A NEW MICROWAVE TRIODE 



537 



the grid itself being grounded. The output circuit in the plate is tuned to 

 the sum or difference frequency, fz ± fv 



By-pass condensers, traps and filters for other frequencies present in the 

 modulator must be considered. Besides the beating oscillator and the 

 signal, their sum and difference frequencies appear in both the input 

 circuit and the output circuit and of course bias voltage on the cathode 

 and plate voltage on the plate must be applied. Some of the traps and 

 by-pass condensers which influence the converter performance are in- 

 dicated in Fig. 8. It is obvious that microwave energy should be kept 

 from flowing into the i.f. signal circuit and vice versa if the highest con- 

 version gains are to be obtained. Both of these conditions are easily 

 achieved. It is not so readily apparent that the components of the wanted 

 and the unwanted sidebands present in the input circuit must be handled 



f f 



f,+f, " f,-f, 



T TTT 



^2~'fl *B + 





Fig. 8. — Schematic diagram of converter with traps and filters for fundamental fre- 

 quencies of signal,/], beating oscillator,/;, and sidebands, /2 ±/i. 



properly. Of these two, the more important is the wanted sideband and 

 the next figure illustrates just how necessary it is to treat it properly. 



The simplest way to keep the wanted sideband component of the input 

 circuit from being absorbed by the beating oscillator branch is to reflect 

 the energy back into the converter by means of a reflection filter. This 

 reflected energy arrives back at the tube and may conspire to reduce the 

 conversion gain of the modulator if the phase is wrong. The phase de- 

 pends upon the spacing along the waveguide between the tube and the 

 filter and Fig. 9 illustrates how badly the gain is affected when the wrong 

 spacing is used. Data for two different tubes are given which indicate 

 that the correct spacing for one tube may be incorrect for another. It 

 should be pointed out, however, that these two tubes were early experi- 

 mental models and that production tubes behave more consistently. 



The i.f. impedance of the modulator is also affected by the filter spac- 

 ing for the wanted sideband on the input. This effect can be utilized to 



