Time Base Generator – Working Principle and Circuit Diagram

For the testing or repairing of analog circuits, function generators, the electronic test equipment, are used as a signal source. For digital circuits, pulse generators are used. Function generators can produce both single-shot waveforms or repetitive waveforms over a wide range of frequencies. For the generation of different types of electronic waveforms, function generators require an internal or externals source. For producing high-frequency sawtooth waveforms, a special type of function generator called Time-Base Generator is used. This also produces a linear time-varying voltage or current signals. Apart from time base signals, this generator also produces different types of waveforms.

What is a Time Base Generator?

Time Base Generator is a specially designed electronic function generator to produce various time base signals by generating varying voltage or current. This generated linear time-varying voltages are used in a Cathode-ray tube to deflect the electron beam in a horizontal direction. This is highly used in Oscilloscopes for generating sawtooth waveforms.

For producing a sawtooth waveform the voltage should have a constant horizontal velocity. Such a voltage is called Ramp voltage. When this voltage rapidly tends to zero, the Sawtooth waveform is formed. In the sawtooth waveform, the sweep time Ts is the time duration of the linear rise of the voltage and the retrace time Tr is the time used by the wave to return to its initial state. Across the CRT screen, the waveform moves from left to right. For such a horizontal deflection Time-Base Generator is attached to the deflection plates.

Circuit Diagram

The circuit of Time-Base Generator consists of a variable resistor -R, which charges the capacitor-C and discharges periodically through a transistor Q1. For producing sawtooth waveforms the sweep rate should be greater than it’s retrace time. The sweep time of the waveform is controlled by the resistor present in the circuit.

Working Principle of Time-Base Generator

The voltage Vcc is applied across resistor R. The capacitor C starts charging. An input signal Vi turns on the transistor Q1. This transistor provides low resistance through which the capacitor gets discharged. If the transistor is not turned on the capacitor will charge exponentially to the supply voltage Vcc. This controlled charging and discharging of the capacitor generates the sawtooth waveform.

Time-Base Generator is used in CRO to produce the sawtooth waveforms. These are also known as Sweep Generators. A CRT consists of an electron gun to produce an electron beam, a phosphorous covered screen for displaying and deflection plates to deflect the beam. In radar systems, a Time-base generator is used with CRT to sweep the target location across the display and to know the target location. The early warning radar system uses Time-Base Generator. Here, the sweep is initiated when the broadcasting signal ends. The beam gets deflected whenever an echo is detected.

Analog television sets have two Time-base generators. One for deflecting the beam horizontally and another to pull that beam down the screen. The oscilloscope uses several time-base generators to display various Time-base signals. What are the types of Time-base generators?