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Extract Features of a Clock Signal

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How sharply does an on/off signal turn on and off? How often and for how long is it activated? Determine all those characteristics for the output of a clock.

Load the signal and plot it. The time is measured in seconds and the level in volts.

load('clock.mat')

plot(tclock,clocksig)
xlabel('Time (s)')
ylabel('Level (V)')

Figure contains an axes object. The axes object with xlabel Time (s), ylabel Level (V) contains an object of type line.

Use statelevels to find the lower and upper levels of the signal by means of a histogram. If you do not specify an output, the function plots the signal, marks the levels, and displays the histogram.

levels = statelevels(clocksig)
levels = 1×2

    0.0138    5.1848


statelevels(clocksig);

Figure State Level Information contains 2 axes objects. Axes object 1 with title Histogram of signal levels (100 bins), xlabel Level (Volts), ylabel Count contains an object of type line. Axes object 2 with title Signal, xlabel Samples, ylabel Level (Volts) contains 3 objects of type line.

Determine how fast the signal rises at each transition. risetime uses the lower and upper levels found by statelevels. It defines the rise time as the time it takes the signal to rise from 10% to 90% of the difference between the levels.

[Rise,LoTime,HiTime,LoLev,HiLev] = risetime(clocksig,tclock);

Levels = [LoLev HiLev; (levels(2)-levels(1))*[0.1 0.9]+levels(1)]
Levels = 2×2

    0.5309    4.6677
    0.5309    4.6677

If you call risetime without outputs, the function draws an annotated plot of the signal. The rise times are shaded, the crossing points are marked, and the levels are displayed. You can use the time vector or the sample rate as input.

risetime(clocksig,Fs);

Figure Rise Time Plot contains an axes object. The axes object with xlabel Time (seconds), ylabel Level (Volts) contains 12 objects of type patch, line. One or more of the lines displays its values using only markers These objects represent rise time, signal, upper cross, lower cross, upper boundary, upper state, lower boundary, upper reference, lower reference, lower state.

overshoot and undershoot show how far the signal deviates from the state levels at each transition. The results are expressed as percentages of the difference between the levels. Further outputs give the actual times and signal values.

overshoot(clocksig,Fs);

[pctgs,values,times] = undershoot(clocksig,Fs);

hold on
text(1.1e-3,2,'     Undershoot','Background','w','Edge','k')
plot([times;1.17e-3],[values;2],'^m','HandleVisibility','off')
hold off

Figure Overshoot Plot contains an axes object. The axes object with xlabel Time (seconds), ylabel Level (Volts) contains 13 objects of type line, text. One or more of the lines displays its values using only markers These objects represent signal, upper cross, lower cross, post-overshoot, upper boundary, upper state, lower boundary, upper reference, lower reference, lower state.

Determine how fast the signal falls using falltime. You can set the state levels and the percentage reference levels manually. You can do the same with risetime.

falltime(clocksig,tclock, ...
    'PercentReferenceLevels',[30 80],'StateLevels',[0 5]);

Figure Fall Time Plot contains an axes object. The axes object with xlabel Time (seconds), ylabel Level (Volts) contains 12 objects of type patch, line. One or more of the lines displays its values using only markers These objects represent fall time, signal, upper cross, lower cross, upper boundary, upper state, lower boundary, upper reference, lower reference, lower state.

Find the period of the signal. By default, the period is defined as the time elapsed between consecutive rising crossings of the reference level halfway between the state levels. You can change the polarity of the crossings, specify the state levels, or adjust the reference level.

per = pulseperiod(clocksig,tclock)
per = 4×1
10-3 ×

    0.4143
    0.4200
    0.4188
    0.4111


pulseperiod(clocksig,Fs,'Polarity','negative','MidPct',25);

Figure Pulse Period Plot contains an axes object. The axes object with xlabel Time (seconds), ylabel Level (Volts) contains 10 objects of type patch, line. One or more of the lines displays its values using only markers These objects represent pulse period, signal, mid cross, upper boundary, upper state, lower boundary, mid reference, lower state.

The duty cycle is the ratio of pulse width to pulse period. Determine it directly or using a dedicated function.

dut = dutycycle(clocksig,Fs);

wdt = pulsewidth(clocksig,Fs);

compare = [wdt./per dut]
compare = 4×2

    0.4862    0.4862
    0.4756    0.4756
    0.4871    0.4871
    0.4886    0.4886

See Also

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