Phase shift vernier for automatic test systems

Abstract

A phase shift vernier for providing an output signal with continuously variable delay based on an input phase delay is disclosed. The apparatus comprises delay value means, a ring oscillator, a multiplexer, a DAC, and a signal combiner. The delay value means is adapted for receiving an input phase delay value, indicating the amount of delay for an output signal. The ring oscillator is adapted for circulating an oscillating signal through multiple differential stages to generate multiple quadrature signals. The oscillating signal has a predetermined frequency and each of the differential stages is connected in series. Each of the stages delays its inputs by a predetermined amount to generate its differential outputs from each stage. The multiplexor is coupled to the ring oscillator and to the delay value means to receive the quadrature signals from the ring oscillator. The multiplexor selects first and second quadrature signals from the ring oscillator in response to the input phase delay from the delay value means. The first and second quadrature signals are offset by 90 degrees. The DAC is coupled to the multiplexor and to the delay value means to generate a first and second currents based on the input phase delay from the delay value means. The signal combined is coupled to the DAC and to the multiplexor for generating the output signal phase-shifted by the input phase delay. The signal combiner multiplies the first and second quadrature signals with the first and second currents to generate third and fourth quadrature signals, respectively. The signal combiner further combines the third and fourth quadrature signals to generate the output signal.