A digital storage oscilloscope samples the input and stores the values, which allows it to
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.
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According to the sampling theorem, a digital oscilloscope must sample at a rate exceeding
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.
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Aliasing on a digital storage oscilloscope shows itself as
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.
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The memory depth of a digital storage oscilloscope determines
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.
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The vertical resolution of a digital storage oscilloscope with an eight-bit converter is about
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.
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A sampling oscilloscope builds up a display by taking
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.
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A Lissajous figure is obtained on an oscilloscope by
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.
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The ratio of the two frequencies producing a Lissajous figure is obtained from
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.
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A Lissajous figure that is a straight line sloping at forty-five degrees indicates two signals of
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.
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A Lissajous figure that is a circle indicates a phase difference of
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.
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