Analysis of a Special Purpose RC Filter Incorporating a Periodically Conducting Bilinear Element

The paper concerns the mathematical analysis of an experimentally-developed, four-terminal network consisting of a resistor-capacitor-chopper combination. The circuit is essentially a simple, low-pass rc filter, modified by the insertion of a periodically-conducting bilinear element. The circuit receives a modulated sinusoidal carrier, filters the modulation, and transmits the resulting signal as a series of amplitude-modulated rectangular pulses. The reed driving coil of the chopper is separately excited by a sinusoidal voltage of carrier frequency so that the pulse repetition rate of the output pulses is equal to the frequency of the input carrier. The analysis takes into account the "dead time" of the chopper and admits an arbitrary choice of phase difference between the input carrier voltage and the chopper-reed excitation voltage. An equivalent circuit is derived from response of circuit to an input carrier of fixed amplitude. A formula is obtained for equivalent circuit-time constant, and a condition for minimum attenuation is also derived. May 10, 1954.