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A microwave interferometer for the measurement of instantaneous frequency is described. It consists of a transmission line circuit used as a frequency modulation discriminator, and an indicator consisting of a microwave detector, video amplifier, and oscilloscope. The discriminator transforms the frequency variation during rf pulses, or that of continuous waves, to an amplitude variation at the input of the indicator. The characteristics of the discriminator are investigated theoretically and conditions are discussed for adjustment of the discriminator in order to minimize linear and nonlinear distortions. Since amplitude deviations of the input wave will simulate frequency variations, a technique which greatly reduces this effect is presented. Analysis and experiments indicate that this method provides a practical technique for measuring the frequency and its stability during a radar pulse.