[time-nuts] Lucent RFTFm-II-XO
scott.j.stobbe at gmail.com
Thu Sep 22 16:05:04 EDT 2016
You may even be lucky enough to see brief periods of phase reversal on some
of the amplifiers on the input chain, adding more edges.
On Thu, Sep 22, 2016 at 3:47 PM, Charles Steinmetz <csteinmetz at yandex.com>
> 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
> 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"
> 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.
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