Shouldn't the output of an FM PLL demodulator be the message signal?

[skatefast08]

Shouldn't the output of an FM PLL demodulator (for a receiver) be the message signal? everywhere I look online, it says that the output is a modulated signal. What is the point of having a modulated signal at the output when you're trying to demodulate the FM signal? I look at all the other discriminator circuits, such as Slope detector, or Foster Seeley discriminator, which are more comprehensible, but what is the PLL accomplishing from demodulating an FM signal, when everyone is saying that the output is a modulated signal?

 

[BigBoss]

If PLL has a narrowband Loop Bandwidth, it will demodulate the FM /PM signals.
Depends on how much Frequency Deviation you selected.

 

[betwixt]

What come out depends on the loop filter characteristics. If they aren't correct you will not recover the modulation.

Explanation:
The PLL contains a VCO which should free run close to your carrier frequency. The phase detector compares the free running frequency (and phase) to the carrier frequency and produces an error voltage if they are different. The error voltage is then filtered by the loop filter components so it is somewhat slower to follow the difference. The VCO frequency is then controlled by the filter output to bring it back into line with the incoming carrier.

If you shift the incoming carrier by frequency modulating it, the VCO will try to track it because the phase comparator will generate a error voltage to keep them at the same frequency. If it did it immediately with no loop filter delay, the VCO wold simply be a copy of the incoming signal. If the filter delay was too long, the VCO would settle at the average of the incoming frequency because it couldn't react quickly enough. If you get the loop filter response just right, it will follow the changes caused by the modulation so the error voltage is the copy of the original modulation you are trying to recover.

Brian.

 

[G4BCH]

It also depends on what you call the output, if it is the VCO control voltage it is the signal's modulation, if it is the VCO output then it is a regenerated version of the modulated input signal.

 

[skatefast08]

What come out depends on the loop filter characteristics. If they aren't correct you will not recover the modulation.

Explanation:
The PLL contains a VCO which should free run close to your carrier frequency. The phase detector compares the free running frequency (and phase) to the carrier frequency and produces an error voltage if they are different. The error voltage is then filtered by the loop filter components so it is somewhat slower to follow the difference. The VCO frequency is then controlled by the filter output to bring it back into line with the incoming carrier.

If you shift the incoming carrier by frequency modulating it, the VCO will try to track it because the phase comparator will generate a error voltage to keep them at the same frequency. If it did it immediately with no loop filter delay, the VCO would simply be a copy of the incoming signal. If the filter delay was too long, the VCO would settle at the average of the incoming frequency because it couldn't react quickly enough. If you get the loop filter response just right, it will follow the changes caused by the modulation so the error voltage is the copy of the original modulation you are trying to recover.

Brian.

So I believe the error signal that has been filtered out by the LPF is the recovered message signal?

 

[betwixt]

Correct - but for best results the loop timing has to be optimal.

Brian.

 

[skatefast08]

Correct - but for best results the loop timing has to be optimal.

Brian.

You also say that the VCO has to run closely to the carrier frequency, but the carrier frequency should be the center frequency (FM signal) coming from the LNA. I believe you mean the center IF frequency after the RF mixer, right? In simple terms, if I was to design a full FM receiver, there should be 2 different oscillators, one at the carrier frequency and the other at the IF frequency (for the PLL)?

 

[betwixt]

The same principle applies in both cases. By carrier I meant the signal carrying the modulation, it could be directly from a LNA or it could already be down-mixed to a lower IF frequency. The VCO should match whatever the frequency entering the PLL is.

Brian.