A Precision Noise-Power Comparator

This paper describes a prototype noise-power comparator based upon a theory given by Allred. Operating at 3 MHz, it is a null-type instrument, the principal components of which are a hybrid four-port, a stable CW generator, a precision attenuator, a dual-channel amplifier and band-pass filter, and an analog multiplier. Unlike other radiometers, no rapid switching of the noise power or reference voltage is performed. The instrument can compare noise powers having effective noise temperatures in the range from below liquid-nitrogen temperature to greater than 30,000°K. Two noise generators having known spectral densities are used to calibrate the comparator. The uncertainty of comparison is less than 1 per cent at 75°K and decreases to 0.2 per cent at 29,000°K.