A Travelling-Wave Parametric Amplifier

Abstract

THE principle of parametric excitation of an oscillatory system has been known for many years. An electric oscillatory circuit can be excited in this way if the condenser plates are instantaneously pulled apart by a fixed amount whenever the voltage reaches a maximum value, and instantaneously restored to their original positions when the voltage is zero. Under these conditions energy is communicated to the circuit when the plates are pulled apart; but none is extracted when they are restored, and so oscillations can be maintained. The capacitance/time graph envisaged in the foregoing is a square wave at twice the resonant frequency of the circuit; but it can be shown that a sinusoidal variation at twice the resonant frequency may be used. Any method of periodically varying the capacitance may be used, or alternatively the inductance may be varied. Moreover, such a circuit can be used as an amplifier, since, at the fundamental frequency, the excitation mechanism is analogous to negative resistance.