Cryogenic wideband LNA design and characterization.

Abstract

In investigating star-burst galaxies in the early universe, a radio telescope requires a large intermediate-frequency bandwidth, to scan the atmospheric window rapidly and examine the galaxy efficiently. Therefore, a cryogenic wideband low noise amplifier (LNA) is designed and characterized to meet this requirement. Since existing LNA design configurations are either narrowband with low noise or broadband but noisy, a new wideband mechanism with low noise is proposed. This utilizes a mix of low-frequency inductive feedback on the source side of the transistor and high-frequency capacitive feedback on the drain side. As no input inductor is required, this new mechanism is also low noise over a wide bandwidth. Both its signal and noise performance are fully analyzed. The superiority of this method over other matching approaches has also been demonstrated. Using a knowledge of the underlying physics, an 8--20 GHz LNA has been designed. By carefully modifying the loading impedance for the first-stage transistor, both small input reflection coefficient and low noise temperature can be achieved. For the cryogenic noise temperature measurement, two popular methods, the variable temperature load and the cold attenuator, are investigated and implemented inside the cryostat. Different factors affecting the measurement sensitivity for each case are also addressed. Moving from noise temperature to noise parameters, since the conventional tuner method is incompatible with cryogenic noise measurement systems, a novel broadband frequency-variation method is proposed for measuring the four cryogenic noise parameters. Here the highly periodic noise temperatures, caused by a long mismatched input transmission line, can be demodulated using the known generator impedances to obtain the four unknowns. After the optimization of the algorithm, a wideband mismatched module using an inductive circuit is constructed. Since the noise parameters of a passive circuit can be calculated solely from its scattering parameters, it can be used to verify this frequency-variation method. Measurements of the noise and scattering parameters of the 8--20 GHz LNA at room temperature, 77 K and 4 K are presented. In summary, this LNA at 4 K environment has -10 dB input reflection coefficient, 10 K noise temperature, and 15 mW power dissipation.