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So my shortwave radio has adjustable AGC constant with settings between 0.1 s to 8 s.

I always find myself using the fastest AGC settings. Often, I experience 20-30 dB fades and if I use the slower AGC settings, what happens is that the station is too weak to be received until the AGC compensates.

On the other hand, I can't seem to figure out why or when would I want to use the slower AGC constant.

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  • $\begingroup$ what are the use cases you're considering? Note that coming from a modern digital comms background, a time constant of 0.1s is "incredibly incredibly slow", not "fast". I was about to write an answer involving symbol timing and channel coherency times, but I found I might just be telling you something completely off-topic $\endgroup$ – Marcus Müller Jun 17 '18 at 11:03
  • $\begingroup$ @Marcus Müller Well, coming from the digital communications background as well (but no professional experience in the field, unfortunately), such answers are interesting to me, but the question itself was mostly based with regards to traditional analogue modulations, such as audio carried over AM or SSB, and very slow digital modes such as A1A telegraphy or RTTY. $\endgroup$ – AndrejaKo Jun 17 '18 at 11:38
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CW is one reason. CW is on-off keying, so if the AGC reacts at a timescale similar to or shorter than the transitions of the signal, it will partially cancel the signal, making it "muddy" and more difficult to copy. 15wpm morse has a dit length of 0.08 seconds, making a dah or an inter-letter space around 0.24 seconds, so a fast 0.1s AGC will do it quite a bit of harm. This doesn't really explain the very long settings, but it does explain why an AGC constant of 1 second or so is useful.

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  • $\begingroup$ Interesting! I'll definitely have to try out different AGC settings with CW. $\endgroup$ – AndrejaKo Jun 16 '18 at 21:14

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