frequency modulator

frequency modulator

Frequency modulator

An electronic circuit or device producing frequency modulation. This device changes the frequency of an oscillator in accordance with the amplitude of a modulating signal. If the modulation is linear, the frequency change is proportional to the amplitude of the modulating voltage.

High-frequency oscillators usually employ either LC (inductance-capacitance) tuned circuits or piezoelectric crystals to establish the frequency of oscillation. This frequency can be controlled by changing the effective capacitance or inductance of the tuned circuit in accordance with the modulating signal. Practical circuits usually employ a varactor diode to change the oscillator in accordance with a modulating voltage.

The oscillators in high-frequency electronic systems, such as frequency-modulating (FM) transmitters, usually employ piezoelectric crystals for precise control of the carrier frequency. These crystals are equivalent to a series LC tuned circuit with an extremely high Q. The crystal holder has a small capacitance which is in parallel with the crystal and therefore causes parallel resonance at a slightly higher frequency than the series resonant frequency of the crystal. The actual oscillator frequency is between these two resonant frequencies and is controllable by the parallel capacitance.

The junction capacitance of a semiconductor diode varies with the diode voltage, and a reverse-biased diode may be used to control the oscillator frequency to produce frequency modulation. Low-loss diodes designed for this service are known as varactor diodes and have trade names such as Varicaps or Epicaps. A basic varactor modulating scheme is shown in the illustration. In this circuit, the transistor that drives the varactor modulator provides reverse bias as well as the modulating voltage v_m. The radio-frequency (rf) choke provides very high impedance at the oscillator frequency to isolate the transistor amplifier output impedance from the oscillator circuit but to allow the modulating signal to pass through with negligible attenuation. Only the frequency-determining part of the oscillator is shown. The symbols C_c, L_c, and R represent the electrical equivalents of the compliance, mass, and loss, respectively, of the crystal; C_h is the crystal-holder capacitance and C_b is a dc blocking capacitor. See Varactor

Basic varactor modulator circuit

The varactor-diode modulator is also commonly used to control the frequency of local oscillators in radio receiving equipment where programmed, push-button, or remote tuning is desirable. In these applications, conventional LC tuned circuits may be used.