I have a test tommorrow on phase lock loops, AM/FM receivers along with several other topics. I need to understand this topic clearly before I take the exam to advance to the next schooling in the Navy training.
As far as I understand, both the AM and FM receivers contain the RF amplifier, mixer and local oscillator as part of their tuner circuit. In FM receivers, the intermediate frequency from the mixer is sent to the IF Amplifier and then to the limiter which clips off noise. The IF Amplifier typically has a transformer that only accepts the IF and rejects all other higher frequencies produced from the local oscillator. In the AM receiver, they have a AM detector which acts like a low band pass filter.
The FM receivers have a de-emphasis stage to correct for the pre-emphasis stage from the FM transmitters. My question is why is the higher frequency audio preferred at the pre-emphasis stage of FM transmitters? The main job of the FM receiver is to de-modulate the reference signal (sent from the transmitter) from the voltage controlled oscillator signal and after that de-modulation there is a de-emphasis stage.
As far as I'm aware the block diagram goes like this:
RF Amp -> Mixer & Local Oscillator -> IF Amp -> Limiter -> Demodulator -> De-emphasis -> Audio Frequency Amp.
I'm unclear as to how phase lock loop fits into this picture. The basic function of a phase lock loop is to compare the phase and frequency of an oscillator with the phase and frequency of an input signal. I remember reading about voltage controlled oscillators, in that the varactor diodes would store a capacitive charge and that their voltage would determine the frequency of an oscillator in a VCO, but I'm not sure where this fits in the big picture. Does the phase lock loop operate only in the local oscillator block in the block diagram above?