Oscilloscope questions - Page 1
Forum section: electronics / general

author: Wawavoun
author_rank: Regular Contributor
timestamp: November 18, 2021, 01:36:42 pm
content: Hello there,I apologize if this question has already been posted here but I did not find it...I recently bought a Hantek DSO2D15.The specifications speak of a 150 MHz scope.Certainly this is (very) optimist but a bandwidth of 50 to 70 Mhz is widely sufficient for me so at the time of the purchase I was not worry about that...The scope seems to work correctly (menus, display etc...).I try to measure a clock signal at 16 MHz at the output of a 74LS04 quartz driven gate.The probe was x1 and 150 MHz bandwith (the one supplied with the scope). Probe calibrating is done but act only in x10 position.The signal look more like a sinus than like a square and has no step part...The peaks stay far from 0 V and from VCC voltage (oscillate between ~1 V and ~4 V)...My poor Hantek 6022 20 MHz usb scope give something very similar but this is more understandable...The electronic assembly containing the oscillator works, this is not the question...The propagation time (in to out) indicated for such a gate is 15 ns but this is not the rise or fall time...1/2 period at 16 Mhz is 31.25 ns so I am in doubt about what I should measure...A square with default but with flat parts near 0 V and near 5 V or something looking more or less like a sinus whitout flat parts ?My strong impression is that the rising time of the scope inputs is too long...Any advices will be appreciated and possible solutions also.Regards.Philippeps : be telorant about my english pls...

author: rsjsouza
author_rank: Super Contributor
timestamp: November 18, 2021, 02:00:38 pm
content: If I understood correctly, you are measuring the 16MHz signal with the 1x probe, is that so? If so, think about what is between the circuit and the oscilloscope itself: the probe. It is probably the limiting factor that is distorting your 16MHz square wave. Can you retry with the probe at 10x? It will usually have much less capacitance and therefore much more bandwidth.Another significant factor that influences this is the location where you are probing - if you have very long connections between the board and the probe points, they will influence the measured signal that arrives at the probe itself. Although 16MHz will not be severely limiting, it might still show distortions if the distance between the alligator clip and the probe tip are very far apart. It might be worth using the small spring accessory that was supplied with your probe. An interesting video is:As a side note, don't forget that a square wave is never perfectly drawn, since it requires an infinite bandwidth to be fully represented, but quite a reasonable approximation can be done with a bandwidth of about 9 times (10 times is a good rounded approximation) of the fundamental frequency - in your particular case, that would require a 145MHz of bandwidth, which is still within the limits of the oscilloscope (assuming it has true 150MHz of analog bandwidth). The reference below contains some interesting aspects about this:https://www.allaboutcircuits.com/technical-articles/what-is-the-fourier-transform/Good luck!

author: tunk
author_rank: Super Contributor
timestamp: November 18, 2021, 02:21:21 pm
content: No expert on this, but I think a 1X probe has a bandwidth of ~5MHz.Maybe this is some help:https://youtu.be/OiAmER1OJh4

author: Wawavoun
author_rank: Regular Contributor
timestamp: November 18, 2021, 02:25:02 pm
content: yes measurement made in x1 position...I have think that this is better that x10 position...Will try in x10 position.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 18, 2021, 02:56:56 pm
content: Quote from: tunk on November 18, 2021, 02:21:21 pmwith some good probes you might reach 15MHz in a 1x probe, but it's in any way not the way go for with high speed logic signal

author: Wawavoun
author_rank: Regular Contributor
timestamp: November 18, 2021, 09:54:46 pm
content: In x10 position the signal look most better !!!Problem solved and one thing more learned today !Thanks for help.Philippe

author: rsjsouza
author_rank: Super Contributor
timestamp: November 18, 2021, 11:04:11 pm
content: Thanks for coming back with the outcome of your situation; not many people do.