[amsat-bb] Re: Turn off AGC when receiving BPSK-1000

i8cvs domenico.i8cvs at tin.it
Tue Aug 16 21:47:48 PDT 2011

Hi Phil, KA9Q

Since modern VHF/UHF receivers do not supply AGC switching
ON/OFF I use and old HF Drake R-4C receiver tuned from 28 to 30
MHz with a 144/146 MHz receiving converter in front of it having
a 28/30 MHz IF

In general the old HF receivers supply AGC slow-medium-fast and

The above setup is usefull as well for Noise Figure measurements.

If a old HF receiver and a 2 meters converter are not available it is
possible to use a modern receiver reducing the RF gain belove the
AGC threshold level.

73" de

i8CVS Domenico

----- Original Message -----
From: "Phil Karn" <karn at philkarn.net>
To: "Amsat - BBs" <amsat-bb at amsat.org>
Sent: Tuesday, August 16, 2011 11:31 PM
Subject: [amsat-bb] Turn off AGC when receiving BPSK-1000

> I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000
> telemetry beacon: turn off your receiver AGC if at all possible. If you
> can only choose between fast and slow, pick slow. If this causes a large
> variation in audio level, reduce the gain to avoid clipping on the
> peaks. A sound card A/D is 16 bits so you have plenty of dynamic range;
> don't be afraid to use it.
> Ideally the background noise level should be constant with the signal
> going up and down.
> This greatly helps the demodulator and decoder to distinguish signal
> from noise. The error correction uses the Viterbi algorithm, and one of
> its big features is the ability to distinguish between "strong" and
> "weak" bits; a strong '1' or '0' is considered less likely to be in
> error than a weak '1' or '0'. The decoder can even accept "I don't know"
> for a limited number of bits.
> The decoder can still fix errors in strong bits. But it can fix more of
> them in the weak bits and still more in the "I don't knows" (known
> technically as "erasures").
> This is especially important when the signal fades deeply, as it often
> does with ARISSat-1. With the AGC off, the audio signal level falls
> during a fade and the decoder can recognize it as a burst of erasures or
> near-erasures.
> As with many questions in life, "I don't know" or "I think it's X but
> I'm not sure" are better answers than being sure of the wrong answer.
> 73, Phil
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