Generate a 100 kHz pulse train triggered by the GPS PPS with the NI-6614 card

[Witschas82]

Dear Labview-Experts, 

I am currently struggling with generating a PPS-triggered reference clock for synchronizing different processes in a wind lidar system that I am currently developing.

Principally, my goal is trivial, but I could unfortunately find no possibility to reach it. 

For the PXIe chassis, I use the 10 MHz ref_clk of the OCXO on-board the NI-6614. Now, I want to use a counter to generate a 100 kHz pulse train triggered by the PPS signal a GPS module (Septentrio PolarRx2e).

The code I am currently using looks as follows:

The frequency of the pulse train is set to 100 kHz.

When I now set the number of finite pulses to be also 100 kHz, I run into a problem that I cannot solve. As the pulse train is not finshed (by a few ns) until the next PPS signal apears, it is not re-triggering the pulse train. 

When I set the number of pulses to be 99999, it is working as it should (blue line = PPS, white line = pulse train):

However, in this case, I am missing 1/100000 counts, which I want to avoid.

Do I have a logic-error in my thinking? Is there any easy work-arround for my problem?

I am happy for any suggestion that I can try and appreciate any reply. 

Thanks a lot in advance and best regards,

Benjamin

[santo_13]

Can you please share a timing diagram of the PPS signal and expected 100kHz pulse relationship that you want to achieve?

[Witschas82]

Dear Santo_13,

thanks again for your fast reply. Sure, I will share my plannen timing diagram and appologize for the ppt-made sketch. If you needed something more professionell, please let me know!

The blue line indicates the pps signal. Triggered by the pps, I want to start a clock signal, e.g. a pulse train with 100 kHz, that devides the 1 second between sucessive PPS in 0.01 ms intervals. After 1 s, when the next PPS is detected, the next 100 kHz train should start.

When I use continuous samples, this partly works. However, after a few minutes, the 100 kHz clock is not in phase with the PPS by a few ms (which would be no showstopper for me, but I do not like the imperfect way).

When I apply the finite 100 kHz pulse train, it is not completely finished until the next pps occurs. 

A workarround in a previous instrument: We had a custum-built OCXO with a poti that enabled us to very slightly accelerate the OXCO frequency, such that we made sure that the OCXO was finished with 100 kHz before the next PPS pulse was detected. But as far as I could read, there is no way of doing something similar with the onboard OCXO. 

I hope all of this makes sense. If not, please let me know.

Thanks for your support and best regards,

Benjamin

[santo_13 · Accepted solution]

So, your concern is that the 100kHz pulse train generated by 6614 in 1s does not perfectly align with PPS signal from GPS. You can fix this only if your 10MHz OCXO is in turn disciplined by the PPS signal.

Now, in your case, 100kHz OCXO's accuracy, drift and phase will not exactly match with PPS' over time though you try to match it using triggers.

An appropriate solution will need a GPS disciplined 10MHz clock (like FS752), which drives the PXI chassis and in turn 6614 uses this 10MHz clock as reference instead of OCXO, this ensures all hardware are in the same/derived clock domain.

[Witschas82]

Dear Santo_13,

thanks a lot for summarizing my needs and for suggesting a solution. Indeed, I though that I can match the de-coupled OXCO to the GPS PPS by retriggering a pulse train although it is not completely finished. But this seems not to work.

I will now test how much the time drift would be during an 8 h measurement flight. If acceptable, I would leave it as is. If not, I will go for an GPS disciplined reference clock.

Thanks a lot for your time and support and best regards,

 Benjamin