Could anyone give me the necessity of automatic gain control in AM and automatic frequency control in FM?

 

Lets say that two stations transmit their signals with different power levels or for some reason the power of the signal received by the antenna varies. If you set the volume to a desired level by the volume control on your radio and for some reason the power of the received signal varies then the volume will vary too, something which will be annoying. For this reason, they implement the automatic gain control as to keep the power of the signal received from the antenna and fed into the radio amplifier constant as not to create these fluctuations in the sound volume.

 

I don't understand....
if that different between AGC AM & AFC in FM.

 

all those gains in a superheterody receiver

 

Well, I dont know much about receiver/transmitter circuits. I just know what is the function of AGC as I described for you and i think it is used in both FM and AM receivers. The AFC is a circuit which searches for radio channels (frequencies) and lock to them if they are valid radio stations (its the scan function of a radio).

Here are links about AFC:

http://hem.passagen.se/communication/afc.html

http://en.wikipedia.org/wiki/Automatic_frequency_control

and about AGC

http://en.wikipedia.org/wiki/Automatic_gain_control

 

AGC (Automatic Gain Control) keeps the signals a consistant amplitute in circuits, so they will treat them the same. In AM this translates as a constant volumn on the audio, in FM it means the FM discriminator will sound the same, otherwise minor changes could cause undesirable distortions.

AFC (Automatic Frequency Control) keeps the tuner of the radio centered on the desired frequency that the radio is tuned to. All circuits have drift, either in the transmitter or the reciever. The current state of the art has reduced this to small numbers on the tranmitters, and done pretty good for the recievers, but small differences in calibration could still be a problem without this circuit. It compensates for the variations. In the beginning days of radios the frequency would actually move around somewhat, so they were even more necessary then.