APPENDIX 



Explanation of operation of Figure 31.5 



Before discussing the detailed operation of this circuit (which is shown 

 in simphfied form) it is perhaps as well to consider why something more 

 elementary would not suffice. The requirement of the device is that upon 

 the receipt at the input terminal of a brief negative-going pulse of above 

 a certain size, a large negative-going square wave of controllable duration 

 shall be generated at that terminal. 



Clearly some kind of flip-flop is required. Suppose we set up the circuit — 

 already discussed in Part I — of Figure 31.10. V^ is normally cut off", V^ 

 conducts heavily and we attempt to trigger the arrangement by a negative 

 pulse applied to F^ anode: then we bump up at once into at least three 

 difficulties. The first is that there is no simple way of varying the threshold 



HT + 



Figure 31.10 



Figure 31.11 



level of the device which does not also interfere with the duration and 

 amplitude of the square wave generated. The second is more serious. The 

 input impedance seen by the trigger pulse has the equivalent circuit of 

 Figure 31.11. The shunt elements R^ and R may be of quite high resistance, 

 but the resistance of the conducting diode (representing the grid and 

 cathode of V^ in series with Rj^ is of the order of a few thousand ohms only. 

 The effective internal resistance of the Geiger tube, regarded as a generator, 

 would be much higher. Consequently only a smafl proportion of the open- 

 circuit Geiger tube output would be available to trigger the circuit, probably 

 insufficient for reliable operation. 



Suppose we return R, not to HT+ but to a potential only just sufficiently 

 above earth to hold V-y properly cut off". It may then be possible to arrange 

 that V2 grid is sufficiently negative with respect to the cathode for inappreci- 

 able grid current to flow, in which case the input resistance of Kg is high 

 and the circuit will trigger satisfactorily. Unfortunately we run into a new 

 difficulty. K2 grid wave form is sketched in Figure 31.12 for the two cases 

 'R returned to HT+' and (the time constant being appropriately modified) 

 'R returned to a less positive potential'. It is clear that variations in the 



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