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When demodulating a phase modulated signal using a PLL it is recommended to use a narrow loop BW, so that the VCO does not really follow the input phase modulated signal but outputs a reference frequency and the error signal represents the demodulated signal

On the other hand and for frequency demodulation it is recommended to use a wide loop BW , so the VCO tracks the input signal and in this case the error signal also represents the demodulated signal.

Why is that? what makes an FM signal different than a PM signal so that for one one uses a wide Loop BW for demodulation and for the other a narrow loop BW?

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  • $\begingroup$ hm, from the top of my head this sounds exactly the wrong way around. Can you refer us to where this is said? Maybe context helps understand the claim! Thank you. $\endgroup$ – Marcus Müller Jan 10 at 20:48
  • $\begingroup$ If I give you the name of the reference(s) you'll probably tell me that you don't have them. A better way of responding to my message is to explain what you understand ( if you understand) even if it is the other way round , as you say. Happy New Year $\endgroup$ – Hatem Tawfik Jan 10 at 21:03
  • $\begingroup$ Because frequency is the derivate of phase over time, a constant post-PLL phase error bias would mean you have a frequency offset, exactly what you want to detect in the FM case. But "bias" implies "averaging", and that's a lowpass, i.e. band-limiting operation. So, please state your sources. I don't think stating your sources should be something that hurts you. I feel like even having to ask twice for sources is kind of unnecessary on a site like this. $\endgroup$ – Marcus Müller Jan 10 at 22:46
  • $\begingroup$ I certainly do not believe that mentioning the sources would help in any way, if you insist here they are: $\endgroup$ – Hatem Tawfik Jan 10 at 23:04
  • $\begingroup$ Phase Locked Loop Circuit Design by Dan Wolaver p. 4 and 5, p192, 202 and Phase lock basics by W.Egan p.111-116 $\endgroup$ – Hatem Tawfik Jan 10 at 23:08

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