Hello everyone!
Welcome to Typekit YouTube channel.
My name is Amirul and in a previous video I talked about what is impedance and how to bridge real and
ideal world to get better results on modeling and simulation.
And we have also discussed why we don't want any impedance discontinuity on our transmission line.
Now further on this today we will talk about what is single ended impedance and how to estimate impedance
profile for one or more than one tracks using cadence cross section editor.
So let's get started.
We will start with very first question.
What is single ended impedance?
Single ended impedance is same as instantaneous and characteristic impedance of a single ended signal,
and single ended signals are signals which are not coupled with any adjacent signal.
Now in the next topic, let's talk about types of single ended signals and their impedances.
First one is microstrip single ended signal, which has signal on the top layer and return plane on
the bottom layer to estimate the characteristic impedance of this type of signal, you can use this
equation or you can use cross section editor to estimate it.
I'll show you later in this video.
Second one is Stripline single ended signals on a strip line.
We have signals sandwiched between two return plane and to estimate the characteristic impedance, you
can use this equation or you can simply use cross section editor.
Apart from these two, there are two more types of single ended signals.
One is embedded microstrip and another one is asymmetric stripline.
For that you can refer this image.
And here they have given their formulas for calculating characteristic impedance.
Let's move to demonstration part of single ended impedance using cadence PCB editor and cross section
editor for that open PCB editor 17.4.
And this is the board file on which we are going to route a couple of tracks.
So as you can see all these unrouted tracks are 50 ohm impedance track here and here.
So we have four tracks that we need to route.
Let's start with width calculation for these 50 ohm tracks for that open cross section editor.
And click over Primary View.
Now if you go to the left side here you will find an tab or column which is signal integrity.
Just click over here.
All right.
And here you will find we have added a impedance value.
Based on that we are getting width for example.
So as you can see here as of now we have added 50 ohm impedance.
If I change it to 100 ohm the width will vary.
So using this calculator we can able to calculate what should be the width of track in a mill for a
particular target impedance.
So in our case we are going to route a 50 ohm track.
So that's why I have noted down these three values for all the 50 ohm impedances on different layers.
Now as you can see for the top layer the value is manufacturable.
The width is manufacturable, but for layer two and bottom layer it is little bit at the high end side,
which is 110 and 124mm.
All right.
So that means this width is too high for routing a track.
So that is how we got to know on which layer we are going to route for a particular stackup.
So we are going to route our 50 ohm track on top layer.
Now we'll click over apply and okay.
Now in the next step we'll going to define constraint set for these four nets.
All right.
To do that open constraint manager.
And here you have to go to physical and all layers.
And under this we have to define the constraint.
To do that you have to right click over DSN.
Go to create and create physical Cset.
You can name it 50 ohm.
And okay.
So after creating constraint set of 50 ohm Under that, we'll get all the layer information that we
have defined during layer Stackup.
But in our case, we have to put all the information for top layer L1 and bottom layer for linewidth
that we have calculated during the impedance calculation.
So I'm just going to put that quickly.
So once you have added all the details for top layer L1 and bottom layer we'll just close it and click
over knit all layers.
Now here we have to find all the four nets that we we are going to route and change its constraint set
from default to 50 ohm.
So there are four nets.
One is P4 and p5.
So these are the two.
And here if you select you will get 50 ohm option.
You have to just select it.
So once you select 50 ohm you will get all the three layer track widths are already defined here.
And same will do for P5.
Five.
Another two nets are net.
R 34.
Underscore two.
So it should be somewhere here.
This is the one and one is R 36.
Underscore two.
All right.
So here we have assigned the correct constraint set to all these nets.
Now let's close it.
And we are okay to route to route it.
First you have to just zoom in a little bit.
Just click over all on select the pad and press F3 on your keyboard.
So it will automatically pick the correct width that we have defined on Constraint Manager.
So this is how we can route a 50 ohm controlled impedance tracks.
I'll put that project link in the description.
You can download it from there and practice whatever we have learned in this video.