Strategy to color crosstalk source

[yefj]

Hello ,given a layout with multilayer traces.On certain trace there is a high distrotion due to cross talk.
Is there a strategy to see from where the cross talk is coming from?
To color different contributions of the spectrum noise and have indiction the source of it?
Is there such a thing?
Thanks.

 

[Baluncore]

Is there a strategy to see from where the cross talk is coming from?

Feed a PRBS or a wide-band noise into one line, while you terminate both ends of the other line, then look for the signal in the other line's terminations.

It seems that you do not have cross-talk between what are assumed to be uncorrelated signals, but coupling between phase locked signals. The cross-talk analysis is not expecting correlated channels, so it might be breaking that assumption.

Keep the signals far apart. Use tracks above and below the PCB, not one track above a shared ground plane.

 

[yefj]

Hello baluncore , I have intresting phenomena as following.
we have three differential lines one victim and two agressors.all of them are terminated at the same end of the line.The cross talk is measured by SDD21 so it crostal from one differential mode to another differential mode.

And I get a very intresting phenomena where the differential line which is far from the victim has the same crosstalk levelel as differntial line which is much closer.
How could it happen?
Thanks.

 

[yefj]

update:
I have a situation as shown below where there are 3 differential lines one is a victim and two agressors.
I have one agressor is far away then the other agressor .
what is the best way to see the crosstalk between the differential ines.
buttom line We have digital data which is getting distorted in the time domain.
Differential mode cross talk is a mode that calculates the crosstalk from differential line.
maybe its better to analise the cross talk by common mode Scc21?
What is the best method to do in VNA?

update :

there are plots shown below how to know what is the dominant factor for distorsion?

 

[Baluncore]

And I get a very intresting phenomena where the differential line which is far from the victim has the same crosstalk levelel as differntial line which is much closer.
How could it happen?

When two tracks are above a ground plane, the crosstalk is reflected backwards into the victim.
Analysis of backward wave couplers comes down to identifying if the odd or even mode of coupling is operating. Maybe you should look at both ends of the victim track, by treating your system as a four port network.

A travelling wave antenna is fed from the back, but terminated in the front, so energy that is not radiated forwards, is absorbed in the front termination resistor. Signals from behind are also absorbed. Maybe the same sort of thing is occurring with your experiment.

Since you will not answer questions, provide an accurate model of the test jig, and do not show where the ground plane is, or define what you mean by a differential line, it is impossible to answer your question.

 

[yefj]

Hello Baluncore thank you for your responce, I did show the system simulation shown below.
I get common mode and differential mode comapison between the far agressor effect ofdifferential line and the close differential agressor line.
few questions?
1. which mode is more dominant in ruining the digital data?
2. could you please give me a manual for the comulative IFFT of the crosstalk which is basickally how much energy traveled at each pulse of the cross talk IFFT time responce?

 

[sophiecentaur]

Analysis of backward wave couplers comes down to identifying if the odd or even mode of coupling is operating. Maybe you should look at both ends of the victim track, by treating your system as a four port network.

I think this is very relevant. All four ports will be involved. If the 'output port' is not terminated well then the reflected wave would appear at the fourth port. Using a range of terminations on the output should show this effect.
I don't know what your simulation involves or what assumptions are made but you can't take anything for granted if you don't specify all the parameters. Look into it a bit closer and you may get a clue.

Presumably the terms 'victim' and 'aggressor' refer to 'wanted signals' and 'interfering signals'(?). Am I correct when I understand that a noise signal is involved? Would it be easier to use a swept cw signal for analysing the performance? Is there a hardware version of the setup?

 

[yefj]

Hello, I have an update with a dilema regarding this cross talk issue.
I have a multilayer PCB with differential traces .
I measured SDD21 of agressor to victim and found a very high cross talk.

Then I though to measure the effective permitivty er threw the phase delay and i saw that the efective permitivy is much higher then what it needs to be.

Few question:
1.when we are designing multilayer PCB do we expect the effective permitivity to be such that we will get 100Ohm characteristic impedance?
2.suppose my core by manufacturer permitivity but I get another effective permitivity is it a sign of trouble?
3.given multilayer pcb what is the transmission line structure name for which we design the 100Ohm carateristic impedance line?
4. can difference in effective perimitivity cause cross talk?
Thanks.

 

[sophiecentaur]

.given multilayer pcb what is the transmission line structure name for which we design the 100Ohm carateristic impedance line?
4. can difference in effective perimitivity cause cross talk?

If you have doubts about the permittivity then you can't be sure of the Impedance so you can't be sure that the lines are terminated correctly. Mis-termination will cause imbalance and you won't know what's going on.
You have to sort things one at a time in order of priorities.
Permittivity is an intrinsic quantity. If you're getting unexpected results then I think you need to find out more about what would cause this suspected different effectivity? Maybe you need to go deeper into the basics as to where the change in performance can be explained in terms of a modification of the permittivity of the material.

 

[berkeman]

1.when we are designing multilayer PCB do we expect the effective permitivity to be such that we will get 100Ohm characteristic impedance?

For transmission line applications and circuits where parasitic capacitance is critical, you must specify the permittivity and the tolerance. Often, we would specify the exact dielectric material that should be used in our PCBs that had strict requirements like that. It should be part of the Fab Drawing in the Notes section, and you should be sure to make it clear to the PCB manufacturer that it is a strict requirement. Obviously the dimensions of the diffferent layers need to be specified and have tight tolerances as well.

4. can difference in effective perimitivity cause cross talk?

Perhaps, but I'd be more concerned with the poor signal quality (ringing, etc.) from the impedance mismatch with your terminations on the TLs.

 

[Baluncore]

1.when we are designing multilayer PCB do we expect the effective permitivity to be such that we will get 100Ohm characteristic impedance?

The man was Ohm, the unit is the ohm.
The term permittivity is spelled with tt, not t.
If you could NOT design a line with Zo = 100 ohm, then there would be a problem.

2.suppose my core by manufacturer permitivity but I get another effective permitivity is it a sign of trouble?

It shows you are using the wrong equation or geometry when calculating permittivity of lines.
Do you use US inches, mil or thou. Or metric, SI millimetres. Please give a reference, or a link, to the equations and line geometry you are using.

3.given multilayer pcb what is the transmission line structure name for which we design the 100Ohm carateristic impedance line?

There are several possible geometries. You have never answered a question we have asked about what geometry or equations you are using. You think we can read your mind when you write "differential lines". Your brightly coloured pictures are useless because you give us no detailed cross-section of the tracks or ground plane, or specify the dielectric material.

4. can difference in effective perimitivity cause cross talk?

No. But if you use the wrong dielectric material or geometry equations, then Zo will be wrong, so you will have a mismatch between the instrument and the lines, which will give you different S parameters, that will confuse you.

A. What is the dielectric material used for your PCB?
B. What are the equations for the Zo geometry you use?
If you do not answer questions, we cannot help you.

 

[yefj]

Hello Baluncore I am using the following calculator taken from the article the impedance .
I am using high speed pcb substrate with permitivity of about 3.1 as the manufacturere and the link below said hope its helping to understand better the situation .
The question is about physics causing parasitic effects which we not taken into account when designing the differtial line.

Suppose we have two edge coupled differential lines which by individual work fine.
However when the manufacturer makes the substrate permitity between the lines higher then usual then we get crosstalk between the differential lines.

few questions:
1.When I calculated the phase delay of the agressor and extracted its individual effective permitivity.
maybe the manufacturer made a situation where the capacitance between the differential lines got much bigger.Why my individual phase delay of SDD22 (agressor) cannot be used as a sign for cross talk?
2.What could be done to reduce this cross talk?

https://www.signalintegrityjournal....-is-differential-impedance-and-why-do-we-care
https://www.signalintegrityjournal....ts-physics-chemistry-of-copper-clad-laminates

 

[Baluncore]

The pictures you show are from the publication. They do not show the section you are actually using. How do you connect to a differential pair, and how do you terminate the pair. How do you maintain phase tracking between the two lines.

Your differential pairs may be too close together. The 3W/5W Rule: To eliminate crosstalk, the spacing between two separate differential pairs should be at least 5 times the internal gap of a single pair.

 

[yefj]

Hello Baluncore, thank you very much for the responce.
I am talking purely on a very delicate issue in physics which I think is playing a role here.
I imagined that the cross talk got stronger because the dielectric constant between the differential lines got bigger or the surface area of the conductors got bigger .basicaly its the capacitor formula shown bellow.

I measured the Phase delay of SDD22 ( the return loss of the differential line agressor).
because its return loss and the delay is twice (round trip) I divided the time results by 2 then I tool the real life length of this line and calculated the effective dielectrics , by the formula

few questions:
1.does the designer planning the effective dielectic constant (or phase delay) when designing a differential line?

3.can it show that if the effective dielectic is not what the designer ment then the manufacturer also ruined the dielectrics between the differential line?
Is there some truth in my method that if the effective dilectric constant is not what the designer ment then the parasitic capacitance between the lines got ruined by the manufacturer also?
Thanks.