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 OFF
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@philkarn.net To: "Amsat - BBs" amsat-bb@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 _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb