Design Ideas: February 15, 1996

To alter the phase response of a signal without changing its amplitude, you commonly use an all-pass filter, as in Figure 1a. The following expression gives the response of this filter:

As the signal frequency varies from dc to infinity, the phase shift varies from 0° to 180°. You can also see that varying R₁ causes the phase shift to vary at a given frequency. If you reverse the positions of R₁ and C₁, as in Figure 1b, the transfer function becomes

and the phase then varies from -180° to 0°. The phase shift this circuit produces at any given frequency is

where [lower case omega] is the frequency in rad/sec, or 2[pi]f. For practical purposes, most circuits of this type can comfortably cover a phase range of about 120° with available op amps and passive components. You can replace R₁ in Figure 1b with a digital potentiometer, as shown in Figure 1c. With this circuit, whose values are optimized for phase shifting in the audio range, the phase at any given frequency is under digital control. You can set the wiper position in the DS1669 by using a single digital control line to cause the wiper to increment at the appearance of each low-going pulse. At the upper limit of the potentiometer's range, the wiper reverses direction and decrements.

If you supply a clock to the digital input (Figure 1c), the phase sweeps constantly, providing an effect commonly used in contemporary recordings. Note that, in this single-supply circuit, the bias to the digital potentiometer is ½V_CC. You can cascade several of these phase-shift sections to obtain greater phase shifts. You can use dual, quad, or hex potentiometers to put all phase-shift sections under control of a microprocessor, as in Figure 2. The DS1806 hex digital potentiometers' wiper positions are controlled by a simple, three-wire interface, which clocks the wiper-position settings into the device over the DQ line when RST [overscored] is high. When RST [overscored] goes low, the wiper assumes its new setting. (DI #1826)