Hi. ,,Having a 'signal'-challenge. Not sure if to call it ringing, or bouncing?

Thread Starter

Rolf(norway)

Joined Jul 9, 2016
6
Hi. ,,Having a 'signal'-challenge.
Not sure if to call it ringing, or perhaps some sort of bouncing? It happens after reaching either the top or bottom. (..see attached picture)

--Signal is around 50kHz on the fastest pulses--

Using a differencial busTranceiver SN75176B (balanced Tx via 120ohm-Load, and ca 120ohm-TransmissionLine), and having a littlebit struggle with the digital output signal.
(Application: The busTranceiver have the task of sending controll-signals to an another controller for a lighting-system, via one pair of balanced TwistedPair-120ohm)

Would be appreciative if someone have some insight to any approach one could take.

..picture attached.
Have nice day/evening
br, Rolf
 

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dl324

Joined Mar 30, 2015
12,871
Welcome to AAC!

Could be ringing caused by inductance in the ground lead on your probe. Does your scope have a bandwidth limiting option? Do you have proper termination on the signal? Where are you taking the measurement?
 

Thread Starter

Rolf(norway)

Joined Jul 9, 2016
6
Where are you looking? Is this the output of the receiver?
Hi Ian! --Oscope probes are connected to the ends of a 30cm twisted pair, after the transmitter(assume those TP are about 120ohm impedance). ..'And' the transmitter have a load-resistor of 120ohm across the D- D+ outputs.('before' the short 30cm twisted pair couple)
 

Ian0

Joined Aug 7, 2020
2,189
Hi Ian! --Oscope probes are connected to the ends of a 30cm twisted pair, after the transmitter(assume those TP are about 120ohm impedance). ..'And' the transmitter have a load-resistor of 120ohm across the D- D+ outputs.('before' the short 30cm twisted pair couple)
Is it a differential scope probe? OR do you have the probe to one lead and the earth clip to the other?
Is your circuit grounded and is your scope grounded? If so you have a short on one output of your 75176.
 

Thread Starter

Rolf(norway)

Joined Jul 9, 2016
6
Is it a differential scope probe? OR do you have the probe to one lead and the earth clip to the other?
Is your circuit grounded and is your scope grounded? If so you have a short on one output of your 75176.
->> --Fantastic-- ThankYou, Ian! ..I will come back with picture-'result' very soon..
...I just now connected the lab-PSU(wich supplying the dut-circuit) , via 1:1 mains separating-transformer to be sure that there was no earth-talking/quarreling between those two. And what a joyfull-result it was.
'Initially' I had earlier checked and ruled-out(..so I thought..) with multimeter for any earth(or otherwise) interaction between DUT(tranceiver) and Oscope, and found nothing then.

--I'm very happy and thankful that you jumped on my question-thread Ian.
Your last notes over here made me do the extra effort and connect my lab-PSU's lineVoltage via a 1:1 transformer.
I will now look into some more permanent solution for the lab-PSU, now that I know about this.

As mentioned, ..I will come back with picture of much nicer Oscope-curves very soon.
 

Ian0

Joined Aug 7, 2020
2,189
You need both probes, one probe on each signal, and the "MATH" function set to "A-B", otherwise the capacitive loads on each signal are not equal. One side is driving the scope chassis to earth capacitance and the other isn't.
 

Thread Starter

Rolf(norway)

Joined Jul 9, 2016
6
You need both probes, one probe on each signal, and the "MATH" function set to "A-B", otherwise the capacitive loads on each signal are not equal. One side is driving the scope chassis to earth capacitance and the other isn't.
Yes, I was thinking about it. But didn't go that far. But learned a lot this evening. ...Have now attached new picture of a much-much better signal :) Still using one probe with the short eart-lead. I think/thought it doesent matter so much on this relatively low Frq, but will certanly do experiments with duo-probe and Math onward.
There is still some 'small' ringing but it has no influence on the operation, since the receiver has a sensitivity of 200mV and 50mV hysterisis.
Very happy now, thanks Ian! ..see attached picture
 

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Thread Starter

Rolf(norway)

Joined Jul 9, 2016
6
Yes, I was thinking about it. But didn't go that far. But learned a lot this evening. ...Have now attached new picture of a much-much better signal :) Still using one probe with the short eart-lead. I think/thought it doesent matter so much on this relatively low Frq, but will certanly do experiments with duo-probe and Math onward.
There is still some 'small' ringing but it has no influence on the operation, since the receiver has a sensitivity of 200mV and 50mV hysterisis.
Very happy now, thanks Ian! ..see attached picture
----
Just hoping now that by attaching yet a 'longer' (10mtr/33feet), hopefully it will have similar/sufficient quality on the arriving signal in the other end. Will see how it goes.
 

Thread Starter

Rolf(norway)

Joined Jul 9, 2016
6
Welcome to AAC!

Could be ringing caused by inductance in the ground lead on your probe. Does your scope have a bandwidth limiting option? Do you have proper termination on the signal? Where are you taking the measurement?
Hi Dennis didn't catch your post earlier, my apology (..everything happened so fast yesterday).
Thanks for your welcome, and also your inputs.
The overall experiencing of this AAC-website, and its forumArea also,..is very impressive!
br, Rolf
(norway)
..
Just some additional thoughts; Very happy that all this things happened yesterday, because now I will be going to look more closely to how my instruments are interconnecting here. ..'Also' in regards to how those are feed from the mains-powerOutlet.
>>Though important in this offcourse, will be to take care that one always do-not-compromize on safetyGround.
Just mentioning, in case some other people too, getting "spring-cleaning" idea of doing something with their mains-power connection for instruments and Psu's.
For my self, contemplating here now on some standAlone 1:1 separation -transformers; 1 dedicated for labPsu-only, and 1 dedicated for Oscope-only. (..OnlyOneAparatus pr. transformer, ..otherwise it will not accommodate electrical-Safety)
//Thanks again.
Have nice weekend to everyone.
 
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