Working Principle and Application Circuit of PM Angle Modulation Circuit

A common method to implement PM is to use a modulation signal to control the reactance or resistance elements of a resonant circuit or a phase-shifting network to achieve PM.

The figure is a single-stage loop varactor diode PM circuit. Inductor L, capacitor C, CC and varactor diode C1 form a parallel resonant circuit. C3, C4, C5 are coupling capacitors, and L1 is a high-frequency choke. The varactor is used as a high-frequency amplifier in the circuit, and the capacitor element in the resonant circuit. When the modulation signal causes the varactor diode junction capacitance C1 to change, the phase shift caused by detuning also changes, thereby achieving PM. Due to the nonlinearity of the loop phase characteristic, a single-stage loop is used to achieve PM. Due to the nonlinearity of the loop phase characteristic, a single-stage loop is used to achieve PM. Only when the modulation index MP﹤N/6 can a PM with better linearity be obtained.

In order to obtain a larger phase shift, that is, to increase the modulation index, a multi-stage varactor PM circuit can be used. Figure 5.5-21B is a three-stage varactor PM circuit. To ensure consistent phase shift, a 22K resistor is connected in parallel to the loop to adjust the quality factor Q value of the loop. The total phase shift is the sum of the three-stage phase shift, so the circuit can obtain linear modulation within a 90-degree range.

Figure 5.5-22 is a circuit that uses a varactor diode to control the reactance of the phase-shifting network to achieve PM. V1 is an inverter, V2 is an emitter follower, and the R of C1 constitutes a phase-shifting network. After analysis, let UO be called φ relative to U1, then when φ≤X/6, φ≈2WCRC1. C1 can be linearly related to the reverse bias within a certain range. Within this range, when the modulation signal is applied to the varactor diode, φ is proportional to the modulation signal, and linear PM can be achieved.