From the documents, I learned that most of the PLL-based FSK demodulators take the control voltage of the VCO as the modulating signal, but do you anyone know any other methods about PLL-based demodulator? I mean any other circuit architecture for this domain using PLL.
For FM demodulation: If modulation index is low (max. phase deviation less than 1 radian), you could use a narrow-band PLL as a phase demodulator. The output is the ouput of the phase detector, that after derivation gives the frequency.
This can work for low-index FM but not for FSK.
Why do you need a method other than take the control voltage of the VCO?
Regards