The intermediate frequency, ( IF ), amplifier is a radio frequency amplifier which processes the output of the mixer. In addition to amplifying the signal, the IF amp usually has some form of bandpass filtering so that only a selected range of frequencies is allowed through. These filters are often of the SAW, crystal lattice, or ceramic varieties. Common, IF frequencies are 70, 45, 21.4, and 10.7 MHz, however there is no restriction to these frequencies. One difference between most radiotelescope IF amps and those found in communications receivers is that communications receivers usually employ some form of automatic gain control, ( AGC ). AGC circuits need to be disabled in communications receivers modified for radio astronomy use as they tend to mask the subtle changes in signal strength we are trying to detect.
The amount of gain needed for the IF amplifier is determined by the signal level exiting from the mixer, the amount lost in the IF amp filter(s) and the appropriate level required for the square law detector which follows. It is sometimes difficult to determine all of these values ahead of time and so a variable attenuator is sometimes introduced at the IF amp output prior to the detector.
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