Question

Justify the need of current time base generator to obtain the specified sawtooth waveform with one example.

Answer 1

Justification:- * Current Time base generator is a circuit where the output current is a linear function of time over a specified time interval. * Time base circuits are used by radar systems to determine range to a target, by comparing the current location along the time base to the time of arrival of radio echoes. * Current Time base generators produce very high frequency sawtooth waves specifically designed to deflect the beam in cathode ray tube (CRT) smoothly across the face of the tube and then return it to its starting position. * To display the variations of a signal with respect to time on an oscilloscope, a voltage/current that varies linearly with time, has to be applied to the deflection plates. This makes the signal to sweep the beam horizontally.

Waveform: 

Example: * A cathode ray tube (CRT) consists of three primary parts, the electron gun that provides a stream of accelerated electrons, the phosphor-covered screen that lights up when the electrons hit it, and the deflection plates that use magnetic or electric fields to deflect the electrons in-flight and allows them to be directed around the screen. * It is the ability for the electron stream to be rapidly moved using the deflection plates that allow the CRT to be used to display very rapid signals. * To display such a signal on an oscilloscope for examination, it is desirable to have the electron beam sweep across the screen so that the electron beam cycles at the same frequency as the carrier, or some multiple of that base frequency. * This is the purpose of the current time base generator, which is attached to one of the set of deflection plates, normally the X axis, while the amplified output of the radio signal is sent to the other axis, normally Y. The result is a visual re-creation of the original waveform. 

OR

* Above Fig. shows a simple circuit of a current time base generator. * Here an inductor (L) in series with a transistor is connected across the VCC supply. * The transistor operates as a switch in the circuit. * The gating waveform at the base operates between two levels. V1and V2 as shown. * The lower level (V1) keeps the transistor in cut-off, while the upper level drives the transistor into saturation. * When the transistor switch is turned ON, then neglecting the effect of small saturation resistance (Rcs), the current through and inductor (iL) increases linearly with the time. * The diode D does not conduct during the sweep, because it is reverse biased.