Synchronously tuned methods of harmonic and intermodulation distortion analysis

The need for an instrument to simplify the measurement of distortion as a function of frequency is discussed and existing systems for making these measurements are reviewed. The new system described in this paper uses a heterodyne wave analyser as the basic unit for carrying out the measurements. By means of a `beat frequency oscillator¿ system using the local oscillator from the wave analyser, crystal-controlled oscillators, frequency dividers, mixers and filters, test signals with the requisite purity of wave-form are generated. Switching permits the selection of harmonics up to the 8th for single signal tests, side bands up to the 4th for the S.M.P.T.E. test and intermodulation components up to the 8th for the C.C.I.F. test. The frequency at which the test is carried out is controlled by the wave analyser tuning. Automatic level control ensures that the level of the test signals is held constant throughout any series of tests. Interesting aspects of the circuits (which employ transistors) used to achieve the necessary signal purity and stability are described and the results on an experimental model are given.