The Use of Cryogenic HEMT Amplifiers in Wide Band Radiometers

Advances in device fabrication, modelling and design techniques have made wide band, low noise cryogenic amplifiers available at frequencies up to 106 GHz. Microwave radiometry applications as used in radio astronomy capitalize on the low noise and large bandwidths of these amplifiers. Radiometers must be carefully designed so as to preclude sensitivity degradations caused by small, low frequency gain fluctuations inherent in these amplifiers. 1 Radiometry Fundamentals The basic goal of microwave radiometry is to measure the total microwave power from a specified source over a defined bandwidth. The simplest radiometer consists of a low noise amplifier connected to a square-law detector. The sensitivity of such a total power radiometer is given by [l] where At is the uncertainty of the measured microwave power (expressed in temperature units), Tsys is the noise temperature of the radiometer system, ∆υ is the RF bandwidth of the system and tint is the duration of the measurement integration. Millimeter-wave radio astronomy applications typically have values of At and Tsys as given in eqn(l), requiring the argument of the radical be < l 0 l 2 . State of the art amplifiers in the mm regime [2] can achieve ∆υ ≈ l0GHz, necessitating tint ≈ 100sec. A problem arises due to the presence of the second term, , which is the effective fractional power gain variation occurring during the integration interval. For a typical integration time tint ≈ 100 seconds, fractional gain variations as small as 10 -6 will substantially degrade performance. These gain fluctuations have a frequency dependence [3] 1, allowing radiometer performance to be characterized by a knee frequency, fknee, the frequency at which the two terms under the radical in eqn(1) are equal. The effect of the gain fluctuations is to limit tint < l/fknee, integration periods longer than this produce little further reduction in At. Unfortunately intrinsic fluctuations of the device parameters cause such gain fluctuations, which worsen as device geometry shrinks, and upon cooling to cryogenic temperatures, both conditions required for low noise, high frequency operation. The nature of the problem can be understood by examining the following relation for the instantaneous power incident on the detector in a total power radiometer.