SINE WAVE OSCILLATORS 



bear in mind that the exact expression for the resonant frequency for a tuned 

 circuit performing sustained oscillations under the action of a valve contains 

 terms involving the valve parameter r^. These expressions are different for 

 the various oscillator circuits or the two-coil circuit discussed 



0) 



res 



and we need not concern ourselves with them except to bear in mind that for 

 constant w, r^ should be constant. With tetrodes it is a fortunate fact that the 

 effects of raising the anode and screen supply voltages produce opposite 

 effects on the /•„, the screen effect being preponderant. Thus if the screen be 

 fed from a suitable fraction of the HT supply it is possible to secure — at 

 least over a range of HT variations — a constant r^. Thus a tetrode circuit to 

 use this phenomenon might have the form of Figure 14.13. The oscillator 

 circuit is supplied from a potentiometer across the HT supply and the potentio- 

 meter is adjusted until the required frequency stability is obtained. Un- 

 fortunately we lose with this circuit the valuable property of isolation between 

 oscillating circuit and load, since in the absence of the suppressor there is 

 capacitance coupling between anode and screen. This difficulty is overcome 

 in the following manner. 



Consider once more the simple Hartley circuit of Figure 14.6. There is no 

 particular reason why earth should be connected to the valve cathode. Let 

 us connect it instead to the anode, and re-draw the circuit as in Figure 14.14. 

 The anode circuit of the valve is now ABC DBF. It does not matter whereabouts 

 in this circuit the HT supply appears, so let us remove it from EF and put it 

 between BC. We now have Figure 14.15. This is still a Hartley oscillator, 

 though possibly in unfamiliar form. 



HI 



D 



Figure 14.14 



B 



HT + 



*HT- 



Figure 14.15 



If now this circuit be used as the triode oscillator section of an electron 

 coupled tetrode circuit, we have the arrangement of Figure 14.16, which is 

 the most satisfactory layout. Independence of frequency on the HT supply 

 is secured by adjusting the potentiometer, as before, and in virtue of the 

 rearrangement of the oscillator section the screen potential is now fixed and 

 serves to isolate the oscillator circuit from happenings at the valve anode. 



Summarizing the findings on LC oscillators, the simple Hartley and Colpitt 

 circuits are characterized by: large voltage outputs at good waveform, and — 

 by making the valve a beam tetrode or other power valve — large output 

 powers. Automatic amplitude stabilization is simply obtained, but only poor 

 stabihzation of amplitude and frequency against variations of load. It is 



214 



