[time-nuts] Sound Card Spectrum Analyzer

Bob Camp lists at cq.nu
Fri Feb 19 02:42:38 UTC 2010


Hi

The only reason the 3561 takes in the beat note is because the front end gain changes automatically. Absolutely no different than switching an external preamp except the calibration sticker says not to worry about it. 

Yes indeed I have had that calibration sticker lie to me on occasion.....

Bob


On Feb 18, 2010, at 9:36 PM, John Miles wrote:

> 
>> 
>> If the noise is "known flat" it's a good way to check system
>> bandwidth. Some means of checking response is indeed very necessary.
>> 
>> Not switching the preamp is indeed a good thing. The sound card
>> does not have the range of the 3561, so with the sound card the
>> beat note absolutely require a switch. The switch adds a second
>> calibration step at audio.
> 
> Maybe not -- the 3561A has nowhere near the dynamic range of the sound card
> (13 bits versus 20+, or about 80 dB versus ~117 dB for a good sound card).
> 
> Normally you do beatnote calibration by injecting a signal at (level of
> DUT - gain of LNA), or 40 dB down for the 11729C.  Say your maximum PN level
> of interest is -50 dBc/Hz at 1 Hz.  If you adjust the level going into the
> sound card to position the full, HPF'ed test signal near the sound card's
> rails, you'll have a theoretical floor near -170 dBc/Hz.  Your signal
> generator might be set for -60 dB relative to the DUT input amplitude in
> this scenario, yielding a clean calibration spur without switching gains
> anywhere.
> 
> The signal generator used to inject the beatnote for calibration doesn't
> need to be anything too fancy, just something with a good attenuator.
> 
>> Many of the free applications that are out there will put a tone
>> out of the card and track it back into the card. That should at
>> least provide a tone to work with. I also should be something
>> that can be fairly easily verified. The issue of mixer output
>> impedance is still a little tricky without RF noise loading.
> 
> It does need to be an RF tone, though, to characterize the whole signal path
> (and even then you'd really like to do it with broadband noise if it were
> practical, which it's not).  If you haven't looked over HP note 11729B-1 (
> http://www.ke5fx.com/gpib/5952-8286E.pdf ) it will give you a good refresher
> on what you're up against.
> 
> -- john, KE5FX
> 
> 
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