Message-ID: <389B289A.6939@xtra.co.nz>
Date: Sat, 05 Feb 2000 08:29:30 +1300
From: "vernall" <vernall@xtra.co.nz>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: 73kHz / Soundcards
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James Moritz wrote:
snip snip 
> Since soundcards and software like Spectrogram have many other
> uses to radio amateurs apart from reading QRSS, it would be
> interesting to know if any data exists about the dynamic range &
> spurious responses of the soundcard/software combination - does
> anyone have any information?

Also bear in mind the performance of the audio stages in the radio
receiver feeding the sound card.  Some time back I was testing IF
filters in a receiver, viewing demodulated output for feeding wideband
noise to the RF input, and the sound card "spectrum analyser" gave a
false indication compared to doing the test at RF.  On investigating,
the explanation was that harmomic distortion in the audio stages was
where the "apparent funny response" was arising.  As the sound card was
intended for "hi fi stereo" it was better performing for distortion and
intermod than my receiver audio lineup.  Also there is unlikely to be no
hum from the receiver PSU being impressed on the audio output.  I came
to a conclusion that the sound card was good at indicating filter
PASSBANDS, but suspect for indicating steep skirts and stop bands. 
Audio noise and intermod can easily fill in deep notches on an IF
response as indicated on the PC display.

I have never been happy about the "lines" on the very low frequency end
of a spectrogram display with no audio fed to a sound card (irrespective
of whether the audio is open, shorted or 600 ohms shielded load).  This
is some sort of low frequency switching breakthrough in the sound card
itself, and varies between specific cards (and probably with sampling
rate on the card?).  As my PC was under warranty I did not fiddle about
with it, but in due course I may try added shielding and filtering of
power supply rails.  Some PCs have the "sound card" implemented on the
motherboard, so fiddling would be more difficult with those.

Sound card application programmes are generally good, but be cautious in
the interpretation of audio domain displays when attempting to use it
for RF testing. 

73, Bob ZL2CA