A testing method or apparatus utilizes multiple frequencies applied to a device under test for measuring newly discovered frequency modulation effects. An embodiment may include a lower frequency signal with a smaller amplitude higher frequency signal to test a dynamic change in frequency response, gain, and or phase. This dynamic test can reveal frequency modulation effects. Another embodiment may include the use of a multiple frequency signal to dynamically induce a time varying phase or frequency distortion for the device that has differential phase distortion. The device's output is then measured with an FM detector to measure a shift in one of the frequencies used in the test signal or to measure frequency modulation effects of any signals, including distortion products, from the device. Yet another embodiment of the invention may include biasing a device with a voltage to span the output voltage range of the device while measuring harmonic or intermodulation distortion or phase or frequency response at the various operating points.

Method and apparatus to measure differential phase and frequency modulation distortions for audio equipment

A method and an apparatus for measuring group delay characteristics of a transmission path is disclosed wherein a test signal is fed to the input of the transmission path and the test signal is amplitude modulated by a split frequency. The carrier frequency of the test signal is interchanged periodically between a measuring frequency and a standard reference frequency. The output measuring and reference signals of the transmission path are sensed by noting the positions in time of specified points of the envelope waveform that occur during the periods of the reference type signal and the measurement type signal. A digital pulse counter is provided for measuring a first time interval between a specified point during a period of a first type signal and a specified point during the subsequent period of the second type of signal, and for measuring a second time interval between the latter specified point and a specified point during the subsequent period of the first type of signal. The pulse counter is bidirectional and is reversed at the end of the first time interval. The counter state at the termination of counting represents the difference between the first and the second time intervals and thus the group delay to be measured.

Method and apparatus for measuring the group delay characteristics of a transmission path

Third order IM distortion in a microwave repeater is measured in situ by applying a small amount of low frequency amplitude-modulation to the payload signal while at the same time adding a test-tone of fixed frequency and amplitude. At the repeator's output, a pair of synchronous detectors, driven in phase quadrature by the test-tone generator, measure the modulation depth of the test-tone. This modulation depth is now a measure of the degree of distortion in the repeater.

Single tone technique for measuring IM distortion

A measure of delay distortion produced in a circuit under test is provided by applying a purely amplitude-modulated carrier to the circuit and detecting the resulting phase modulation.

AM-PM Conversion for measuring delay distortion

A method and apparatus for recording directly group delay measurement of a system under temperature and stress tests employing a modulated carrier frequency swept over an S or X-band of interest and applied through a reference path and a test path to separate detectors for group delay measurement using a power divider, e.g., a directional coupler or a hybrid-T junction. A phase comparator is initially balanced, and then the modulated carrier is swept in frequency over the band of interest for different conditions of temperature and/or mechanical stress to obtain a family of characteristic group-delay curves for the system under test.

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Swept group delay measurement

This method and apparatus measures conversion of amplitude modulation to phase modulation and is useful for any application wherein resultant unwanted phase modulation is to be measured, but is mainly intended for testing devices used in frequency modulated multichannel communication systems having amplitude modulation to phase modulation conversion which causes distortion of a desired frequency modulated signal.

Michael Crabtree

Papers

Method and apparatus for measuring conversion of amplitude modulation to phase modulation

Schaltungsanordnung zum messen der umwandlung von amplituden-modulation in phasen-modulation