CATHODE RAY TUBE 



50'^, the sweep will undergo severe brightness modulation at 50'-^ unless 

 the supply is exceptionally well smoothed. 



A better method of achieving trace-brightening is to employ an L-C 

 oscillator, which may conveniently operate at radio-frequency, which is 

 switched on by the square wave from the time-base generator. The oscilla- 

 tions are picked off inductively (Figure 32.41) or capacitively {Figure 32.42) 

 and rectified to produce a positive-going square wave at the grid of the 

 cathode ray tube. The oscillator may conveniently be switched by returning 



HT + 



6 in of twisted 

 flex used as 

 small high- 

 voltage caoacitor 



from 



T.B. 



generator 



EHT- 



Figure 32.42 



the grid leak to a point which is earthy during the forward strokes of the 

 time base, but negative to earth (so that the valve is cut off) at other times. 



Hybrid cathode ray tubes 



These are a recent development in which the deflection is electromagnetic 

 and the focusing is electrostatic. This retains the advantage of magnetic 

 tubes, in that they are short in comparison with their screen size, and that 

 good focus and brightness are possible ; the design is such that the requisite 

 focusing anode potential with respect to the cathode is quite low, about 

 300 V, enabling both the focus and brightness potentials to be derived from 

 potentiometers across the HT supply, and a negative-earthed EHT supply is 

 used across which no resistor chain is necessary. Hence the drain on the 

 EHT generator is low. 



Electrostatic focusing has an advantage over permanent magnet in that 

 remote adjustment does not require complicated mechanical linkages, and 

 over electromagnetic in that almost no power is consumed. It is certainly 

 lighter and it is claimed that it is cheaper. 



Multi-channel cathode ray tube displays 



It is often valuable to be able to display two varying quantities simul- 

 taneously, and we have discussed one method of doing this — the Cossor 

 double-beam tube. Another is to have a number of electrode systems in one 

 glass envelope, all projecting on to one screen; this scheme is feasible with 

 electrostatic tubes but not with magnetic. Two-channel and four-channel 

 tubes may be bought from stock, and eight-channel models to special order 



469 



