[time-nuts] Lucent RFTFm-II-XO

Charles Steinmetz csteinmetz at yandex.com
Thu Sep 22 15:47:33 EDT 2016


Bill wrote:

> If you could,  would you see if you can take the pps out of your lucent box and use it to sync the pc sound card clock via continuous calibration for  using SL for fx measurements.  Maybe by lucent box pps output is bad.

The PPS directly out of most GPSDOs is too fast for sound cards.  But 
once you stretch it, there is another problem.  Assuming 5v logic, and 
knowing that the sound card is AC coupled, the leading edge of the 
(assumed positive-going) pulse will produce a 5v spike that decays over 
a time period that depends on the post-capacitor DC resistance inside 
the sound card.  It might decay fully (back to 0v) during the pulse, or 
less than that.

The falling edge of the pulse will generate a negative-going spike with 
a peak negative voltage of [decayed voltage after leading edge minus 
5v].  This could be anywhere from just slightly negative to the full 
logic supply (if the post-capacitor voltage decays fully before the 
falling edge occurs).  The same would be true for 3.3v logic, 
substituting "3.3" for "5" above.

These peak voltages are way outside the input range of most sound cards, 
so the positive spike is almost certainly going to overload the ADC, 
with unpredictable (and, very likely, unfortunate) results.

At the very least, you will need some sort of sound-card oscilloscope 
program so you can see what the sound card is actually capturing.  Then 
you can start shaping your trigger pulses (with voltage clamping and 
possibly LP filtering) to give predictable and stable results.

Best regards,

Charles





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