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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <AANLkTiksDowcKxyJ86R=p-4SqvwcbAQBjQrGY1efm_jN@mail.gmail.com> <003201cbde72$606d6090$0401a8c0@xphd97xgq27nyf> <543492.45357.qm@web28102.mail.ukl.yahoo.com> <998C9385-2AEE-4C44-91FE-86FE85F09BA9@gmail.com>
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Date: Thu, 10 Mar 2011 01:28:16 -0000
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Subject: LF: Re: VLF Stability and soundcard locking
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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