Cathode Ray Oscilloscope MCQ Quiz - Objective Question with Answer for Cathode Ray Oscilloscope - Download Free PDF

The instrument used to generate a signal of various frequency contents is an Arbitrary waveform synthesizer.

Explanation:

• Arbitrary waveform synthesizer: This instrument can generate virtually any waveform, which inherently means it can create signals with various and precisely controlled frequency components. It can synthesize complex waveforms by combining different frequencies, amplitudes, and phases, as specified by the user.
• Spectrum Analyzer: This instrument is used to measure and display the frequency spectrum of an electrical signal, showing its component frequencies and their amplitudes. It does not generate signals.
• Logic Analyzer: This instrument is used to capture and display digital signals and their timing relationships, primarily for debugging digital circuits. It does not generate analog signals with various frequency content.
• Frequency Counter: This instrument is used to measure the frequency of a periodic electronic signal. It does not generate signals.

Concept:

When a Lissajous pattern is formed on a Cathode Ray Oscilloscope (CRO), the ratio of frequencies of the vertical and horizontal signals can be determined from the pattern.

The general formula is:

\( \frac{f_y}{f_x} = \frac{\text{Number of vertical tangencies}}{\text{Number of horizontal tangencies}} \)

Given:

Reference frequency (horizontal) \( f_x = 10~kHz \)

Lissajous pattern shows 2 vertical loops and 1 horizontal loop ⇒ \( \frac{f_y}{f_x} = \frac{2}{1} \)

Calculation:

\( f_y = 2 \cdot f_x = 2 \cdot 10~kHz = 20~kHz \)

Answer:

Option 3) 20 kHz

Explanation:

In the context of phase angle measurement using an oscilloscope, T_d represents the time difference between corresponding points of two signals. This time difference is crucial for determining the phase angle between the two signals, which is an essential parameter in analyzing the relationship between periodic waveforms.

Phase angle measurements are typically performed using an oscilloscope by displaying two signals simultaneously and analyzing their behavior. The time difference between corresponding points of the two waveforms (e.g., peaks, zero crossings, or any other identical points) is measured. This time difference is used to calculate the phase angle (ϕ) using the formula
ϕ = (Td / T) × 360°

Where:

• T_d: Time difference between corresponding points of the two signals.
• T: Time period of the waveform.
• ϕ: Phase angle in degrees.

The correct answer is 4) Time multiplexing

Solution:

• In a multiple-trace oscilloscope, the technique commonly used to display two (or more) signals on the screen simultaneously is time multiplexing.
   

• The oscilloscope rapidly switches (multiplexes) between the input signals at a high rate.

• This switching happens so fast that to the human eye, it appears as if both signals are being displayed at the same time.

Additional Information

• 1) Voltage division: Used for attenuating signal amplitude, not for displaying multiple signals.

• 2) Beam splitting: Not practical in oscilloscopes — they use a single electron beam.

• 3) Signal amplification: Used to strengthen a weak signal, not to display multiple signals.

Explanation:

Oscilloscope Horizontal Deflection Plates

Definition: An oscilloscope is a device used to observe the varying signal voltages, typically as a two-dimensional graph of one or more electrical potential differences plotted as a function of time. The horizontal deflection plates of an oscilloscope control the movement of the electron beam along the x-axis (time axis) of the screen, enabling the time-based representation of the input signal.

Correct Option Analysis:

The correct option is:

Option 1: Sawtooth waveform

The horizontal deflection plates of an oscilloscope require a sawtooth waveform voltage to achieve the uniform and continuous sweeping motion of the electron beam across the screen. This waveform is essential for the proper functioning of the oscilloscope, as it ensures that the beam moves linearly from left to right during the "sweep" period and quickly returns to the starting position during the "flyback" period.

Applications:

• The use of a sawtooth waveform in the horizontal deflection plates is fundamental for oscilloscopes to accurately depict waveforms in various applications, such as signal analysis, troubleshooting, and testing electronic circuits.
• It ensures that the time base of the oscilloscope remains consistent, which is critical for measuring parameters like frequency, period, and phase shift of the input signal.

Concept-When both pairs of the deflection plates (horizontal deflection plates and vertical deflection plates) of CRO (Cathode Ray Oscilloscope) are connected to two sinusoidal voltages, the patterns appear at CRO screen are called the Lissajous pattern. Shape of these Lissajous pattern changes with changes of phase difference between signal and ration of frequencies applied to the deflection plates (traces) of CRO.

Case – 1: When ø=0 or  ø=360

when the angle is   ø = 0 or   ø = 360, the Lissajous pattern is of the shape of straight line  passing through origin from first quadrant to third quadrant.

Case – 2: When, 0 < ø < 90 or 270 < ø <360 : –

when the angle is in the range of 0 < ø < 90 or 270 < ø < 360, the Lissajous pattern is of the shape of Ellipse having major axis passing through origin from first quadrant to third quadrant.

Case – 3: When ø=90

when the angle is   ø = 90  the Lissajous pattern is of the shape of circle.

Case – 4: When 90  < ø < 180 or 180 < ø < 270

when the angle is in the range of 0 < ø < 90 or 270 < ø < 360, the Lissajous Pattern is of the shape of Ellipse having major axis passing through origin from second quadrant to fourth quadrant.

Case – 5: When ø=180

when the angle is   ø = 180  the Lissajous pattern is of the shape of straight line  passing through origin from second  quadrant to  fourth quadrant.

Solution:-

Let at horizontal plate voltage graph is

Option-1:- It is of case (1) so at vertical plate voltage graph will be with phase difference ϕ = 0 or 360°

Option-2:- It is of case (2) so 0 < ϕ < 90° so at vertical plate voltage graph will have phase difference of 0 < ϕ < 90°

Option-3:- It is of case (3) so ϕ = 90° ↑ vertical voltage graph of have ϕ = 90° w.r.t horizontal voltage graph

Option-4:- It is of case (4) so 90° < ϕ < 180°, vertical voltage graph will have phase difference 90° < ϕ < 180° w.r.t. horizontal voltage graph

Concept:

When two sinusoidal signals of the same frequency and magnitude are applied two both pairs of deflecting plates of CRO, the Lissajous pattern changes with the change of phase difference between signals applied to the CRO

When 0 < ϕ < 90o or 270o < ϕ < 360o 

The Lissajous pattern is of the shape of an Ellipse having a major axis passing through the origin from the first quadrant to the third quadrant.

 When 90o < ø < 180o or 180o < ø < 270o

The Lissajous Pattern is of the shape of an Ellipse having a major axis passing through the origin from the second quadrant to the fourth quadrant.

Explanation:

The given inputs are: Em sin (ωt) V, Em sin (ωt + 30°) V

The phase difference between both the inputs = 30°

The Lissajous pattern on the screen of a CRO is an ellipse with a major axis in quadrant 1 and quadrant 3. 

Important Points

The correct answer is option 4): (200 Hz)

Concept:

The given time base is set at 1 ms per division.

Also, for one complete cycle, the given sinusoidal signal occupies five horizontal divisions. This is as shown

The time period of the sinusoid will be: T = 5 × 1 ms = 5 ms

With f= \(1\over T\), the frequency of the sinusoid will be:

f = \(1 \over 5\times 10^ {-3}\)

= \(1000\over 5\) Hz

f = 200 Hz

Aquadag: 

• The bombarding electrons, striking the screen, release secondary emission electrons
• These secondary electrons are collected by an aqueous solution of graphite called Aquadag which is connected to the second anode
• It is the conductive coating on the screen
• Collection of secondary electrons is necessary to keep the CRT screen in a state of electrical equilibrium

Formula:

BW × rise time = 0.35

Given that, bandwidth (B.W.) = 10 Hz

Raise time is, \({t_r} = \frac{{0.35}}{{B.W.}} = \frac{{0.35}}{{10}} = 0.035\ s=0.035\times1000m = 35\;ms\)

Aquadag: 

• The bombarding electrons, striking the screen, release secondary emission electrons
• These secondary electrons are collected by an aqueous solution of graphite called Aquadag which is connected to the second anode
• It is the conductive coating on the screen
• Collection of secondary electrons is necessary to keep the CRT screen in a state of electrical equilibrium

Concept:

CRO is a cathode-ray oscilloscope used to measure amplitude, frequencies, and phase angles of sinusoidal signals.

The display screen of the Oscilloscope is as follows:

Graticule: graticule is the grid on the display screen of an oscilloscope that comprises the horizontal and vertical axes. The graticule is used to visually measure waveform parameters.

Important points:

Focus in CRO: The electron beams entering the field at angles other than the normal to the equipotential surfaces will be deflected towards the normal and the beam is thus focused towards the center of the tube axis.

Aquadag: The aquadag coating has two functions: it maintains a uniform electric field inside the tube near the screen, so the electron beam remains collimated and is not distorted by external fields, and it collects the electrons after they have hit the screen, serving as the return path for the cathode current.

Intensity control: Intensity control is provided for adjustment of the brightness of the spot on the screen. It is accomplished by varying the voltage between the first and second anodes.

Cathode-Ray Oscilloscope (CRO):

CRO is a very fast X-Y plotter that shows the input signal versus another signal or versus time.

• Intensity control is used to adjust the brightness of the waveform. As the sweep speed is increased, there is a need to increase the intensity level.
• Focus control is used to adjust the sharpness of the waveform
• A trace control is used to rotate the trace on the CRO screen
• Synchronizing control is used to lock the display of the signal

Applications:

The CROs are used to analyze the waveforms, transient, phenomena, and other time-varying quantities from a very low-frequency range to the radio frequencies.

Time Base Generator:

• It generates an output voltage or current waveform, which varies linearly with time.
• The horizontal velocity of a time base generator must be constant.
• The displayed signal should be varied with time.
• It makes the signal to sweep the beam horizontally across the screen.
• Then the displayed signal varies linearly with time.
• Hence the voltage is called Sweep Voltage.
• The Time Base Generators are called Sweep Circuits.

The time base signal in CRO is a Sawtooth signal.

Lissajous pattern:

• The lissajous figure is the pattern that is displayed on the CRO when sinusoidal signals are applied to both horizontal & vertical deflection plates of CRO.
• The lissajous figures are used for the measurement of frequencies, and phase differences of the sinusoidal signals.
• The table of lissajous figures for different phase angles is given below:

• CRO is a very versatile instrument in the laboratory for measurement of voltage, current, frequency and phase angle of any electrical quantity.
• We cannot directly measure power by using CRO.
• The current is measured indirectly by measuring voltage drop across an impedance.
• A Lissajous figure is a pattern which is displayed on the screen when sinusoidal signals are applied to both horizontal & vertical deflection plates of CRO.
• These are used to measure the frequency of the given signals and phase difference between the signals.
• From the shape of the Lissajous pattern displayed on the CRO screen information about relative phases of signals and the frequency ratio of signals can be determined.
• It is not used for precise measurement; it depends on the type of signals.
• If one frequency is an integral multiple (harmonic) of the other, the pattern will be steady. If not, the figure will not be stationary.
   

Important Points

Common Lissajous pattern observed when two signals with different phase angle are applied to the two plates is as shown: