3.4 centimeter cyrogenic parametric amplifier system for radio astronomy
Halpain, Joe Lee
Walters, G. King; Blackwell, L. A.
Master of Science
A low-noise microwave receiver was designed and fabricated to provide the necessary sensitivity to detect the helium 3 line. In order to achieve ultra-low noise amplification in the input states of this receiver, cascaded non-degenerate cryogenically cooled parametric amplifiers were used. These amplifiers were followed by a tunnel diode amplifier and a mixer-preamplifier which provides a total system gain of approximately 60 db. The parametric amplifiers were cooled to the cryogenic temperature by a closed-cycle gaseous helium refrigerator. The ambient operating temperature provided by the refrigerator was approximately 18°K. The parametric amplifiers were pumped with a Ka-band klystron which provides a signal-to-idler ratio of approximately 0.38. Gain stability was achieved by using waveguide ferrite current variable attenuators which were used to control the pump power as required to maintain a constant varactor current. The system was designed to consist of two major assemblies. One of the assemblies is packaged in a 30 x 30 x 60 inch weather proof enclosure for mounting at the focal point of the National Radio Astronomy Observatory 140 foot radio telescope. This enclosure includes a thermoelectric cooling and heating system to maintain a constant temperature inside the assembly under all weather condition. The second assembly was packaged in a standard relay rack type cabinet. This unit contains all of the receiver control, monitoring and special remote test circuits. The two assemblies were designed to operate with connecting cable lengths of up to 500 feet. During the first observing period, the noise temperature performance of the receiver was about three times that expected, i.e., 100°K. However, the stability of the system was sufficient to allow long integration periods so that usable data was obtained. To correct the noise temperature performance, the parametric amplifiers were redesigned, particularly in the area of thermal conductivity. In addition, the manufacturer and package style of the varactor was changed to terminate the high failure rate that had occurred in the diodes. In addition to the low-noise temperature and excellent gain stability achieved by this receiver, several new concepts were utilized in the design of the system which greatly improved its mechanical and thermal properties. One of these is a special non-contacting choke flange on the input line to the parametric amplifiers which allows the input line to be short and unrestricted in location. The system will be utilized again in the summer of 1970. The present performance of the system should provide data which is far superior to that obtained in the last observing period since for the same signal-to-noise ratio, the integration time should be reduced by a factor of four or more.