I ordered a Velleman HPS10SE from kitsusa.net and it’s having trouble calibrating.
It keeps giving me a side unperfect square like this
What is going on? That’s at 2v/div at 1mSec/div
Is my unit defected?
I’ve tried different probes
Sorry, but the picture you’ve added does not allow us to judge if there is a problem or not.
Please post a screenshot.
How do I post a screenshot?
My set did not come with computer connectors. Can you direct me a step-by-step on how to post the screenshot?
I just got this unit brandnew.
Unit is not equipped with PC interface.
I was thinking of a picture of the screen.
This will allow us to judge the behaviour you’ve noticed.
This is what I get when I try to calibrate it to a square (this is the best that it can do at this voltage/division)
http://vietpho.net/~images/vellemanproblems/1g.jpg
This is what it looks like when I zoom in the voltage/division:
http://vietpho.net/~images/vellemanproblems/2g.jpg
This is what it looks like after I’ve tried to flatten the top of the previous image at a new voltage/division:
http://vietpho.net/~images/vellemanproblems/3g.jpg
See? Nothing I do will give me the perfect square calibration.
You are getting a perfect calibration, it’s the top of the waveform that matters. not the sides…
The effect you are seeing is typical of digital all scopes, and is only more pronounced on the HPS10SE
because of its 128 pixel wide display and 10Mhz sampling rate.
While a perfect square/rectangular wave would have 0.0s rise and fall times, the reality is that
there’s no such thing as a perfect square wave, they all take time to change from high to low voltage and vice-versa.
What you are seeing is that the calibration signal fell from its maximum level to its minimum level over a period
of two samples made by the scope. One was made while the signal was high, the other while it was low,
since the two samples represent different points in time the display of the falling signal is “stepped”.
This is the same effect as the horizontal steps seen in the top and bottom of the “peaked” and “rolled” waveforms,
the difference being that the horizontal steps represent the scope’s vertical (voltage) resolution while the
vertical steps represent the unit’s horizontal (time) resolution.
Here is a screen shot from my $3200 Lecroy WJ-322 scope showing the same effect, deliberately exaggerated
through selection of a slow rise/fall time wave. This was recorded at a 500MHz sample rate.
In the top trace the red arrows show the steps created by changes in level
that fell between samples. The bottom trace is a 10x zoom of the selected portion of the top trace, showing
the “real” shape of the falling level. Note however that there are still steps, just smaller ones.
Short story, there’s nothing wrong with your 'scope…
You’re only showing the top 1/2 of the waveform.
But please, I an too old and have been repairing and calibrating
test equipment for too many years, to care to argue any of this.
I have just tried to help you understand the entry level instrument
you have purchased. There is nothing wrong with it, you just have to
understand it’s limitations–which all instruments have, as the are all imperfect…
Believe whatever you wish…
Hello moh.vze.com
Actually, this is quite normal and it is explained through the Gibbs phenomenon. When there’s a rising or a falling edge on your trace, there’s is a mathematical discontinuity (a moment in time " t " where the signal connot converge to a precise value). In fact, the signal is calculated with Fourier series and the discontinuity at that time " t " introduce a “kink” in the waveform, the scope not being able to converge to a specific value. It is indeed quite complex to explain mathically because you need to understand Fourier analysis and discreet signal mathematics and functions. The kink would be on each corner of the waveform on your picture if the HPS10 was as fast during its falling edge as it is during its rising edge. You shouldn’t worry… Expensive scope will hide this behavior to you by doing some more complex math that are well too advanced for a HPS10. If you know Fourier mathematics and related stuff there’s a wikipedia page explaining it very well :
[quote=“neptuneturf”]Hello moh.vze.com
Actually, this is quite normal and it is explained through the Gibbs phenomenon. When there’s a rising or a falling edge on your trace, there’s is a mathematical discontinuity (a moment in time " t " where the signal connot converge to a precise value). In fact, the signal is calculated with Fourier series and the discontinuity at that time " t " introduce a “kink” in the waveform, the scope not being able to converge to a specific value. It is indeed quite complex to explain mathically because you need to understand Fourier analysis and discreet signal mathematics and functions. The kink would be on each corner of the waveform on your picture if the HPS10 was as fast during its falling edge as it is during its rising edge. You shouldn’t worry… Expensive scope will hide this behavior to you by doing some more complex math that are well too advanced for a HPS10. If you know Fourier mathematics and related stuff there’s a wikipedia page explaining it very well :
http://en.wikipedia.org/wiki/Gibbs_phenomenon[/quote]
Well stated and put forth!
Thank you…
-cliff-