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

The correct answer is: 1) The propagation speed of the signal in the line

Explanation:

In an electrical delay line used in oscilloscopes, the delay time is determined by:

• The physical length of the delay line

• The propagation speed of the signal through the line (which depends on the medium's properties, such as its dielectric constant and inductance/capacitance per unit length).

Option Analysis

2. The resistance of the delay line → Affects signal attenuation, not the delay time.

3. The frequency of the input signal → The delay line is typically designed to be frequency-independent for a wide range of signals.

4. The speed of the electron beam → This relates to the CRT display, not the delay line.

Thus, the correct choice is 1) The propagation speed of the signal in the line.

Explanation:

Measuring Time Delay in an Oscilloscope

Definition: An oscilloscope is an electronic instrument used to measure and visualize varying electrical signals. It displays the waveform of the signal, allowing for analysis of its properties such as amplitude, frequency, and time delay. The time delay in an oscilloscope refers to the interval between specific points in the waveform, such as between peaks or zero crossings.

Working Principle: Oscilloscopes work by sampling the input signal and plotting it on a screen as a function of time. The horizontal axis represents time, while the vertical axis represents the signal’s amplitude. By analyzing the waveform, one can measure various parameters, including the time delay.

Correct Option Analysis:

The correct option is:

Option 1: Measuring the time between two successive peaks of the signal.

To measure the time delay in an oscilloscope, the most straightforward and accurate method is to observe the time interval between two successive peaks of the signal. This approach utilizes the waveform's visual representation to determine the period or frequency of the signal. By identifying the time difference between consecutive peaks, one can accurately measure the time delay.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Measuring the amplitude of the signal.

This option is incorrect for measuring time delay. The amplitude of the signal refers to its strength or magnitude, not the time interval between points in the waveform. While amplitude is an important parameter, it does not provide information about the time delay.

Option 3: Counting the number of weak signals.

This option is also incorrect. Counting the number of weak signals does not relate to measuring time delay. Weak signals might refer to low amplitude portions of the waveform, but this does not help in determining the time interval between specific points in the signal.

Option 4: Measuring the decibel of the signal.

This option is incorrect as well. Decibels are a unit of measurement for the intensity of the signal, typically used in audio and sound engineering. Measuring the decibel level of the signal does not provide information about the time delay.

Conclusion:

Understanding the correct method to measure time delay in an oscilloscope is crucial for accurate signal analysis. By measuring the time between two successive peaks of the signal, one can effectively determine the time delay. Other options such as measuring amplitude, counting weak signals, or measuring decibels do not provide the necessary information for time delay measurement. Proper analysis and understanding of the waveform are essential for utilizing an oscilloscope effectively in various applications.

Explanation:

Why is shielding used in oscilloscope probes?

Definition: Shielding in oscilloscope probes refers to the use of conductive materials to encase the probe and its components to protect against external electromagnetic interference. This interference, often coming from various electronic devices and environmental factors, can adversely affect the accuracy and reliability of measurements taken by the oscilloscope.

Working Principle: Shielding works on the principle of electromagnetic compatibility (EMC). It ensures that the probe is less susceptible to external electromagnetic fields, thereby maintaining the integrity of the signal being measured. The shielding typically involves a conductive material, such as metal, which encloses the probe and acts as a barrier to electromagnetic interference.

Correct Option Analysis:

The correct option is:

Option 2: To reduce external electromagnetic interference.

This option correctly describes the primary purpose of shielding in oscilloscope probes. The shielding effectively minimizes the impact of external electromagnetic interference, ensuring that the oscilloscope provides accurate and reliable measurements.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: To control the electron beam in the oscilloscope.

This option is incorrect because shielding is not used to control the electron beam in the oscilloscope. The electron beam is controlled by the internal components of the oscilloscope, such as the electron gun and deflection plates, which direct the beam to create the visual representation of the signal on the screen.

Option 3: To increase the probe’s sensitivity.

This option is also incorrect. While shielding can help improve the accuracy of measurements by reducing interference, it does not directly increase the sensitivity of the probe. The sensitivity of an oscilloscope probe is determined by its design and the quality of its components.

Option 4: To amplify weak signals.

This option is incorrect as well. Shielding does not amplify signals. Signal amplification is typically achieved through the use of amplifiers within the oscilloscope or external amplifier circuits. Shielding is solely for the purpose of reducing interference.

The correct answer is: 2) Create a sweeping movement of the beam from left to right

Explanation:

In a CRT (Cathode Ray Tube) oscilloscope:

• Horizontal deflection plates are responsible for moving the electron beam left to right (and back) in a controlled sweep. This generates the timebase (X-axis) of the displayed waveform.

• The sweep can be triggered (for stable displays) or free-running (for repetitive signals).

Option Analysis

1. Focus the beam → This is done by the focusing anode, not the deflection plates.

2. Control the electron gun → The electron gun emits the beam, but deflection plates only steer it.

3. Display the vertical signal → The vertical deflection plates (Y-axis) handle this, not the horizontal plates.

Thus, the correct choice is 2)

The correct answer is: 4) Using a single electron gun with alternating sweeps

Explanation:

In a multiple-trace oscilloscope, two or more signals are displayed simultaneously using:

• A single electron gun that rapidly switches (alternates) between input channels.

• Time-division multiplexing: The beam alternates between traces so quickly that the human eye perceives them as continuous.

• Chopping/Alternate Mode:

  • Chopping: Fast switching (at ~100 kHz) draws small segments of each waveform alternately.

  • Alternate: Completes one full sweep of the first signal, then the next (used for slower signals).

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:

Lissajous Figure:

• When two identical sinusoidal voltages with 0° or 360° phase differences are applied on X and Y plates of a CRO, the Lissajous figure obtained will be a straight line with a positive slope.
• When two identical sinusoidal voltages with 30° and 330° phase differences are applied on X and Y plates of a CRO, the Lissajous figure obtained will be an ellipse with the rotation in the clockwise direction. 
• When two identical sinusoidal voltages with 90° or 270° phase differences are applied on X and Y plates of a CRO, the Lissajous figure obtained will be a circle.
• When two identical sinusoidal voltages with 120° or 240° phase differences are applied on X and Y plates of a CRO, the Lissajous figure obtained will be an ellipse with the rotation in the anti-clockwise direction.