Measurement of Nonlinear Phase Distortion in Microwave Limiters under Dynamic Conditions

Nonlinear phase distortion (amplitude-modulation to phase-modulation conversion) in microwave limiters can be different under static and dynamic conditions. A technique for measurement of dynamic phase distortion is described in which a two-tone test signal is used to simulate amplitude modulation with modulating frequencies of the order of megahertz. The two-tone test signal is generated by linearly summing two equal-amplitude sine waves. The result is a constant-frequency amplitude-modulated signal whose envelope varies as a rectified cosine wave at the beat frequency of the two sine waves. There is no phase deviation except at the envelope zeros, where the phase jumps 180°. Measurements are made by splitting the output signal from the limiter into two identical signals and delaying one relative to the other by approximately an odd multiple of one-half the envelope period. The two signals, when displayed on an X-Y cathode-ray tube, produce a pair of filled-in ellipses with major axes at 45° and and 135°. Peak phase deviation is obtained by direct measurement of either ellipse. Measurement data in E band are given for two tunnel-diode amplifier-limiters, a balanced mixer-limiter, and a traveling-wave-tube amplifier followed by a balanced mixer-limiter.