556 



BELL SYSTEM TECHNICAL JOURNAL 



is too long, resonances will occur which can absorb power and otherwise 

 aflfect the cavity impedance. 



When properly adjusted and sweeping, the output is continuous and 

 the frequency varies approximately sinusoidally back and forth over the 

 band of interest. The width of the sweeping band depends upon the ac 

 current in the voice coil of the speaker drive, and the center frequency 

 depends upon the mean position of the tuning plunger. The latter can be 

 adjusted mechanically by loosening the clamping screw and raising or lower- 

 ing the sweeping mechanism by hand. It is also possible to make small ad- 

 justments of the center frequency electrically by adding a dc component to 

 the voice coil driving current. 



5 20 



O 16 



5 



o 

 a 14 



3600 3700 



3800 3900 4000 4100 4200 4300 

 FREQUENCY IN MEGACYCLES PER SECOND 



4400 4500 



Fig. 2. — Power output curves at 115, 170, and 200 Volts on the anode, for a mean 

 anode current of 25 ma. 



Typical curves of power output vs. frequency, taken at different anode 

 voltages, are shown in Fig. 2. The flattest curve requires a voltage con- 

 siderably lower than the tube rating. The feedback phase is not optimum 

 for best power output, a larger phase shift being desirable in this oscilla- 

 tor. The lowered voltage helps in this regard, increasing the electron 

 transit time in the tube and thereby increasing the phase shift. Efficiency 

 was sacrificed in this design to increase the tuning band. A longer feed- 

 back path would increase the power output, but would tend to narrow 

 the band over which oscillation could be obtained by a single tuning 

 adjustment. 



The anode power supply should be variable between 100 and 250 volts, 

 but need not be regulated because this voltage is not critical. A rheostat 

 is used for cathode self-bias. The cathode heater and the sweeping mech- 

 anism are supplied from a single 6.3 volt filament transformer, with a 

 potentiometer control to vary the sweep range. 



