Keyed AGC
Hi,
Because I am not an expert/professional on radio receiver design I can only offer some links to study this topic and hopefully most of your questions will be covered in them.
First there is a good book on communications receiver design from Ulrich Rohde, you can download it here, he is considered an authority on it:
Also, the IF AGC circuit design was a topic here:
and go down and look for the Ham Radio Magazine AGC articles.rar file uploaded by flatulent in 2003.
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"But, I still don't fully understand why we need both WBAGC and NBAGC... "
My own explanation for them is that for a given IF amplifier stage with a given filter bandwidth, there exists a possibly optimized AGC circuit/operation which is designed into it by all means. If this receiver includes another IF stage with another filter bandwidth, then it should be treated again with another optimized AGC circuit in itself, the previously mentioned AGC circuit parameters will not serve optimally the other IF stages. In the wider bandwidth IF stage, which is usually needed for the high quality demodulation, the possibility of interfering signal to appear is more likely and it would be undesirable if the WBAGC were acting upon the (stronger) interfering signal and the gain for the wanted signal would be lowered, so I think this is the first explanation for the NBAGC.
Receiving signals with AM modulation and with FM modulation in the same receiver demands for the different bandwidths IF stages and this is usually solved by cascading the two IF stages (full needed IF gain can be divided and summed), this is also a consideration.
I hope this topic is more understandable now, sorry for not having more info and
I do not claim everything you asked for is covered in the above downloadable materials. Maybe others here can be of further help on your specific questions.
rgds
unkarc