Electrical Engineering ⇒ Topic : CRT Circuits
A free running oscilloscope is one in which the sweep circuit is not synchronized with the input signal frequency. This involves adjusting the sweep oscillator until the two are locked and the display is steady. This is an adjustment of nuisance value as it goes out of synchronization often.
To avoid this, the frequency of the input signal triggers the sweep oscillator so that the two are in unison. Synchronization means the frequency of vertical signal input is an integral multiple of the sweep frequency.
fin = nfs ............. (1)
This gives a steady display.
A delay line of 200 ns is introduced in the vertical amplifier circuit to the Y plates. As part of the input is fed to the trigger to start the sweep generator before application to the X plates and this operation takes about 80 ns, a delay line is introduced in the Y circuit. The delay line is introduced after the main vertical amplifier.
Regarding voltages, the voltage of the deflection plate is kept as low as possible. This is done by applying negative voltages to cathode, grid, and focussing anode and the accelerating anode kept at low positive voltage. As the post deflection voltage is very high, the deflection plates are virtually at ground potential. This prevents the output of the vertical amplifier from being at a high potential and simplifies the design of the circuit. Thus, it provides safety to the operating personnel.
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