Low-Noise Amplifiers for Centimeter and Shorter Wavelengths

A number of new techniques for detecting signals at centimeter and shorter wavelengths are making possible far better noise performance than ever before attainable. Over the past ten years, noise temperatures have been reduced from around 3,000°K to less than 5°K. The low-noise devices which are currently competitive or which show future promise include traveling-wave tubes, parametric amplifiers, tunnel diodes, masers, photon counters, and photosensitive detectors. This paper discusses the various techniques for attaining low noise and summarizes the achievements relative to each technique. Effective noise reduction in traveling-wave tubes is accomplished by making the beam flow through an extended low-velocity region. Noise temperatures as low as 250°K have been measured on traveling-wave tubes at S band. Low noise has been attained in parametric devices using both electron beams and semiconductor diodes. Some of the lowest noise temperatures measured on any unrefrigerated microwave amplifiers are those for electron-beam parametric amplifiers. Refrigerated semiconductor-diode parametric amplifiers have given even lower noise temperatures. The tunnel-diode amplifier is the most recent negative resistance microwave device. It seems to have limitations as far as extremely low-noise performance is concerned, but nevertheless it is a good competitor for many low-noise applications. So far, the maser is the best in microwave low-noise amplification. Intrinsic maser noise temperatures of a few degrees are readily attainable. Photon counters and photosensitive detectors are inherently even quieter and give promise of considerable future potential.