Time Base Generator

Definition: The time base generator is the special type of an electronic generator which is used to generate the high-frequency sawtooth waves. These waves are used in cathode ray tube for deflecting the beam in a horizontal direction. The time base generator is also used in a radar system for determining the range of the target.

The oscilloscope has generally used a waveform that varies with the time. For accurately reproducing the waveform it must have a constant horizontal velocity. The beam velocity operates with the deflecting voltage which is increased linearly with the time. The voltage having such characteristic is called ramp voltage. If the voltage decreases rapidly to zero with the waveform repeatedly reproduced as shown in the figure the pattern is generally called a sawtooth waveform.

saw-tooth-wave-2During the sweep time, Ts the beam moves from left to right across the CRT screen. The beam is deflected to the right by the increasing amplitude of the ramp voltage and the fact that the positive voltage attracts the negative electrons.

During the retrace time or fly back time Tr the beam returns quickly to the left side of the screen. The control grid is generally gated off, which blanks out the beam during the retrace time and prevents an undesirable retrace pattern from appearing on the screen.

The sweep rate of the wave can be adjusted, and it can be changed by switching a different capacitor in the circuit. The front panel control for this adjustment is marking time/div or Sec/div. The sweep rate can be adjusted in minor ways by making the resistor R a variable resistor.Consider a simple sweep circuit in which the capacitor C charges through resistor R. The capacitor discharges periodically through the transistor Q

Time Base Generator Circuit

Consider a simple sweep circuit in which the capacitor C charges through resistor R. The capacitor discharges periodically through the transistor Q1, which causes the waveform shown in the figure below to appear across the capacitor.

saw-tooth-generatorThe signal Vi, which must be applied to the base of the transistor to turn it on for a short time interval is also shown in the figure. When the transistor is turned completely on it presents a low resistance discharge path through which the capacitor will discharge quickly.

sawtooth-waveformsIf the transistor is not turned on the capacitor will charge exponentially to the supply voltage Vcc according to the equation.

time-0base-generator-equation-1where, Vo = instantaneous voltage across the capacitor at time t
Vcc = supply voltage
T= time of interest
R = value of series resistor
C = Value of capacitorThe instantaneous voltage  and

The instantaneous voltage and the supply voltage is measured in volts, the time is measured in second, the resistance is measured in ohm, and the capacitor is measured in Farad.

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