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" <k...@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 _______________________________________________ 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
