Hi everyone.
Welcome to Steam Academy.
My name is Avril.
Today we are going to talk about what is crosstalk in a transmission line in terms of aggressor and
victim nets and what are the sources of crosstalk in a design.
Then we will discuss what is near end and far end crosstalk, and how to reduce it in a transmission
line, along with a couple of simulations in security Aurora 17.4 and Topology Explorer.
So let's get started.
We will start with first question what is crosstalk in a transmission line?
As its name suggests, crosstalk is transfer of an unwanted signal from one net to adjacent net, and
it occurs between every pair of net which are at close proximity of each other.
The term net we are using for signals and return path both.
We will understand that in more detail while discussing ground Now here we are using few terms for the
source of noise.
We call the aggressor net or active net.
For the net on which noise generated due to crosstalk, we call them quiet or victim nets.
As you can see on your screen.
Now let's talk about what is the origin of crosstalk, how this coupling occurs between two parallel
running transmission lines, and what is the role of fringe field which cause capacitive and inductive
coupling?
First we will talk about fringe field.
If you can recall from return path video.
There I have explained how electric field and magnetic field line is spread between signal and return
path conductors.
These field lines are not just confined to return path.
This spread of field lines affect the surrounding conductors as well.
We call them fringe fields.
As you can see on the screen, if we put quiet net closer to active net, the fringe feel effect will
be more, but it will increase the pitch between these two parallel running tracks.
The effect of fringe field will reduce.
So we can say that the effect of fringe field is inversely proportional to pitch between parallel running
tracks.
The only way noise will be picked up by victim net or quiet line is when there is change in voltage
or current in aggressor or active net.
And that change in voltage and current cause change in electric and magnetic field, which will induce
current in adjacent conductor.
So as a result these fringe field arise small capacitance and inductance between two parallel running
tracks.
And we call them mutual capacitance and mutual inductance.
Solution.
To avoid fringe field noise or to reduce mutual capacitance and inductance, we have to place nets far
from each other.
In the next step, I'll give you brief about the simulation of crosstalk in Aurora 17.4.
And then we will talk about concept of near and far end crosstalk.
So let's open security Aurora 17.4.
And here I have done the simulation already.
As you can see I have simulated for one byte group from Parallel Data Bus zero to Parallel Data Bus
seven.
And the simulation highlighted all the part which has maximum crosstalk.
And as you can see, parallel data seven and parallel data five are highlighted.
We have simulated for parallel data bus seven.
So that means parallel data seven is the victim net and parallel data five is the aggressor net or active
net.
From this plot also we can verify the parallel data.
Bus seven has maximum crosstalk of 110.3 millivolt.
And if I plot for parallel data five as well to see why there is a crosstalk, you can see there is
a clear transition of 1 to 0, and due to that there is a current induced on parallel running tracks.
Data seven.
Similarly, you can plot for other victim rates.
And here we will get all other segments which are running parallel to parallel data.
Seven.
So in case of XU2 where we have all the list of nets.
And similarly for u seven.
So by selecting each net you can verify how much crosstalk is there because of this aggression net.
Another way to verify is just hover your cursor there.
So it will give you all the information for each segment.
So if you want me to create a detailed video of step by step process, how I have done this simulation
for any layout, let me know in the comment section.
Now we are good to go to discuss about near end and far end crosstalk.
Next let's talk about near end and far end crosstalk between two parallel running active end coordinates.
So let's assume a setup where we have active net and quiet net.
And here both ends of the quiet net are terminated to avoid any reflection on the line.
And similarly for active net, the other end of the signal is terminated to avoid reflection on that.
So in this setup the measured noise voltage has a very different pattern for the noise in both ends
of the victim nets.
So to distinguish between these two ends, we label the end nearest to the source as near end of the
victim net and the noise induced here we call it near end crosstalk.
And similarly the end farthest from the source we call it far end of the victim net.
And the noise induced there we call it far end crosstalk.
Similarly we can define near end and far end in terms of direction of signal traveling Far end will
be at the forward direction of signal propagation and near end as the backward direction of signal propagation.
Let's talk about probing for near end crosstalk and far end crosstalk on victim nets.
So when we connect oscilloscope on near end to observe the crosstalk we'll see a pattern like this.
The near end noise rise up quickly and stays there for few nanoseconds and return back to zero.
So on the active net, if we are sending 200 millivolts of signal at quiet net, the level is approx
13 millivolts and which is about 6.5% of total signal level.
On the other side at the far end will get really strange response.
So if we connect crow and observe the noise for same active net parameters, the noise on quiet net
will be in different shape and its amplitude will be approx 16 millivolts.
So there will see a sharp dip which is 30% of the signal level on active net.
Now I'm going to tell you some equations to estimate near end crosstalk and far end crosstalk.
We can estimate the ratio of voltage on quiet line and voltage on signal line in terms of mutual capacitance
and mutual inductance.
So the equation will be voltage on quiet line divided by voltage of signal on active track is equal
to one by four CML divided by CL plus LX divided by LX.
So here CML and Llml are mutual capacitance and mutual inductance per unit length of coupled transmission
line.
And CL and LX are as you know, it's a capacitance and inductance per unit length of active line.
So the ratio of voltage on quiet line divided by voltage on signal line of active net.
We represent it with KB, which is known as backward coefficient or Near-end crosstalk coefficient.
And similarly you can estimate the far end crosstalk coefficient, which is represented by KF, and
it is the ratio of voltage at far end of quiet line and voltage at signal line of active net.
It will be equal to one upon two v.
So here v is the speed of signal on active net multiplied by CML divided by CL minus L divided by L.
So these are mutual capacitance and mutual inductance and capacitance and inductance per unit length.
And here V is the speed of signal on active net.
So this is how you can estimate the far end crosstalk coefficient.
And from that you can calculate the far end crosstalk.
All right.
So now I'm going to summarize this video by telling you couple of ways or methods we can use to reduce
crosstalk between parallel running tracks.
And the first one is you have to increase the pitch between signal tracks.
Second is you have to use solid planes without any cutouts on the return path to avoid any ground bounce.
Third one is keep coupled length short.
So there will be very minimum crosstalk between those.
Fourth is you have to route on strip line layers if you can.
Fifth is do not share return pins in package or connectors because it will also lead to crosstalk.
And last but not the least in case of high isolation required route tracks on different layers.
So that's it for this video.
In case you want me to create a video where I cover step by step process of simulations, how I have
done the simulation on Aurora 17.4.
Let me know in the comment section.
See you in the next video.