Basic introduction of series-connected base frequency modulation circuit

A circuit that causes the instantaneous frequency of the modulated wave to vary with the modulation signal. Frequency modulators are widely used in electronic devices such as FM radio, TV sound, microwave communication, phase-locked circuits and sweepers. The basic requirement for the frequency modulator is that the frequency shift is large, the frequency modulation characteristics are good, and the parasitic amplitude modulation is small.
The frequency modulator is divided into two types: direct frequency modulation and indirect frequency modulation. The latter uses an integrating circuit to integrate the modulated signal so that its output amplitude is inversely proportional to the modulation angular frequency Ω, and then the phase modulator is phase-modulated. At this time, the output of the phase modulator is the desired frequency-modulated signal uf(t). The advantage of indirect frequency modulation is that the carrier frequency is relatively stable, but the circuit is more complicated, the frequency shift is small, and the parasitic amplitude modulation is large. Usually, multiple frequency doubling is required to increase the frequency shift. Indirect FM modulators are not subject to DC voltage modulation and cannot be used in phase-locked loops and automatic frequency control loops.

Serial-connected base frequency modulation circuit diagram