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# Deflection Oscillators

Under consideration represents a deflection coil, a trapezoidal voltage must be applied across it. Note that for a negative going sawtooth current, the resulting trapezoid will naturally have an inverted polarity as illustrated in Fig.

DEFLECTION OSCILLATORS

Under consideration represents a deflection coil, a trapezoidal voltage must be applied across it. Note that for a negative going sawtooth current, the resulting trapezoid will naturally have an inverted polarity as illustrated in Fig.

As explained above, for linear deflection, a trapezoidal voltage wave is necessary across the vertical deflecting coils. However, the resulting voltage waveform for the horizontal yoke will look closer to a rectangular wave shape, because voltage across the inductor overrides significantly the voltage across the resistance on account of higher rate of rise and fall of coil current. Effect of Driving Source Impedance on Wave shapes.

In deflection circuits employing vacuum tubes, the magnitude of R is quite large because of high plate resistance of the tube. Therefore, voltage wave shape across the vertical deflection coils and that needed to drive the vertical output stage is essentially trapezoidal. However, in a horizontal output circuit employing a tube, the wave shape will be close to rectangular because of very high scanning frequency.

When transistors are employed in vertical and horizontal deflection circuits, the driving impedance is very low and equivalent yoke circuits appear to be mainly inductive. This needs an almost rectangular voltage wave shape across the yoke.

To produce such a voltage wave shape, the driving voltage necessary for horizontal and vertical scanning circuits would then be nearly rectangular. Thus the driving voltage waveforms to be generated by the deflection oscillator circuits would vary depending on deflection frequency, device employed and deflection coil impedance.

## Deflection Oscillators

In order to produce a picture on the screen of a TV receiver that is in synchronism with the one scanned at the transmitting end, it is necessary to first produce a synchronized raster. The video signal that is fed to the picture tube then automatically generates a copy of the transmitted picture on the raster.

While actual movement of the electron beam in a picture tube is controlled by magnetic fields produced by the vertical and horizontal deflection coils, proper vertical and horizontal driving voltages must first be produced by synchronized oscillators and associated wave shaping circuits. As illustrated in Fig.

For vertical deflection the frequency is 50 Hz, while for horizontal deflection it is 15625 Hz. The driving waveforms thus generated are applied to power amplifiers which provide sufficient current to the deflecting coils to produce a full raster on the screen of picture tube.

Free running relaxation type of oscillators are preferred as deflection voltage sources because these are most suited for generating the desired output waveform and can be easily locked into synchronism with the incoming sync pulses.

The oscillators commonly used in both vertical and horizontal deflection sections of the receiver are:

Blocking oscillator,

Multivibrator,

Complementary pair relaxation oscillator,

Overdriven sine-wave oscillator.

·         It may be noted that complementary pair circuits are possible only with transistors while all other types may employ tubes or transistors.

As explained earlier, both vertical and horizontal deflection oscillators must lock with corresponding incoming sync pulses directly or indirectly to produce a stable television picture.

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Television and Video Engineering : Monochrome Television Transmitter and Receiver : Deflection Oscillators |

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