SWEEP CIRCUITS 249 



Since d, x, and D are all fixed by design in the tube and collected with e 

 into the constant K', we have 



. . . K' K 



bensitivity = -, 5 = — 



2™*V V a 



The sensitivity or ease with which the electron beam is deflected by 

 the electric field of the deflection plates, varies inversely with the axial 

 component of the kinetic energy of the electron leaving the electron gun. 

 To increase the deflection, the time of flight of the electron must be 

 increased as it passes through the field; i.e., the accelerating potential 

 V a on the electron gun must be decreased so that the axial velocity v x 

 of the electrons is small. 



The reciprocal of the sensitivity is the deflection factor; it is usually 

 expressed in d-c volts per inch. Thus as the potential of the (accelerat- 

 ing) electrode changes from 2000 to 1500 to 1000 volts, the deflection 

 sensitivity increases from 0.23 to 0.30 to 0.45 mm/d-c volt (Du Mont, 

 Type 208). 



The intensity of the fluorescent spot increases, however, with the 

 kinetic energy of the electrons. In order that a bright fluorescent spot 

 may be obtained, and a high deflection sensitivity be retained, the elec- 

 trons can be accelerated after they have passed through the deflection 

 plates. This may be accomplished by introducing an electrostatic 

 electron lens between the deflection plates and the screen (see " Electron 

 Microscope "). This post accelerating electrode is called an intensifier. 



Sweep Circuits 



In modern cathode-ray oscillographs of the self-contained commercial 

 form, a linear time-base generator has been built into the cabinet. The 

 output voltage of this generator may be connected to the x deflec- 

 tion plates. In order to obtain a linear displacement of the fluorescent 

 spot from left to right, the potential of the deflection plates must rise 

 linearly with time and then at a given value drop to zero voltage so 

 rapidly that the trace of the beam on the return movement is so faint 

 that it cannot be confused with the fluorescent spot moving forward. 



The device most commonly adopted for this purpose is the relaxation 

 oscillator. It generates distorted currents in which the variations are 

 due to the charge and discharge of a condenser through a resistor. 



A relaxation oscillator circuit in its simplest form is shown in the 

 insert of Fig. VI-22. It may be formed with a neon-gas-filled glow dis- 

 charge tube G, in conjunction with a variable high resistor R, a variable 

 condenser C, and a d-c voltage source. The discharge tube is connected 

 across the frequency-controlling condenser C. 



