Role of Time Base Generator:

• Application of CRO involves measurement of a waveform that varies with time. Therefore, the electron beam spot should move across the screen with constant velocity.
• The deflection of the electron spot should increase linearly up to the other end of the screen and must retrace back to the starting point after reaching the edge of the screen or after the completion of one cycle.
• Since the deflection is proportional to the deflecting voltage, the voltage applied to theX-plates of the CRT should increase linearly and suddenly drop to zero.
• Hence, the time base waveform being given to theX-plates must be a saw-tooth waveform.
• The horizontal amplifier accepts either a run-up or run-down ramp voltage and supplies both waveforms to its push pull/output circuit which in turn gives it to the horizontal deflection plates.

• The circuit that produces the saw-tooth waveform is called atime base generator or asweep generator. The output of a sweep generator is called thesweep voltage.

   

Basic operation:

The basic sweep generator uses the charging characteristics of a capacitor to generate linear Rise time voltages. Linearly increasing ramp which becomes zero with very short duration of time ensures that the spot is visible from left to right and invisible from right to left.

Figure shows a capacitor charged from a constant current source.

• When switch S1 is closed, S2 is open and capacitor charges to produce linear ramp at the output. The sweep rate can be controlled by changing the value of capacitor or charging current.
• On reaching the maximum value of ramp voltage, the switch S2 is closed and S1 is open. Thus capacitor gets discharged through the resistance R. This is called fly back or retrace.
• The time t1 is called sweep time.
• The circuit is a sort of relaxation oscillator which generates saw tooth waveform.
• But this circuit has less accuracy. The bootstrap techniques allow much greater linearity but the techniques are much more costly.

• During the sweep time, the spot moves from left to right. During retrace, the screen is blank and spot comes back to its starting level but its movement from right to left is invisible.

   

Requirements of time base:

The time base requirements are:

1. Sweep time variations from 10nsec to 5 sec per division. (is it sec or nsec?)
2. Time accuracy is better than 3%. (better than what?)
3. Linearity better is better than 1% across the cathode ray tube. (better than what?)
4. Ten times expansion in the horizontal amplifier which allows 1 ns/division displays for very high speed transients.
5. The speed of the spot should be constant across the entire screen.
6. The spot should be invisible while tracing from right to left and should be visible only from left to right