Cathode Ray Oscilloscope Questions and Answers

Answer: A

Changing the sweep speed changes the sample rate, and since the alias frequency is the difference between the signal and a multiple of the sample rate, the displayed frequency shifts. A genuine signal would keep its frequency and merely occupy a different number of divisions.

Answer: B

Chop is preferred at slow speeds because alternate mode flickers there, not because the switch is slow. The switch runs at hundreds of kilohertz; at fast sweeps the trouble is that its rate becomes comparable with the signal and the trace breaks into dashes.

Answer: C

Correct answer: 1-Q, 2-P, 3-R

Three controls acting at three different points. Intensity sets the control grid potential and so the beam current and brightness. The trigger level sets the signal voltage at which each sweep is started. And time per division sets the sawtooth slope, and hence the horizontal sweep speed.

Answer: D

The line, the ellipse and the circle are the three cases of one family: zero phase collapses the ellipse into a line, quadrature with equal amplitudes opens it into a circle, and anything between gives a tilted ellipse.

Answer: A

Intensity first, because nothing else can be seen without it; then automatic triggering to force a sweep whether or not the signal triggers; then centre what appears; and only with a visible trace is there anything to set the trigger level against.

Answer: B

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: the grid controls brightness, and a general purpose instrument deflects electrostatically - magnetic deflection belongs to television tubes, where large angles are needed. Statement 3 misattributes the aquadag: the conductive graphite coating collects the secondary electrons knocked off the screen and provides the final anode, while focusing is done by the electrostatic lens in the gun.

Answer: C

Single-shot capture and converter-limited resolution are both correct. The third is false: aliasing depends on the sampling rate and not on the analogue bandwidth, and a wide bandwidth with a slow sample rate is exactly the condition in which aliasing occurs.

Answer: D

Correct answer: setting the brightness of the trace

Brightness is set by the intensity control acting on the grid, not by the time base. The time base generates the sawtooth for the horizontal plates, starts a sweep when triggered, and supplies the blanking pulse for the retrace - the last of these being the one that is easily forgotten, since the blanking is applied to the grid but originates in the time base.

Answer: A

A Lissajous figure discards time altogether, plotting one signal against the other, so nothing about the shape of an edge survives in it. Rise time is measured against a calibrated time base, which is precisely what the figure dispenses with.

Answer: B

A thirty per cent loss is precisely the three decibel point, so the signal frequency has reached the bandwidth of the instrument. Amplitude measurements should be made well below the bandwidth, typically at a fifth of it or less.