CATHODE RAY TUBE 



Considering that the time-base output is also asymmetric, these tubes repre- 

 sent, in their freedom from distortion, a remarkable achievement. 



Tube supply networks — A typical ancillary circuit for feeding a conven- 

 tional electrostatic cathode ray tube is shown in Figure 32.17. The deflection 

 voltages are capacitor-coupled to the plates, so the time constant of the 

 signal path circuit, CR, must be adequate to transmit the lowest waveform 

 frequency it is required to examine. Since the X plates are also capacitor 

 coupled to the time-base generator, it is unlikely that sweep times longer 

 than about I sec would be feasible, since an extremely large value of coupling 

 time constant C R' would be necessary to transmit the triangular waveform 



Y shift 



X shift 



R'^ ^R' 



—r- — EHTf 

 Astigmatism 



Figure 32.17 



without serious distortion. Such a circuit is not therefore of much use in 

 electrophysiological work, but it illustrates the kind of network to be found 

 in certain commercial oscilloscopes. Notice how the shift potentials are 

 applied in push-pull; the sliders move up or down each ganged potentio- 

 meter together. Notice also the astigmatism potentiometer for finding the 

 optimum focusing conditions. The cathode ray tube heater is fed from a 

 special winding on the mains transformer, insulated to withstand the EHT 

 voltage. The capacitor across the 'brightness' variable resistor is important; 

 the degree of smoothing provided on most EHT power supplies is sufficient 

 to steady the inter-anode potentials, but the spot brightness is extremely 

 sensitive to variations in grid-cathode potential, and additional smoothing 

 is invariably required here. About 0-5 ^F is generally sufficient. 

 The calculation of the values required in the resistor chain is quite easy. 



457 



