Cathode Ray Oscilloscope

CATHODE RAY OSCILLOSCOPE

The cathode ray oscilloscope is a device that allows the amplitude of electrical signal, whether they be voltage, current, power and etc, to be displayed primarily as a function of time. The oscilloscope depends on the movement of an electron beam, which is then made visible by allowing the beam to impinge on a phosphor surface, which produces a visible spot. If the electron beam is deflected in either of two orothogonal axes, such as the familiar X and Y axes used in conventional graph construction, the luminous spot can be used to create two‒dimensional displays.

Various blocks and Working of Oscilloscope

The major block circuit of general purpose CRO is shown in Fig. 6.23.

1. Cathode Ray Tube

A CRO consists of cathode ray tube (CRT) which is the heart of the oscilloscope, and some additional circuitry to operate the CRT. The major parts of the CRT are

(i) Electron gun assembly

(ii) Deflection plate assembly

(iii) Fluorescent screen

(iv) Glass envelope

The cathode ray tube is shown in fig. 6.24.

The electron gun assembly produces a shortly focused beam of electrons which are accelerated to high velocity. This focused beam of electrons strikes the fluorescent screen with sufficient energy to cause a luminous spot on the screen.

After leaving the electron gun, the electron beam passes through two pairs of "Electrostatic deflection plates". Voltages applied to these plates deflect the beam. Voltages applied to one pair of plates move the beam vertically up and down and the voltages applied to the other pair of plates move the beam horizontally from one side to another. Focusing anode is used to focus the beam on the screen, and accelerating anode makes the electron beam to move with high velocity.

2. Vertical Amplifier

The vertical amplifier consists of several stages, with fixed overall sensitivity or gain expressed in v/divs. This is a wide band amplifier used to amplify signals in the vertical section as shown in Fig. 6.25.

The first elements of the pre amplifier is the input stage, often consisting a FET source amplifier whose high input impedance isolates the amplifier from the attenuator. The FET input stage is followed by a BJT emitter follower to match the medium impedance of FET output with the low impedance input of the phase inverter.

The phase inverter provides two antiphase output signals which are required to operate the pushpull output amplifier. The push pull output stage delivers equal signal voltages of opposite polarity to the vertical plates of the CRT.

3. Delay line, Horizontal and Vertical deflection circuit

Delay line is used to delay the signal for some time in the vertical section. The horizontal signal is initiated or triggered, by a portion of the output signal applied to the vertical CRT plates. Signal processing in the horizontal channel consists of generating and shaping a trigger pulse that starts the sweep generator; whose output is fed to the horizontal amplifier and then to horizontal deflection plates. This whole process takes time in the order of 80 ns. To allow the operator to observe the leading edge of the signal waveform, the signal drive fore the vertical CRT plates must therefore be delayed by atleast the same amount time. It is fixed by delay line.

4. Horizontal Amplifiers

The horizontal amplifier basically serves two purposes.

(i) When the oscilloscope is used in the X‒Y mode, the signal applied to the horizontal input‒terminal will be amplified by the horizontal amplifier.

(ii) When the oscilloscope is used in the ordinary mode of operation to display a signal applied to the vertical input, the horizontal amplifier will amplify the sweep generator output.

5. Time base Generator

It is used to generate the saw tooth voltage required to deflect the beam in the horizontal section.

6. Trigger Circuit

This is used to convert the incoming signal into the trigger pulses that the input signal and the sweep frequency can be synchronized.

7. Power Supply

Power supply circuit has two power supplies, a negative high voltage (HV) supply and a positive low voltage (LV) supply. Two voltages are generated in the CRO. The positive voltage supply is from +300 to 400V. The negative high voltage supply from ‒ 100 to ‒1500 V. This voltage is passed through a bleeder resistor at a few mA.