REFLEX OSCILLATORS 



595 



a potentiometer properly ganged with the transmitter in radar systems 

 would provide optimum output throughout the band. T his is an advantage 

 over electronic tuning, since the signal to noise performance of the receiver 

 depends in part on the beating oscillator power into the crystal. 



In a thermally tuned tube it is necessary to provide safeguards against 

 excessive power input to the tuning strut since this might produce a per- 

 manent deformation and impair tuner operation. A simple method for 

 obtaining such protection is to use low-frequency cathode feedback produced 

 by a cathode resistor. With a cathode biasing resistor of 725 ohms the 

 grid may be held 15 volts positive with respect to the more positive end of 

 the cathode biasing resistor indefinitely without damage to the tuner when 

 the normal plate voltage of 300 volts is applied. 



The grid control characteristics for a typical 2K45 shown in Fig. 88 were 

 obtained while using the cathode biasing resistor. These characteristics 

 may be given in two ways. In one case the repeller voltage is held fixed 

 and the characteristics are given over a range between half power points. 

 It will be observed in this case that the characteristics are discontinuous 

 because of the electronic tuning resulting from the repeller voltage shifts 

 between ranges. For the other case the repeller voltage is maintained at 

 its optimum value at each frequency. 



In either case, one striking feature is the essential linearity of the variation 

 of frequency with grid voltage. This is of considerable importance in many 

 frequency stabihzing systems and represents an advantage of thermal tuning 

 over electronic tuning. In the case of electronic tuning, as shown in Section 

 VII, the rate of change of frequency with repeller voltage varies rapidly 

 as the repeller voltage shifts away from the optimum value. Since frequency 

 stabilization is essentially a feedback amplifier problem in which the rate of 

 change of the frequency with the control voltage enters as one of the factors 

 determining the feedback, it is apparent that the frequency stabilization 

 will vary as the repeller voltage is shifted. In contrast, for the case of 

 thermal tuning, because of the linearity of frequency with grid voltage, the 

 stabilization will be independent of the frequency. It should not be for- 



2K45 Operating Conditions 



Heater Voltage 



Resonator Voltage .... 

 Klystron Current .... 

 Repeller Voltage Range 



Tuner Current 



Tuner Power 



Th (9660-8500 Mc/s) . . 

 Tc (8500-9660 Mc/s) . . 



