Lars DIY GPSDO with Arduino and 1ns resolution TIC

[thinkfat]

Did you by chance edit the series in TimeLab? Like, remove linear drift or drift curve? The MDEV graph looks a bit indecisive here, after 5 days of collecting samples, you'd expect a very clear uptick that signifies the OCXO drift. But it's going down and further down only.

Yes I subtracted the global linear frequency trend. Here is the unedited plot.

Mike

Aha. Well, that's more like it. You know, by removing the linear frequency trend you edited out the very thing you've been looking for :-)

Now, how I'd interpret the graph is that this OCXO should be good up to a time constant of 2000 seconds, which is pretty impressive, especially since it is not protected against temperature changes at all.

PS: Note that the curve shows a clear drift behavior only beyond 20000 seconds, which is a bit too good to be true. That would be more in line with a Rubidium standard, but not with an OCXO.

[Dbldutch]

Mike,

The DAC plot looks reasonable. There are no significant corresponding changes in the ns plot, although the excursions are a bit large. Due to the saw-tooth effect, this should create about a 20nsec "band", which it typical and sufficient.  Your setup is not bad at all, most of the saw-tooth excursions are in a 15nsec band, which is very good. From your data, it looks like that the DAC just moves with the temperature.

Due to the temperature induced changes on the DAC (room temperature changes and draft), it makes sense to add OCXO temperature tracking. With that you can compensate the DAC with the temperature effects and try to eliminate them.

This is important because these temperature induced changes in the DAC values "falsify" a TimeLab plot based on the DAC values and therefore cannot show the "real" OCXO performance. This DAC-based TimeLab method is a poor man's way of looking at the OCXO performance, so keep that in mind.

[iMo]

When looking at the pictures in Lars' docs (page 19-21) he got something like ~1E-12 at 1000secs (in "the hold mode", not sure it is an identical measurement process, however)..

[Mike99]

Thanks everyone, that's all very educational!

Now, how I'd interpret the graph is that this OCXO should be good up to a time constant of 2000 seconds, which is pretty impressive, especially since it is not protected against temperature changes at all.

The problem with such a long time constant would be the initial lock time (I think 2000 seconds would take around 3 hours with the warmup?) as I don't plan to run this continuously. However I will try it out of curiosity (this project is becoming addictive!).

I will add temperature compensation once I put it in an enclosure.

My Trimble OCXO is due to be delivered today, and yesterday I bought a Vectron oscillator on eBay from a UK seller (just £10 and it's new!) so I will have plenty to play with soon.

Mike

[Dbldutch]

However I will try it out of curiosity (this project is becoming addictive!)

  Welcome to the club!

[0xFFF0]

Would you post photos of your device? A little hardware porn...

[Mike99]

Would you post photos of your device? A little hardware porn...

Of course, here it is.

I'm running it off a 15V wall wart which supplies an LM7812 for the OCXO, then an LM7805 for the logic/Arduino/GPS and a REF02EZ for the OCXO control voltage.

All unused tracks on the stripboard have been removed, and I've built a transistor buffer for the 10MHz output.

The GPS antenna is an active puck mounted on a ground plane on the house wall with about 180 degree view of the sky.

Mike

[Mike99]

My Trimble OCXO is due to be delivered today, and yesterday I bought a Vectron oscillator on eBay from a UK seller (just £10 and it's new!) so I will have plenty to play with soon.

Mike

A quick update on this. The Trimble OCXO is tatty but working, although very power hungry. It draws 1.9A on startup before settling down to around 200mA.

The output looks a bit noisy. This is with an open circuit control pin.

The good news is that the Vectron device I ordered is an MX2001X4 which appears to be what is fitted inside the Trimble module 

Mike

[0xFFF0]

The assembly is quick and clean. What is the REF02 for? If the GPS fails?

[Mike99]

The assembly is quick and clean. What is the REF02 for? If the GPS fails?

No, it's a precision 5 volt reference that feeds the resistive divider connected to the OCXO voltage control.

Mike

[0xFFF0]

The assembly is quick and clean. What is the REF02 for? If the GPS fails?

No, it's a precision 5 volt reference that feeds the resistive divider connected to the OCXO voltage control.

Mike

The OCXO voltage control comes from Arduino PWM.

[Mike99]

The OCXO voltage control comes from Arduino PWM.

Yes, but in order to reduce the tuning range of the OCXO to get the gain up to around 500 I had to add a resistor network and a 5 volt supply. I couldn't do it with resistors alone.

The circuit is attached. The 10M resistor has been reduced to 4M7 and the gain is now 585.

Mike

[0xFFF0]

The OCXO voltage control comes from Arduino PWM.

Yes, but in order to reduce the tuning range of the OCXO to get the gain up to around 500 I had to add a resistor network and a 5 volt supply. I couldn't do it with resistors alone.

The circuit is attached. The 10M resistor has been reduced to 4M7 and the gain is now 585.

Mike

Ok, now I understand. A kind of limiter/bias. The REF02 can drift around 0.5mV, but I think that doesn't matter.

[rpetitho]

I posted this on another thread, but wanted to show the difference between an indoor and outdoor antenna on my GPSDO (very similar to Lars' design). This was the same antenna, but ran it for a few hours outside, then a few hours inside.

[0xFFF0]

I posted this on another thread, but wanted to show the difference between an indoor and outdoor antenna on my GPSDO (very similar to Lars' design). This was the same antenna, but ran it for a few hours outside, then a few hours inside.

(Attachment Link)

Would be interesting to know how much dB U-Center shows indoor and outdoor.

[Johnny B Good]

I posted this on another thread, but wanted to show the difference between an indoor and outdoor antenna on my GPSDO (very similar to Lars' design). This was the same antenna, but ran it for a few hours outside, then a few hours inside.

(Attachment Link)

 Proof positive that it's vital to locate the antenna outside with as clear an all round view of the horizon as you can achieve! It doesn't matter how good the gpsdo is, a poorly sited antenna will seriously degrade the phase and frequency stability. There are enough deficiencies in the whole GPS system to start with without adding yet another deficiency in the form of a poorly located antenna.

John

[Johnny B Good]

I posted this on another thread, but wanted to show the difference between an indoor and outdoor antenna on my GPSDO (very similar to Lars' design). This was the same antenna, but ran it for a few hours outside, then a few hours inside.

(Attachment Link)

Would be interesting to know how much dB U-Center shows indoor and outdoor.

 It's a surprisingly small 10 to 15 dB drop in the C/NO figures!    I was testing the second of the last two 24 quid M8T modules I'd purchased some five weeks ago despite a seemingly minor "NAV5" bug, that turned out to be a USB2 winXP VM driver issue I'd discovered only a a day ago, in the first 41 quid one I'd purchased some weeks earlier from the Amazon seller "Amoq313"  (the bad news is that he's now bumped the price to £59.19 - still relatively cheap, mind you) using the supplied IPX connected active 25mm square ceramic patch antenna on my bench, some 15 feet away from a lead roofed first floor bay window, when I wanted to force LoS to avoid wearing out the IPX connector so I could verify the 4Hz 50% unlocked PPS output I'd programmed (set to 10Hz 50% when locked for the purposes of this test).

 Optimistically, I rather though that placing a small empty peanut tin and then a taller coffee powder tin over the antenna might have cut off the signals, especially if filled with steel wire wool to act as an "RF absorber". Fat chance!

 Just as on all previous attempts to provoke a LoS situation, human flesh still proves the most effective attenuator (using my hand to cover the antenna) even if it still wasn't enough to provoke an unlocked state (I wasn't about to risk damaging the module by sitting on it to degrade both the signal and my dignity any further).

 Even disconnecting the antenna to invoke a LoS condition to lose (NOT "Loose"! ) GPS lock can take several minutes to remove all trace of the SV signals from the sky map plot. These M8 u-blox modules (both N and T types) can maintain an extremely tenacious grip on its acquired constellation of SV signals unltil they all finally dip below the 5dB C/NO threshold for more than a few seconds.

 It's an impressive 'Party Trick' to my mind, considering that even with a good antenna, the signals are still below the background noise, but it does make simulating a LoS condition a lot more tricky to achieve when you want to avoid undue wear and tear on the antenna connectors. However, I've just remembered that in the case of active antennas, an effective way to obtain another 20 to 30dB loss is simply to disable the active power in the ANT CFG settings (has no effect of course with a passive patch antenna).  Dang it! I clean forgot that fact! 

John

[Mike99]

 ... when you want to avoid undue wear and tear on the antenna connectors.
John

That's easy, just fit SMA connectors to the pads provided on the PCB.

Mike

[Johnny B Good]

 ... when you want to avoid undue wear and tear on the antenna connectors.
John

That's easy, just fit SMA connectors to the pads provided on the PCB.

Mike

 That's all fine and dandy after you've verified that the delivered module is working properly before tainting it with the leaded goodness of 'proper solder' to avoid accusations by the seller, in the event you have to return it for a refund or replacement, that you've damaged it by careless soldering. 

 I've soldered edge contact SMA sockets to these modules in the past to avoid fiddling around with those pesky IPX connectors any longer than needed to do the initial test, using clip on connectors to the Vcc, GND and TTL serial contacts which have been more trouble than they're worth (I use an elastic band with a  4 pin header to provide a more reliable temporary connection for these tests now).

 The MK I gpsdo build used a right angled edge soldered SMA socket to attach the module to the rear panel for additional support but the current MK II which replaced it makes use of an IPX ended panel mount SMA socket pigtail, mainly it has to be said due to the M8T module's location and orientation within the case.

 Regardless of the connector type, it's still good practice to avoid a high connect/disconnect cycle count to keep wear and tear to a minimum even with the more robust SMA sockets if you can help it. 

 John

[Fennec]

@Johnny B Good

Pls can you make some pictures from your "version". U know, pix say more than 1000 words..

@all  (esp. Dbldutch)

Pls upload PCB files (projects) for maybe Eagle, Target something like that, so it make it way easier to change the PCB design. Just the Gerber files are hard to use.
Also pls upload more pictures, configs, wich OCXO you use, and the source code. It's way easier than to read the whole thread.

kr

[Dbldutch]

Fennec,

I'm posting most of my stuff on my own blog to not pollute this thread more than needed, and through that you can get to my Github site. I think I mentioned that in an earlier thread.

https://github.com/paulvee/Lars-GPSDO

[cdev]

I have one of the exact same GY-GPSV3-NEO "blue boards" which Lars mentions in his original description, the one where to connect the 1PPS you need to connect to the high side of the LED resistor.

The GPS board is thinner than the typical PCB size and I had to use a shim on an SMA to use mine with the SMA connectors I had at that time. Now I have some that work without a shim, of course.

But to test the GPS with any SMA antenna just plug the antenna cable with its SMA edge mount in without soldering it.

You may have to use gravity to presss the tinned area - prop it up a bit at an angle to get it to make the connection adequately. It takes a little less than 15 minutes to work from a cold start.

Additionally, in my experience if one is careful the (male?) IPX connectors may last years and can be used dozens of times. You buy them premade. The other end is tinned.

[Johnny B Good]

@Johnny B Good

Pls can you make some pictures from your "version". U know, pix say more than 1000 words..

@all  (esp. Dbldutch)

Pls upload PCB files (projects) for maybe Eagle, Target something like that, so it make it way easier to change the PCB design. Just the Gerber files are hard to use.
Also pls upload more pictures, configs, wich OCXO you use, and the source code. It's way easier than to read the whole thread.

kr

 Well, I'm a little flattered that you think "my version" will be of much interest here since... well I'll quote this from the leapsecond.com article on a GPSDO performance comparison here:- http://www.leapsecond.com/pages/gpsdo/ since it's essentially a re-spin of the following using a modern u-blox timing GPS rx module in place of the Jupiter-T, running at 100kPPS to simplify the divider and operate at a better optimised phase locking input frequency.

"James Miller Simple GPSDO

This is about the simplest design possible for a GPSDO; with no DAC, no Vref, no time interval counter, no microprocessor. Just an XOR gate acting as a phase comparator aginst the 10 kPPS output of a Jupiter GPS engine. In spite of this simplicity, the performance is amazingly good."

 Not only does it lack all the essential component parts of the Lars design, it also lacks sensitivity to ambient temperature variations (just thought I'd toss that in  ). So posting pictures of "my version" and hand drawn circuit sketches here would, I suspect, be regarded as "off topic" and earn me a 

 I've been following this thread simply because it relates to a cost effective DIY GPSDO project and hasn't withered away like 99% of all other GPSDO related threads that I've been able track down. Also, of course, I'm doing this with a view to finally making use of one of my collection of three Nano3 boards, the first of which didn't get extracted from the anti-static bag it had been delivered in some 18 or months ago, until just a few months back to try it out with the "Blink" test program... er, sketch?

 I've only made contributions where I've believed they might help, either in regard of more general problems that apply to all designs of GPSDO or where they might just prompt a fresh look at the problem. I know just how easy it is to become blind sided to a problem that ultimately in hindsight proves to be blindingly obvious now!

 I'm currently trying to build myself an RFS from an Efratom LPRO-101 rubidium frequency standard which means mounting it in an enclosure where I can stabilise the base plate temperature and compensate for the effects of changes in atmospheric pressure.

 It turns out that it's not only ambient temperature variations you have to account for with a RFS but barometric variations as well and whilst I had hoped I could keep it simple and use a barometric sensor with analogue output which, with some pre-conditioning, could be applied directly to the freq tuning input terminal, it turns out that the most cost effective way is to use a cheap digital sensor and, you've guessed it, a nano3, so it looks like I'll be diving much deeper into programming the nano3 rather sooner than I thought.

 If my nano3 based environmentally compensated RFS project succeeds, the next obvious project is going to be this Lars gpsdo one (leaving me just one spare nano3 to find a use for). Ultimately, if I'm going to discipline my RFS to a GPS based frequency standard, the Lars design is the most cost effective way to smooth out the minute to minute and hour to hour phase variations by averaging days' worth of timing data with a Kalman filtering algorithm to compensate for the ageing driven frequency drift of the RFS. It might be some time before I'm ready to make any experience based contributions to this topic though.

John (formally JBG)

[Dbldutch]

Update on item #561: root cause of OCXO temperature jumps

Contrary to what I first believed the root cause for the OCXO temperature jumps was, it is not caused by the USB power connections (+5, GND or shield), but the data lines (D- and D+). I figured this out by using a break-out board contraption after I isolated the +5V line earlier.
 
If either one or both of the data lines are connected, both temperature measurements in the Lars report jump by about 0.5 degrees up when connected and down when not.

I can't explain that yet, can anybody? 

[Fennec]

Dbldutch
Johnny B Good & all

Very interesting to watch/read both of you at work. You're doing a great job. Thanks a lot for this.