Electronic Design

Variable-capacitance diode phase modulator

This circuit varies the phase between two squarewaves through at least 180°. This capability finds application in fixed-frequency, phaseshift, resonant-mode converters. ICs such as the UC3875 usually only work up to about 500 kHz, whereas this circuit can be extended up to tens of megahertz. In addition, the circuit shown uses low-cost components. This example was used for a high-efficiency 2-MHz RF power supply.

The signal is delayed at each gate by the RC network formed by the 4.7k input resistor and capacitance of the 1N4003 diode. The capacitance of the diode, and hence delay, can be varied by controlling the reverse dc bias applied across the diode. The 100k resistor to ground at the input to the second stage corrects a slight loss of 1:1 symmetry. The fixed delay for output A adjusts the phase to be approximately in phase at a 5-V bias. Note that the control voltage should not drop below approximately 3 V, because the diodes will start to be forward-biased and the signal will be lost.

See associated figure

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