A Wide Range Microwave Sweeping Oscillator 



By M. E. HINES 



{Manuscript Received July 24, 1950) 



1. Introduction 



A SWEPT frequency oscillator is a useful laboratory tool for testing 

 wide-band circuit components. It permits an oscillographic display 

 of a frequency characteristic, avoiding much of the labor of point-by- 

 point testing at discrete frequencies. There was a particular need in the 

 Bell Telephone Laboratories for a sweeping oscillator to cover the com- 

 munications band between 3700 and 4200 megacycles to facilitate the 

 testing of components for radio relay repeaters. 



This paper describes one type of oscillator designed to satisfy this 

 need. It utilizes the BTL 1553 (or the Western Electric 416A) micro- 

 wave triode. The tuning is accomplished mechanically so that the fre- 

 quency varies continuously back and forth over the band at a low audio 

 frequency rate. Continuous oscillations have been obtained over a 900 

 megacycle band from 3600 to 4500 megacycles. 



2. Circuit Structure 



Basically, the rf circuit consists of a tunable cavity for a grid-anode 

 resonant circuit, a means for feedback to an untuned grid-cathode circuit, 

 and a means for coupling the cavity to a waveguide output. The grid- 

 anode cavity is the only sharply tuned circuit, and it was found that oscil- 

 lations could be obtained over the entire band by changing the resonant 

 frequency of that cavity alone. In this application, the electronic conduct- 

 ance between the grid and cathode is so high that this portion of the 

 circuit has an inherent broad band such that separate tuning is unneces- 

 sary. 



The necessity for continuous, rapid tuning virtually requires that there 

 be no sliding contacts in the tuning mechanism. A type of cavity was 

 chosen so that tuning could be accomplished by a simple variable capaci- 

 tor of the non-contacting type. Reduced to its simplest elements, it con- 

 sists of a short coaxial line, resonant in the half-wave mode. Actually the 

 line is much shorter than a half wavelength because of excess capacitance 

 at both ends. At one end is the capacitance of the grid-anode gap, and at 

 the other end is the variable capacitor used for tuning. 



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