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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?

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    \$\begingroup\$ Your description of FM and Am receivers is lacking in accuracy in some places and then you go on to ask a seemingly unrelated question about PLLs. You also make errors in understanding the varactor diode. Why don't you ask the basic question without making imperfect assertions about receivers - this makes it easier to answer. \$\endgroup\$ – Andy aka Dec 18 '14 at 8:31
  • \$\begingroup\$ For example, 'low band pass filter' is a contradiction in terms. \$\endgroup\$ – user207421 Apr 20 '15 at 21:39
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The varactor diode acts like a voltage controlled variable capacitor which is part of the VCO. The phase lock loop locks to the input's phase and frequency. The VCO and the voltage controlling the VCO is part of the loop. And that same voltage is essentially the recovered, demodulated signal. The PLL is essentially the demodulator.

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The PLL (Phase Lock Loop) generates the local oscillator frequency. This frequency is then mixed with the incoming signal - the mixer creates two output signals: the sum of the two frequencies and the difference between the frequencies.

We are interested in only the difference signal. It is that signal that is sent to the IF stages for further amplification. Then, as you said, to the limiter and demodulator.

Because this is a FM signal, the demodulated signal actually comes from the control signal that closes the control loop of the PLL. That is: the output of the limiter stage is fed back to the PLL so as to adjust the local oscillator frequency to keep the PLL locked.

As the incoming frequency varies according to the modulation, the PLL has to track those frequency changes to stay locked. That means that the PLL frequency control signal also has to follow the modulation. That is your recovered signal.

Finally, de-emphasis is applied to the recovered signal.

The reason that pre-emphasis is applied to the transmitter is to improve the signal-to-noise ratio.

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