2-50 Figure 2-46.—Parametric amplifier. PARAMETRIC FREQUENCY CONVERTERS.—Parametric frequency converters, using varactors, are of three basic types. The UPPER-SIDEBAND PARAMETRIC UP-CONVERTER produces an output frequency that is the SUM of the input frequency and the pump frequency. The LOWER-SIDEBAND PARAMETRIC DOWN-CONVERTER produces an output frequency that is the DIFFERENCE between the pump frequency and the input frequency. The DOUBLE-SIDEBAND PARAMETRIC UP-CONVERTER produces an output in which both the SUM and the DIFFERENCE of the pump and input frequencies are available. Parametric frequency converters are very similar to parametric amplifiers in both construction and operation. Figure 2-47 is a functional diagram of a parametric down-converter. The parametric frequency converter operates in the same manner as the parametric amplifier except that the sideband frequencies are not reapplied to the varactor. Therefore, the output is one or both of the sideband frequencies and is not the same as the input frequency. The output frequency is determined by the cavity used as an output. For example, the idler cavity in figure 2-47 could be replaced by a cavity that is resonant at the upper-sideband frequency (22 gigahertz) to produce an upper-sideband parametric up-converter. Since input and output signals are at different frequencies, the parametric frequency converter does not require a ferrite circulator. However, a ferrite isolator is used to isolate the converter from changes in source impedance. Figure 2-47.—Lower-sideband parametric down-converter.

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