Hi everyone, welcome to PCB Academy.
My name is Avril.
In today's video we are going to talk about what are modes in a coupled transmission line, some application
of modes and waveform patterns of differential and common signals.
Then we will quickly go through a couple of equations to estimate differential and common impedances,
even an odd mode impedances along with their voltage components.
So let's get started.
We will start with very first question what are modes in coupled signals?
To understand it better, we'll see three cases of differential pairs where we will change input of
coupled line and try to understand the effect on signal received.
Let's see the first case here we have two signal lines.
Let's say signal one and signal two.
Signal one has voltage level 0 to 1 traveling down the transmission line, and signal two stays at zero
volt.
Now, as soon as signal starts traveling, there will be crosstalk because of voltage change.
And due to that, we'll see this type of response at t one time.
At t two time it will be more steep and will see more voltage transfer between signals, and at T3 and
T4 the crosstalk will increase.
Now this is happening because of far end crosstalk and near end crosstalk.
If you wanted to understand this concept, I have already created a video on this.
You can click on I button and watch it.
But at this stage, this point is not important.
In case one.
As we have excited on signal line and zero volt constant at second coupled transmission line, we have
observed whatever the signal we have sent on SG one or line one, it is not equal to the signal received
at the receiver side.
In the next step, I am going to discuss two very special cases and those will define these modes.
So let's see those.
Case two is when we apply same voltages on both each coupled transmission line at the same time.
Let's say it's signal one and signal two and voltage applied v1 and v2.
So in this case V1 should be equal to v2.
Since signals are traveling together.
So there will be no change in voltage or current with respect to each other.
If we probe at the both input and at the receiver side, we'll observe after t time delay receiver's
voltage should be equal to v1 and v2.
This is a special case.
We'll talk about it more later in this video.
Let's move to another case.
Now let's talk about another case where we are sending 0 volt to 1V on first line and 0 volt to -1V
on second line, and the condition is same.
Line one and line two are charge coupled.
So as signal propagate down the transmission line, both signals will generate noise on each other but
opposite in polarity.
So you can say the resultant noise at the receiver side will be negligible and the voltage pattern will
be undistorted.
So now what will be net voltage.
We'll see at the other side of coupled transmission line.
And that will be one volt minus -one volt.
And net voltage will be two volt as you can see in the figure.
So this is another special case that we are going to refer to define modes for these two special cases
case two and case three.
The voltage pattern will get at.
The output will be undistorted.
And we call these special state of signals.
Mode of pairs.
When a differential pair is excited into one of the two modes, either case two or case three, signal
follows special properties and remains undistorted.
So, as we have already seen, when we drive same voltage on both lines, we call it even mode, and
when we drive opposite voltage on both transmission lines, we call it odd mode.
As we have already discussed, odd and even mode are special case which propagate signals undistorted
on the line.
For example, as we have seen in case of differential signal and impedance.
Tutorial.
Impedance.
A signal sees depends on coupling of adjacent trace and voltage pattern on the other line as well.
Now, using odd and even mode terminologies, we can refer to these impedances.
For example, we can refer a impedance of one line as OD mode impedance.
If signal follows, od mode states.
Similarly, if signal follows even mode states, we call impedance of one signal line.
Even mode impedance.
Now things will become more complicated when the geometry of each coupled transmission lines are not
identical.
So that will be our advanced case.
We will talk about it in later videos.
Now in the next step I am going to talk about how to estimate even and odd mode impedances.
So as we have already talked about differential signals before, we have seen that differential impedance
is series combination of impedances of each line with respect to return path.
So as you can see in the diagram here z equivalent is equal to z differential.
And where z differential is two times z odd.
Z differential is Differential is differential impedance and z od is characteristic impedance of one
line.
When pair is driven in OD mode and from there we can define z od is half of z differential.
Next we will see what is common and even mode impedance.
So as we all know, the common signal is average voltage between signal lines.
And when a common signal drives a coupled pair, we call it even mode state and the characteristic impedance
of one line.
When pair is even more driven, we call it even mode impedance.
As you can see in the diagram here, z equivalent is z even parallel to z even where z even is characteristic
impedance of one line.
And when we further solve it, we'll find out Z even is two times of z common.
So we can use this relation to find out the even impedance or the even mode impedance of common signals.
Now let's take a quick example.
If we have two 50 ohm edge coupled transmission line.
In first case, if they are driven by event states then z event will be 50 ohm and z common will be
25 ohm.
But if they are driven by odd states, then z odd will be 50 ohm and z differential will be 100 ohm.
All right.
So it is that easy to find out even an odd mode impedances.
Now at last we will see how to find out v common and v differential.
So v common is average of voltages on each coupled transmission line and V differential is the differential
voltages.
So let's say we have a coupled transmission line which has V1 and V2 voltages on each line.
Then V common will be one by two summation of v1 and v2 and v differential is v one minus v two.
So that's it for this video.
I hope you got the concept of even and odd modes.
See you in the next video.