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Hello
AGC (Automatic Gain Control) is used in Recievers mainly to stablize output level of the recieved signal. there are some types of AGC such as Normal AVC and delayed AVC.. Normal AVC just apply a function that have shape like Vo=(1-exp(-AVin)) i.e. this function will cause small signals to be aggressively enlarded whil large signals are less enlarged.. wherease delayed AGC enalrges all signals smaller than certain level with constant gain and work as normal AGC for signal higher than this limit.
I hope this helps
Tell me if something isn't clear.
BR
Ahmad,
Hello
AGC (Automatic Gain Control) is used in Recievers mainly to stablize output level of the recieved signal. there are some types of AGC such as Normal AVC and delayed AVC.. Normal AVC just apply a function that have shape like Vo=(1-exp(-AVin)) i.e. this function will cause small signals to be aggressively enlarded whil large signals are less enlarged.. wherease delayed AGC enalrges all signals smaller than certain level with constant gain and work as normal AGC for signal higher than this limit.
I hope this helps
Tell me if something isn't clear.
BR
Ahmad,
Let me summarize your statement, two kinds of AGC are here, which are
1)Normal AGC and 2)Delayed AGC.
Normal AGC works as Amplitude Loop Lock, maintain signal in a constant level.
Delayed AGC works as ALL if the signal level is higher than a threshold level.
But, I still didn't get any relation between your description and my question?
Anything I missed?
So far I have only guessed what WB, NB and keyed AGC may mean and to clarify I did some search. Certain radio services need high quality AM or FM receptions and receivers designed for this purpose include multi conversion solutions (at least they have 2 IF stages at different frequencies). Usually their first IF stage includes a filter for the wider bandwidth requirement and their second IF stage (after the second mixer) includes the narrow band filters.
So both first and second IF filter and amplifier stage are designed to have their corresponding AGC control, INDEPENDENTLY from each other, called Wide band for the first and Narrow band AGC for the second IF stages. Each must have a different attack and decay time. See a data sheet for a (professional) AM/FM VHF transceiver, page 4, receiver unit specifications:
**broken link removed**
There are other FM receiver examples intended for broadcast services like at
**broken link removed** for the AF210 type where WB and NB
AGC also employed.
Regarding the keyed AGC, it is a different approach. In very noisy enviroment with much pulse-like interference signals being present the keyed AGC solution proved the best and showed great immunity to such disturbing signals. It was developed for television receivers first and is nicely explained here (see text under PART 2):
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.
Basically, modern radios need "headroom" to operate, that is X dB of room between the actual rf output power at each active device and its compression point. If you are building a receiver with a big dynamic range, that does not get that range via many bits in the ADC, then you need either:
1) really high compression point components in the receiver chain
2) more reasonable compression point components with 1/3 of the AGC done at the front end, 1/3 of the AGC done at the 1st IF, and 1/3 of the AGC done at the 2nd IF--or some variation thereof.
If you are using a modulation with a lot of amplitude peaking, like QAM or OFDM, this becomes very important.
An example of keyed AGC can be found in analog television; where in, the sync pulse only is used to determine the gain level rather than the full video level.
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