[time-nuts] Lucent RFTFm-II-XO
bill.riches at verizon.net
Thu Sep 22 17:02:53 EDT 2016
Thanks for info - makes sense. With SL Lucent pulse did not work - Jackson unit did with no mods. Guess I was lucky!
From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Charles Steinmetz
Sent: Thursday, September 22, 2016 3:48 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Lucent RFTFm-II-XO
> 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.
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