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

[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)...