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I need to demodulate an FM signal and I want to use a quadrature demodulator for this. The signal is 16±0.15 MHz, 3.3V output from CMOS oscillator. I know that Gilbert cell mixers are most commonly used for this but I don't have any of them around yet I have some other mixers and parts that can be used as ones:

  • ADE-1, diode ring mixer
  • SN74LVC1G3157, SPDT analog switch
  • BF1105 and BF998 dual-gate MOSFETs
  • SN74AHC1G86, XOR gate with Schmitt trigger inputs

Which one of those methods can achieve highest SNR and lowest distortion (<3% is okay)? How good is the performance compared to Gilbert cell mixer? What are the main requirements for choosing a mixer for quadrature demodulation?

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  • \$\begingroup\$ Read datasheet on NE604. \$\endgroup\$
    – analogsystemsrf
    May 24, 2018 at 2:38
  • \$\begingroup\$ @archimedes is there a reason you deselected my answer as accepted? \$\endgroup\$
    – Andy aka
    May 27, 2018 at 8:43
  • \$\begingroup\$ @Andyaka yes, the circuit you suggested is not working. The phase-shifting circuit removes DC level from the signal and I get weird output. I lowered the oscillator frequency and used 4046's phase comparator since it has self-biasing amplifier for AC signals. I tried to put a DC blocking capacitor between LC tank and ground and replaced C1 with resistor to keep the DC level but this didn't work either. I also plan to experiment with other types of mixers and write an answer with the results I got, I just needed a quick answer back then, now I have time for experimentation. \$\endgroup\$
    – Archimedes
    May 27, 2018 at 9:36
  • \$\begingroup\$ @Archimedes the thing is that I use this circuit and it works so instead of rejecting an answer as wrong by unaccepting you should post your circuit and raise a comment asking for help. Doing what you have done means you get no help from me in understanding your problem and getting to a solution. I know the answer to be correct and it is likely that the problem you experience is in your implementation. Yes, you need to bias the output of the phase shifter to mid-logic-rail. \$\endgroup\$
    – Andy aka
    May 27, 2018 at 9:44
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    \$\begingroup\$ Instead of the inductor going to ground - use a potential divider to lift it to midrail but make sure that you use a ceramic capacitor (much bigger than C2) from the midrail to ground so that the AC tuning of the quadrature tank is unaffected. The 4046 phase comparator is likely a good choice for the exor. I've used that exor for this very circuit (running at 11 MHz). \$\endgroup\$
    – Andy aka
    May 27, 2018 at 11:21

1 Answer 1

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Given that your signal is already a logic level and fairly tame in frequency I would look no further than using an exclusive OR gate as the mixer. Originally Gilbert cells were designed as a solution to making an exor gate so it would be almost rude not to use a regular exor chip as it was originally intended.

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Picture source

Make sure the 90 degrees signal from the tank is schmitt triggered back to a reasonable square wave and you should be set to run. Example for 455 kHz: -

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Circuit from here

Functionally and performance wise all the options you state are going to be about the same but the exor is going to be more tolerant of any rogue amplitude modulation caused by the tank.

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