fsk demodulator pll
The basic theory is pretty easy to understand. The VCO has a tuning voltage that is proportional to its output frequency, so you just look at the tuning voltage when the pll is locked and determine the input frequency.
That is where the simplicity ends, however. In the real world, there are things like noise and jamming signals. What stops the PLL from locking onto the wrong signal? Your IF Filter? Then it had better be a brickwall filter, because most IF filters I have seen can let in a little out-of-band signal, especially if you consider LO drifts, etc.
Also, what happens when there is no signal present? The VCO typically tunes to one end or the other of the power supply rail. Then lets say a weak received signal shows up. What exactly makes the VCO suddenly tune the the middle of its band and lock up? That weak signal burried in noise? You might have to add a sawtooth sweeping circuit on the VCO tune line just to assure re-acquisition.
So unless you have a very strong signal, very good signal to noise ratio, etc, a PLL FM demodulator can be a pain in the butt.