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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)?

PLL-Based FM Demodulator

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.

Any explanations?

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No, you've misunderstood how a PLL demodulator works. The NCO (or VCO in an analog system) must track the instantaneous frequency of the incoming signal, not just its average value. If it does this accurately, then then the control value is an accurate replica of the original modulating signal.

If the oscillator only tracks the average frequency, then you have a PM (phase modulation) demodulator, which is related, but different. With wideband FM (such as used for FM broadcast), the instantaneouis phase error will be much greater than 360° at times, and the output of your phase detector will "wrap", createing a lot of distortion.

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  • \$\begingroup\$ Ok that's great... what do you mean by "the average value of the incoming signal" in the first paragraph, and "the average frequency" in the second one? Are they same? Do you mean that when I leave the NCO running on the centre frequency ONLY, the PLL tracks the so called "average frequency"? \$\endgroup\$ – Siraj Muhammad Nov 26 '14 at 14:38
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    \$\begingroup\$ Well, no, not exactly. If you have no feedback at all, the oscillator doesn't track anything. But it if happens to be aligned with the center frequency of the signal, it will function as a phase detector, but not as a frequency demodulator. But if you have feedback, then you have a choice: If the loop filter bandwidth is wide enough to pass the modulating signal, then the NCO/VCO will track it, and the control value will represent the original modulation -- an FM demodulator. If the loop filter bandwidth does NOT pass the modulating signal, then the oscillator will only track its average ... \$\endgroup\$ – Dave Tweed Nov 26 '14 at 15:01
  • \$\begingroup\$ ... frequency and the output of the phase detector upstream of the filter will represent the instantaneous phase of the signal -- a PM demodulator. \$\endgroup\$ – Dave Tweed Nov 26 '14 at 15:05
  • \$\begingroup\$ Very good, but I still don't understand the meaning of the average frequency. What do you mean by this? \$\endgroup\$ – Siraj Muhammad Nov 26 '14 at 15:26
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    \$\begingroup\$ The center frequency of the transmitted signal; i.e., the unmodulated carrier frequency. \$\endgroup\$ – Dave Tweed Nov 26 '14 at 17:21
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If the input signal drifts off in frequency, without having a mechanism that tracks the input centre frequency, eventually performance will degrade and demodulation will fail. It is imperative that the phase comparator has two inputs whose average frequency is the same.

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