VISUAL INDICATORS 



used for time-base waveform production, generate also a positive-going 

 square wave at som.e point in the circuit. If this wave can be coupled to the 

 grid of the cathode ray tube the required beam suppression is achieved ; the 

 brightness control is set so that only during the forward stroke of the time 

 base is the grid bias of the cathode ray tube sufficiently reduced for beam 

 current to flow. 



The coupling between the source of the square wave and the C.R.T. grid 

 requires comment. If a magnetic tube is used, the — ve side of the EHT 

 supply is usually earthed and it is not too difficult to devise a direct coupling 



From time base 

 generator 



Brightness 

 control 



EHT- 



Figure 32.40 



scheme employing a neon lamp or potential divider. With electrostatic 

 tubes, the +ve side of the EHT supply is at or near earth potential, and the 

 C.R.T. grid is about 2 kV below earth. 



A possible method is to employ resistance-capacitance coupling, remember- 

 ing that the capacitor must be rated to withstand the EHT voltage {Figure 

 32.40). The technique is open to two serious objections: 



(1) The leakage resistance of C must be much greater than the resistance 

 R, otherwise it will not be possible to suppress the beam. This may restrict 



HT+ 



From 

 T.B. 

 generator 



HT- 



EHT- 



Figure 32.41 



the upper possible CR product to a time constant insufficient to pass the 

 brightening-up waves accompanying slow sweeps. It is difficult to brighten 

 up a trace of duration greater than about 100 msec by this method. 



(2) The ripple on the EHT supply is now applied to the cathode of the 

 C.R.T., but not to the grid. If the EHT is generated by rectification from 



468 



