[time-nuts] Venus timing receiver PPS output

[Mark Sims]

I recently add the ability to Lady Heather to monitor a PPS output and apply sawtooth correction sto the measured PPS and log the results. 

The Venus timing receivers tout a PPS output with +/- 6 nsec PPS jitter and sawtooth corrections.  Alas, the PPS output does not appear to meet this claim.

The latest receivers that I tested were the Venus/Navspark/Sayer timing receivers that use the Venus GPS timing receiver chip.   I was measuring PPS jitter on the order of -26/+32  nsec, which a +/-6 nsec sawtooth correction message has little useful effect on.  Plotting a histogram of the measured PPS values shows several humps in the histogram that are spaced 12 nsec apart.  It seems that their PPS generator has some issues deciding  just which clock cycle to trigger on.
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