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  Horizontal Deflection Oscillator

Introduction to Horizontal Deflection Oscillator:

A multivibrator is other type of relaxation oscillator that uses two amplifier stages, in which the output o one is coupled to the input of another. As a result in overall positive feedback and the circuit operates like that while one stage conducts, it forces another to cut-off. As Soon the stage which cuts off returns to conduction to force the first stage to cut-off. This sequence repeats to produce rectangular or square output with a frequency which is controlled through the coupling networks among the two amplifier stages. Like in the case of a blocking oscillator the multivibrator is employed like a controlled switch to charge a capacitor by a resistance to produce the needed saw tooth wave output. The amplifier might use tubes or transistors as active devices.

Multivibrators might be categorized as constable, bitable, and actable. To complete one cycle of oscillation a bitable multivibrator contain two stable stages and requires two external trigger signals. The constable type contains one stable stage and completes one cycle of output with just one external pulse. Though, as table multivibrator is a free running type and does not require any external trigger pulse for normal operation of it. This kind of multivibrator which is used like a deflection oscillator and its frequency is synchronized with the horizontal AFC Voltage or vertical synchronize pulses. Multivibrators can also be categorized on the basis of coupling among the stages. The two kinds which are employed in TV receivers are plate (or Collector) coupled and cathode (or emitter) coupled.

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The circuit configuration displayed in diagram is of a free running collector coupled multivibrator in which two common emitter amplifiers are cross coupled to give positive feedback. Note: the base resistance the circuit operation can be simply described if the sequence of operations is followed from the instant while one transistor now conducts and another goes to cut-off.

Diagram demonstrates the collector and base voltage waveforms for one whole cycle. At the instant marked t0 transistor Q2 that is just conducts to saturation and transistor Q' returns to cut-off. As this occurs, the increasing voltage at the collector of Ql charges the capacitor C2 to Vcc. Because the, VB charging current of C2 flows by the base of Q2 to complete its circuit. RB2 is elected to give enough current from Vcc to the base of Q2 to remain it in saturation, even while the charging current of C2 becomes zero and C2 charges to Vcc.

In an identical way C1 would have got charged to Vcc in the preceding cycle when Q was in saturation. In fact at t = t0 the capacitor C1 that was charged previously to Vcc gets  earthed with positively charged plate of it in the direction of ground, the moment, Q2 goes into full conduction. As displayed in diagram, is then in parallel along with emitter-base junction of the 'off' transistor Qr This places a reverse bias on Ql equal to -Vcc at t = t that is well across cut-off bias of the transistor.

The capacitor CI now begins .charging from -Vcc towards +Vcc as displayed in diagram. At t -t, the negative voltage across C, decreases to zero and allows base current flow in the transistor Qr This action is regenerative and Q, instantaneously goes to saturation that in turn cut-off Qv The time constant RB1 controls the off period (t, - tQ) of Q, and can be set equivalent to the retrace period of the needed saw tooth wave.

The moment Q | goes into saturation state the positively charged plate of C2 is efficiently grounded by Q,. It then starts to charge in the direction of +Vcc at a rate determined through the time constant RB, C. Again while VC7 = 0, the second transition occurs to complete one cycle of operation. This cycle then repeats and allows the circuit to function like a free running multivibrator.

 The time constant RB2C2 can be completed equal to the trace period. As displayed in the circuit, RB contains Rl 4 and R in which R2 is a potentiometer to adjust the retrace period and thus controls the frequency of the multivibrator.

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