SQUARE WAVE OSCILLATORS 



property of the (relatively stable) passive jR's and C's as it was with the RC 

 sine wave oscillators, but is also dependent on the HT — which can be 

 stabilized, though it is a nuisance to have to do so — and the valve parameters, 

 liable to progressive alteration as the valves age. For this reason the multi- 

 vibrator is not used as a source of known and stable frequency, it being better 

 where frequency is important to square up the output of a sine wave oscillator, 

 as mentioned earlier. 



Pentode multi-vibrator 



The rapidity of transition between the two states depends on the grids 

 having low capacitances to earth, and being driven from low source resis- 

 tances {Figure 15.8). The capacitances can be greatly reduced by using 



n 



Input 



capacitance 

 of Vy 



Input capacitance 

 of Vi 



Figure 15.S 



pentodes instead of triodes, and if pentodes are used the source impedances 

 are substantially the anode loads. The circuit has the appearance of Figure 

 15.9. The screen decoupling time constants must be long compared with the 

 time of a cycle of oscillation. If this is not possible because the latter is 



HT* 



Figure 15.9 



itself low, the screens will require stabihzation, usually, by a soft diode. 

 The time constant of the transitions of the circuit, t in Figure 15.10, will 



Figure 15.10 



be R]^ times the total shunt capacitance across each grid, including strays. 

 If it can be assumed that the bulk of this is composed of Cg^., then for a 



230 



