Measuring the distortion or linearity of FM demodulators is usually performed with low-distortion frequency modulators as test sources. Such modulators are expensive and there is no assurance that the distortion of the test signal is lower than that of the demodulator being tested, especially if that demodulator is state of the art. This paper describes a technique which heterodynes two FM generators of modest performance to produce a highly accurate SMPTE or CCIR intermodulation test signal. It then describes an extension of the technique for testing frequency modulators directly, and describes a system using the technique actually implemented to confirm the performance of an FM generator for automatic testing.
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