Strange oscilloscope overshoot - Page 1
Forum section: electronics / beginners

author: N3xus
author_rank: Contributor
timestamp: March 15, 2016, 09:17:51 pm
content: 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 regardsArmin

author: focovario
author_rank: Contributor
timestamp: March 15, 2016, 09:57:22 pm
content: 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

author: homebrew
author_rank: Frequent Contributor
timestamp: March 16, 2016, 07:49:52 am
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: March 16, 2016, 09:17:32 am
content: 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.

author: N3xus
author_rank: Contributor
timestamp: March 16, 2016, 09:31:14 am
content: Oh, I'm really sorry, I didn't saw that it was flipped by 180°.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!

author: tggzzz
author_rank: Super Contributor
timestamp: March 16, 2016, 11:54:47 am
content: Quote from: N3xus on March 16, 2016, 09:31:14 amWe all make mistakes; the trick is to makenewmistakes.Glad the problem is sorted. Understandingwhythis 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.

author: danadak
author_rank: Super Contributor
timestamp: March 16, 2016, 12:11:15 pm
content: This might help -[/http://www.tek.com/document/application-note/how-oscilloscope-probes-affect-your-measurementurl]Regards, Dana.

author: tggzzz
author_rank: Super Contributor
timestamp: March 16, 2016, 01:19:39 pm
content: Quote from: danadak on March 16, 2016, 12:11:15 pmSince this is a common problem, I've collected my list of good theory/practice/use/safety references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/