Limiters; What and Why

An FM receiver has several stages to process the signal and decode it for use by the operator. The first stage would normally be a preamp followed by an RF amplifier feeding a mixer. The output of the mixer goes to an IF stage or two that, in turn, feed the limiters. The output of the limiter is fed to a detector to decode the intelligence from the signal. The detector, or demodulator or decoder, is looking for a signal devoid of any amplitude variation and consisting only of a frequency or phase shift that contains the information, be it voice, data, music or whatever.

Note that the Limiters are in the IF stages prior to the detector and are for the express purpose of removing any amplitude component from the received signal prior to detection.

Most signals on the repeater are what we lovingly call Full Quieting. This means that the limiters are functioning because the strength of the incoming signal is such that the threshold of the limiting circuits has been met and the amplitude component is being stripped from the IF signal. Therefore, the amplitude component does not reach the detector and has become a non-issue.

Conversely, if you hear a hiss or crackling on the signal it is most likely caused by a signal that is not strong enough to make the limiters go to work and the amplitude component is reaching the detector. Generally, this component is decoded as a hiss, static or whatever the detector thinks it is. The end result is a less intelligible signal; or, in the worst case, the signal isn't strong enough to open the squelch and nothing gets through.

Remember that when evaluating a signal through a repeater, there are two receivers involved. The repeaters and yours. You have limiters too; and, for the same reason,they yield the same results.

73, de N1TF-Fred