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Horizontal Deflecting System

Horizontal Deflecting System:

The Horizontal Deflecting System consist of a Time Base Generator and an output amplifier.

Sweep or Time Base Generator

Horizontal Deflecting System

A continuous sweep CRO using a UJT as a time base generator is shown in Fig. 7.8. The UJT is used to produce the sweep. When the power is first applied, the UJT is off and the CT changes exponentially through RT The UJT emitter voltage VE rises tow­ards VBB and when VE reaches the peak voltage VP, as shown in Fig. 7.9, the emitter to base ‘1’ (B1) diode becomes forward biased and the UJT triggers ON. This provides a low resistance discharge path and the capacitor discharges rapidly. The emitter voltage VE reaches the minimum value rapidly and the UJT goes OFF. The capacitor recharges and the cycle repeats.

Horizontal Deflecting System

To improve sweep linearity, two separate voltage supplies are used, a low voltage supply for UJT and a high voltage supply for the RTCT circuit.

RT is used for continuous control of frequency within a range and CT is var­ied or changed in steps for range changing. They are sometimes called as timing resistor and timing capacitor respectively.

The sync pulse enables the sweep frequency to be exactly equal to the input signal frequency, so that the signal is locked on the screen and does not drift.