A radiometer for measurement of the noise temperature of low-noise microwave amplifiers

The noise temperature of a low-noise microwave amplifier can be measured by connecting the amplifier input in turn to thermal noise sources at different physical temperatures. The conventional technique is limited by the necessity for a very stable amplifier and system gain during the measurements. The paper describes a manually-balanced radiometer circuit designed to overcome this limitation, and intended to measure amplifier noise temperatures in the range below 500°K. Examples of experimental measurements on a maser and on uncooled and cooled parametric amplifiers are discussed.