Dear Andy, LF Group,
Not sure if the first mail I sent on this topic ever got through - not to
me, anyway...
I have been using a USB sound card for VLF reception with my lap-top PC,
and, like all the PCs/sound cardsI have tried, it shows what appear to be
transient changes in the sample rate from time to time. It uses an
integrated codec/usb interface chip similar to the device Andy mentioned,
with a 12MHz clock crystal. I measured the actual clock frequency, which is
about +47ppm from nominal. With a suitable stable calibration source, and
48kHz sample rate, Spec Lab indicated a sample rate error of -87ppm. The SR
compensation facility in Spec Lab makes the indicated frequency on the
spectrogram correct to within a fraction of 1ppm. The sample rate calibrator
reflects exactly the slow thermal drift of the sound card crystal, less than
1ppm, but always with the much larger and constant offset of about 130ppm in
total from what you would expect. Changing the sample rate to 44.1kHz gave
an indicated sample rate error of -115ppm. I found by accident that plugging
a USB memory stick into another USB port on the laptop caused the sample
rate to change as well - just plugging it in gave a brief glitch of several
ppm, but opening the folders on the USB stick resulted in an
additional -81ppm shift, which remained until the USB stick was unplugged
again, when the SR returned to the previous value. The actual 12MHz clock
frequency was not noticeably affected by any of this activity.
In addition to these apparently stable shifts in sample rate, transients
occur now and again of several ppm, aparently at random, maybe once an hour
on average. These give rise to glitches in a strong trace on a spectrogram
with resolution about 1mHz. But on weak amateur signals these would probably
not be visible, since the transient sidebands mostly only last for a few
pixels and are 20 or 30dB below the main spectral line, which remains on the
correct frequency. However, I think when one gets into the microhertz
resolution range, it would be a problem, because the transients occur often
enough to merge together and smear out the spectral line.
Cheers, Jim Moritz
73 de M0BMU
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