Cathode Ray Oscilloscope Questions and Answers

Answer: C

Since samples are continuously written into a circular memory, the trigger merely marks a point in the record and the samples already stored can be displayed. Pre-trigger view is impossible on an analogue instrument, where nothing exists until the sweep starts.

Answer: D

Twice the highest frequency is the theoretical minimum for the waveform to be recoverable. In practice a digital instrument samples at five or ten times the bandwidth so that the reconstructed display looks like the waveform rather than merely determining it.

Answer: A

Sampling too slowly makes the samples fall on successive cycles at slowly changing phase, and the reconstructed waveform is a spurious slow beat. It is dangerous because it looks entirely plausible; changing the sweep speed and watching whether the frequency changes reveals it.

Answer: B

Record length is memory depth divided by sample rate, so a deep memory allows a long window without dropping the sample rate. It is the specification that decides whether a slow event can be captured with fine detail still resolved within it.

Answer: C

Eight bits give \( 2^8 = 256 \) levels across the full screen height, so about 0.4 per cent of full scale, or a fortieth of a division. This is why a digital instrument is no better than an analogue one for seeing small detail on a large signal.

Answer: D

Taking one sample per cycle, each a little later than the last, assembles a picture of the waveform at an effective rate far above the real sampling rate. It reaches enormous bandwidths but works only on a strictly repetitive signal.

Answer: A

With the internal time base disconnected, the horizontal plates carry a second signal and the spot traces the locus of one against the other. The shape of the closed figure reveals the ratio of the two frequencies and their phase relation.

Answer: B

Counting the points at which the pattern touches a horizontal line and a vertical line gives two integers, and the ratio of the vertical to the horizontal frequency is the ratio of the horizontal tangencies to the vertical ones. A simple ratio gives a stationary figure.

Answer: C

Correct answer: equal frequency and zero phase difference

With equal frequencies and no phase difference the two deflections are proportional at every instant, so the spot traces a straight line; its slope at forty-five degrees means the two amplitudes are equal as well. Adding a phase difference opens the line into an ellipse, and ninety degrees with equal amplitudes gives a circle.

Answer: D

The parametric equations of a circle are a sine and a cosine of equal amplitude, which is exactly two signals of the same frequency and amplitude in quadrature. The sense in which the spot travels round distinguishes a lead from a lag.