The backward wave converter

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This paper describes the development of a backward wave converter which is a hybrid device that combines the essential elements of an electronically swept receiver in a single vacuum envelope. Early work at ITTL was undertaken on the development of an electronically swept receiver which used a backward wave amplifier, a backward wave oscillator and a crystal mixer. Near the end of this development, work was reported by Gray of the University of California on the possibility of combining these three functions in a single vacuum envelope. Encouraged by the success in tracking the two backward wave tubes to provide a constant I-F in the receiver development, we undertook the development of a backward wave converter, as the device was called, for the frequency range of 500 to 1000 Mc. This device provides a number of advantages when compared to the conventional crystal mixer, such as, (1.) Elimination of crystal burnout, (2.) Isolation of L. O. power from the antenna, (3.) Preselection, (4.) Voltage tunability at high rates over a large frequency range. These early backward wave converters, which could be swept at rates up to 1 kc over an octave band, had a preselection bandwith of 10 mc, a dynamic range of greater than 70 db and an I-F output at 60 inc. Noise figures, when the tube was adjusted for octave tuning operation, ranged between 13 and 19 db. Much better noise figures than this (6-8 db) could be obtained at spot frequencies by optimizing the voltage profile between cathode and helix. Limitations of length required the use of higher beam current (6-10 mA) than is desirable for good low noise performance. Thus if these limitations are removed a considerable improvement can be expected. A range of converters are now being developed extending to the 12 Gc region. Results on these later tubes will be given. Possibilities and expectations of extending this technique to the mm wave region will also be reviewed.