Lars DIY GPSDO with Arduino and 1ns resolution TIC

[thinkfat]

A linear regression. Plot DAC output vs. temperature, feed it into a linear regression solver, this gives you the coefficient. make sure your correlation "r" is high enough, > 0.9 would be perfect.

[Mike99]

A linear regression. Plot DAC output vs. temperature, feed it into a linear regression solver, this gives you the coefficient. make sure your correlation "r" is high enough, > 0.9 would be perfect.

Perfect, thank you  .

Since I optimised the time constant (now 300 seconds) the DAC is much better behaved which should make it easier.

Mike

[MIS42N]

Choosing the optimum time constant depends entirely on the stability of the oscillator's behaviour. You might expect an OCXO to be pretty immune to external temperature variations but once you start chasing after parts per trillion accuracy, you soon discover the effect of temperature on the humble OCXO leaving you hankering after a high quality DOCXO upgrade or even a suitable RFS.

 You mentioned the expense of a good voltage reference (MAX6350CPA+) so, despite the fact that you've already got one on order, I think it's worth considering the temperature stabilised voltage reference available from the better quality OCXOs which provide this on their Vref pin. Even if this comes from a humble LDO supplying the oscillator and the oven controller, it's worth considering that, once warmed up to temperature, it's supplying a constant current and its internal reference is temperature stabilised making it only second in stability to that of an expensive lab standard (at least for use with a low constant current external load such as the 1mA of my reference voltage supply).

By going down the non PLL route, there is no optimum time constant. The processor collects noisy error information every 1ppS, averages it, and determines if the error is 'big enough' to require a correction. 'Big enough' starts off large (my current version starts at around 1Hz error from 5 samples) and becomes smaller as more samples are collected (down to .0003Hz from 2000 samples). This is consistent with applying statistics to the short term jitter of the 1ppS from a cheap GPS module. Any misbehaving by the oscillator is caught in the process.

As I mentioned in previous post, I will be using a used Morion MV89 DOCXO in the next version. It was used in my first attempt at a GPSDO. Its performance is way better than OSC5A2B02 as you would expect. The whole point of the circuit I published was to show how few low cost components could still make a working GPSDO. I have no real interest in using a GPSDO, I like the technical challenge of trying different strategies to see what can be achieved.

I couldn't find any details of the stability of the Vref pin on the Morion. The MAX6350CPA+ claims "Ultra Low, 1ppm/°C Max Tempco". The sensitivity of the OSC5A2B02 is determined as approx 0.1V/Hz with a lock voltage of around 2V. The Morion is advertised as 1V/Hz and its lock voltage was around 2V also. So the MAX6350CPA+/MV89 combo should have a sensitivity of around 2*10^-13/°C which may be good enough. I don't have the equipment to determine if this is better or worse than using the Vref.

[Mike99]

I have some questions for the experts please. Sorry if they are a bit basic 

I insulated my OCXO with closed cell foam and ran the GPSDO for 5 days, results below.

(The glitch at 1.35x10⁵ seconds occurred when I fired up my 100 watt HF transmitter in close proximity  ).

Here are my questions:

• The MDEV and Frequency Difference plots were derived from diff_ns. Is that correct?
• Any comments on the performance (good/average/try again)?
• I presume the downwards DAC drift is due to aging?
• It looks to me as if the swings in diff_ns might be inversely proportional to temperature but there's no correlation with the DAC, so how do I determine tempCoeff?

Thanks!

Mike

[nealix]

I have finally started building my first unit (after being FAR too busy at work for several months).

QUESTION:   Has anyone else built a unit with sinewave 10MHz output?   I wanted to use the Morion MV85 that @Thinkfat suggested might be a good first choice.
But since it is sine instead of square, I can't use a digital output buffer like the 74HC14 or 74AC04.    I am guessing I need a high speed op-amp buffer circuit?
Does anyone have an example output buffer for sine that is known to work well?
[If not, I also have several sqaure wave OCXO, so I could start there if needed]
Opinions, ideas?

Thanks,

Neal

[cncjerry]

I have finally started building my first unit (after being FAR too busy at work for several months).

QUESTION:   Has anyone else built a unit with sinewave 10MHz output?   I wanted to use the Morion MV85 that @Thinkfat suggested might be a good first choice.
But since it is sine instead of square, I can't use a digital output buffer like the 74HC14 or 74AC04.    I am guessing I need a high speed op-amp buffer circuit?
Does anyone have an example output buffer for sine that is known to work well?
[If not, I also have several sqaure wave OCXO, so I could start there if needed]
Opinions, ideas?

Thanks,

Neal

http://www.wriley.com/A%2010%20MHz%20OCVCXO%20and%20PLL%20Module.pdf

Take a look at some of the buffers in the doc above.

[thinkfat]

• The MDEV and Frequency Difference plots were derived from diff_ns. Is that correct?
• Any comments on the performance (good/average/try again)?
• I presume the downwards DAC drift is due to aging?
• It looks to me as if the swings in diff_ns might be inversely proportional to temperature but there's no correlation with the DAC, so how do I determine tempCoeff?

Thanks!

Mike

Use TIC data instead of diff_ns for the MDEV. I cannot say a lot about the performance until I see that graph. The diff_ns MDEV indicates though that it's in a very acceptable range regarding long-term stability. Nice straight descend into the 1e-13 range with not much ripple. TIC value range looks good, too, +/- 20ns is absolutely normal with a timing receiver, which I believe you are using.
The DAC curve indicates still accelerated aging, yes. OCXO has still not settles, I think, but keep in mind that you have a very sensitive instrument to measure it.
I cannot see first glance a temperature dependency between the DAC and the temperature. Maybe it's still shadowed by the OCXO aging/settling curve. At least it is not very pronounced.

[thinkfat]

I have finally started building my first unit (after being FAR too busy at work for several months).

QUESTION:   Has anyone else built a unit with sinewave 10MHz output?   I wanted to use the Morion MV85 that @Thinkfat suggested might be a good first choice.
But since it is sine instead of square, I can't use a digital output buffer like the 74HC14 or 74AC04.    I am guessing I need a high speed op-amp buffer circuit?
Does anyone have an example output buffer for sine that is known to work well?
[If not, I also have several sqaure wave OCXO, so I could start there if needed]
Opinions, ideas?

Thanks,

Neal

I'm using the attached circuit derived from discussions with @imo. Basically a biased inverter gate with a tuned tank circuit. R6 is 49.9 Ohm actually, not 0 Ohm.

[Mike99]

Use TIC data instead of diff_ns for the MDEV. I cannot say a lot about the performance until I see that graph. The diff_ns MDEV indicates though that it's in a very acceptable range regarding long-term stability. Nice straight descend into the 1e-13 range with not much ripple. TIC value range looks good, too, +/- 20ns is absolutely normal with a timing receiver, which I believe you are using.
The DAC curve indicates still accelerated aging, yes. OCXO has still not settles, I think, but keep in mind that you have a very sensitive instrument to measure it.
I cannot see first glance a temperature dependency between the DAC and the temperature. Maybe it's still shadowed by the OCXO aging/settling curve. At least it is not very pronounced.

Thank you! 

Yes I am using a NEO-M8T. Here's the MDEV plotted from TIC.

Mike

[thinkfat]

OK, it seems like the TIC data is smoothed, is that correct? So it makes the curve look better than expected, for small "tau". But nevertheless, the long-term stability looks pretty good. diff_ns looks worse for small tau; I guess it isn't as heavily filtered. It's closer to the GPS.

Of course the proof will be measurement against a very stable external source, with an instrument that has a noise floor in the 1e-12 range @1s at least. If you want to go that route, look into DMTD systems. If not, it's probably the end of the journey for you

[Miti]

I have finally started building my first unit (after being FAR too busy at work for several months).

QUESTION:   Has anyone else built a unit with sinewave 10MHz output?   I wanted to use the Morion MV85 that @Thinkfat suggested might be a good first choice.
But since it is sine instead of square, I can't use a digital output buffer like the 74HC14 or 74AC04.    I am guessing I need a high speed op-amp buffer circuit?
Does anyone have an example output buffer for sine that is known to work well?
[If not, I also have several sqaure wave OCXO, so I could start there if needed]
Opinions, ideas?

Thanks,

Neal

Take a look at replies # 70, 73 & 131 in this thread and here:

https://gerrysweeney.com/build-a-10mhz-rubidium-frequency-standard-and-signal-distribution-amp-for-my-lab/

[Mike99]

No I didn't apply any corrections to the MDEV.

Mike

[Mike99]

OK, it seems like the TIC data is smoothed, is that correct? So it makes the curve look better than expected, for small "tau". But nevertheless, the long-term stability looks pretty good. diff_ns looks worse for small tau; I guess it isn't as heavily filtered. It's closer to the GPS.

Of course the proof will be measurement against a very stable external source, with an instrument that has a noise floor in the 1e-12 range @1s at least. If you want to go that route, look into DMTD systems. If not, it's probably the end of the journey for you

Sorry I meant to Quote instead of Reply!

I didn't apply any correction to the MDEV.

My use for this is occasional calibration of a frequency counter and HF radio equipment, so perhaps it's good enough?

Mike

[Johnny B Good]

====snip====

My use for this is occasional calibration of a frequency counter and HF radio equipment, so perhaps it's good enough?

Mike

 Seeing as how nobody bothered to answer this (and I'm now at a bit of a loose end awaiting the imminent delivery of an SSA3021X+), I'll chuck in my tuppence worth:

 If you're only going to use it as a calibration test signal, even just the 10MHz output on the PPS pin of a u-blox M8N will more than suffice on its own.

 For calibrating an HF receiver or transceiver, even better would be to tune into the third harmonic of the 10MHz square wave output onto the PPS pin which will be of much better quality, despite the non-integer divide jitter from the M8N's 48MHz clock and the sawtooth correction clicks that afflict the raw PPS signal, than what you can ever hope to get from WWV 10MHz in the UK and Europe during this period of low sunspot activity (2020AD).

 For that matter, it'll provide a better calibration source even during sunspot maxima simply because the jitter/sawtooth noise is a much better trade off against the random skywave propagation delays and associated phase cancellation effects inherent to all such long distance HF broadcast reference standards.

 You only need to bother disciplining an OXCO with a timing GPS receiver module when you need an accurate and stable lab reference to lock T&M kit clocks to or when using transverters to work the GHz bands using SSB or the even more critically narrower BW data modes (or when partaking of the simple pleasure of time nuttery  ).

John

[Mike99]

 If you're only going to use it as a calibration test signal, even just the 10MHz output on the PPS pin of a u-blox M8N will more than suffice on its own.

 For calibrating an HF receiver or transceiver, even better would be to tune into the third harmonic of the 10MHz square wave output onto the PPS pin which will be of much better quality, despite the non-integer divide jitter from the M8N's 48MHz clock and the sawtooth correction clicks that afflict the raw PPS signal, than what you can ever hope to get from WWV 10MHz in the UK and Europe during this period of low sunspot activity (2020AD).

 For that matter, it'll provide a better calibration source even during sunspot maxima simply because the jitter/sawtooth noise is a much better trade off against the random skywave propagation delays and associated phase cancellation effects inherent to all such long distance HF broadcast reference standards.

 You only need to bother disciplining an OXCO with a timing GPS receiver module when you need an accurate and stable lab reference to lock T&M kit clocks to or when using transverters to work the GHz bands using SSB or the even more critically narrower BW data modes (or when partaking of the simple pleasure of time nuttery  ).

John

Thanks for that John.

I can receive WWV in the early hours of the morning, but as you say the atmospherics limit its usefulness.

I'm aware that the u-blox timing signal can be reprogrammed to 10MHz but was put off by the reports of jitter which is when I came across the GPSDO concept, and was taken by the simplicity of the Lars design. I proceeded to ignore the warnings about how addictive the search for perfection would become ...

So yes, it's more than I need but it's been an interesting and educational experience. I shall stop when I've cracked the temperature compensation 

Mike

[thinkfat]

By definition, if you cannot "see" the GPSDO instability or accuracy with the test equipment you have, it's axiomatically good enough.

[MIS42N]

I have finally started building my first unit (after being FAR too busy at work for several months).

QUESTION:   Has anyone else built a unit with sinewave 10MHz output?   I wanted to use the Morion MV85 that @Thinkfat suggested might be a good first choice.
But since it is sine instead of square, I can't use a digital output buffer like the 74HC14 or 74AC04.    I am guessing I need a high speed op-amp buffer circuit?
Does anyone have an example output buffer for sine that is known to work well?
[If not, I also have several sqaure wave OCXO, so I could start there if needed]
Opinions, ideas?

Thanks,

Neal

In my first attempt at a GPSDO, I used a sine wave output OCXO, and wanted to use the output as a clock to the microprocessor. This circuit worked because there's no input hysteresis. The 74LS04 switches around 1.1V and for a 50/50 duty cycle the output should average around 2.5V. Hence the string of resistors.

[MIS42N]

I posted a circuit a few days back, maybe people would like to know how it performs. Better than I expected. The OCXO is allowed to drift in the short term but the control voltage is altered to bring it back into phase at intervals. I've attached a days worth of adjustments, a lot of the time it jumps around (due, I think, to GPS having a poor view of the sky) but sometimes (e.g. 172518 UTC to 182501 UTC) it doesn't need adjusting and one assumes is accurate to 10 digits. I have an external antenna on order, see how it goes with that fitted. The 1ppS is captured by a comparator and timer latch in the PIC16F1455, it is good to 25nS. The timer is 16 bit so captures any reasonable frequency swing. The software steers the phase to try and see jitter in the 1ppS, and cheap GPS modules oblige. The processor decodes all the NEMA messages from the GPS, mainly to see if the GPS data is valid. But it also allows the output to be timestamped. The control voltage is dithered at 40kHz so emulates a 24 bit DAC, The output impedance is low (12k ) which should be low enough to minimise noise pick up and drive most OCXOs.
The adjustment list is a sample of the output from the serial port. It actually logs a lot more data, I extracted the adjustments. The method used requires that the sensitivity of the OCXO to changes of control voltage be known. This is achieved in a calibration run of a couple of hours then written to permanent memory for use on subsequent starts. Knowing the sensitivity means the logged adjustments can be reported by the processor in ppb (1 part in 10^-9).
This is still a work in progress. I hope to interest some local radio amateurs who may come up with a PCB design. And a scheme to output the 10MHz.

[Mike99]

I have attached two sets of results, first for the GPSDO with no temperature compensation, the second with an initial stab at compensation.

The second certainly looks more stable apart from during the overnight temperature drop. Any suggestions for improving this?

Mike

[Dbldutch]

I highly recommend you use a small PID controlled fan to control the temperature of the enclosure as I described many posts ago now.
It can keep the inside temperature within fractions of a degree, thereby removing room temperature effects on the OCXO. With that, there is no need to embark on the compensation, which I found rather cumbersome and indirect.

Details can be found on my own blog : http://www.paulvdiyblogs.net/2020/07/a-high-precision-10mhz-gps-disciplined.html
The modifications for the Lars firmware to do that are available on my Github, the link is on the bottom of that Blog.

Enjoy!

[Mike99]

Hi Paul, I have read your blog and I thought you might pop up with this advice 

However given that my needs for this device are not too onerous I'd like to avoid further complication, power consumption and fan noise if possible.

Maybe I'll just avoid using it during the hours of darkness 

Mike

I highly recommend you use a small PID controlled fan to control the temperature of the enclosure as I described many posts ago now.
It can keep the inside temperature within fractions of a degree, thereby removing room temperature effects on the OCXO. With that, there is no need to embark on the compensation, which I found rather cumbersome and indirect.

Details can be found on my own blog : http://www.paulvdiyblogs.net/2020/07/a-high-precision-10mhz-gps-disciplined.html
The modifications for the Lars firmware to do that are available on my Github, the link is on the bottom of that Blog.

Enjoy!

[Dbldutch]

I know you did Mike 
My advice was more for those that didn't go through the whole Blog (yet).

[MIS42N]

The second certainly looks more stable apart from during the overnight temperature drop. Any suggestions for improving this?

I noted somewhere in your previous comments you were looking for "good enough". I have played with this for a few years now, my conclusion is just about any reasonable setup will lock the frequency within 1 part in 10^-9 (or 1Hz/GHz). I talked to some amateur radio people and they said that was good enough for digital working at GHz frequencies. The next step is 1 part in 10^-10 and with a bit of care and decent oscillator is not too hard. This is equivalent to +- 360 degrees phase shift in about 17 minutes. A cheap GPS module will claim most 1ppS within 100nS (equivalent to a spatial fix within 30m) so not unreasonable. And most reasonable OXCO will show negligible drift in that time.

Getting below 1 part in 10^-10 is not so easy. Temperature plays a part and I found that different components appeared to react differently to the rate of change of temperature. So compensating for a particular temperature was partially successful but not the whole story. The answer of course is to ensure the whole kit runs at constant temperature. Then there's the power supply. In my previous rig I used a 17V switch mode supply (old laptop supply) then linear regulators from that to feed various sections of the circuit. Also careful earth wiring, there was a single earth connection between the oscillator and its 12V supply and the microprocessor and its supply. This seemed pretty stable. The rig I described above uses a 5V switch mode but was never intended to be a high performing device - 10^-9 territory.

And unless you have the capacity to average a day's worth of GPS data, a special GPS is highly desirable. My understanding is the same satellites are in the same position in 1 sidereal day (about 23 hours 56 minutes) so one day should be similar to another.

My current thinking is if something consistently better than 1 part in 10^-11 is desired, then run 3 oscillators and have an arbitration system to pick up oscillator glitches. It is unlikely two oscillators glitch at the same time so it should be clear if a glitch is due to the GPS or the oscillator. Glitches occur in the first few months of running a really good double oven OCXO, changes in the mounts, lost mass from the crystal etc. After that they have a fairly consistent ageing which requires a tweak of the control voltage at intervals. I had the job of correcting a standards laboratory oscillator, every week it was calibrated for an hour against a cesium beam. Each day an adjustment was applied on the basis of known drift. The difference at the start of a new week between the adjusted oscillator and the beam was usually below 10^-13.

So that's my thoughts. What's your target? what are you going to be happy with?

[thinkfat]

Hi Paul, I have read your blog and I thought you might pop up with this advice 

However given that my needs for this device are not too onerous I'd like to avoid further complication, power consumption and fan noise if possible.

Maybe I'll just avoid using it during the hours of darkness 

Mike

I highly recommend you use a small PID controlled fan to control the temperature of the enclosure as I described many posts ago now.
It can keep the inside temperature within fractions of a degree, thereby removing room temperature effects on the OCXO. With that, there is no need to embark on the compensation, which I found rather cumbersome and indirect.

Details can be found on my own blog : http://www.paulvdiyblogs.net/2020/07/a-high-precision-10mhz-gps-disciplined.html
The modifications for the Lars firmware to do that are available on my Github, the link is on the bottom of that Blog.

Enjoy!

What I do as a sanity check is run the DAC values through timelab and check the ADEV or MDEV. While this doesn't yield "real" stability values for the output, it gives some kind of "noise floor" for your system. In other words, whatever comes out of the GPSDO, it will never be _better_ than what you see in the DAC ADEV. So, if you have something in the 10^-10 range @1s, you know that your output will never reach 10^-11.

[Mike99]

So that's my thoughts. What's your target? what are you going to be happy with?

I'm using a timing receiver and external antenna which give me a positional accuracy of better than 1 metre. The OCXO is a new old stock Vectron that has been running for about 3 months. It claims +/-10ppb stability over 0-70degC and +/-1ppb aging per day.

The most accurate piece of kit I have is a 9-digit Racal 1991 frequency counter, so calibrating that would be my ultimate goal. I would then be using that to calibrate HF transmitters and receivers, so nothing too demanding.

Mike