Doc Ewen: The Horn, HI, and Other Events in US Radio Astronomy

Slide 31: Microwave and Millimeter Wave Applications in the 1970s and 1980s

40-50 GHz cooled receiver functional diagram. The signal passes through a quartz vacuum retaining window and enters a scalar feed-horn. The feed-horn consists of two sections: an outer section containing the quartz window which is at or near ambient temperature, and an inner section located within the dewar at a nominal temperature of 100K. The two sections are separated by a vacuum gap approximately 20 mils wide. This spacing is referred to as a "thermal gap" which minimized the conduction of ambient energy into the interior of the dewar via the feed-horn structure.

The functional block diagram is essentially the same for all 8 of the cryogenically cooled receivers. Differences are associated with the difference in the RF frequency, i.e. type of RF mixer, waveguide, local oscillator chain, etc. For most receivers, the feed is interconnected to the mixer via bandpass consisting of a combination of low pass and high pass filters. The principal function of these filters is to reduce the radiation and reception of spurious LO signals. The mixer is mounted on a copper coldplate which is engaged by the helium-cooled cold-finger. The 8-10 GHz low noise pre-amplifier is attached to the opposing side of the coldplate. The local oscillator signal is derived from a microwave phase locked oscillator, which is multiplied to the appropriate LO frequency. The output of the cooled low noise pre-amplifier is fed to a post-amplifier and bandpass filter located in the non-cooled lower section of the receiver. The receiver output is an undetected 8-10 GHz signal.

A closed cycle helium system developed by Philips, and distributed through Magnavox, was the principal cooling unit. The use of in-line linear reciprocating pistons significantly reduces the vibration level and contributes to long life. To be effective, the chamber within which cooling occurs must be evacuated. This is accomplished by a combination of a rough vacuum pump followed by operation of a vacion pump. Each receiver is equipped with its own vacion pump which is located in the lower section of the cylindrical enclosure. Only one rough pump is used for all 8 receivers. The heat generated by the helium condenser and compressor units is removed from the system by means of a compressed air heat exchange system. Each receiver is equipped with its own air cooling compressor, which is located external to the receiver. Credit: Photo courtesy of Doc Ewen.

Slide 32

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