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