Varactor Diode Modulator

Fig (b) shows the basic concept of a varactor frequency modulator. The L₁ and C₁ represent the tuned circuit of the carrier oscillator. Varactor diode D₁ is connected in series with capacitor C, across the tuned circuit. The value of C₂ is made very large at the operating frequency so that its reactance is very low. As a result, when C₂ is connected in series with the lower capacitance of D₁, the effect is as if D₁ were connected directly across the tuned circuit. The total effective circuit capacitance then is the capacitance of D₁ in parallel with C₁. This fixes the center carrier frequency.

The capacitance of D₁, of course, is controlled by two factors: a fixed dc bias and the modulating signal. In Fig b, the bias on D₁ is set by the voltage divider which is made up of R₁ and R₂. Usually either R₁ or R₂ is made variable so that the center carrier frequency can be adjusted over a narrow range. The modulating signal is applied through C₃ and the RFC. The C₃ is a blocking capacitor that keeps the DC bias out of the modulating signal circuits. The RFC is a radio frequency choke whose reactance is high at the carrier frequency to prevent the carrier signal from getting into the modulating signal circuits. The modulating signal derived from the microphone is amplified and applied to the modulator. As the modulating signal varies, it adds to or subtracts from the fixed bias voltage. Thus the effective voltage applied to D₁ causes its capacitance to vary. This, in turn, produces a deviation of the carrier frequency as desired. A positive-going signal at point A adds to the reverse bias, decreasing the capacitance and increasing the carrier frequency. A negative-going signal at A subtracts from the bias, increasing the capacitance and decreasing the carrier frequency.