Direct frequency modulation circuit of relaxation oscillator Figure 7-21 is a block diagram of a frequency modulation triangle wave generator. The modulation signal controls the constant current source generator. When the modulation signal is zero, the constant current source output current is I; when there is a modulation voltage, the output current is I + ΔI (t), which is proportional to the modulation signal.
Figure 7-21 Triangle wave frequency modulation block diagram 
Figure 7-22 Hysteresis characteristics and input and output waveforms of the voltage comparator 
Fig. 7-222 Transformation characteristics of triangle wave into sine wave
The output of the voltage comparator is a frequency-modulated square wave voltage. If you want to get a sinusoidal FM signal, you can add a waveform conversion circuit or filter at its output. Figure 7-23 shows the characteristics of a converter that changes from a triangular wave to a sine wave. It is a nonlinear network and its transmission characteristics are
Second, indirect frequency modulation circuit The key to indirect frequency modulation is phase modulation. The following introduces commonly used phase modulation circuits.
1. Loop parameter phase shift circuit Figure 7-23 is a varactor diode phase modulation circuit. It uses the varactor controlled by the modulation signal as a component of the oscillation circuit. Lc1 and Lc2 are high-frequency choke coils, which prevent high-frequency signals from entering the DC power supply and modulation signal source.
The phase shift between the voltage and current of the high-Q parallel oscillation circuit is:
Figure 7-24 Three-stage loop cascaded phase shifter
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