How to Tune a PLL Based FM Demodulator?

[explosifiy]

Hi, I am designing an FM receiver and I was planning on building a PLL to demodulate the signal. I think I understand the working principle of a PLL, correct me if I am wrong, but the raw input and the error signal is put into the phase comparator, the output is fed through the loop filter, and that goes into the VCO which produces the error signal which is also the output. But how would you tune it? For example: If the PLL locks on to a station at 90 MHz and I want to change to a station at 100 MHz how would I go about it?

EDIT

Turns out I was a bit uninformed. Thanks for the clarification. I thought that the loop filter of the PLL somehow sorted out the frequencies. I now know that the tuning is done in the IF phase and that the audio signal is not the error signal, but the VCO control frequency.

[Kim Christensen]

You'd have 2 PLLs. One to generate the local oscillator for the first mixer. (This is the one that tunes in different stations)
The 2nd PLL is tuned to the center frequency (fixed frequency) of the last IF stage and does the demodulation.

[wasedadoc]

.. the raw input and the error signal is put into the phase comparator, the output is fed through the loop filter, and that goes into the VCO which produces the error signal which is also the output..

You appear somewhat confused.

The inputs to the phase detector are the signal to be demodulated (FMed IF frequency irrespective of transmitted frequency as pointed out above) and the VCO frequency (also FMed at IF frequency).  The output of the phase detector is used to vary the VCO frequency.  The VCO control voltage is the demodulated audio.

[DimitriP]

You'd have 2 PLLs.

Well...maybe ...eventually ...at some point in the future but certainly not right now.

For example: If the PLL locks on to a station at 90 MHz and I want to change to a a station at 100 MHz how would I go about it

One would go about it by either studying or asking about how receivers work ( or how do receivers "tune" different stations?), instead of "designing a receiver".

You could start here:  https://en.wikipedia.org/wiki/Superheterodyne_receiver 

Enjoy the ride and welcome to the forum.

 

[explosifiy]

You appear somewhat confused.

The inputs to the phase detector are the signal to be demodulated (FMed IF frequency irrespective of transmitted frequency as pointed out above) and the VCO frequency (also FMed at IF frequency).  The output of the phase detector is used to vary the VCO frequency.  The VCO control voltage is the demodulated audio.

Thanks for the help! I knew something was wrong, but I couldn't find out what it was.