Evaluation of crosstalk

[Tomsik]

I've designed PCB for testing crosstalk, there are some traces like aggressor/victim with multiple gap cases.
Stackup up of PCB is standard JLC7628 from JLCPCB manufacturer.

The first idea is to compare software calculator "PCB Toolkit" with my test PCB.
I've got calculated coupled voltage 1.65V, I wanted to measure this voltage on my test PCB.


There is a test connection. All oscilloscope channels have 50Ohm impedance. Test signal has 10Mhz and approximately 5ns edges.

On scope, I've got 116mV pk-pk as NEXT and just 15mV pk-pk as FEXT.
I just want to see a correlation of my measured result with the calculated coupled voltage. I can't imagine what coupled voltage means, how to measure it that I can get the notion.

Is there anyone who can explain what I calculated and what I measured?
Because calculated 1.65V looks very horrible but measured 116mV pk-pk is correct in my opinion...

btw. I've tried to change impedance in the oscilloscope channels but the measured pk-pk voltage is still under the 200mV.

Thank you

[Terry Bites]

The current coupled by both magnetic and electrostatic effects will be very small. Lets say there is 10pF between adjacent traces, a current of less than 500nA will  will be flowing@10MHz. 500nA through 500Ohms is 0.25mV. I doubt the scope can resolve that A low noise high gain AC coupled amplifier needs to be placed between the termination reistance and the scope.

[Electro Fan]

Very cool experiment / project 

[jmelson]

The current coupled by both magnetic and electrostatic effects will be very small. Lets say there is 10pF between adjacent traces, a current of less than 500nA will  will be flowing@10MHz. 500nA through 500Ohms is 0.25mV. I doubt the scope can resolve that A low noise high gain AC coupled amplifier needs to be placed between the termination reistance and the scope.

His simulation shows a trace-trace spacing of 0.2mm which is REALLY close for 50 Ohm traces.  magnetic coupling will dominate.
A 3.3 V pulse into 50 Ohms will deliver current of 66 mA!  I was just working with a system using fast 1.5 V photomultiplier pulses and having quite a bit of crosstalk, with signals never closer than .1" and mostly a lot farther away.  I think a lot of the crosstalk is on the coax grounds with the currents I was using (about 20 mA).
Jon

[Tomsik]

The reason of the experiment was comparing calculated results and measured voltage on victim line. I wanted to see correlation between this...
In fact, I don't know what does calculated coupled voltage mean. I want compare it in real measurements.

[Tomsik]

I'm just reviving the topic. Is there anybody who can explain it?

[eTobey]

I am really surprised, that i cant find much information about crosstalk in this forum.

Additionally to the wrong calculation of the above software, i found crosstalk calculators (at least 2 of them - https://www.eeweb.com/tools/microstrip-2/), that have wrong values below 16ns rise time. The value just stays at the value of 16ns.

And about the values: I found this:
https://www.eevblog.com/forum/eda/crosstalk-analysis-altium-vs-the-world/msg941984/#msg941984
The same value as here (in the 2nd picture of that thread)

I guess noone really talks about crosstalk because its not a problem, or has been identified as a problem?

Here is a good video for it:


@Tomsik:
Have you gotten further with your tests?