Low-Noise Temperature Measurement Methods in the Radio Astronomy

The noise temperature is one of the most important performance indicators of the receiver and low-noise amplifier of a radio telescope. The precise measured noise temperature is key for evaluating whether the system works properly. Such a noise temperature of a receiver or low-noise amplifier is becoming increasingly lower with the fast development of electronic technologies, so that it has become very difficult to be measured accurately and rapidly. In this paper, we describe six noise-temperature measurement methods. These measurement methods have the advantages of being accurate, reliable, simple, and easy to implement. All of the methods employ the Y-factor measurement. The first method is the Liquid-nitrogen/ambient-aperture alternate loading method. This method is very accurate, and only requires relatively simple measuring equipments, so it is often used for high-precision noise-temperature measurement. The second method is the cold-sky/ambient-aperture alternate loading method, which uses the cold sky and an ambient-aperture load of normal temperature. It must be carried out outdoors where the sky is unobstructed. It is very accurate for frequencies from 1 to 12GHz. The third method is the liquid-nitrogen/normal-temperature load-alternating method. Two separate loads and a mechanical switch are used in this method. It is often used to measure the noise temperatures of LNAs. The principle of this method is as same as the first method but the precision is not as good though. The fourth method is the noise-source load method. This method uses a commercial noise figure meter in conjunction with a calibrated diode noise source. It is not only the standard method but also the simplest method. The fifth method is the cryogenic-attenuator associated noise-source load method where a cold load is provided when the diode noise source is off, and a hot load is provided when the diode noise source is on. In the off-to-on transition the power and impedance changes of the noise source are attenuated almost to insignificance by the cryogenic attenuator. In addition, mechanical switches are not needed in this method, so it can measure the noise temperature accurately and rapidly. The sixth method is the variable-temperature load method, where a controllable heater is used to heat a load and adjust its temperature. The temperature change of the load leads to a corresponding change in noise power measured at the output. There is a high-degree of repeatability in this approach.We subsequently give some measurement results with the methods. We also discuss in detail the easily overlooked factors that influence the measurement precision.