[sdiy] VCO Jitter, Slop ... (was: Re: Smith's Evolver Desktop's spikey VCO waveforms)
ijfritz at comcast.net
Tue Jun 11 20:26:22 CEST 2013
For another look at that old Moogy Boogy jitter/drift question, I have just
done a test on one of my analog VCOs. This is the Dial-a-Tempco VCO, which
has trimmer adjustments to tune out both exponential and linear temperature
Recently someone posted a question as to why analog VCOs drift so
much. Pressed on this, he said that he could hear an obvious short term
drift of ~2 cents on a module he was using. A 2-cent drift is about 0.1Hz
on a 1kHz signal. This much change can easily be resolved on the frequency
meter of a good DVM.
So I measured the frequency output of the DaT VCO at 1kHz for two hours
using a B&K 2890A multimeter. The update rate on the frequency counter is
1.2sec, so any variations on the time scale of 2sec to 2hr would be
observable. First I watched the meter constantly for a few minutes, then I
took readings every 10min.
The result? The frequency counter *always* read *exactly* 1.0132kHz. I
never saw even a flicker in the last digit. Since the last digit represent
0.2 cents, it is clear that there was nothing like a 2 cent variation (this
would be a digit in the 4th place of the 5 place display).
So, just one result on one VCO. Conclusions? I'll let you draw your own. :-)
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