[time-nuts] Question about SoundCard stability?

[Mark Spencer]

Just curious, if you have set your radio to AM mode to remove any variations due 
to wonder in the radio LO, how would any minor deviations in the TCXO of the 
radio affect the measurement ?  If you are looking at the difference between a 
10.000000 mhz carrier and a 10.000100 mhz sub carrier wouldn't the difference 
always appear to be 100 hz in AM mode even if your TCXO drifted slightly ?

You might be seeing the affects of varrying doppler shift between WWV and WWVH 
(although I'd expect the variation to be larger.)

Sorry it's been a long day for me and I may be missing something obvious (:

All the best Mark Spencer




----- Original Message ----
From: David McClain <dbm at refined-audiometrics.com>
To: time-nuts at febo.com
Sent: Tue, October 12, 2010 4:48:56 PM
Subject: [time-nuts] Question about SoundCard stability?

Hi,

I have a Flex-3000 receiver, running freely on its internal TCXO (0.1 ppm). I 
have been recording the reported deviations in the measurements of the 100 Hz 
sizeband of 10 MHz WWV all day long. I do this in AM detection mode, to remove 
any variations due to the wander in the radio LO. Recording from the 48 kHz 
audio stream and into SpectrumLab for analysis.

After taking out the measured frequency error in the "soundcard" (which I 
believe is the Flex Radio internal CODEC), of 17.5 mHz, I'm seeing frequency 
deviations of 0.2 mHz RMS, and +/- 1 mHz p-p, with no measurable long term 
drift. The FFT uses a bin size of 11.44 mHz. SpectrumLab interpolates to the 
peak assuming high SNR and a Hann window, with no nearby interference.

This implies that my "soundcard" is giving me a stability of 0.004 ppm, or 
around 10^-9. How can this be? I already know that the TCXO wanders about by as 
much as +/- 1 Hz at 10 MHz due to temperature variations in the room. (10^-7). 
If that TCXO were used to derive the soundcard (CODEC) clock then I should be 
seeing variations of 25x larger. And I can assure you the CODEC is *not* being 
driven by an OCXO or GPSDO of any kind.

Anyone?

Dr. David McClain, N7AIG
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ  85750

email: dbm at refined-audiometrics.com
phone: 1.520.390.3995
web: http://refined-audiometrics.com



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