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Re: SNR measurements

Hi Lee,
Stan had some good comments on S/N measurements, mostly paralleling what I 
would say.
I am not familiar with the "Spectrian" product/program, but unless it's 
directly calibrated/programmed to read in S/N, then comparing the peak of a 
narrowband signal to your (relatively broadband) passband noise is not the 
same as S/N.  The only way that your measurement matches S/N is if the 
passband is bandwidth-matched to the signal you're measuring.
And of course, if Spectrian does read directly in S/N or SINAD, then my 
comments are not applicable.

More on Stan's comments...you don't really need to modulate an SSB signal 
with a 1kHz tone; a CW tone with the downlink frequency tuned such that the 
demodulated audio is 1kHz will do the same thing :-)  Although then you get 
into the question of what filter options are available with which modes for 
your particular radio.  Make sure what you test matches your normal 
operating setup.

I have measured my "total station S/N" using a soundcard program that 
includes a specific algorithm for displaying SINAD.  That's partly why/how 
I came up with my modification to Gene's link budget spreadsheet...I can 
input my station parameters along with a maximum TX power (how big my amp 
is), and the modified spreadsheet will calculate S/N for me at maximum TX 
power (subject to LEILA).  I have matched up the calculated S/N to the 
soundcard program measurement +/-1dB for a variety of orbital 
configurations (range/squint/elevation).

At 09:05 2003-02-17, Lee wrote:
>I've been trying to make signal to noise measurements on AO-40 over the
>weekend and seem to be getting numbers better than I would expect.  As a
>result I'm inclined to believe that my technique is in error since sun &
>ground noise measurements result in fairly reasonable values for system NF
>and gain.  What would be the best approach or software to use for making SNR
>measurements?  I would particularly like to find a technique for measuring
>the SNR of the MB's BPSK signal since that eliminates a lot of uplink
>related variables.
>Using Spectran, I tried measuring from the null in the center of the middle
>beacon to the peak during the idle period between blocks and get a value of
>30 dB when the transponder is on and 33 dB during the mode-K window from MA
>124-126.  While I suspect the absolute numbers, the change is consistent
>with the drop in noise floor that I see when the receivers are disconnected
>from the S2 tx.
>Following the mode-K break I looked at CW carriers at two power levels. 14w
>EIRP resulted in Spectran showing a carrier peak 11 dB above the noise
>floor.  Increasing the power to 140w EIRP resulted in an increase of the
>peak to 16 dB. During the measurements the U-Rx AGC varied from 6 to 13 dB
>with an average value of 10 dB.
>Comments and suggestions would be greatly appreciated.
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Scott Townley NX7U
Gilbert, AZ  DM43di

Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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