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Abstract

Insertion loss measurement at microwave frequencies is achieved using an improved IF substitution technique wherein the IF test signal is applied to a feedback loop which generates a feedback signal to null out the test signal. A precision calibrated attenuator in the path of the feedback signal is adjusted to achieve the null, and the difference between attenuator settings with the device under test in and out of the microwave test signal path is a measure of the insertion loss of the device under test. In one embodiment the difference signal between the IF test signal and the fed back null signal is divided into amplitude and phase components which are integrated and used to control the attenuation and phase shift, respectively, of a reference signal at the IF test signal frequency, the controlled reference signal comprising the fed back null signal. In another embodiment the amplitude and phase components are filtered at d.c., restored to IF, and recombined before being fed back as the null signal. Still another embodiment steps the difference signal is stepped down to the audio frequency range in successive steps and is filtered at the audio level before being stepped back up to IF and fed back. Any of these embodiments can be employed in a dual channel system, one of which contains the device under test, wherein the fed back null signals are compared in phase to determine the phase shift introduced by the device under test. A novel phase shifter is disclosed wherein the 360.degree. limitation of unambiguous phase detection is overcome.

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First Claim