TRIANGULAR WAVE GENERATORS 



reduced at the flyback this field collapses., causing the back e.m.f. to reverse 

 and tend to maintain the anode current by driving the pentode anode very 

 positive. In view of the high rate of change of anode current this e.m.f. is 

 very large and may damage the valve or the coil insulation unless limited. 



The procedure is to use a diode as a switch to connect a resistance auto- 

 matically across the coil during the flyback, so that the magnetic energy is 

 dissipated as quickly as possible in the resistance as heat. If R is neglected, 

 the coil and its stray capacitance form a simple parallel-tuned circuit, and the 

 resistance wiU introduce shunt damping. Reference to Chapter 5 shows us 

 that the energy will be removed quickest when the circuit is critically damped, 

 i.e. when R' = ^{L/OK The existence of R means that some series damping 

 is already present, and the critical value of R' will be somewhat higher. The 

 scheme is shown in Figure 16.32. 



Source of triangular wave — Any of the triangular voltage wave generators 

 may be used to feed the pentode, but as their outputs are mostly negative- 

 going, a buffer amplifying stage to provide phase-inversion is generally 

 required. 



Constant-voltage approach 



In this case the deflector coil may be fed from a cathode follower as in 

 Figure 16.33, the appropriate penthouse wave being applied to the grid. It 

 is in generating this appropriate wave that a complication with this method 

 arises, since a different amplitude of square wave component is needed for 



HT + 



Figure 16.33 Figure 16.34 



each sweep speed. Mixing resistors may be used to combine the outputs of a 

 flip-flop, and a Blumlein integrator controlled by it, in correct proportions, 

 and it is wise not to aim at continuously variable speed control but to be 

 content with switched speeds in discrete steps. The same control can then 

 switch in different mixing resistors as required. Alternatively, approximate 

 penthouse waveforms can be generated from a square wave by a circuit of 

 the type of Figure 16.34. If the time constant C{R^ + R^ is much longer than 

 /, the penthouse rise is nearly linear, and then 



square wave component = Fin did 



triangular wave slope = „ . _ ^ d \ 



C(i<i + K2) 



251 



