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The characteristics of a backward-wave circuit as a beam modulator and as a beam demodulator are investigated theoretically and experimentally. A basic modulator-demodulator configuration, consisting of two periodic circuits (e.g., helices) separated by an arbitrary transducer section, constitutes the basis for a new class of backward-wave tubes. These "cascade backward-wave amplifiers" behave as narrow-band amplifiers whose pass bands can be tuned electronically over a wide range of frequencies. They overcome most of the inherent disadvantages of the simple single-circuit backward-wave amplifier and feature high gain well removed from the oscillation region, high off-signal rejection, internal circuit terminations which smooth out variations in operating characteristics with frequency, and provision for adjusting bandwidth electronically. The characteristics of an experimental model at S-band are presented and compared with theory. Analysis indicates that a minimum noise figure in the same range as that of a conventional traveling-wave tube, i.e., about 6 db, should be attainable.