Cathode Ray Oscilloscope Questions and Answers

Answer: B

The rise time of the signal is about 11.18 nsec.

When a signal passes through an amplifier, their rise times combine as the square root of the sum of squares: \(t_{obs}^{2} = t_{sig}^{2} + t_{amp}^{2}\).

\[ \begin{aligned} t_{sig} &= \sqrt{15^{2} - 10^{2}} \\ &= \sqrt{125} = 11.18\ \text{nsec} \end{aligned} \]

18.02 nsec adds the squares instead of subtracting them, 15 nsec ignores the amplifier, and 10 nsec is the amplifier's own rise time.

Answer: D

The correct matching is a-ii, b-i, c-iv, d-iii.

• An electron microscope focuses its beam with a condensing magnetic lens (ii).
• An oscilloscope (cathode ray tube) produces its beam with an electron gun (i).
• A galvanometric strip-chart recorder uses a drive motor to move the chart paper at a fixed speed (iv).
• A magnetic recorder writes and reads the tape through a recording head (iii).

Both instruments in the first two rows have an electron gun and focusing, so the pairing turns on the part each is best known for: the electron microscope's distinctive element is its magnetic lens system. The codes giving the oscilloscope a recording head or a drive motor do not fit.

Answer: D

A DSO cannot be used for signal generation.

A digital storage oscilloscope samples an input signal, digitises it with an ADC and stores the samples in memory, so it can display, monitor, store and analyse the signal, including measurements and FFT spectra.

It is a measuring instrument with inputs only; it does not generate signals. Signals are produced by function generators, signal generators or arbitrary waveform generators. Signal analysis, monitoring and digitisation are all things a DSO does.

Answer: B

A spectrum analyser is used for identifying dominant frequencies and harmonics.

A spectrum analyser displays signal amplitude against frequency, so it shows the frequency components of a signal: the fundamental, its harmonics, spurious signals and the noise floor.

Visualising the time variation of a waveform is the job of an oscilloscope. A spectrum analyser displays the noise floor but does not introduce it. Impedance is measured with an impedance analyser or LCR meter.

Answer: D

The Lissajous figure for a phase difference of 3π/4 is (D), an ellipse tilted to fall to the right.

With equal frequencies and amplitudes on both axes, the pattern is a straight line rising to the right at zero phase difference, an ellipse rising to the right between 0 and π/2, a circle at π/2, and an ellipse falling to the right between π/2 and π, closing to a falling straight line at π.

3π/4 (135°) lies between π/2 and π, so the ellipse leans the other way, as in figure (D). Figure (A), a rising line, is 0 phase difference; figure (B), a circle, is π/2; figure (C), an ellipse rising to the right, is π/4.

Answer: D

The instrument is the DSO, the digital storage oscilloscope.

A DSO samples the input with an analog-to-digital converter and stores the samples in memory. The stored waveform can then be displayed, measured, processed (zoomed, averaged, transformed) and recalled or transferred to a computer later, even for a single event that does not repeat.

An analog CRO displays the waveform only while it is present and cannot store it. A DMM measures single values such as voltage and current, not waveforms. A spectrum analyser displays the frequency content of a signal rather than storing the waveform itself.

Answer: D

The sweep generator produces a sawtooth voltage for the horizontal deflection of the electron beam.

The time base must move the spot across the screen from left to right at a constant speed, so that horizontal distance represents time, and then return it quickly to the left to start again. A sawtooth does exactly this: a linear ramp for the trace and a fast drop for the retrace. It is applied to the horizontal (X) deflection plates.

The vertical plates receive the signal under observation, from the vertical amplifier, not the sweep. A sinusoidal sweep would move the spot at a varying speed and distort the time scale, so it is used only in special cases such as Lissajous patterns.

Answer: A

The heart of a CRO is the CRT, the cathode ray tube.

In a cathode ray oscilloscope, the CRT produces the display: an electron gun generates and focuses a beam, deflection plates move it horizontally and vertically according to the time base and the input signal, and the beam strikes a phosphor screen to draw the waveform. All other sections exist to drive the CRT.

The sweep (time base) generator provides the horizontal deflection voltage, the trigger circuit synchronises the sweep with the signal, and the vertical amplifier scales the input signal for the deflection plates. Each is essential, but they serve the CRT.