[time-nuts] Sound Card Spectrum Analyzer

Bob Camp lists at cq.nu
Fri Feb 19 12:38:24 UTC 2010


Hi

A lot would depend on the phase noise of the source you were looking at. Without amplification the 346 is going to drop -170 dbc/Hz noise onto a 10 dbm source.  Best guess is that you need 30 or 40 db more noise than that. As soon as you start adding amps keeping things accurate gets increasingly difficult. You need enough noise at the carrier, but you don't want the total RMS delivered to the mixer to be to crazy. 

Calibrate with dual slopes, frequency response check with an RF carrier out of a signal generator. Does the job, works with basement level gear.

Bob


On Feb 19, 2010, at 2:35 AM, Gerhard Hoffmann wrote:

> Bob Camp 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. 
>> To calibrate level, the levels of both the noise and carrier need to be well known. For a "basement system" the measure the slope as it crosses zero is likely more accurate. That of course assumes that you do the audio gain and response stuff properly. 
>> 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. 
> 
> One of those 346 A/B/C noise sources with 15 dB excess noise should be about right
> to check the noise floor.
> 
> <http://freenet-homepage.de/dl4yhf/spectra1.html>
> looks like a good FFT program.
> 
> regards, Gerhard
> 
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