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I have to demodulate a square wave FM+PWM modulated signal, so for the FM demod I'm using a PLL with a positive edge sensitive comparator to lock on frequency only (CD4046BE). Carrier frequency is 100Khz, fmax 120Khz and fmin 80Khz. What kind of loop filter should I use? Should it allow the modulating signal BW to pass? (LPF with fc=40Khz in this case?) I tried anything from fc=infinity (no loop filter at all) from fc=150hz.. the oscillator locks in with the FM signal but the VCO controlling voltage (i.e. my demodulated output) is a mess and it's changing in amplitude with the frequency. (Not only the DC part of it, as I would expect)..

Any ideas? Thank you very much!

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  • \$\begingroup\$ Can you be very clear about what the modulated signal is and explain how PWM plays a role? If the baseband signal you are wishing to demodulate is up to 40 kHz and your carrier is only 100 kHz, you will need a decent multi-order low pass loop filter I suspect. Post the circuit you tried also. \$\endgroup\$
    – Andy aka
    Jan 13, 2020 at 14:30
  • \$\begingroup\$ A simple alternate demodulator: Since you already have digital input, a monostable chip followed by a RC low-pass filter should provide linear demodulated output. \$\endgroup\$
    – glen_geek
    Jan 13, 2020 at 14:40
  • \$\begingroup\$ Type II have high phase noise, but lock onto any VCO error, Type I needs stable tuned Voltage with small control range from filter to VCO will have less phase noise so f error must be << loop filter to reject 2f noise with R+10RC filter in order to capture and lockon Some use analog switches with dual BW to lock on then have low phase noise. Active Filters can be design to reduce 2f noise on VCO Vc, to extract data but if binary needs to be raised cosine type. \$\endgroup\$ Jan 13, 2020 at 14:42

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Thank you! I solved the FM demodulation using a monostable and a lowpass filter. My problem is that in simulation I can recover my PMW modulating signal using only a lowpass filter, but in reality I also get the FM modulating signal. They can be the same frequency (both 20 to 20Khz-ish)

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