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Rise Time


Question

Hello,

I was wondering how would be the simpliest way to determine a capacitor value with the help of the scope. I connected a 2.2 uF capacitor in serie with a 990 Ohm resistor, negative port of capacitor to GND, positive port of capacitor to the resistor, and the other leg of resistor to the wavegen1 with a square signal at 20 Hz / 1V amplitude, the scope channel 1 being connected accross the capacitor. My first tought was to create a short script, calculating the rise time at 63% of the value of the voltage (1V) and then dividing this time by the resistor value. I'm not experienced enough to do that with script (it's probably very basic, if someone could give me some advice about this scripting, would be cool...). The other way I found was to play with the cursors ans manually place one horizontal cursor at 63% of 1V (0,63 V) and then to place a set of verical ones in order to determine the rising time from 0V to 0,63V...this is fair enough for me, I found roughly 2.07 ms, which corresponds to a capacitor value of 2.09 uF (I measured the capa at 2.09 uF with my multimeter). Then I saw that in the measurements there is a RiseTime calculation available...but I cannot figure how this value is calculated : it is from 0% to 100% of the signal, or it is from 0 to the triggering value which is set in the trigger value ? The value calculated gives 3.89 ms....

Thanks for your support

Capture d’écran 2018-01-21 à 14.51.57.png

Edited by Odolc
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Yeah, I can confirm that there is some issue with that (3.7.5), or we are using the measurement in a wrong way...:

image.thumb.png.f2d7456ae58178620e7a6fc198f58c0e.png

@attila - I have a suggestion for further releases - maybe add measurement display, like in Agilent's scopes - as you enable measurement on the waveform there are cursors showing where and how measurement was taken.

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I would no say that there's an issue...after spending some time to look at the functions, I've found the explanation, the RiseTime measurement computes from 10% to 90% of the signal, you can see that by editing the measurement, there's a comment saying : "// The average rise time for 10-90% of low-high level."

Coming back to my original need, I created the following custom measurement. It computes the time constant of the capacitor charge, based on the RisTime measurement provided.

var resistance = 990
var rise = Scope1.Channel1.measure("RiseTime");
(rise / (logn(0.9)-logn(0.1))) / resistance

Works fine ! The capacitor value is calculated

Capture d’écran 2018-01-21 à 20.23.27.png

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Hi @Piotr Rzeszut

The rise and fall times are calculated based on high and low values established from the histogram peaks.
A triangle signal does not have histogram clear peak so the high/low values and subsequent calculations might be wrong.
This approach will work with real world signals, which are something between sine and square or triangle with flattened peak.

image.png.e9545ee4f1faf50c5d96c64e485ee932.png

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@attila thank you for the explanation of the method used for Rise/Fall time calculation. I have been thinking of it, and the way of calculation you presented can deal very well with over- and undershoot. At the other for beginners (and some other unaware people like me ;) ) it can lead to some confusions, so maybe in such situations some warning/info message should be displayed? Or some cursors presenting where the measurement was taken?

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