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I have the following circuit which is an AM demodulator (I think). RF input is between the diodes left:

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Plotting the bode plots for the negative and positive input of the comparator, I get a lousy allpass for the positive input (with a small notch at 10kHz) and a multi-order lowpass for the negative input.

  • What is R1+C6 for? If V+ should have the unfiltered input, why not directly connect RF?

  • If V- should be lowpass filtered, why not choose the components such that we get actual 2nd order rolloff rather than having the poles so widely spaced (or the zero in between)

  • At the input of the comparator is a high frequency signal which would toggle the bits at a high rate. Now I get it that the comparator is probably too slow for that but I feel it's odd relying on parasitic elements rather than actually creating the desired behavior.

  • Does the circuit work for for low modulation index, i.e. when I encode bits such that 1 represents a full carrier wave and 0 represents 20% attenuated carrier wave?

  • Anything else to say about this circuit? Is it well known?

I also simulate this system in MATLAB, creating a 1GHz RF signal with 10kHz baseband data, demodulating the RF signal with "rf*sign(rf)" and filtering with the two calculated filters and comparing the values. I cannot get meaningful results.

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  • \$\begingroup\$ Plotting the bode plots for the negative and positive input of the comparator Bode plots are only for linear circuits, a comparator isn't linear so making a Bode plot is meaningless. \$\endgroup\$ – Bimpelrekkie Nov 12 '19 at 7:45
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    \$\begingroup\$ R2 C7 R3 do quite a bit more than JRE says. C7 charges to the signal mean DC value and set a ref point for the comparator to toggle around. R3 sets V_C7 means at about 90% of the mean DC value giving a positive bias to the data out when 0/1 ratio is constant. \$\endgroup\$ – Russell McMahon Nov 12 '19 at 7:48
  • \$\begingroup\$ @Bimpelrekkie No it is not meaningless because the ideal comparator is the function "Vdiff = Vp-Vm > 0" with Vp=Hp(Vrf) and Vm=Hm(Vrf). \$\endgroup\$ – divB Nov 12 '19 at 14:36
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What you have there is a classic, simple data slicer.

D2 and \$C_{p2}\$ form an AM demodulator. What goes to C6 and R1 is no longer RF.

C6 and R1 form a sort of high pass filter/differentiator.

R2, C7, and R3 are there to slightly delay the edges of the detected low frequency signal.

U3 is a comparator. It compares the signal (V+) with the delayed signal (V-.)

When the signal is higher than the delayed signal (that's the rising edge,) the comparator's output snaps to high.

When the signal is lower than the delayed signal (that's the falling edge,) the comparator's output snaps to low.

The output is a series of pulses that follow the originally transmitted on-off keying from the transmitter.

This simple data slicer will output random "data" when there's no received signal. The link I posted goes into some detail on how that is avoided with better designs. It also includes a section on the simple data slicer, though.

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