I need to demodulate an FM signal (4 MHz max deviation) with a strong center frequency drift (550-800 MHz). I've built an AFC circuit to track the drift and now I need to build a tunable quadrature detector to demodulate the signal itself and I'm having trouble making it tunable since the Q of RLC network (C18, R12, L5) will change if I change the capacitance to adjust the center frequency and I need to compensate this Q change somehow.

Here's a schematic with limiting amplifier (left), wideband quadrature detector for AFC (center) and tunable quadrature FM detector (right).

enter image description here

UPD1: I'm thinking towards electrically controlled inductors because Q change is small enough if L is adjusted instead of C but I don't see any of those commercially available.

UPD2: I looked in the datasheet of the inductor I'm using and noticed that its Q factor increases with frequency and I think it should compensate tank Q reduction with frequency.

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    \$\begingroup\$ This is one reason that most FM receivers use a superheterodyne architecture, in which the detector itself does not need to be tunable. What are you trying to accomplish here? \$\endgroup\$ – Dave Tweed Mar 27 '18 at 14:33
  • \$\begingroup\$ I'm trying to simplify the circuit and avoid using a superhet architecture to reduce power consumption \$\endgroup\$ – Archimedes Mar 27 '18 at 14:37
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    \$\begingroup\$ Can't you just lower the Q to produce a bandwidth that is wide enough for the signal across the range of frequencies you are interested in? If not then please post a schematic. \$\endgroup\$ – Andy aka Mar 27 '18 at 14:44
  • \$\begingroup\$ @Andyaka I can't do that because the sensitivity will be unacceptable. Max signal deviation is 4 MHz and center frequency can be in range of 550-800 MHz. By the way, AFC works exactly the way you described. \$\endgroup\$ – Archimedes Mar 27 '18 at 14:50
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    \$\begingroup\$ @Archimedes oh ok then please post a circuit. Where did I answer regards AFC? \$\endgroup\$ – Andy aka Mar 27 '18 at 15:04

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