On Jul 8, 2009, at 5:04 PM, Robert Jefferis wrote:
It seems there is more than one mis-named control. I am still trying to figure out just what the AGC-T is really doing. Seems as though it might be Pwr SDR version dependent? Really, I am not whining, I would just like to get a good handle on things. I like my fairly new F5KA very much. I am actually considering the sale of a boat-anchor or two (gasp! - perish the thought!).
Let me take a stab at it and I am looking for correction because I am guessing that there are errors in what I am about to write.
First, a receiver has an intrinsic gain. It amplifies the low-level RF signal so that it produces a baseband (audio) signal that is some number of dB stronger than the input signal. OTOH, The problem is, this gain is probably good when the signal is weak and way too much when the signal is stronger so AGC comes into the play to reduce the overall gain of the receiver after a certain level is reached. Up to that threshold, the strength of the baseband (audio) signal increases at the same rate (constant gain) as the input signal until one reaches the AGC threshold value.
At the threshold value the AGC begins to reduce the gain of the receiver. How much it reduces the gain is dependent on the transfer function chosen by the designer of the receiver. Some people like a very flat baseband output, i.e. the gain of the receiver is reduced by exactly the same amount as the signal is stronger than the threshold. The result is an baseband output that is constant regardless of signal strength. I would think of this as a flat AGC characteristic or having a slope of zero. Some people like the idea of having a non-zero slope such that the output increases by say 1dB for every 10dB that the signal increases above the threshold. I would think of this as a 10:1 slope for the receiver gain. In essences the AGC reduces the receiver gain by 9dB for every 10dB increase in signal strength. This makes stronger signals louder but not a lot louder. Just enough so you can tell by ear which signal is strongest but there is no threat to your hearing should a really strong signal pop up.
So you have two very important controls: threshold and slope. You set the slope to your preference and leave it alone. The threshold probably needs to be set depending on band conditions. You probably want to set the AGC threshold to about 10dB above the noise so that weak signals are allowed to use the full gain of the receiver prior to AGC starting to act. Since the noise floor varies from band to band and hour to hour, you need to reset the threshold in order to allow AGC to begin to act when S:N reaches a certain point.
Based on the suggestions here and in the KB article, I have taken to reducing the AGC-T control to just start to attenuate the noise and then increase it just a bit so that the noise is right on the threshold. That seems to give the best result for weak signals and yet the AGC quickly brings stronger signals into line so I don't need to ride the AF gain at all.
-- 73 de Brian, WB6RQN/J79BPL Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/ Message delivered to arch...@mail-archive.com
