A study of the noise temperature and bandwidth of microwave parametric amplifiers

Abstract

This study has arisen because of the requirement for a 4 CHz, 500 MHz bandwidth, parametric amplifier with a noise temperature of less than 20ºK, for the proposed satellite communication system and Is concerned entirely with non degenerate parametric amplifiers using variable capacitance (varactor) diodes. Although all the measurements involved were at 4 GHz, much of the theory and most of the conclusions of the study can be applied to any part of the micro wave frequency range (i.e. from 1 to at least 40 GHz).A theoretical study of the limitations on bandwidth has been made and good agreement between the measured and theoretical bandwidth of a single diode amplifier is observed, A novel form of liquid helium cooled broadband amplifier, using two single tuned, single diode amplifiers, separated by a quarterwave section of transmission line, has been realised. This type of amplifier has the advantage of requiring no external mechanical tuning readjustments on cooling to liquid helium temperatures. The gain (^1)/(_4). bandwidth product measured for this device is twice the gain (^1)/(_2); bandwidth, of a single tuned amplifier, as predicted from the theory. A 4 GHz amplifier of this type producing a noise temperature of less than 20% over a 250 MHz bandwidth at 30 dB gain or over a 500 MHz bandwidth at 15 dB gain, is described. Greater bandwidths should be available using the same broadbanding technique on two balanced diode amplifiers. The magnitude of shot noise in parametric amplifiers has been determined by measurements on two single diode amplifiers at room temperature and the factors influencing amplifier noise at very low temperatures, have been investigated by a series of noise measurements on two liquid helium cooled devices. Finally, some possible future trends in amplifier design are considered.