Display: time, split, spectrum or spectrogram

Screen

• Time shows the signal over time, on the whole screen.
• Split shows the signal on the top half and its spectrum on the bottom half.
• Spectrum gives the whole screen to the spectrum.
• Spectrogram shows how the spectrum changes over time, above the live spectrum. See Spectrogram.

Keyboard: F steps through the four.

Drawing

How the trace joins the samples the board measured:

• Sin(x)/x (the usual choice) rebuilds the wave between the samples, as a bench scope does. A sine with only a few samples in each cycle is drawn as the sine it is, rather than a jagged pattern or a slow swelling “beat” that is not in the signal. It is right up to about 0.4 of the sample rate (2.5 samples a cycle); closer to half the rate, raise the rate. A square wave is different: its sharp corners need many harmonics, above half the rate when there are few samples a cycle, so sin(x)/x rounds it and it rings past its top and bottom. For a square with fewer than about 20 samples a cycle, choose Lines or raise the rate.
• Lines joins the samples with straight lines.
• Dots shows only the samples, nothing between them: exactly what was measured.

With many samples to each pixel, all three look the same.

Alias check

A signal faster than half the sample rate does not disappear: it shows as a false, slower wave, clean and steady, that one capture cannot tell from a real one. With Alias check on (the usual choice), every few seconds the scope takes one capture at a slightly lower rate, which you do not see. A real signal reads the same at both rates; a false one moves. If it moves:

• a warning under the measurements says the signal is faster than half the rate, and gives its likely true frequency;
• Aliased shows on the strip above the screen.

Raise the rate above twice that frequency to see the signal truly. If the board cannot go that fast, the signal is beyond what it can show.

The check only judges a steady signal, so noise does not set it off. Turn it off if you do not want the board’s rate changed for a moment every few seconds.

More display

Open More display for persistence, waveform averaging and high-resolution smoothing on the time view.

Persistence

Keeps earlier traces on the screen behind the newest one, coloured by how often the signal visits each pixel: dim where an occasional glitch passed, through warmer tones to bright white where the trace stays.

• Off shows only the newest capture.
• 0.5 s and 2 s fade older traces out over roughly half a second or two seconds.
• Infinite keeps every trace until you press Clear persistence or change the scale.

Use persistence to catch rare glitches, timing jitter or an unstable envelope.

Averaging

Averages 2, 4, 8, 16 or 64 triggered captures sample by sample, lined up on the trigger point. Repetitive parts of the signal stay while random noise averages away. Untriggered captures are left out so the wave does not blur, and the strip above the screen shows how many captures are in the average so far.

Hi-res

Averages 2, 4, 8 or 16 neighbouring samples within each single capture. Unlike multi-capture averaging, Hi-res works on a single-shot capture and on untriggered signals: it trades horizontal bandwidth for lower high-frequency noise and finer vertical steps.

Reading the spectrum

The spectrum shows how strong each frequency in the signal is. Frequency runs along the screen and level up it, in dBV (decibels relative to 1 V RMS) unless you choose another unit. Every 20 dB down is a tenth of the voltage. To zoom in, change the axis or the level scale, see Spectrum.

• A pure sine wave shows one peak, at its frequency.
• A square wave shows a peak at its frequency and smaller ones at 3, 5, 7 times it, and so on.
• Noise shows as a rough floor along the bottom.

The measurements list gives the strongest frequency (Spectrum peak) and its level (Spectrum peak level). In Spectrum mode they are listed first.

Spectrogram

The spectrogram turns each capture’s spectrum into one row of colour: dark where a frequency is quiet, through blue and red to white where it is loud. The newest capture is the row at the bottom, just above the live spectrum; older ones rise and leave at the top. So a steady tone is a straight line up the screen, a tone that drifts is a slanting line, and one that comes and goes is a broken one.

• It shares the spectrum’s frequency axis and level scale, so Span, the frequency axis, the window and the level unit change both. The key on its right gives the levels at the ends of the colour scale.
• It keeps the last 100 captures. When you stop, it stops too, and stepping back through the captures steps it back.
• Each row is one whole capture as it came, even when Average, a hold or Gate to cursors is chosen for the spectrum below.
• Rest the pointer on it, or tap it on a phone, to read a frequency, its level and which capture it comes from. Markers reach up through it.