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Fwd: Re: Receiver performance



Hello,

Below is a good tip for doing noise measurements.  The advantage
of using CoolEdit on a captured sound sample is that you can
get a power level reading averaged over as much time as you want,
to reduce the random variation of the signal level.  I captured
30 seconds of sky and sun noise signal at a time, and the variation 
was something like 0.01 or 0.02 dB.  If you average only one
second, there is about a 0.1 dB uncertainty.

If you are a perfectionist, you could also use Cool Edit to
filter your sound sample to a 300-2500 Hz bandwidth, to eliminate
errors due to soundcard DC offset, AC hum, out-of-band audio
noise, etc.

With my 2-foot dish I see 1.0 dB of sun noise.  Ed Cole's g-t.xls
spreadsheet says I should be getting at least 2dB.
I think I will try making one of those cupped-reflector helix feeds,
and seeing if it helps.

Everybody should do the sun-noise test.
It does not lie.  Estimating your system performance by measuring
the AO-40 beacon strength is a much less accurate and repeatable
technique, since it depends on the satellite's squint angle, 
transmitter output, etc.

Doug
NA1DB

>CoolEdit will do RMS averaging. Check under Analyze | Statistics
>
>http:// www.syntrillium.com
>
>73,
>Mike, N1JEZ

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