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Re: AO40.XLS and Receiver Bandwidth



As long as there is no AGC pumping in the radio's IF, the receiver bandwidth
of your receiver IS the DSP filter's bandwidth...at least roughly.

So...if you have a 6 khz wide IF filter followed by a 400 hz dsp filter and
there is no "interference" and there is no AGC pumping, then yes. you could
enter 400 hz and it would be right.

The spreadsheet is absolutely correct. Your receiver bandwidth with a dsp
filter is whatever the dsp filter is...no magic in this.

Wanna make your station look really good? Remember what CW is? Put in a rx
bandwidth of 250 hz instead of 2500 hz and you will see an improvemnt of 10
dB in MDS.

Want to evaluate the effect of various rx bandwidths:

10*log(bw1/bw2) = dB change. Compare 2500 hz to 1800 hz for a narrow ssb
filter. The narrower filter will always produce an improvement in MDS. To
evaluate systems and  especially to make comparisions with others, it is
VERY important to keep several things constant or we will have no idea what
we are "comparing".

Range: 65,000 km
Freq: 2400 mhz
Squint: 0
Rx NF: 13
Rx Bandwidth: 2500

These are the defaults for the program, I think. Basically at apogee with no
squint and a typical multi-mode radio with ssb filter.

73

hasan schiers, NØAN
schiers@netins.net

----- Original Message -----
From: "Douglas Braun NA1DB" <doug@dougbraun.com>
To: <amsat-bb@AMSAT.Org>
Sent: Saturday, May 26, 2001 10:05 PM
Subject: [amsat-bb] AO40.XLS and Receiver Bandwidth


> Hello,
>
> I was playing with the AO40 Spreadsheet, and I noticed that the
> Beacon S/N value is affected by the choice of receiver bandwidth.
> This would make sense if you wanted to compare the signal level with
> the total noise level, but that's not really what we want to do, right?
> For the beacon, shouldn't the true receive bandwidth be determined not by
> the receiver bandwidth, but by the bandwidth of the filters
> in the demodulator (either hardware or DSP-based)?
>
> I guess that the effective bandwidth of a 400-baud PSK demodulator is 1kHz
> or so.  If you replace the 2500 Hz default value in the spreadsheet
> with 1000 Hz, you get almost a 3 dB change in beacon S/N!
>
> I noticed that if I switch my receiver from its 3 kHz to its 6 filter,
> the quality of by beacon reception is not affected.  Also when I
> switch filters, the spectrum display in AO40rcv does not show a change
> in the SNR of the beacon itself;it simply shows more or less noise at
> frequencies outside of the range of the beacon's bandwidth.
>
> Doug
> NA1DB
>
> ----
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>

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