LC OSCILLATORS 



twice the HT voltage, which is large enough for most purposes. The output 

 impedance, or impedance 'looking back' into the oscillator, will be the dyna- 

 mic resistance of the tuned circuit in parallel with the r^ of the valve. Other 

 output voltages and impedances may be had by inductive couphng and 

 correct choice of the number of turns on the coupling winding, according to 

 transformer theory. 



The snag with simple schemes such as these is the upsetting effect upon the 

 oscillator performance caused by variations in the nature of the load. If the 

 latter is resistive it will affect the Q of the tuned circuit and the amplitude of the 

 oscillation*. If the load is also reactive it will affect the frequency of oscilla- 

 tion as well. What is wanted is some form of 'buffer' or isolating device 

 between the two: this may take the form of another valve, preferably a 

 pentode {Figure 14.11), or resort may be had to a useful scheme known as 

 'electron coupling'. 



Electron coupled oscillator 



The electron coupled circuit is shown in its most simple form in Figure 14.12. 

 It will be seen that a pentode is employed whose control grid and screen are 



HT+ 



HT+ 



Figure 14.12 



Figure 14.13 



used to form a triode Hartley oscillator. R^ and Q supply a direct decoupled 

 screen potential. The suppressor is earthed in the usual manner, and the load 

 is connected between anode and HT. Thus the oscillating triode section is 

 screened from the anode and the effects of diverse anode loads by the suppres- 

 sor. A small proportion of the electron stream passes to the screen and serves 

 to sustain oscillation, the rest passes through to the anode to develop power 

 in the load. The anode current is far from sinusoidal and a further tuned 

 circuit usually constitutes the load in order to recover a pure waveform. 



The pentode electron coupled oscillator frequency is thus independent of the 

 load, but unfortunately it is not independent of variations in the HT supply. 

 In the simplified description of LC oscillator operation, the tuned circuit was 

 merely said to be 'resonant' but nothing was said about precisely how the 

 resonant frequency is found. It is certainly not far from m = {XjLCy^^ and 

 for practical purposes this is sufficient, since LC will be made adjustable and 

 will be set up to give the frequency required. However, it is important to 



* Also the frequency, sUghtly. 

 15 



213 



