Hi everyone, welcome to SDM, PCB Academy.
My name is Amirul.
In this video we are going to talk about what is high speed AI diagram.
We will see what is the purpose of using it and how to do different measurements on AI diagram.
For example jitter I height, width or the amount of distortion etc..
We will also discuss what is eye mask along with couple of simulations on cadence Sigrity topology explorer
17.4.
So let's get started.
We will start with very first question.
What is an eye diagram or eye pattern?
An eye diagram is used to get a good idea of signal quality in digital domain.
In other words, we can say eye diagram is superposition of all the data bits on an z or z channel.
Using eye diagram we can do following measurements like amount of distortion, time variation at zero,
crossing Best or worst signal to noise ratio.
Sensitivity of timing using I slopes, etc..
We'll understand these in more detail later in this video.
But first let's talk about the formation of an eye pattern from data Bits.
Now, before plotting an eye diagram, I just want to explain a few terminologies or basic terms we
use.
Let's say this is a crow screen.
And here on the vertical axis we have amplitude or voltage.
And on the horizontal axis we have time.
And the area where we plot eye diagram we represent it with TB or symbol duration.
We also call it a unit interval or we represent it with one UI.
So I diagram should be within this area.
Now let's say I'm sending following serial bits on a channel which is a polar Non-return to zero channel
for that.
Let's say the voltage scale is plus minus x volt.
Then one bit means we send or receive x volt, and zero means we'll send or receive minus x volt.
All right.
So till here we have drawn a polar energy channel for a particular serial data base.
Now let's plot an eye diagram for a crow for one time duration or one symbol duration.
We represent it with one TB, which will be nothing, just the overlap of all the data bits from polar
energy channel.
So let's do that.
For one it will go to plus x volt, and for zero it will go to minus x volt and all the symbols will
be overlapped.
So that's it.
This is the eye diagram or eye pattern that we have created from energy channel.
And here it is fully open.
Or you can say an ideal eye because there is zero distortion.
or you can say no AC problems.
Similarly, we can create an AI diagram using polar channel in polar return to zero channel.
We represent one with plus x volt and zero with minus x volt.
But the difference is signal should start from zero and return to zero within one sample duration or
one TB.
As you can see in your screen for creating an AI pattern using AWS representation, the process is similar
to NS.
We have to overlap bits from AWS channel.
For example, one will go like this and zero will go to minus x volt and return to zero.
So this is also an ideal AI diagram because of no distortion or no AC problems.
Now I am going to give you one example of non-ideal AI diagram using polar channel.
So as you can see on the screen we have ISI problem on the channel which is represented with highlighted
part.
Now in this case, if I will overlap bits, it will either start before the symbol duration or finish
after symbol duration.
And in result we can see clearly the distortion on the eye pattern.
So this was ISI problem on the channel.
All right.
In the next step I am going to demonstrate a good and bad eye pattern on Security Topology Explorer.
And then we will talk about measurements in detail.
For that let's open Topology Explorer 17.4.
This is LVDS topology.
And I am following all the design guidelines for the signaling scheme.
So let's run the transient analysis and expected eye pattern should be clear with minimum distortion.
So let's click here to check the eye pattern.
And as you can see the distortion is very minimum here.
On the other hand I am I am going to simulate a topology with impedance discontinuity.
So let's do that as well.
As you can see here in this topology we have impedance discontinuity on the transmission line.
And we have added a VI as well.
So let's run the transit analysis and see the results.
So as you can see the waveform is also not good.
Let's plot for eye diagram.
And as expected the eye is collapsed due to impedance discontinuity.
Now in the next step I am just going to talk about all the measurements that we can do on eye diagrams.
In this section I'm going to talk about different measurements we can perform on eye diagram.
First one is amount of distortion.
So distortion on eye diagram is due to ISI or due to noise appears as closure of eye patterns.
So it should be less on eye diagram as you can see on your screen.
Another measurement that we can perform on eye I diagram is time variation at zero crossing.
We also call it timing jitter which measures the time deviation from ideal timing of a data base.
So we represent it with delta t.
As you can see on the diagram for calculating the timing jitter peak to peak percentage we use a formula
delta t divided by TB.
So here TB is symbol duration multiplied by 100.
Third one is signal to noise ratio at sampling point.
So it is nothing but the ratio of desired signal level 0 to 1 divided by levels of background noise,
plus any distortion on it.
So either you can use this formula to calculate the signal to noise ratio at sampling point, or you
can directly measure it on I diagram as shown in the figure.
Fourth, measurement that we can perform on I diagram is sensitivity in timing errors.
It is the rate at which eye closes and the sampling time varies.
Using this rate we got to know the know the possibility of timing errors on the channel.
Measurements of sensitivity in timing errors is very simple.
We need to measure this slope.
As you can see in the figure.
To estimate timing errors and error increase as slope becomes more flat.
Next is mean of signal.
It is half of ideal signal or peak to peak amplitude.
As you can see in the eye diagram.
Another measurement that we can perform on eye diagram is eye width.
So if width is very important measurement, it tells us the time over which signal can be successfully
sampled.
From this measurement we can verify the measure time with minimum sampling time given on the data sheet
of receiver.
Last but not the least is amplitude jitter or amplitude noise, which is the noise at one crossing.
As you can see in the eye diagram.
So that's it for this video.
In the next video I will talk about eye mask and how to create and apply an eye mask using data sheet.