Abstract
Two RC circuits based upon all-pass phase-shift networks are described. They are useful as electronically tunable audio oscillators, selective amplifiers, or bridges. Change of resonant frequency is accomplished by varying amplitude of transmission in one or more circuit branches by means of vacuum-tube modulators. In theory, both circuits may be electronically tuned from zero to infinite frequency, and their feedback-loop gain at resonance should be independent of the frequency to which they are tuned. However, the frequency ratio conveniently obtainable in practice, before appreciable changes in gain occur, is about four to one. This limitation is in part a consequence of the decrease in effective Q of the frequency-controlling portion of the circuit when they are tuned far from center frequency.
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