Basics of Oscilloscopes | Electrical and Electronic Measurements - Electrical Engineering (EE) PDF Download

Block Diagram of CRO

Cathode Ray Oscilloscope (CRO) consists a set of blocks. Those are vertical amplifier, delay line, trigger circuit, time base generator, horizontal amplifier, Cathode Ray Tube (CRT) & power supply. The block diagram of CRO is shown in below figure.

The function of each block of CRO is mentioned below.

• Vertical Amplifier: It amplifies the input signal, which is to be displayed on the screen of CRT.
• Delay Line: It provides some amount of delay to the signal, which is obtained at the output of vertical amplifier. This delayed signal is then applied to vertical deflection plates of CRT.
• Trigger Circuit: It produces a triggering signal in order to synchronize both horizontal and vertical deflections of electron beam.
• Time base Generator: It produces a sawtooth signal, which is useful for horizontal deflection of electron beam.
• Horizontal Amplifier: It amplifies the sawtooth signal and then connects it to the horizontal deflection plates of CRT.
• Power supply: It produces both high and low voltages. The negative high voltage and positive low voltage are applied to CRT and other circuits respectively.
• Cathode Ray Tube (CRT): It is the major important block of CRO and mainly consists of four parts. Those are electron gun, vertical deflection plates, horizontal deflection plates and fluorescent screen.

The electron beam, which is produced by an electron gun gets deflected in both vertical and horizontal directions by a pair of vertical deflection plates and a pair of horizontal deflection plates respectively. Finally, the deflected beam will appear as a spot on the fluorescent screen.
In this way, CRO will display the applied input signal on the screen of CRT. So, we can analyse the signals in time domain by using CRO

Measurements by using CRO

We can do the following measurements by using CRO.

• Measurement of Amplitude
• Measurement of Time Period
• Measurement of Frequency

Now, let us discuss about these measurements one by one.

1. Measurement of Amplitude
CRO displays the voltage signal as a function of time on its screen. The amplitude of that voltage signal is constant, but we can vary the number of divisions that cover the voltage signal in vertical direction by varying volt/division knob on the CRO panel. Therefore, we will get the amplitude of the signal, which is present on the screen of CRO by using following formula.
A = j × n_v
Where,
A is the amplitude
j is the value of volt/division
n_v is the number of divisions that cover the signal in vertical direction.

2. Measurement of Time Period
CRO displays the voltage signal as a function of time on its screen. The Time period of that periodic voltage signal is constant, but we can vary the number of divisions that cover one complete cycle of voltage signal in horizontal direction by varying time/division knob on the CRO panel.
Therefore, we will get the Time period of the signal, which is present on the screen of CRO by using following formula.
T = k × n_h
Where,
T is the Time period
j is the value of time/division
n_v is the number of divisions that cover one complete cycle of the periodic signal in horizontal direction.

3. Measurement of Frequency
The frequency, f of a periodic signal is the reciprocal of time period, T. Mathematically, it can be represented as
f = 1/T
So, we can find the frequency, f of a periodic signal by following these two steps.

• Step1: Find the Time period of periodic signal
• Step2: Take reciprocal of Time period of periodic signal, which is obtained in Step1