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Analyzing a Pulse

chiques

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Hello Community,
I am trying to learn how to analyze the quality of pulse and from what I understand the 'ripple' or 'overshoot' is considered noise. I have a pwm that I sourced (somewhere, I can't recall) and I have a cheap function generator. I configured both so they are generating the same pulse. I see the following differences, can anyone give me some insight on what the differences are?

1621386021605.png


1621386030840.png


I circled the differences in I saw in the peaks, not sure what they mean though.

Thank you for any insight.
 

danadakk

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The damped oscillatory waveform you see in first waveform
is fundamentally a LC resonance in system. Typically distributed
L and C in the signal path. Possible scope probe ground lead L
can aggravate. Or transmission line issues.




The PWM waveform looks like Vdd noise on the logic level output
of the PWM. Possible clock noise bleed thru as well. Eg, internal
parasitic C and common mode noise due to switching activity.

Look at spectrum to get clues as to frequency components.

When looking at noise to get worst case pk-pk put your scope on infinite
persistence and watch the sampling process aggregate noise generated.

Bypass caps critical in fine grained suppression, not all caps, for equal C,
yield same esr behaviour -

1621391198355.png


Generally supply pins both bulk and ceramic in parallel. Chip datasheets usually have
recommendations.


Regards, Dana.
 
Last edited:

c_mitra

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Hello Community,
I am trying to learn how to analyze the quality of pulse and from what I understand the 'ripple' or 'overshoot' is considered noise. I have a pwm that I sourced (somewhere, I can't recall) and I have a cheap function generator. I configured both so they are generating the same pulse. I see the following differences, can anyone give me some insight on what the differences are?

First of all, you are trying to produce a square pulse.

They are very useful in switching applications; kind of on and off states.

Mathematically, they are discontinuous functions. The rising and falling parts of the square wave are not well-behaved functions. In most diagrams, however, you will see that they are drawn with a slope (finite rising and falling times) and that avoids one problem.

Next you will see the corner (where the vertical line joins the horizontal line and vice-versa)- these are also singular points.

xfourier_04.png

The ringing at the corner point is called the Gibbs' phenomena. It can be reduced but cannot be eliminated. To get another idea, see

The higher the bandwidth, the cleaner it looks. But whatever you may do, the horizontal part of the square wave will always appear noisy.

In short, your signals are fine.
 

Easy peasy

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how you probe these waveforms makes a huge difference to what you see on a scope, soldering the 1:1 scope lead to the signal is the only way to get real measurements for rise times < 1nS - the ripples you see may well be artifact of the terminations you are using ...
 

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