[time-nuts] Re: Compairing the phase drifts between two PPS signals

Wed Oct 5 08:47:31 UTC 2022

Hi Mayukh,

I implemented an SDR-based (USRP N210/X310 w/ LFRX daughterboards) 
solution for simultaneous measurement of sine and pulse signals ~5 years 
ago. It achieved 360 fs 1\sigma accuracy for 10 MHz signals and 16.6 ps 
1\sigma for pulse signals. [1]

We also work with the RFSoC. I believe you should be able to implement a 
similar, sampling-based solution with it if you have ADC channels to 
spare. If your RFSoC board implements multi-tile synchronization (MTS) 
you could coherently sample the 1PPS, your down-mixed interferometry RF 
signal, and any associated reference signals. With a shared, low phase 
noise sampling clock that approach should enable the best possible 
accuracy and precision compared to using additional external sensors.

Best regards,
Carsten

[1] https://arxiv.org/abs/1803.01438

On 04.10.22 22:29, Mayukh Bagchi via time-nuts wrote:
> Hey Thomas and Others,
>
> Thanks for your input!
>
> I could explain my background and goals a bit more in-depth.
>
> So we here at my university are building a balloon-borne VLBI station that will observe a bright radio source at K-band simultaneously with a large ground-based radio dish. Our balloon will be flying 35-40kms in the stratosphere.
>
> To do a correlation of the signals later on( and hence interferometry/VLBI)  we would need to be able to track the position of our balloon to a fraction of the wavelength at which we are observing (which is 1.3cm). Our positional tracking requirements are down to a few millimeters. For such position tracking requirements, we are using a high-precision GPS unit with differential correctional services (ex: Hemisphere GNSS, etc). For the shorter timescales, we are using a Kalman filter to fuse accelerometer and gyroscope solutions.
>
> Of course, a key aspect of doing interferometry would be to be able to timestamp our data very precisely. For that reason, we are using a pretty expensive RAKON OCXO(RK409) having an ADEV of about 6*10^-11.
>
> Another important requirement will be to have excellent phase offset information for our own clocks. The plan over here is to be able to measure and record the phase drifts between our onboard OCXO with the GPS. Initially, I was planning to implement this in our FPGA board that we are already using for our IF signal processing (we are using the RFSoC). However, a lot of people quickly pointed out that this could be a problem as I would need to manually override the internal clocks of the RFSoC FPGA, and even then there might be jitters.
>
> To completely avoid that I wanted to use a TDC solution. Hence I thought of asking you all!
>
>  From what I understand going through Thomas's (and John's) email and please correct me if I am wrong, I would be using our stable OCXO clock to run the TICC and then feed that same signal(10 MHz) into one of the input signal ports. In that case, would I also need to square the sine wave output of my OCXO? for that purpose would an LTC 6957 (with a CMOS logic output) be useful?
> And do I just insert the 1 PPS from the GPS into the other signal port of the TICC? Won't there be an issue as I am trying to measure two signals with different frequencies i.e 10MHz and 1 PPS? I guess I didn't quite understand that well.
>
> Also, do you think overall the TICC would fit my case? Would I be able to measure the phase drift information well enough using this setup?
>
> I would then just feed this phase drift information through my FPGA and simply store them in the disks.
> Also, any input on the position tracking system would be also highly appreciated!
>
> Thanks for your time and help!
>
> Regards,
> Mayukh
>
> Mayukh Bagchi (He/Him), MSc
> Graduate Student
> Department of Physics, Engineering Physics, and Astronomy
>
> mayukh.bagchi at queensu.ca<mailto:mayukh.bagchi at queensu.ca>
> [Queen's University Logo]<https://www.queensu.ca/>
> ________________________________
> From: Thomas Abbott <thomas at reversebiased.com>
> Sent: Tuesday, October 4, 2022 2:16 AM
> To: Mayukh Bagchi <mayukh.bagchi at queensu.ca>
> Subject: Re: Compairing the phase drifts between two PPS signals
>
> Hi Mayukh,
>
> I've done this recently and can make a few suggestions.
> But first - what's your real goal here?
>
> If you're looking to get good data about the OCXO or the GPS and have a several thousand dollar budget, then you should just buy the necessary professional equipment. My previous company did this:
> - OCXO and divider logic
> - GPS receiver in a case, with PPS & 10 MHz coaxial output
> - Pendulum or Keysight time interval counter
> - GHz oscilloscope for sanity check of evrerything
> - Laptop etc.
>
> But if you're looking for the minimal set of equipment to do some basic time-nut experiments, (more like me now), then you can learn a lot by hooking up
> - 10 MHz OCXO
> - basic ebay / amazon GPS board
> - some sort of time interval counter / timestamping counter
> - computer for logging things.
> - 100 MHz oscilloscope
> In this case you'll spend much more time debugging the equipment and its issues, than characterising the OCXO and/or the GPS.
>
> I've used both the TDC7200 Click demo board, which is fine but you have to deal with its quirks, and also Marek Dorsic's PicoPET<https://can01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fdorsic%2FPicoPET&data=05%7C01%7Cmayukh.bagchi%40queensu.ca%7Cb89cdbac409e49a92b4e08daa5d00352%7Cd61ecb3b38b142d582c4efb2838b925c%7C1%7C0%7C638004610069460879%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=%2B96GgMXvg%2BXc7m2M71EMXhP21g%2Ft2wdZjR9pUC2Rf%2FQ%3D&reserved=0> (see the list from a year ago) which is a raspberry pi pico modified to accept an external clock and used to timestamp the GPS PPS signals. You need some electronics common sense to avoid issues like spurious signal pickup (false PPS signals) and dropped clock pulses, and also coupling between channels leading to pulling of the TDC results. Breadboard construction is OK to get started but will drive you crazy when trying to get real data, it will b
>   e full of glitches when someone turns on a printer two offices away.
>
> I recommend clocking the PicoPET (or the TICC) directly from the 10 MHz OCXO, not dividing it down to 1PPS and then capturing the timestamps of the 1PPS of the GPS. Otherwise the two PPSs drift apart until you have whole microseconds of difference, and then the accuracy of the TIC clock starts to matter too. Or worse, the pulses cross over and the TIC stops counting because Stop happens before Start.
>
> If you use a TDC7200, I suggest squaring up the OCXO if it isn't already a square wave, and using it as the "stop" channel. Then you have a Stop signal every 100 ns, and it will count from the GPS pulse to the soonest available clock edge (within its limits), This way it's never counting too long, so its timebase doesn't matter too much. I got to this point when I realised the clock sine wave wasn't triggering the TDC reliably, and that I had bad coupling between channels, leading to very non-uniform distribution of time values. Hence the recommendation about soldering everything and using proper ground planes and metal boxes.
>
> Finally you need to capture and use the TIM_TP messages from the GPS, telling you how far ahead/behind the true 1 second edge the next PPS will be.
>
> I plotted everything in Timelab, after some manual repair of the files, glitch removal etc, in python. It's not difficult to learn, accepts all kinds of input formats and plots many files easily.
>
> Hope this helps for starters, I'm happy to answer more questions off-list.
>
>
> Regards, Thomas
>
> Thomas Abbott
> thomas at reversebiased.com<mailto:thomas at reversebiased.com>    +1 604 365 7671
>
>
> On Mon, 3 Oct 2022 at 13:43, Mayukh Bagchi via time-nuts <time-nuts at lists.febo.com<mailto:time-nuts at lists.febo.com>> wrote:
> Hello,
>
> I am trying to compare the phase drifts between a 1PPS signal generated by a GPS receiver with a 1PPS from an OCXO( using a frequency divider to convert 10MHz to 1 PPS).
> I was thinking of using a TDC like the texas instruments TDC7200 boards.
> Do any of you have an idea for doing this?
>
> Let me know if you need more background on the kind of work I am trying to achieve.
>
> Thanks,
> Mayukh
>
> Mayukh Bagchi (He/Him), MSc
> Graduate Student
> Department of Physics, Engineering Physics, and Astronomy
>
> mayukh.bagchi at queensu.ca<mailto:mayukh.bagchi at queensu.ca><mailto:mayukh.bagchi at queensu.ca<mailto:mayukh.bagchi at queensu.ca>>
> [Queen's University Logo]<https://www.queensu.ca/>
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