[amsat-bb] SNR measurements

["Lee" <lee-fl@xxxxxxxxxx>]

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.

73,
Lee-KU4OS

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