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Re: MB signal-to-noise ratio evaluation



>        In the process of setting my AO-40 downlink, I have a little
dilemma:
>how to evaluate the signal-to noise ratio of the received beacon signal.
Using
>the ao40 excel evaluation spreadsheet, the predicted value should be around
23
>dB. AO40Rcv decoder gives me an aproximate value of 21 dB in the graphical
>display ( my evaluation ) and a value of 15 dB at best as the new SNR
facility
>added in release 2.0.
>        My question is: which one of the last values are correct?
>        My setup:       - 90 cm no-name offset dish
>                        - 5.25 turn helix feed
>                        - AIDC3731, 1 dB NF, 37dB gain
>                        - IC706Mk2G as telemetry receiver ( will be
replaced by
>an IC820 when antenna setup will be finished).
>        My feeling is that somewhere, something in my setup is not working
>properly, but I don't know what. Any sugestions are welcome.
>
>        73' de yo3gcf, Dragos

The SNR value displayed in AO40Rcv 2.0x should be viewed very cautiously.
It is derived from some BPSK signal parameters and is only valid at low S/N
values.  It clamps at a fairly low level.  It also is dependent on IF
passband shape, doppler, audio levels, AGC and probably a bunch of other
things.  Its purpose was to show the effectiveness of the FEC at low signal
levels and not to judge overall system quaility.  It may be of some use for
relative measurements but is of little use as an absolute value to compare
with other setups.

Moe, ae4jy
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