14 

 SINE WAVE OSCILLATORS 



Oscillators producing a sine wave output occur frequently in electrobiological 

 work, important applications being the production of time markers from an 

 alternating source of accurately known frequency, and the generation of test 

 signals of variable known amplitude and frequency for measuring the 

 performance of amplifiers and recording gear. There are three important 

 types: (1) LC oscillators; (2) Crystal oscillators; and (3) RC oscillators. 



LC OSCILLATORS 



LC oscillators are suited to the production of frequencies above, say, 1 kilo- 

 cycle (where the amount of capacitance and inductance required to achieve 

 resonance is not inconveniently great) provided that the range of frequency 

 adjustment required does not exceed about 2 to 1. The reason for this 

 limitation is that the amplitude of oscillation depends on the Q of the resonant 

 circuit — (LlCR^y^. Thus for constant amplitude over the range of frequencies 

 required the ratio L/C ought to be held constant, while the product LC 

 (remember resonant frequency equals 1/LC for series tuned circuit and 

 approximately 1/LC for parallel tuned circuit) is varied. This requires 

 ganging of variable inductors and capacitors, rather an elaborate business. 



If a parallel tuned circuit be used as the load for a valve whose anode 

 current contains an alternating component of frequency co and of instanta- 

 neous value /, and if the tuned circuit is resonant at the frequency co, the valve 

 sees an a.c. load which is pure resistive and is equal to LjCR, and a circulating 

 current yQ/ flows round the tuned circuit. If a second inductance be loosely 

 mutually coupled to the tuned circuit inductance, then the e.m.f. induced in 

 the second inductance is jojM(jQi) = —coMQi. If this e.m.f. is appHed to 

 the grid of the valve — due regard being paid to the sense of the connections — 

 the feedback is positive and without phase shift, which means that if M, Q and 

 the va'lve gain are great enough the circuit will generate continuous oscillations 

 at frequency co (Figure 14.1). 



Now this is a feedback system of the kind we have become familiar 

 with, where the loop gain is clearly AB — g„fi)MQ for pentodes and 

 fjLl{{LjCR) + rj (dMQ for triodes. The criterion for oscillation is that 

 AB=\. If ^5 becomes < 1 the oscillations die away and '\iAB> 1 they will 

 — theoretically — grow to infinity. Thus the production of oscillations of con- 

 stant amplitude requires that AB should be held exactly equal to 1 at all 

 times. It is in the nature of practical things that this sort of state of aff'airs, 

 even if it could be temporarily estabhshed, would not persist for very long 

 and in general sine wave oscillators require some sort of automatic regulating 

 device to maintain the amphtude of the output. 



In the LC oscillator this is simply achieved by the addition of a capacitance 

 and a high resistance or 'grid leak', and leaving out any form of grid bias 



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