Horizontal Deflection System in CRO

The horizontal deflecting system of the cathode ray oscilloscope (CRO) consists of the following stages.

1. Sweep generator or time base generator.
2. The horizontal amplifier,
3. The trigger circuit.

For an oscilloscope to display the waveform under study a voltage which is linearly increasing with time is required. Also the voltage has to fall in its amplitudes at time periods that are equal or multiples of the input signal time periods. This voltage which is of the saw tooth waveform is applied to the horizontal deflecting plates to sweep the beam horizontally. Hence the name sweep voltage or time base voltage.

A sweep generator is used to produce this sweep voltage. The sweep voltage will be employing an R.C. charging circuit. The capacitor after acquiring a predetermined value of charge will be discharged using a voltage operated switch. Charging of the capacitor is done through a constant current generator. The sweep voltage waveform is shown below Figure. 
The linearly increasing portion of the voltage is called 'ramp' voltage, The beam is swept from left of the screen to the right during the rise period of the sweep voltage Ts. The time taken by the voltage to fall to the initial value from the maximum, Tr is called the retrace period of fly back period. During this retrace period the electron beam is cutoff, in order to blank the display of the retrace on screen.

The sweep generators may employ vacuum tube circuits, transistors or ICs. Thyratrons, gas diodes were used as the voltage operated switches in vacuum tube type circuits. Transistor circuits use UJT for this purpose. IC packages are available that generate the ramp voltage. 555 IC can be used for generating the sweep voltage.

(a) Sweep Generator Using UJT :

The circuit of a sweep generator using a UJT is shown below in Figure.

The RC circuit consists of the resistor RT and the capacitor CT. The UJT is connected as a voltage operated switch across the capacitor. When supply is switched on the capacitor gets charged through the resistance RT. As the emitter and base one (1) of the UJT are across the capacitor CT the emitter voltage goes on increasing with the charge of the capacitor. When the voltage across the capacitor that is the emitter voltage on the UJT reaches a value slightly greater than the peak voltage of UJT, the emitter base one (1) diode conducts heavily. This discharges the capacitor. As the voltage at the emitter is lost the UJT ceases to conduct. Again the capacitor gets charged.
This process of charging and discharging repeats itself as long as the supply is present. The voltage across the capacitor gives the required sweep voltage. These circuits that generate non-sinusoidal waveforms are called the relaxation oscillators. The frequency of this sweep generator can be varied in steps by using number of capacitors (CT) connected as required using a switch. This gives coarse frequency variation and provides required number of ranges marked as TIME/DIV on the selector switch. Varying the value of the resistor also varies the frequency. This makes the fine frequency control of the sweep generator. By changing the value of the capacitor and the resistor we are changing only the time constant of the RTCT the timing resistor and timing capacitor of the circuit. Two supplies can be used for the above circuit to improve the linearity of the sweep circuit. One a high voltage supply for the timing resistor, capacitor and the other for the UJT.

The other stages of Horizontal Deflection System in CRO are as follows.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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