SINE WAVE OSCILLATORS 



point at which to introduce them. They are a means of generating an exceed- 

 ingly brief pulse, say, less than 1 microsecond, or they may be used to generate 

 approximately triangular waves. Suppose we take an ordinary LC oscillator 

 of the type first discussed and transpose the pick-up coil and the stabilizing 

 RC network, as shown in Figure 14.17. This has no effect on the operation of 



^f 



T+ 





Figure 14.17 



Figure 14.18 



the circuit. Now suppose we increase CR. The time taken for the bias to 

 adjust itself to a small reduction in the loop gain, perhaps caused by a small 

 decrease in the HT supply, becomes longer till a stage is reached at which the 

 oscillations apparently die away spontaneously. If now, to counteract this, 

 we increase the loop gain, usually by increasing M, we find the amplitudes of 

 oscillations become themselves oscillatory. Oscillations die away as before, 

 then the bias leaks away, whereupon fierce oscillation becomes once more 

 possible. Oscillations rapidly build up, generating a bias which eventually 

 rises to a level too high to sustain them, then they die away once more 

 (Figure 14.18). This state of affairs is called 'squegging' and is the halfway 

 house to the production of a blocking oscillator. To achieve the latter we go 

 on increasing the loop gain, and the effect of this is to cause the oscillations 

 to grow so fast that the capacitance is charged extremely quickly. Eventually 

 a state is reached where the valve is cut off after only half a cycle of oscillation : 

 this is the blocking oscillator. In that half-cycle enough grid current flows to 

 charge the capacitor to a bias voltage which will prevent a further cycle of 

 oscillation. The circuit is quiescent until this bias voltage has leaked away 

 sufficiently for oscillation to recommence. The waveform across R and C 



Anode 

 potential 



T 



Potential 



at top of 



/?and C 



Figure 14.19 



is therefore quasi-triangular. At the anode a train of negative-going pips is 

 available, of duration approximately half a cycle of the frequency at which 

 the tuned circuit is resonant {Figure 14.19). 



216 



