Nick Ierfino, IGS Technologies, Montreal, PQ, Canada; Edited by Charles H Small and Fran Granville -- EDN, 6/12/2008

Phase shifters find use in a variety of circuits, but variation in amplifier and capacitance tolerances usually makes it difficult to control the exact phase shift that precise control circuitry requires. The circuit in Figure 1 can control the phase shift from input to output by using IC₃, an AD5227 64-step-up/step-down control digital potentiometer, to replace the value for the resistance.

The formula of the centre frequency of the output is 1/(2×π×R×C). Different ranges of resistance are available for the AD5227. This example uses a 10-kΩ value. By stepping through the 64 points, the 720-kHz input sine wave rotates several times from 0 to 360°. The AD5227 acts as a potentiometer, in which A and B are the extremes and W is the wiper.

This example uses IC₂, a PIC16F84 microcontroller with a crystal frequency of 20 MHz. This microcontroller has a theoretical potential performance of 5 MIPS and should serve many purposes in PLL (phase-locked-loop) circuitry. You could use any microcontroller or even an FPGA to control the AD5227.