I am trying to build FM Demodulator in FPGA. The architecture I am trying to use is PLL-based FM Demodulator like the one below. I searched for the working principle of this demodulator but there is one thing I couldn't understand and I couldn't find an answer to. Why do we need this feedback to the NCO (d(t) in the diagram)?
I think it doesn't make sense because the NCO should run on the centre frequency, that is the carrier frequency of the received signal, why should I change this running frequency while detecting the phase, thus extracting the information signal?
I have implemented this demodulator, and simulation assures my point of view. The output signal of demodulator with feedback to NCO is distorted and has no relevance to the input signal (of the modulator), on the other hand, the output signal without feedback to NCO is very similar to the transmitted signal.