TRIANGULAR WAVE GENERATORS 



accompanied by continued rise in screen potential and holding off of suppres- 

 sor bias until the run-down can proceed no further. Suppressor bias then 

 re-asserts itself and the anode current is cut off; flyback occurs. 



A time base using a triggered Miller transitron has the advantages of 

 simplicity and the ease with which it can be made a 'free-running' generator 

 of continuous triangular voltage oscillations, merely by reducing the sup- 

 pressor bias. Its snag lies in its rather long refractory period between strokes ; 

 this is bound up with the ratio of R'C to RC. If R'C is large enough to 

 ensure the continued holding off of suppressor bias, then after flyback, 

 when the screen moves negative once more, there is a rather long period of 

 extreme suppressor negativity which the trigger pulse will be unable to over- 

 come. This is clear from the waveforms in Figure 16.26. 



Screen 



Suppressor 

 potential 

 necessary 

 to initiate 

 run -down 



Suppressor 



This one can 

 This trigger 

 is unable to 

 initiate run-down 



Figure 16.26 



Trace brightening — It is worth noting that all the triggered triangular wave 

 generators listed generate somewhere a positive going square wave. Where 

 these circuits are used for cathode ray time bases this wave may usefully be 

 applied either through a CR circuit or through an RF coupling device (see 

 Chapter 32) to brighten the beam during its traverse, the beam being 

 suppressed at other times. 



Triangular current wave generators 



In this section we are concerned with the production of triangular waves 

 of current in an inductive device such as a beam-deflector coil for a cathode 

 ray tube. Such a coil comprises inductance and resistance in series {Figure 

 16.27), the resistance being of course the inevitable resistance of the wire 

 with which the coil is wound. 



If it is required to produce a current which rises at a steady d//d/ = /, then 

 the potential difference across the coil terminals required to produce it has 

 the 'penthouse' form of Figure 16.28 and comprises two components: a 

 square wave to overcome the back e.m.f., of magnitude Li, plus a triangular 

 wave of slope tR, to overcome the drop across the resistance. 



There are thus two approaches to the problem of triangular current 

 generation : to supply a current wave of the form of Figure 16.28a from a 

 constant-current generator, or to supply a voltage wave of the form of 



247 



