Audio Card Sampling errors

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W8TOD
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Last seen: 3 years 11 months ago
Joined: 2012/03/04 - 09:52
Audio Card Sampling errors

I posted this on the Softrocks forum and am re-posting here for WSPR ops who use SDR's such as the Softrocks RXTX:

For anyone else in search of precision frequency performance from the Softrocks
radios I would like to share some work I have been doing on sampling rate errors
in the audio cards.

There is a very easy way to check your audio card for sampling rate errors.
Using HDSDR tune WWV or your calibration signal with the LO set as close as
possible to the tune frequency and still avoid the "hump". I use 9.999 for the
LO. Calibrate your Si5170 here.

Then move the LO as far as possible away from WWV so you can just see the signal
at the edge of the spectrum. I did about 9.978 which put the 10 MHz signal just
inside the edge of the display. Now check your 10 MHz calibration again. Any
difference is the result of PC/Soundcard rate errors.

Once you know you have an error it is easily compensated for. You just need to
be sure you calibrate with the offset you normally use. The error amount
increases as you move away from the center frequency.

My beloved iMic showed a total error of more than 3 Hz between the center and
edge points. My E-MU showed about 200 millihertz which would not be resolvable
normally.

The point of this is, for any real precision frequency measurements, you must
account for this sampling error. If you are using the RXTX for WSPR you should
calibrate the Si5170 with a 13500 Hz offset as this is where it would normally
operate on this mode. Simply assuring your radio is calibrated and on frequency
is not sufficient and can easily result in you being 1-2 Hz off, even when you
think you are precisely on frequency.

I have been using an RXTX referenced to GPS for 20 meter precision WSPR work and
finding I still had 20 millihertz of unexplainable error.

Thanks to Light Coder from the HDSDR development team in helping me to
understand this came from the sound card and guiding me through correcting it.