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Electronics => Beginners => Topic started by: N3xus on March 15, 2016, 09:17:51 pm

Title: Strange oscilloscope overshoot
Post by: N3xus on March 15, 2016, 09:17:51 pm
Hello all!

I learned to work with oscilloscopes in school and already have one at home so I'm quite familiar using an oscilloscope.
Today I recognized very strong behavior during a measurement.

Measurement setup:
Red Pitaya acting as Function generator, generating a rectangular signal of 20 kHz.
Furthermore I had to test two analog oscilloscopes that are connected directly to the output of the Red Pitaya.
In a second setup I used the probe to measure the voltage.

The strange behavior: When I connect both devices via BNC cable there is some overshoot after each positive and negative slope. See pictures below.
If I measure the voltage with the probe there is no ringing at all.

Does anybody know why this happens? Is it because of the wave impedance of the coaxial cable or is the probe damping the signal so that it is not recognizable anymore? Or is the reason something complete different?

Maybe another interesting information: With a digital scope I am not able to reproduce this behavior with the BNC cable.

Best regards
Armin
Title: Re: Strange oscilloscope overshoot
Post by: focovario on March 15, 2016, 09:57:22 pm
N3xus, this is my first post also!

You might be having an impedance match issue with the BNC cable.

Assuming a 50 ? cable, you should terminate the cable at the scope with 50 ?.

The oscilloscope probes most probably are correctly matched to the scope.

Keep posting :-+
Title: Re: Strange oscilloscope overshoot
Post by: homebrew on March 16, 2016, 07:49:52 am
Yes, probably exactly that ...

What you see are signal reflections at the end of the cable.

Took me a while to grasp that the images are rotated 180 degrees. Otherwise the waveform wouldn't make any sense ...

Just put a 50 Ohm terminator with a T-Connector to the last connection of the cable.
Title: Re: Strange oscilloscope overshoot
Post by: tggzzz on March 16, 2016, 09:17:32 am
When you see something peculiar or unexpected in a waveform, use the scope to diagnose the cause. In this case zoom in on the transitions.

Apart from that, in general construction and measurement techniques and conditions are highly important, and need to be outlined. A photo of a circuit being tested and the scope front panel settings often indicate the cause of the surprise.
Title: Re: Strange oscilloscope overshoot
Post by: N3xus on March 16, 2016, 09:31:14 am
Oh, I'm really sorry, I didn't saw that it was flipped by 180°.  |O

Yes you both were exactly right. Attaching a termination resistor solved the problem and led to a beautiful rectangular signal.
Now I have to read myself into the topic to understand the reason why this happened.

Thank you for your fast answers!
Title: Re: Strange oscilloscope overshoot
Post by: tggzzz on March 16, 2016, 11:54:47 am
Oh, I'm really sorry, I didn't saw that it was flipped by 180°.  |O

Yes you both were exactly right. Attaching a termination resistor solved the problem and led to a beautiful rectangular signal.
Now I have to read myself into the topic to understand the reason why this happened.

Thank you for your fast answers!

We all make mistakes; the trick is to make new mistakes.

Glad the problem is sorted. Understanding why this happened plus other diagnostic techniques you could have used will be very helpfiul to you in the future. Good to see someone that takes an opportunity to teach themselves something new.
Title: Re: Strange oscilloscope overshoot
Post by: danadak on March 16, 2016, 12:11:15 pm
This might help -

[url][/http://www.tek.com/document/application-note/how-oscilloscope-probes-affect-your-measurementurl]

Regards, Dana.
Title: Re: Strange oscilloscope overshoot
Post by: tggzzz on March 16, 2016, 01:19:39 pm
This might help -
/http://www.tek.com/document/application-note/how-oscilloscope-probes-affect-your-measurement

Since this is a common problem, I've collected my list of good theory/practice/use/safety references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ (https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/)
Title: Re: Strange oscilloscope overshoot
Post by: blueskull on March 16, 2016, 02:02:14 pm
Oh, I'm really sorry, I didn't saw that it was flipped by 180°.  |O

Yes you both were exactly right. Attaching a termination resistor solved the problem and led to a beautiful rectangular signal.
Now I have to read myself into the topic to understand the reason why this happened.

Thank you for your fast answers!

20kHz might sound like a low frequency signal that can not generate enough reflection, but that is simply not true. How your reflection screws up your signal depends on how sharp the rising edge and the falling edge are, so if you use a 20MHz capable output to generate 20kHz SQUARE WAVE signal, you will get a the same rising edge as if it is generating a 20MHz square wave. For the exact reason, when working on signal integrity issues, never assume low frequency signals, especially switching signals, to be problem free. The rule of thumb is 1ns/inch, so if the transmission line is 1 inch long, your Tr/Tf can be 1ns. The faster the rising/falling edge, the shorter the transmission line it can tolerate without proper termination. Say, for a 20MHz analog BW device, the rule of thumb Tr/Tf is 400/f=20ns (some people use 350 some others use 450, but it should be in the range between 350 to 450, depending on the analog and digital filter characteristics of the particular device), therefore if you have a longer than 50cm cable (if it is a Y cable, the total length), you might have some troubles.