Some Analyses of Wave Shapes Used in Harmonic Producers

HE heterodyne method of frequency comparison requires, except in the case of the comparison of nearly identical frequencies, the generation of harmonics of either the unknown, or of the standard frequency or of both. These harmonics may be generated directly in the modulator which produces the difference frequency, or "beat note", or may be generated in an entirely separate circuit before the frequency is applied to the modulator. An example of the latter is the multivibrator circuit often used in connection with a frequency standard to produce a series of harmonics of this standard frequency. The design of harmonic generators for frequency measuring equipment presents a different problem from the design of equipment for producing a single harmonic such as doubler or tripler stage in a radio transmitter. In the latter case the amplitude of the one harmonic and the efficiency are of primary importance. In frequency measuring equipment, although a large amplitude of each harmonic is desirable, it is of greater importance that each harmonic within the range to be used, which may be up to the 100th or 150th harmonic or even higher, be present and that the amplitude of nearby harmonics be of the same order of magnitude. Unless the latter conditions are met, there is a danger that the beats obtained with a weak harmonic will either be entirely overlooked or mistaken for a higher order modulation product. The generation of harmonics is usually accomplished by the distortion of the wave shape in some nonlinear circuit element such as a vacuum tube.