[Ham-Mac] Accuracy of PSK Offset Cluster Spots

[Chris Smolinski]

>After reflecting on the conversation, I was became concerned that something
>may be wrong with my rig, the cocoaModem software, the Mac G4's sound card A
>to D converter or the operator (me).

D) All of the above (well, I can't speak for the operator ;-)

You are always going to have sampling rate errors in any computer 
sound card, even a Mac. Your radio is not going to accurate to 0.001 
Hz across all of HF.

You can determine your sound card sampling rate error by say tuning 
into WWV when it is transmitting a known audio tone, and verify 
against what an FFT program claims the tone frequency is. Assume for 
the moment that the error is fixed and does not drift (lots of hand 
waving here).

You can then determine the frequency error of your radio at various 
standard frequencies, using WWV, CHU, and even a few other stations 
if you dare, again using an FFT/waterfall function. MultiMode OSX 
works well for this.

You will probably find (in modern rigs anyway) that the frequency 
error is not constant, nor is it a function of the frequency. There 
are probably one or more PLLs in your radio, and perhaps a DDS as 
well. It will likely vary up and down as you tune to different 
frequencies.  If you can gain access to a well calibrated RF 
generator, you can measure the errors on various frequencies of 
interest. Lots more hand waving to assume that it does not drift over 
time.

The real question to ask is: "Is it good enough?" That will of course 
depend on what you're trying to do with your setup.


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Chris Smolinski
Black Cat Systems
http://www.blackcatsystems.com