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I don't really understand how FM narrow band is created, and at the same time I can't figure out how using this simple circuit of varactors (varicap diodes) we narrow the bandwidth.

schematic

simulate this circuit – Schematic created using CircuitLab

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Narrowband FM transmissions have a bandwidth that is about the same as the bandwidth of the modulating signal (the so-called modulation index is lower than 1). Wideband FM transmissions exceed the bandwidth of the signal (broadcast FM is 150kHz wide).

To narrow the modulation bandwidth for this circuit, you might reduce the influence of the varactor on the total capacitance of the resonant circuit.

You could:

  • Add a capacitor in series between the varactors and C1
  • Increase the varactor bias voltage (increasing the width of the depletion region, lowering the change of capacitance with voltage).
  • Make C1 bigger
  • Lower the modulation voltage (the AC signal superimposed on the varactor bias voltage)

The first three options require retuning of the circuit. So the simplest way to do it is to attenuate the modulation signal to bring the modulation index below 1.

Please note: Using the given component values the sweep will not be very big anyway. The few pf's from the varactors will not substantially change the total capacitance because C1's capacitance is almost a million times larger.

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