't say it was a GOOD system, just what is in there  It should have at least given me volts on the Antenna feed if there wasn't an internal fault.

The VCC_RF looks like a bit of a waste of a pin to be honest, but I suppose it might save a couple of external passives in very space constrained designs (mind you, those would probably be using the NEO anyway).

Agreed on the resistor dissipation, a little better at 3V3 but not by nearly enough. I think the intention is probably that something notices (don't know if the internal  f/w has this capability) and turns it off before cooking becomes a problem, not exactly failsafe though. I note that the datasheet max V_ANT current is 100mA.

My current solution seems fine anyway, I have no need to switch the antenna supply and the external inductor is just taking the place of the internal one. The 3V3 regulator current limit and thermal shutdown should provide a worst case blunt short protection.

Chris

Hi

Coming up with an inductor that is really good at 1.5 GHz is not as easy as one might think .... Then you have to get it onto a trace that does not mess up the RF matching.

Bob

[Vgkid]

Has anyone delved into the manual to determine how to interface with the chip(using usb) while the chip is already powered up. I have just plugged a usb cord into the module, and I reported usb power errors.

[DimitriP]

It seems to me the device is either powered from the USB port, or not.
There is some hint of an explantion in section 1.6.2.1 in the Hardware Integration Manual  and it sounds like an either/or but not both kind of situation.
As for having it powered on and then connecting it to a USB port..I guess it depends on how the board you have is wired up.
What's the  error ?

[Gyro]

The Hardware Integration Manual section 2.1.2 gives a further hint:

2.1.2 Design considerations
For a minimal design with a u-blox 6 GPS module the following functions and pins need to be considered:
•  Connect the Power supply to VCC.
•  VDDUSB: Connect the USB power supply to a LDO before feeding it to VDDUSB and VCC. Or connect to
GND if USB is not used.

It looks to me as if in Self-powered applications, VCC is powered and VCCUSB becomes a 'USB present' indication (though still needs to be driven by an LDO).

There probably needs to be a diode-OR from the USB supply to the input of the main VCC LDO so that both VCCs still get powered if the USB is plugged in while the unit is turned off - I assume it is illegal to Power up VCCUSB while VCC is powered off.

Chris

[Gyro]

Hi

Coming up with an inductor that is really good at 1.5 GHz is not as easy as one might think .... Then you have to get it onto a trace that does not mess up the RF matching.

Bob

I'm sure it isn't. As all my smd inductors are either power or FB I salvaged this one from an old mobile phone PCB. It has 10 turns 1:1 spaced on a 1x0.5mm plastic / ceramic former - I assumed that anything with a permeable core would be bad news at 1.5GHz. I've no idea of the value even** - too low for me to measure, the only indication is a yellow dot. There were lower values (red or brown dot) but the only higher value (blue dot) was close-wound so I assumed less suitable.

I'm definitely working on a 'good enough' basis here*. There will inevitably be impedance mismatch caused by the soldered coax termination in addition to any inductor induced effect. Who knows, they might even partially cancel (but almost certainly not!).

Edit: *And beyond my ability to accurately measure.
** Approx 30nH?

[uncle_bob]

Hi

Coming up with an inductor that is really good at 1.5 GHz is not as easy as one might think .... Then you have to get it onto a trace that does not mess up the RF matching.

Bob

I'm sure it isn't. As all my smd inductors are either power or FB I salvaged this one from an old mobile phone PCB. It has 10 turns 1:1 spaced on a 1x0.5mm plastic former - I assumed that anything with a permeable core would be bad news at 1.5GHz. I've no idea of the value even** - too low for me to measure, the only indication is a yellow dot. There were lower values (red or brown dot) but the only higher value (blue dot) was close-wound so I assumed less suitable.

I'm definitely working on a 'good enough' basis here*. There will inevitably be impedance mismatch caused by the soldered coax termination in addition to any inductor induced effect. Who knows, they might even partially cancel (but almost certainly not!).

Edit: *And beyond my ability to accurately measure.
** Approx 30nH?

Hi

The nutty part when designing the front end for one of these modules is finding a choke with a SRF that is high enough. Again, that seems easy but ... not so much. The "easy way" is to design a bandpass filter instead. You will not have a narrow enough passband to do anything useful selectivity wise. It makes the parts easier to select (lower values so higher SRF). The other thing it lets you do is protect the input to the device from lightning a bit better. A "real world" lightning pulse has an energy distribution that is heavier as you go down to VHF. Anything you can do to reflect that energy is a good idea.

Bob

[Gyro]

Thanks Bob,

From a bit of searching It looks as if they're Coilcraft 0805 ceramic core, there are four families of these but taking worst case - next higher yellow dot value above 30nH (they repeat) and worst family the minimum SRF would be 1.8GHz but probably higher. I might drop the the next lower inductance value available on my donor board in that case.

Chris

[Gyro]

Has anyone delved into the manual to determine how to interface with the chip(using usb) while the chip is already powered up. I have just plugged a usb cord into the module, and I reported usb power errors.

Hi,

After working through the data a few times I've got a scheme that works well for me. The Hardware integration manual advice is a bit strange - it recommends a second LDO to derive VDDUSB (3V3) from the USB Vbus (5V). This seems a little strange because it also points out that in self-powered mode the USBVDD input current is only approx 1mA. It also recommends that the LDO has an enable pin so that it can be disabled when the LEA-6Ts VCC is powered down.

This all seems over the top, the 5V USB Vbus is high enough to be able to drive an NPN transistor to switch VDDUSB directly from VCC. Here's the schematic:



When the USB is disconnected VDDUSB is at 0V, when plugged in VDDUSB gets switched to VCC - transistor Vce sat (approx 3.2V), and the LEA is recognized. If USB is plugged in with the unit turned off, VCC is at 0V so the only voltage into VDDUSB is the current through the transistor base resistor and transistor Vbe junction divided by the pulldown resistor on the VDDUSB pin (few hundred mV). When power is restored VDDUSB goes high again the unit is recognized by by the PC.

I've tried lots of power cycling and USB plugging and the unit is recognized every time by u-center without having to restart anything so it it seems to be working fine.

I've applied the mod to my ebay LEA-6T module. To do this I've changed the function of connector Pin 1 from RxD1 to USB Vbus. This involved cutting the track between Pin 1 and pin 4 of the module. I also cut the track connecting the module's VDDUSB pin and associated decoupling cap to VCC and added the switching transistor and resistors.



I can't see that I'll need the missing RxD. NEMA data is automatically output on TxD1 for use with a micro / display and any module config changes can be done over USB.

I hope this helps.

Chris.

P.S. @Bob: I've been reading the LEA-M8T datasheet and HIM. They've changed to using an external antenna supply choke in most of their sample circuits. The choke they specify is a Murata 27nH with a SRF of 1.7GHz and impedance at 1.5GHz of 500R. It looks as if I was lucky with my salvaged Colcraft part to hit it pretty much on the nose. It doesn't change the termination characteristics of the soldered coax of course - that 'is what it is'.

[uncle_bob]

....
 This seems a little strange because it also points out that in self-powered mode the USBVDD input current is only approx 1mA.

....

Hi

The *maximum* current limit in "power down mode" is 500 ua. It's best to stay below about half this limit to get around aggressive USB disconnect schemes.

Bob

[Gyro]

Hi Bob,

Do you have a reference for that?

The datasheet is light on Absolute Maximum Ratings, but it does specify that the limits on VDDUSB are -0.5 to +3.6V (rather than -0.5 to VCC). The HIM gives the reason for disabling the USBVDD regulator as:

If the USB device is self-powered it is possible that the power supply (VCC) is shut down and the Baseband-IC core is not powered. Since VBUS is still available, it still would be signaled to the USB host that the device is present and ready to communicate. This is not desired and thus the LDO (U1) should be disabled using the enable signal (EN) of the VCC-LDO or the output of a voltage supervisor.

This indicates that VDDUSB could actually be powered all the way up without VCC powered, the host just wouldn't be aware of the situation. It indicate "not desired" rather than 'not permitted'.

With the circuit as shown the current into VDDUSB is approx 300uA when USB is connected with VCC off. the pin sits at ~600mV so isn't stressing any protection diodes if present.

Chris

[uncle_bob]

Hi Bob,

Do you have a reference for that?

The datasheet is light on Absolute Maximum Ratings, but it does specify that the limits on VDDUSB are -0.5 to +3.6V (rather than -0.5 to VCC). The HIM gives the reason for disabling the USBVDD regulator as:

If the USB device is self-powered it is possible that the power supply (VCC) is shut down and the Baseband-IC core is not powered. Since VBUS is still available, it still would be signaled to the USB host that the device is present and ready to communicate. This is not desired and thus the LDO (U1) should be disabled using the enable signal (EN) of the VCC-LDO or the output of a voltage supervisor.

This indicates that VDDUSB could actually be powered all the way up without VCC powered, the host just wouldn't be aware of the situation. It indicate "not desired" rather than 'not permitted'.

With the circuit as shown the current into VDDUSB is approx 300uA when USB is connected with VCC off. the pin sits at ~600mV so isn't stressing any protection diodes if present.

Chris

Hi

Head off to USB.org and they have far more tech details on all the nonsense associated with the various versions of the USB bus than you would ever want to wade through. On top of that you have the way various chip sets actually do this or that and the degree to which a given manufacturer decides to enforce the rules. You mostly see the power down nonsense in portable devices. There is a major fight between "I have a very small battery ... go away !!" and "this thing does not work .. I want a refund".

Mr Google comes up with:

http://www.ti.com/lit/an/slyt118/slyt118.pdf (shows the 500 ua limit)
http://www.usb.org/developers/docs/usb20_docs/ (spend a few years sorting through all that ...)

Most of what you find looks at maximums rather than the other end of things. The limit (generally with no numbers) gets mentioned down in the guts of a lot of USB IC's. FTDI is one example of this.

Bob

[Gyro]

Ah, ok, got you. I thought you were talking about a power down current limit on the LEA-6T pin, not the USB spec!

Yes, sure, you need to be careful not to exceed the USB quiescent current spec. The circuit should be ok for that... 10k at 3.4V approx after Vbe and input drops. If necessary the base resistor and pulldown resistor values can be increased a bit. 

EDIT: Just to be clear, the 1mA I was talking about is the active VDDUSB supply being pulled from VCC via the turned-on transistor, Not USB Vbus (transistor base) current.

[edavid]

I noticed that these modules are available at Goodluckbuy for a little less than on eBay:

http://www.goodluckbuy.com/ublox-neo-6m-flight-controller-gps-module-pixhawk-interface-with-compass.html

[Gyro]

Not sure if there's a useful error there - the link says neo-6m but the description says LEA-6T. They do show a NEO-6M (non-timing) version for $5 more in another listing.

[uncle_bob]

Not sure if there's a useful error there - the link says neo-6m but the description says LEA-6T. They do show a NEO-6M (non-timing) version for $5 more in another listing.

Hi

There are multiple versions of the "white blob" out there. The early ones had LEA-6T's in them. The later ones have who knows what. The best guess is that they make them out of whatever surplus parts are running around when it's time to do another run. The info in the seller's ads does not track the internals very well. For the intended purpose (non timing) drone navigation, they all work equally well.

Bob

[Vgkid]

On a related topic, anyone familiar with the v.kel / beitian units?

[uncle_bob]

On a related topic, anyone familiar with the v.kel / beitian units?

Hi

I've seen them, but never bought one. The specs are quite close to the ones that advertise uBlox modules. They seem to put their own sticker on the module. My guess has always been that it gives them more freedom in sourcing the modules. If they have real uBlox this week and "clones" next week .... no need to re-shoot the pictures.

Bob

[Gyro]

On a related topic, anyone familiar with the v.kel / beitian units?

I hadn't heard of those - presumably enough of a clone to still work with ucenter s/w? I guess it depends what you want to use them for - no indication of them being suitable for timing application (presumably not) and less board space to get at pulse outputs, if included.

[uncle_bob]

Hi

I think the key point is that there is indeed a very real market for knockoff GPS modules. There are a handful of chips in a module and a bunch of IP. A good chunk of the money you pay goes for the IP. If the label on the part does not say "uBlox" the infringement police have less reason to chase you down.

One thing you *are* getting with the real part - very good assembly technique on the module. That may not be the case with a knock off ...

Note - I have absolutely no idea if the part in the picture *is* a perfectly legit uBlox with a sticker over the label. It may well be. It could also be a legit product from another manufacturer that uses a similar chip set but different IP. With no documentation there is no way to know. It's a "buy one and tear it down" sort of thing, even if you want to know the basics of how to use it.

I would pretty much guarantee that the module will work with any of the multitude of NMEA based programs. It's a rare module these days that will not. Also it would not work in the advertised application if it did not.

Bob

[Gyro]

Those don't look like re-labeled* ublox modules to me... they don't have the corners of the can soldered down like that and you can't see the termination pads, as you can on the ublox. They have put the screening can over something directly on the board.

Edit: * or should I say, cloned

[uncle_bob]

Those don't look like re-labeled* ublox modules to me... they don't have the corners of the can soldered down like that and you can't see the termination pads, as you can on the ublox. They have put the screening can over something directly on the board.

Edit: * or should I say, cloned

Hi

With all of these gizmos, the one in the picture may have no resemblance to the one you get. At this price point, about all you can reasonably expect is that they function. If you want a set of units that are all the same, your best bet is to buy them all on the same day at the same place.

This is not limited to things like GPS modules. I have an embedded processor board that is very picky about flash memory. I have to buy one, try it and then buy a batch each time we do a run of them. In some cases, the memory I get four days later is not at all the same as what I got as a sample. It's the same label, same manufacturer, same part number. The guts aren't the same ...

Lots of fun.

Bob

[Gyro]

For the moment though, there still seems to be a good chance of picking up a genuine LEA-6T from the ebay listings if you want one for a fixed location timing use. I doubt if the window will last long as the 6T isn't best suited for navigational use, somebody must have found a batch of them.

It was the same story with the Trimble Resolution T a year or so back, and doubtless will be with the Symetricon GPSDOs.

[Robaroni]

Well, I got the LEA 6T, pulled it apart and ran a couple of jumpers for power and the one second pulse. It's just sitting on the bench run into my Milliron 260 OCXO driven 53132A counter. The Milliren was recently disciplined from my rubidium clock so all should be pretty accurate.
I'm getting 0.999,999,999.40 out of the counter - ten minute gate time. It runs just about right on the .40, maybe an occasional .41 or 42 so it'a good enough for most of my work now and its main purpose was to check to see if my rubidium was in the ball park without building PPL or uC translators. Fast and simple, just what I wanted!

So thanks to everyone who directed me to the LEA 6T. I'll mount it permanently but haven't decided whether to run it inside or outside yet.

Rob

[bitseeker]

Rob, it sounds like your use case is similar to mine. I've been reading various threads about GPSDO, but all I need is a simple reference to occasionally adjust my OCXO-based counter to. It sounds like the LEA-6T module is fine for this purpose.

My OCXO is manually adjusted, so I'd just hook up its 10 MHz clock output and the LEA-6T (set to 10 MHz, or is 1 or 2 MHz more stable?) to my scope and minimize the drift between them over a period of minutes or an hour if I'm feeling ambitious. The 0.6 Hz difference at 10 MHz isn't an issue for my use.

[Gyro]

Rob, thanks for reporting in. It's good to have a confirmation that the module is as stable (and useful) as it seems.

bitseeker, you will see significant jitter at 10MHz compared to 1 or 2MHz because it is a non integer divide of the LEA's 48MHz internal TCXO. Having said that, I found no difficulty in visually comparing it against my 10MHz OCXO output and adjusting for zero drift.

[Koen]

On a related topic, anyone familiar with the v.kel / beitian units?

Hi

I've seen them, but never bought one. The specs are quite close to the ones that advertise uBlox modules. They seem to put their own sticker on the module. [...]

The main chip is uBlox, the format and pinout are identical to uBlox, the board routing and components are different.

[bitseeker]

Thanks for the confirmation on 1 & 2 MHz being the stable choices, Gyro. I recalled that being the case, but after reading so many threads, wasn't sure where I had seen it.

10 MHz and 2 MHz should be easy enough to tune on the scope as it's only 5:1 ratio.

[Robaroni]

Guys,
I'm the one who should be thanking! It was you guys on EEVblog that pointed me to this GPS and I can't believe how well it brings in satellites just sitting on my bench!

Bitseeker,
I think we are basically doing the same thing and I encourage anyone who needs to check their time base to look into this simple solution.

Rob

[DrTune]

Ha I just scored three of those LEA-6T modules off ebay ($30 a pop - bargain!), because I'm interested in a very accurate GPS clock for (ironically) playing with radio multilateration as a fun pet project..

I also have some of those V.KEL modules in the mail ($8 on Aliexpress!) which apparently contain a ublox 8 - UBX-M8030-KT chip, ~30ns timing, but only up to 1khz time output.  We shall see how they do.

I was interested in these LEA-6T modules b/c it has dual time outputs allegedly configurable up to 10Mhz (but in practice the u-center software will let you go up to 24mhz, which works fine!)

The appnote on LEA-6T timing is a good read https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_%28GPS.G6-X-11007%29.pdf

They mention the internal osc runs at 48mhz so you pick an integer factor of this to get a relatively jitter-free output (e.g.6mhz, 8mhz, 12 etc)

Because I have several of these units (and no proper time source other than that) I figured I'd play one off against the other - I hooked two of them up, set them to 3Mhz output (48mhz/16) and put them on two scope inputs. As I write this now they've both oddly decided they can't get a satellite lock but earlier today they did lock fine, and there was roughly 30ns constant jitter between the two clocks, and no drift at all.  The phase jumped around a bit as the units improved their satellite lock, the best I saw was in the region of 20-40ns offset between the edges, the worst was a few hundred ns. (this was measured with cables of exactly the same length from each unit to the scope).

The ~30ns constant jitter figures because their internal 48Mhz clock has a granularity of 20.8ns, so one naturally expects random jitter of up to 41.6ns when measuring between two of them. Eyeballing it, it looked slightly little better than that, but close enough.

I thought that was pretty good - these two GPSes were right next to each other - if there were e.g. 20miles apart they might have more variation in their outputs, but for $30 a pop it doesn't suck at all.  Next stop is to use it as a ref clock for a PLL, I see there's a cheap board on ebay with a ADF4351 that can run off a 10mhz reference and generates 35Mhz - 4Ghz (but only a sine output >=2Ghz else it's a square, meh).  Obviously I could get a real GPSDO or whatever but I'm just playing around here.

I'll shoot a bit of video of the two GPS outputs next to each other on a scope for you.. once I figure out why now they suddenly won't get a sat fix (they're in same position near a window that they were earlier.. weird).. when they don't have a fix, the two outputs drift from each other quite rapidly as you'd expect; right now I'm seeing about 200ns/sec of continuous drift between the two time pulse edges.

EDIT: I'm pretty sure the reason they couldn't get a fix just now is because I had them both generating a 6Mhz clock temporarily driving long, unshielded wires... As soon as I turned the timepulse down to 10hz they suddenly both started seeing a ton of satellites and got a nice fix. The rise/fall time on the clock is ~2v in 10ns so at 6Mhz that'd be quite enough mush radiated to screw them up (esp as they only have a moderate view out of a side window). Note - use shielded cable with fast clocks to avoid screwing up the incredibly weak GPS signals!

Anyway, pretty sweet modules for $30.  Planning to use a 2mhz output from the LEA-6T into the OSC_IN input of an STM32F103, which can PLL it up to 26mhz and output it again to run a CC1101 radio, the second internal PLL can clock the CPU core at 78Mhz, and if I'm not mistaken the whole thing should then be nicely synchronous together (gps+radio+cpu) meaning I can hopefully transmit radio packets with the only jitter being the ~20ns from the GPS module itself.

Coming up next, some $8 VK2828U7G5LF GPS modules (which according to the spec at https://github.com/CainZ/V.KEL-GPS/blob/master/VK2828U7G5LF%20Data%20Sheet%2020150902.pdf look pretty well suited as a regular GPS unit for a different project I'm pondering)...

[bitseeker]

Those results sound great, DrTune. More than good enough for my needs. Looking forward to your vids.

[DrTune]

Vid!  Two LEA-6T's head to head. Looks a bit rough but IMO it's a pretty decent result. The jitter is pretty much down to the 48Mhz clock granularity.

 


10Minute persistance view (after GPS units had been on overnight)

[Gyro]

At last, someone with the wealth to buy 2.

Good comparison data, thanks. 

BTW I'm in the process of developing a very basic GPSDO based on these modules, in case you haven't seen it...

https://www.eevblog.com/forum/projects/my-u-blox-lea-6t-based-gpsdo-%28very-scruffy-initial-breadboard-stage%29/

[Robaroni]

Gyro,
Love to see what you come up with. I've been watching this on the bench for the last few days and it is rock solid to my rubidium.
Rob

[splin]

You said that the unshielded cables were screwing up GPS reception (thus  explaining the > 30ns rms and 99% <60ns specifications for timing error between the two) but the Ublox software seemed to show at least one was seeing quite a number of satellites and thus working ok. Did you see that the second wasn't locked? Does the LEA-6T have an output that shows when the timing output is locked and valid?

At the end of the video when you have the timebase set to 10ns/div the 2nd trace shifts by approx 10ns at a time. How can it do that when its clocked at 48MHz?

[jpb]

Does the LEA-6T have an output that shows when the timing output is locked and valid?

According to the Ublox white paper on timing you can set the time signal amplitude to be zero when it is not locked so the signal is only present when it is locked. (It is shown in one of the figures but I don't have it to hand to be more precise.)

I have a couple of these Ublox 6T modules (the cheap e-bay ones) and also a couple of older GPS timing modules with 10kHz output and at some point I'd like to compare them. Unfortunately at the moment I keep moving between rented places whilst I try to sell my house after a job move so I no longer have a GPS antenna setup.

The timing module has a correction mode to correct for the 48MHz jitter (quantization). In the white paper they show results with rms on the timing pulse of only 6.6 nsecs uncorrected and only 3 nanosecs corrected.

A quick back of an envelope calculation showed this was consistent. The spec in the datasheets is much wider (30 nanosecs and 15 nanosecs) to allow for low to the horizon satellites and atmospheric effects (according to the white paper). You can send a command to tell it to ignore low angled satellites.

James Miller (of the simple GPSDO fame) told me he has his GPS antenna in a chimney pot (mainly to avoid spoiling the looks of his house I guess) but such an arrangement where it is forced to look up more might work very well for timing.

[DrTune]

You said that the unshielded cables were screwing up GPS reception (thus  explaining the > 30ns rms and 99% <60ns specifications for timing error between the two) but the Ublox software seemed to show at least one was seeing quite a number of satellites and thus working ok. Did you see that the second wasn't locked? Does the LEA-6T have an output that shows when the timing output is locked and valid?

Yeah I wasn't being super scientific with the unshielded cable comment - the ublox s/w was showing quite a lot of sats but it was also saying "no fix" in red (instead of 2d or 3d fix in green) and kinda alternating between no fix/fix. There's a sat signal strength waterfall and that wasn't looking very good when I started shooting the video.   I could run the test again OR I could simply not be a dumbo and use shielded cable. When the unit is trying to receive -160dbm or whatever GPS is, having two long bits of wire radiating squares wave edges 12 million times a second probably doesn't help.

Yes I'm pretty sure it does have a 'locked' output you'd have to check the datasheet but I think so; obviously it does tell you over the serial/usb output - one really nice feature is that the clock period/duty cycle can be set to two different values for "locked" and "not locked" so for example you could have a 20% duty for locked and 50% for unlocked and it'd still be a useful signal, I thought that was a neat idea.   It's not at all hard to solder extra wires onto the Ublox module (see previous pics in this thread by other folks).

At the end of the video when you have the timebase set to 10ns/div the 2nd trace shifts by approx 10ns at a time. How can it do that when its clocked at 48MHz?

I think that's just an illusion; obviously at 1hz the update is pretty slow so you don't have many samples to look at; if you look at the other faster clock stuff earlier in the vid you can see the jitter is evenly distributed (as it should be, there's no physical connection between the two clocks, the position of the jitter just depends on the phase of the two TCXO's in the GPS modules).  If I put the clock on say 10 or 100hz it looks as random as you'd expect it to.  It'd actually be quite a technical feat if somehow there was granularity in the jitter!

[Gyro]

Yes, you can configure the outputs to generate a constant low, constant high, or free run clock of any frequency before lock. After lock you can, if you want, output a constant high or low too instead of a clock as a permanent lock signal... or any duty cycle or pulse interval between these states. It's pretty flexible.

I'm running mine with the timing app note recommended settings: SBAS turned off and reject low angle satellites (it's 5 degrees by default). At 45 degrees I could drop down to two or even 1 satellite briefly at my location and indoor antenna. 30 degrees seems like the best compromise for me at the moment (at least 3 satellites used at all times). Of course all satellites are still tracked, it just selects the ones being used to calculate the time fix.

[DrTune]

The timing module has a correction mode to correct for the 48MHz jitter (quantization). In the white paper they show results with rms on the timing pulse of only 6.6 nsecs uncorrected and only 3 nanosecs corrected.

Mmm I thought that was interesting, it looks like you basically switch it to a low frequency output (e.g 1hz) and then "after the fact" of each 1hz tick you read out over UART/whatever an error value in nanoseconds.   Personally I'd rather they just had e.g. a 200mhz clock inside the GPS but hey, I'm nitpicking :-)

[Vgkid]

I have my small taoglas gps antenna in my window, i have no problems there. With the 6T , it stays locked to atleast 8 sats at all times.

[jpb]

I have my small taoglas gps antenna in my window, i have no problems there. With the 6T , it stays locked to atleast 8 sats at all times.

For timing using just one satellite is better according to Ublox. I guess though the problem is when the unit needs to switch between satellites.
Multiple satellites is better for navigation of course.

[Gyro]

I guess though the problem is when the unit needs to switch between satellites.

Yes, that's the one that's puzzling me. I'm working on the basis that at least 3 high angle satellites might be the best compromise for switching effects but I don't know if that's the case.

P.S. I haven't found a setting in ucenter to specify the number of satellites rather than the angle, that would probably be more useful.

[DrTune]

Setting # of satellites to use:

It appears from looking at the protocol spec that you can limit the number of satellites (a.k.a. "SV" = Space Vehicles) using the NAVX5 setting;
From page 196 of the u-blox6_ReceiverDescrProtSpec document:

"The minimun number of SV default setting is set to 1 in a firmware with the timing premium feature enabled (LEA-6T)"
which totally figures, the default minimum for non-timing GPS modules is 3.
There's also "Maximum number of SV" = 16, the GUI will let you set this as low as 2.

Furthermore on the "NAV5" setting there is "Min SV elevation" (default 5 deg) described as "Minimum elevation of a satellite above the horizon in order to be used in the navigation solution. Low elevation satellites may provide degraded accuracy, due to the long signal path through the atmosphere."

Not tested either but that looks promising if you want to limit angles/and #SV's.

I like "SV", it sounds kinda techy and also suitably open-ended for future enhancements, i.e. if in future instead of satellites we launch, say, Donald Trump into a 12,550 mile high orbit.

[Gyro]

Well there were some of the old Routemasters that had clocked up close to a million miles. 

I think I tried the NAVX5 settings page without much success. It's a bit confusing because the min #SVs setting also looks like a min C/N. The Min SV elevation setting certainly works.

[Robaroni]

I'm setting mine for min angle.
Rob

[edpalmer42]

Normally, you should set your minimum elevation according to your local conditions i.e. what angle do you need to clear nearby obstructions like trees and buildings.  This lets the receiver reject signals from satellites that will be distorted by travelling through trees or bounced off buildings.

Artificially restricting the number of satellites you use isn't usually a good idea.  Yes, you may see a glitch when a satellite joins or leaves the constellation, but you minimize that by having a good location fix and you will probably get your best fix with lots of satellites.  The timing receivers have extra smarts to select the best satellites at all times.  If you restrict the number of satellites used, it could impact the performance of those features.  I've never tried something like that and I suspect that each receiver would react differently.  It could cause 'churn' where the satellites used change frequently to always choose the best of the limited number you allow.  If not, you could find yourself using a few bad satellites as long as they are in sight.  Tread carefully!

Ed

[jpb]

The Ublox white paper says

If the position is known, the receiver can provide an accurate time solution by tracking only one satellite.

This is very ambiguous. I took it to mean that you got better results from one satellite, but it equally could be a statement of the fact that you can still get time from one just satellite but that it is still better to look at more satellites (excluding low angle ones).

But the next paragraph (in the white paper) refers to the UBX-CFG-NAVX5 message for reducing the number of satellites. If the hardware is clever enough to select best satellites why allow the user to reduce the number? All very confusing.

I guess the best way to resolve it is to experiment - I wish I had more time!

[uncle_bob]

The Ublox white paper says

If the position is known, the receiver can provide an accurate time solution by tracking only one satellite.

This is very ambiguous. I took it to mean that you got better results from one satellite, but it equally could be a statement of the fact that you can still get time from one just satellite but that it is still better to look at more satellites (excluding low angle ones).

But the next paragraph (in the white paper) refers to the UBX-CFG-NAVX5 message for reducing the number of satellites. If the hardware is clever enough to select best satellites why allow the user to reduce the number? All very confusing.

I guess the best way to resolve it is to experiment - I wish I had more time!

Hi

A *normal* GPS will stop giving you time information when it drops below 4 locked on sats. Some units have a feature that helps in this situation. That is what uBlox is talking about when they refer to 1 sat timing. As long as you are doing autonomous timing, the more sats you can see the better. The errors will sort of go down by the square root of the number of sats in view (it's a lot more complex than that...).

The feature to mask out satellites is there so you can take out a "known bad" SV. At various points over the last few decades this might have been useful.

The stuff about elevation mask is there to reduce multi path. For timing (as opposed to survey) you don't need the widely dispersed constellation. You can also reduce ionosphere issues this way.

Most of the above *assumes* you have a very well surveyed location. The fast survey done by the module is pretty crude. If you are really going to use this as a reference, you should do some longer term data collection.

Bob

[DrTune]

A touch off topic but I got my $8 Aliexpress GPS modules today - V.KEL VK2828U7G5LF
Datasheet:
https://github.com/CainZ/V.KEL-GPS/raw/master/VK2828U7G5LF%20Data%20Sheet%2020150902.pdf

Aliexpress e.g.
http://www.aliexpress.com/item/Smart-Electronics-VK2828U7G5LF-GPS-Module-with-Antenna-TTL-1-10Hz-with-FLASH-Flight-Control-Model-Aircraft/32634474566.htm
also on Banggood.com

Rather than turning it on, I took it apart (well, peeled off the can)...
So it's legit - but only kinda - the datasheet claims "Chip UBX-M8030-KT"
But my microscope says otherwise... I see "G7020-KT" on the board.

Other than that I'm a happy camper for $8.50 or so; has onboard flash, battery backup, remote power-off, etc.

The SPI flash chip on there (for autonomous data logging! sweet!) is this 512kbyte part...
http://www.elm-tech.com/en/products/spi-flash-memory/gd25q41/gd25q41.pdf

Also rather handily this chip (like the LEA-6T) has PPS output that goes up to 10mhz...  (if it's like the 6T it actually will go up to 24Mhz)
[edit] - ooh! actually it has TWO timepulse outputs (like the 6-T) which is super-handy. Getting to the second one (4th pin from the right on the top edge in my pic below) will require a steady hand with the iron :-)

When I have a sec I'll head-to-head it against the LEA-6T (which is $30 vs this being $8)...  it's a GPS BATTLE! :-)

[Koen]

The product is correct, the datasheet is wrong. U7 are G7020, U8 are M8030.

Quick note : the M8030 offering is limited but both chips are drop-in-compatible so you can order an U7 module with an M8030 chip.

Anyway VKel modules are a great alternative to uBlox modules distributors and sample shop idiotic prices.

[bitseeker]

That's very interesting indeed. Looks like it uses the same 48MHz internal clock. So, same caveat on choice of frequency output as with the LEA-6T.

http://innovictor.com/pdf/UBX-G7020-Kx_DataSheet_%28GPS%20G7-HW-12001%29_Confidential.pdf

I'm looking forward to the shootout!

[uncle_bob]

A touch off topic but I got my $8 Aliexpress GPS modules today - V.KEL VK2828U7G5LF
Datasheet:
https://github.com/CainZ/V.KEL-GPS/raw/master/VK2828U7G5LF%20Data%20Sheet%2020150902.pdf

Aliexpress e.g.
http://www.aliexpress.com/item/Smart-Electronics-VK2828U7G5LF-GPS-Module-with-Antenna-TTL-1-10Hz-with-FLASH-Flight-Control-Model-Aircraft/32634474566.htm
also on Banggood.com

Rather than turning it on, I took it apart (well, peeled off the can)...
So it's legit - but only kinda - the datasheet claims "Chip UBX-M8030-KT"
But my microscope says otherwise... I see "G7020-KT" on the board.

Hi

One of the things you get with a "real" LEA-6T is very good workmanship on the board. They also use very good components on the board. The boards made by "some guy named Bob" may not have as good parts (TCXO and SAW matter a lot). The one you shared the pictures of has some obvious workmanship issues.

How much is that all worth? Certainly not the delta between the eBay "guy named Bob" prices and the single piece price direct from uBlox ...

Bob

[Koen]

I have both and more, it's completely worth it. And ease of order is another big plus.

[uncle_bob]

I have both and more, it's completely worth it. And ease of order is another big plus.

Hi

I have both as well. I just would not pay the $280 each that they want when you buy and ship one.

Bob

[bitseeker]

So, Bob, you're in favor of the cheap modules? At first it sounded like you were against them. Re-reading your prior post again, I can see how it could read the other way. I agree that $8-30 vs. $280 is quite a premium for buying directly from uBlox.

[uncle_bob]

So, Bob, you're in favor of the cheap modules? At first it sounded like you were against them. Re-reading your prior post again, I can see how it could read the other way. I agree that $8-30 vs. $280 is quite a premium for buying directly from uBlox.

Hi

I'd much rather buy the real thing for about $40 or so....If I can't get them, then the "no name" parts would be my second choice. Buying in low quantity from uBlox would be a distant third.

Bob

[bitseeker]

Ah, OK. I'm clear now. Thanks, Bob.

[DrTune]

It's my decidedly uninformed opinion that this $8 V.KEL module "doesn't suck".
Specifically I mean by looking at what's on the board compared to the hardware integration manual
https://www.olimex.com/Products/Modules/GPS/MOD-GPS/resources/UBX-G7020_HardwareIntegrationManual_-GPS.G7-HW-10003-_Confidential.pdf
..there's a number of things in there which are more expensive than the "Cost optimized circuit" reference design; specifically

(absurdly large pic of the antenna input circuitry here

)
a) External LNA and what I assume is a SAW filter
b) TCXO - couldn't identify the brand but looking at the use of only XTAL_I pins it's not a regular xtal, they don't seem to be using the LDO_X_OUT internal LDO though, looks like a nonpopulated stuffing option and they're using the inductor near the marking "G" to filter some other power supply, probably the 3v3 LDO on the board)
c) Has an RTC xtal (as opposed to not having one)
d) Has 512KB SPI flash which isn't required, and
e) some sort of supercap for battery backup
f) the DC-DC converter option is in use (w/inductor etc),

looks like it wouldn't be hard to switch it to USB not UART by populating a couple of 0402's that have pads on there.

Don't know what the performance is like in absolute terms but I was surprised that they didn't cut half as many corners as they could have, given that it's very nearly the cheapest GPS module out there.

[bitseeker]

That's a very good first impression. From skimming the docs, it seems comparable to other modules, though it's a nav/position GPS, rather than a timing one. How much does that matter for occasional reference adjustments, though?

Maybe I should get one before they decide to remove "non-essential" parts.

[DrTune]

There's really an embarrassment of cheap GPS riches out there right now; choose a timing-rated LEA-6T for $30 or a mere $8.50 for the V.KEL (which is the next generation chip and picks up other sat systems like GLONASS);  both with 10Mhz (actually up to 24Mhz) clock output. (Two on the 6T, and two on the V.KEL if you can manage to get a wire soldered on the right pin of that QFN).  Stupendous value IMO. If you can't wait for China shipping the V.KEL can be got for $10 with 3-day shipping in the US (from Banggood, there's where I got mine, I have some more coming from China)

I've got three of the 6T's and am going to acquire a small bag full of the V.KELs (I have a GPS project in mind). 

It looks like the VKEL can be run at quite low voltages (assuming the TCXO doesn't drop out); the module datasheet says 3.3-5v but from the chip datasheet it looks like it will go a bit lower. It's pretty awesome for my purposes that the VKEL can even do autonomous position logging to its SPI flash.

When I have a sec (maybe in a week or so) I'll head-to-head the VKEL and the 6T time outputs. My hunch is that when configured the same they're both equally good.. but we'll see..

[bitseeker]

Is the back of the VK module the antenna?

[DrTune]

Yep, that's all that's on there. Like any other ceramic antenna..

[bitseeker]

Very good. That's what it looked like, but I wanted to be sure. Thanks!

[Robaroni]

I'm wondering how these $30 LEA 6T discs hold up to the elements. I'd like to make a new case and run it outside my lab window but we get hellacious cold tough winters here where it can get to 20F or 30F below. The simplest solution is to put inside the window but of course I'd get more coverage outside.

I just checked and the PCB will, with a little machining, work inside a 2" PVC pipe so I might build a holder to see how it goes.
Rob

[uncle_bob]

I'm wondering how these $30 LEA 6T discs hold up to the elements. I'd like to make a new case and run it outside my lab window but we get hellacious cold tough winters here where it can get to 20F or 30F below. The simplest solution is to put inside the window but of course I'd get more coverage outside.

I just checked and the PCB will, with a little machining, work inside a 2" PVC pipe so I might build a holder to see how it goes.
Rob

Hi

I would make sure it is well sealed. You would do much better to deploy an antenna and keep the electronics indoors.

Bob

[texaspyro]

I got in one of these pucks off of Ebay from a US seller (3 days, $40).  It works fine.   I wired it to one of my RS-232 level shifter / regulator boards  (https://oshpark.com/shared_projects/YPvKgMYa).   The pucks speak 9600:8N1 serial without having to mod the USB power connection.  Also you can drive them with 3.3V instead of 5V.   

Attached is a plot of the thing in action.  It's plotting the sat count,  PPS sawtooth,  PPS accuracy,  and fractional clock bias.  The plot in the lower left corner is a signal level map (but with not enough data collected yet to show much of interest)

[texaspyro]

BTW,  the firmware in these units supports the RAW data message so you can do carrier phase processing.

I suspect the reason that they are using these high-end timing receivers in a simple drone oriented device is that they bought a bunch of them as surplus items from a defunct company or discontinued product line... and when they are all used up, there will be no more of them with the 6T modules in them.

[texaspyro]

I got in a couple of the V.KEL Ublox based boards in.   They seem to be based upon a Ublox-7 chip... hardware id reports 00070000.   Performance seems good.  I am testing it indoors away from a window.  It's tracking 14 sats at the moment (including 3 SBAS sats).  It does not support position-hold or survey mode like the Ublox timing receivers do.

Putting it through the Lady Heather ringer, I noticed that the Timepulse 2 output is limited to 1000 Hz.  It will accept any value (it accepted 1 GHz) for the PPS1 output,  but my freq counters are tied up at the moment, so I don't know what the actual maximum is.

The firmware only has the new CFG_TP5 message, it does not support the older CFG-TP message.

[Ebel0410]

Hi
It seems that it is now not easy to purchase cheap u-Blox LEA-6T or NEO-6T .
Just ordered a GPS module with LEA-6T from aliexpress, 25$ (the same described here at the begining of that thread), where you can find NEO-6M from less than 10$.
Keep you posted.

[uncle_bob]

Hi

The "6" series parts have been out of production for a while now. The 8 series is on the market at low prices. I can't see a really good reason to go with anything other than the newer parts.

Bob

[cdev]

Unfortunately the NeoM8T and Lea 8T are consistently priced at $65-70 or more each.  For that though you get both timing and RTK, plus, they support that use, so you can use it as the base for a multi-constellation RTK capable system. The "T" models have two timing pulses, and its adjustable.

Hi

The "6" series parts have been out of production for a while now. The 8 series is on the market at low prices. I can't see a really good reason to go with anything other than the newer parts.

Bob

[uncle_bob]

Unfortunately the NeoM8T and Lea 8T are consistently priced at $65-70 or more each.  For that though you get both timing and RTK, plus, they support that use, so you can use it as the base for a multi-constellation RTK capable system. The "T" models have two timing pulses, and its adjustable.

Hi

The "6" series parts have been out of production for a while now. The 8 series is on the market at low prices. I can't see a really good reason to go with anything other than the newer parts.

Bob

Hi

If you are willing to pole the device, all of the timing information is there on the < $20 versions of the 8 series.....

Bob

[cdev]

Could you explain what you mean a bit more?  I would love to be able to build a GPSDO out of a cheap ublox module. There is a library on github that might be useful.

Hi

If you are willing to poll the device, all of the timing information is there on the < $20 versions of the 8 series.....

Bob

The M8 brings multiple GNSS constellations to the table for better navigation accuracy/fewer cycle slips..

 But, I think that they may still derive their timing just from one GPS system?  They may use multiple systems to tell the GPS what time/date and where it is at the beginning.

I think having multiple systems introduces some variations as to naming of satellites and NMEA sentences.

There may be advantages to using multiple GNSS systems for timing I don't know about but my gut feeling is that picking one system to standardize on would be the only way to go lacking some NTP-like setup for satellite systems that could sanity check all of them and reject "false tickers".

(You could likely with Ublox M8 or other newer GPS receivers now have several GNSS receivers each with its own NTP instance that used it in its refclock -so basically each with a different GNSS system as its timing source.)


They might disagree by some fixed offset or it might vary?

[uncle_bob]

Hi

If you pick the right part, they have the pps output and the sawtooth correction information. The T versions will output sawtooth once a second without any "prompting" from the user. The non-T versions need to have a request sent to get the sawtooth correction data. Of course if you are not using sawtooth, then there really is no difference at all ....

The Neo-M8N specs are online. Mr Google is quite happy to dig them up. The PPS output is on pin 3 .....

Bob

[cdev]

It's great the way that you can extract the data by multiple means. I really do need to get some more experimenter friendly ublox HW.

Do you know the name of the relevant messages so I can graph it to see what is happening there? I love being overloaded with useless information.

[cdev]

Have you ever measured the stability/jitter of its 1KHz ouput?

I got in a couple of the V.KEL Ublox based boards in.   They seem to be based upon a Ublox-7 chip... hardware id reports 00070000.   Performance seems good.  I am testing it indoors away from a window.  It's tracking 14 sats at the moment (including 3 SBAS sats).  It does not support position-hold or survey mode like the Ublox timing receivers do.

Putting it through the Lady Heather ringer, I noticed that the Timepulse 2 output is limited to 1000 Hz.  It will accept any value (it accepted 1 GHz) for the PPS1 output,  but my freq counters are tied up at the moment, so I don't know what the actual maximum is.

The firmware only has the new CFG_TP5 message, it does not support the older CFG-TP message.

[uncle_bob]

Have you ever measured the stability/jitter of its 1KHz ouput?

I got in a couple of the V.KEL Ublox based boards in.   They seem to be based upon a Ublox-7 chip... hardware id reports 00070000.   Performance seems good.  I am testing it indoors away from a window.  It's tracking 14 sats at the moment (including 3 SBAS sats).  It does not support position-hold or survey mode like the Ublox timing receivers do.

Putting it through the Lady Heather ringer, I noticed that the Timepulse 2 output is limited to 1000 Hz.  It will accept any value (it accepted 1 GHz) for the PPS1 output,  but my freq counters are tied up at the moment, so I don't know what the actual maximum is.

The firmware only has the new CFG_TP5 message, it does not support the older CFG-TP message.

Hi

You use the 1 pps output for a GPSDO ( 1 pulse per second). The jitter is mainly caused by the way the pulse is quantized. That's where the sawtooth correction comes in. It's in the 10 ns range. For a reasonably quiet GPSDO, you run filters with time constants out past 500 seconds or so. That pretty much puts you into the digital filter world .....

Bob

[texaspyro]

Have you ever measured the stability/jitter of its 1KHz ouput?

Nope...

[texaspyro]

Do you know the name of the relevant messages so I can graph it to see what is happening there? I love being overloaded with useless information.

Lady Heather will do that...

[mark03]

I'm still curious about this comment (above) that the non-T versions are almost the same as the -T version in the 8 series.

I don't know the correct terminology here, but I think it used to be the case that the timing firmware would support a process where you (1) run like a normal GPS for some day(s), averaging to get a super accurate position, then (2) switch into a timing mode where the 3D position is taken as a *given*, and all the module solves for is time.  This was supposed to cut down on jitter, I believe, by letting you tell the GPS engine that it is firmly affixed to an immovable object.  (Obviously neglecting plate tectonics here...)

Can you do this on the non-T modules now?  Or was this never as useful as it was made out to be?

[texaspyro]

Thats pretty much what a timing receiver does.  With a known fixed position, it can also output time solutions with just one satellite in view.   Timing mode receivers can also operate without a fixed location.  The firmware may have tweaks to optimize the GPS solutions for improved time accuracy at the expense of position.  Timing firmware may have a message that sends 1PPS output sawtooth error corrections that normal firmware does not have.

With the proper firmware any GPS receiver can be a timing receiver... its just that the receiver makers tend to double the price for timing firmware.  Ublox configures their receivers so you can't load timing firmware on modules that were not originally sold as timing receivers.

[mark03]

Right, so the claim several posts back about the non-8T modules being essentially the same as the 8T modules, except you have to query for sawtooth data instead of having it spit out automatically, is incorrect.  That's what I was attempting to clarify.

[texaspyro]

Most of the Ublox messages can be configured to be output at a specified interval.  The "T" versions might default to being output... I don't know for sure if they are.

[cdev]

Does the output of ntpq serve any useful purpose? This is my non-timing receiver currently sitting on my living room coffee table with an indoor passive antenna.  Is the clock jitter low or is that not the meaningful figure. Its taken a while to tweak the settings to get it this low but then again, as I dont really understand what the numbers mean, I could be making mistakes and I would never know..

I have no idea what the settings mean. The documentation isnt very helpful at all.

I wish the NTP documentation was better. It would be a golden opportunity to teach people all sorts of interesting things.

ntpq -c rv -c pe -c cv -c kern -c as
associd=0 status=0415 leap_none, sync_uhf_radio, 1 event, clock_sync,
version="ntpd 4.2.8p10@1.3728 Fri Jul 14 05:12:26 UTC 2017 (1)",
processor="armv6l", system="Linux/4.1.19+", leap=00, stratum=1,
precision=-19, rootdelay=0.000, rootdisp=450.030, refid=GPS,
reftime=dde3060d.0e333675  Mon, Dec 18 2017 22:29:17.055,
clock=dde30610.07937c4a  Mon, Dec 18 2017 22:29:20.029, peer=33092, tc=3,
mintc=3, offset=0.000651, frequency=-35.014, sys_jitter=0.001907,
clk_jitter=0.002, clk_wander=0.000, tai=37, leapsec=201701010000,
expire=201806280000
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
oGPS_NMEA(0)     .GPS.            0 l    3    8  377    0.000    0.001   0.002
+clock.nyc.he.ne .CDMA.           1 u    1   16  377   13.832    3.419   1.643

XXXXXXXXXXXXXX
seven servers omitted for brevity
XXXXXXXXXXXXXX


-time-a-g.nist.g .NIST.           1 u   15   32  377   42.130   -6.978   6.841
associd=0 status=0062 6 events, clk_bad_format,
device="NMEA GPS Clock",
timecode="$GPGGA,032920.000,____.____,N,_____.____,W,1,10,1.1,94.0,M,-34.3,M,,0000*__",
poll=10786, noreply=0, badformat=6, baddata=0, fudgetime2=150.000,
stratum=0, refid=GPS, flags=9
associd=0 status=0415 leap_none, sync_uhf_radio, 1 event, clock_sync,
pll offset:            0.000591
pll frequency:         -35.0139
maximum error:         0.4515
estimated error:       1e-06
kernel status:         pll nano
pll time constant:     3
precision:             1e-06
frequency tolerance:   500
pps frequency:         0
pps stability:         0
pps jitter:            0
calibration interval   0
calibration cycles:    0
jitter exceeded:       0
stability exceeded:    0
calibration errors:    0

ind assid status  conf reach auth condition  last_event cnt
===========================================================
  1 33092  971b   yes   yes  none  pps.peer clock_alarm  1
  2 33093  941d   yes   yes  none candidate              1
  3 33094  9414   yes   yes  none candidate   reachable  1
  4 33095  9314   yes   yes  none   outlier   reachable  1
  5 33096  9314   yes   yes  none   outlier   reachable  1
  6 33097  9314   yes   yes  none   outlier   reachable  1
  7 33098  9414   yes   yes  none candidate   reachable  1
  8 33099  9414   yes   yes  none candidate   reachable  1
  9 33100  9314   yes   yes  none   outlier   reachable  1
 10 33101  9314   yes   yes  none   outlier   reachable  1

[Leo Bodnar]

Have you ever measured the stability/jitter of its 1KHz ouput?

Everything that runs on Ublox chip is clocked from 48MHz system clock, inlcuding both timepulse outputs.  Thus +-10.4ns quantisation which is just the nearest 20.83ns clock.  Ublox is basically Cortex-M3 with RF bodged onto it.

Leo

[cdev]

So Ublox is really not so different than Skytraq which uses a Leon processor (a SPARC core originally designed for the EU space effort) but I think Skytraq has around an 80 MHz clock