REFLEX OSCILLATORS 537 



X. Variation of Power and Electronic Tuning with Frequency 



When a reflex oscillator is tuned through its tuning range, the load 

 and repeller voltage being adjusted for optimum efficiency for a given drift 

 angle, it is found that the power and efiiciency and the electronic tuning 

 vary, having optima at certain frequencies. 



When we come to work out the variation of power and electronic tuning 

 with frequency we at once notice two distinct cases: that of a fixed gap 

 spacing and variable resonator (707A), and that of an essentially fixed 

 resonator and a variable gap spacing (723A etc.); see Section XIII. 

 Here we will treat as an example the latter case only. 



The simplest approximation of the tuning mechanism which can be ex- 

 pected to accord reasonably with facts is that in which the resonator is 

 represented as a fixed inductance, a constant shunt "stray" capacitance 

 and a variable capacitance proportional to 1/rf, where d is the gap spacing. 

 The validity of such a representation over the normal operating range has 

 been verified experimentally for a variety of oscillator resonators. Let 

 Co be the fixed capacitance and Ci be the variable capacitance at some 

 reference spacing di . Then, letting the inductance be L, we have for the 

 frequency 



CO = (L(Co + Ci d,/d))K (10.1) 



Suppose we chocse di such that 



Co = Ci. (10.2) 



Then, letting 



d/di = D (10.3>) 



a'l = (ILCor = 27r/i (10.4) 



w/a;i = IF. (10.5) 



IF = 2'(1 + \/D)~K (10.6) 



We find 



This relation is shown in Fig. 51, where D is plotted vs TF. It is perfectly 

 general (within the validity of the assumptions) for a proper choice of refer- 

 ence spacing di . We have, then, in Fig. 51a curve of spacing D vs re- 

 duced frequency IF. 



The parameter which governs the power and eflicency is Gn/ye . We 

 have 



Cs/jc = (G«/i8')(2Fo//o0). (10.7) 



As Fo , /o and 6 will not vary in tuning the oscillator, we must look for varia- 

 ton in Gu and (3'^. 



