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Date: Thu, 10 Mar 2011 18:55:17 +0100
From: wolf_dl4yhf <dl4yhf@freenet.de>
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Subject: Re: LF: Re: VLF Stability and soundcard locking
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Hello Jim, Andy, and group,

Many thanks for the detailed analysis - having an easy-to-build GPSDS 
(GPS-disciplined soundcard) would have been really nice.
Anyway, if you have a GPS receiver with 1-pps-output, it can be 
connected to the 2nd input of the soundcard. By examining the 
pps-signal, a software could detect 'missing samples', and -if the NMEA 
sentences are fed to the same input- even compensate dropouts which last 
longer than a second by simply inserting 'silence' into the sample 
stream which enters the FFT. Not perfect, a bit tricky in software, but 
minimum hardware requirements.
For example, the Garmin GPS 18x "VLC" which I use here at the moment 
sends the NMEA string in the middle of the second-period, so both 
(serial data and sync pulse) could be added with a passive network. I am 
still working on the best way to reconstruct the pps-position with a 
resolution better than the sampling interval. At the moment, the pulse 
is turned into a ramp with a simple RC lowpass, but I'm not sure if this 
is the best way to do it.

All the best,
   Wolf .

Am 10.03.2011 02:28, schrieb James Moritz:
> 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
>
>
>