TRIANGULAR WAVE GENERATORS 

 and because t ^ RC{A + 1) in the working range, and A ^ 1 



dF, . tE 



dt • RC 

 So the time to generate a given output ampHtude Fout is ? = Ci? VonijE. 



HT+, 



HT+, 



HT* 



Figure 16.17 



Figure 16.18 



If £ is derived from the HT supply and is, say, 250 V, and Fout is, say, 200 V, 

 and / = 300 seconds 



300 X 250 ^^^ ,. ■ r J 



CR = ;rzrz: — =375 megohm-microfarads 



The solution is of course to reduce E, but if we do this we reduce the virtual 

 charging voltage AE and spoil the linearity. The Gibbs and Rushton refine- 

 ment makes use of a 'bootstrap' circuit {Figure 16.18). Any cathode-follower 

 cathode will set itself up a few volts positive to its grid. Applied to this circuit, 

 these few volts constitute our charging voltage, E. Current flows into C via R 

 from Fi cathode, but the concomitant small rise in grid potential as the 

 stroke proceeds does not reduce the rate of charging because the grid potential 

 rise is transferred to F^ grid and produces a similar increase in E, maintaining 

 the charging current. If we can by this method, without impairing the 

 linearity, reduce E from 250 V to 2-5, we can secure a 5 minute sweep with 

 CR = 3-75 megohm-microfarads, say C = OT microfarad and R = 37-5 

 megohms, both reasonable values. 



Blumlein 

 integrator 



Figure 16.19 



Control of Miller run-down — We have now to discuss how to derive the 

 square wave which allows the Miller run-down to occur. There are a number 

 of possible ways, depending on what is wanted. 



If the requirement is for independent control of the duration and slope of 

 the wave (from which it follows that the amplitude will vary) then all that is 

 necessary is to precede the Blumlein integrator with a flip-flop. The flip-flop 



243 



