PCB form-factor requirements are getting smaller and smaller, which means
we're designing our layouts with traces closer and closer together. But if we put
them too close, they can interfere with each other causing the receiver to get
incorrect information. In this episode we will be walking through how the effects
of crosstalk can help you make important design and redesign decisions.
Hi, I'm Melissa and welcome to Scope's University.
If you have traces too close together they can interfere with each other. Serial data can disrupt power
lines, power lines can disrupt serial lines, and serial lines can interfere with
each other. Think of a self-driving vehicle. What if a serial packet was sent
that was meant to turn a vehicle left by 90 degrees, but because of interference
from another trace, the receiver saw this. And what if this packet means to turn
the car right by 90 degrees. Obviously some protocols have more strict
margins than others, but you get my point. We want to send accurate signals.
When signals on a board interfere with each other, we call this crosstalk.
Luckily, you don't have to wait for your components to malfunction or your
self-driving prototype to drive itself into a lake to find out if you have an
issue. I'm going to show you how you can use Keysight's crosstalk application to
find crosstalk and model how much better your designs could be without it.
The crosstalk app can analyze up to four signals at once, so, if you have
multiple traces close together you can test them all. On my board, I have three
serial data lines that are very close together and I think that trace in the
middle is at risk with interference from the traces on either side of it. My
middle trace is the victim signal and the ones on the outside are called the
aggressor signals. I've connected the victim to channel 1 and the aggressors
to channels 2 and 3. First I'll create an eye diagram to see what my margins look
like and find out if there is really a problem. I've centered my waveforms on
screen and vertically scaled them. So now I'll go to analyze, choose quick eye
diagrams, and channel 1. This shows me what the eye diagram is of
the victim signal. You can see the eye opening is very small which means I do
have errors and could be sending false packets, potentially sending my drone
right into my coworker instead of up into the sky.
But redesigning and building a new PCB board can be extensive, a lot of hard
work, and very expensive. So to find out if it is worth it to go through all of
that, I could have my crosstalk application quantify the amount of
crosstalk on my victim, then remove it so I can view how my signal would look
if I had a better layout. To you set up my crosstalk application I'll go to the
analyze menu and choose crosstalk. I want my channel 1 signal to be set up
as the victim and I'll have the scope automatically configure my thresholds
and clock recovery. I'm going to set channel 2 as an agressor signal. It's
serial data and again I'm going to have the scope Auto configure that. And
channel 3 is also an aggressor so I'll just go through the same steps. Now going
back to signal 1, I'm going to remove crosstalk from signal 2, signal 3, and any
unknown crosstalk or noise that could be occurring. Now I'll just enable this
application and let the tool run. This red trace is now signal 1, the victim
signal, with all of the crosstalk from channels 2 and 3 and any unknown
crosstalk removed. Already we can see improvement. I'll create an eye diagram
of this function to find out just how much it can be improved. To create an
eye diagram on a function I'll go back to my analyze menu and I'll choose
gallery, and I'll scroll down to this real-time eye, and here's my function
crosstalk 1 and I'll turn on the eye diagram for that. Now you can see just
how much more open the eye actually is compared to our original signal as is.
To quantify how much more open the eye is, I can add some measurements.
We'll go to the measurement menu, add measurements, and over here I'm going to
choose eye, and then we'll add eye height and eye width. Down here are my
measurements so my eye width is 74 ps and my eye height is only
27.7 mV. Let's see how that compares to the
signal with crosstalk removed. I'll go to measure, add measurement, and we'll add
eye height - first I have to choose my source - so do this on crosstalk 1. We'll
apply eye height and we'll apply eye width. If I redesign my board to
improve the layout and get rid of the crosstalk my eye width could potentially
be 125 ps compared to the 88 ps we have now.
Additionally the eye height is 109 mV, whereas the eye height as it
is in this design is only 13 mV. After seeing how much better my eye
diagram is with the crosstalk removed, I think in this case it might be worth it
for me to redo my layout and find a better route for these traces and I can
use these measurements to justify the extra budget I need for a redesign. If
you think this analysis tool would be useful to you, I encourage you to check
out the link in the description below.
This episode concludes the first
season of scopes University. Thanks to everyone who tuned in and watched. If
you're excited about the next season of scopes University or if you have any
questions we can address in future videos please comment below .Until then, I
hope you've learned a lot here at Scopes U and we'll see you next season