ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
INTRODUCTION TO ADS
For the MsC Courses led by the
Centre for High Frequency Engineering
Dr Roberto Quaglia
quagliar@cardiff.ac.uk
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Scope of this lecture
To import simulation results into ADS circuit simulation; to improve the accuracy of EM simulations.
Objectives
- Simulate network parameters and plot them
- Create a layout using the PDK
- Setting a Momentum simulation
- Launch a simulation, look at results, use the results in circuit simulation
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
EM Simulations
The previous lecture arrived at this point. We have generated a layout of a MIM capacitor by using the
Create Layout function; the layout was generated because the design kit has a predefined link
between the circuit element and its layout drawing. Remember to put ports in!
Drawing layers can be associated to EM simulation layers in Momentum using the Substrate
Definition.
We defined the calibrated ports (TML) and the EM simulation setup (frequency range, mesh) and we
launched the simulation. We obtained a plot of scattering parameters.
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
The questions were:
Do they make sense?
Is the result the same as before?
In reality we were not too concerned with S-parameters, but we mainly cared about our capacitor
showing a desired capacitance value at a target frequency. How can we see if the value makes sense?
One option is to calculate the value of capacitance directly in the data-set. This operation is not
straightforward, though, since the capacitor has been simulated as a 2-port in the EM, while we
simulated as 1-port in the circuit.
However, by assuming that the dominant capacitance is the one we are simulating and the capacitance
to ground is negligible, we can calculate the equivalent capacitance by first transforming the S matrix
into a Y matrix, and then the admittance of the capacitance is the parameter -Y12:
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
So, this is a possibility of check your EM simulation result of the capacitance.
Using EM results in Circuit Simulation
It might be handy to be able to use the EM simulation results in your circuit simulation; like having a
subcircuit that instead of having the components inside it, has the EM simulation values that are the
network parameters.
I can think of at least three ways of doing it.
Data export using Touchstone file
This method is useful since it can be used also for network parameters coming from other simulators
or measurement systems.
First, let’s create the Touchstone file from the simulation results. In the data display window of the
EM simulation, you can click on the Data File Tool Button:
This opens the window where you can select you want to export a file, select the dataset you want to
export (the name of your EM layout followed by _MomUW_a) and then a name for your output file:
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
By default, the file will be saved into the data folder of your workspace folder.
The Touchstone file is a standard data format for exchanging network parameters data. The default
file extension is of the type .s2p; “s” stands for scattering, “2” because it’s a 2-port, “p” for parameters.
If you open the file with any text editor, you can see it is indexed vs. frequency, and each pair of
columns represents a network parameter (in a format defined in the header).
Software like ADS, MWO, and measurement instruments are able to import/export Touchstone files
with basically no effort.
How do you use the file in the circuit simulator?
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Let’s Save As your “CapacitorSimulator” schematic where you were simulating the PDK capacitor at
circuit level in another file “CapacitorSimulator_CompareEM”. Then let’s put another Term:
Then we need to place a component that allows us to use the Touchstone file as it was a Subcircuit.
This component is the part of the Data Items components and it is called SnP:
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Double-click on the component and its window opens:
You can browse and select the simulation file with suffix .s2p; it will automatically recognize it’s a
Touchstone and it has 2 ports, then click Apply and OK; connect the component to the Term2 and to
ground:
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Let’s simulate, and calculate the capacitance in the circuit and EM cases (consider that Term1 is not
simulating the circuit model, while Term2 the EM “subcircuit”):
You can see that the results are very close, not perfect, but not too bad!
Remember, if you are re-simulating the EM changing some parameters in the simulation, you will need
to export a new Touchstone file and point to it in the simulation.
Data export using Dataset
If you are within ADS, you do not really need to export to Touchstone before using network
parameters inside your simulation. From the SnP component window, select as File Type the Dataset,
then browse and select the Momentum simulation file with suffix .ds:
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Click Apply, OK and repeat the simulation; the results should not change from before.
Remember, with this method, if you re-simulate the layout component, you will overwrite the dataset,
therefore a new circuit simulation will use the new results.
Data export using EM Component Model
A third method uses a Momentum feature. Once the EM simulation is finished, you can go on the
Layout page and do EM→Component→Create EM Model and Symbol
How do you use it in the schematic?
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
You must insert the CapacitorEM as a subcircuit; go to the CapacitorSimulator_CompareEM
schematic, and do Insert→Component→Component Library and look for CapacitorEM; drag and
drop it into the schematic:
It will appear as a rendered version of the layout with pins available where the ports are.
Substitute it to the SnP component.
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
As it is, this is just a symbol for a subcircuit; we need to tell ADS we want to use the EM results of this
component; right click on the component, and do Component→Choose View For Simulation
Then select emModel and OK:
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Run the simulation, it will be the same as before!
Remember, if you re-simulate the EM component and want the changes to reflect in the circuit
simulation, you must do EM→Component→Create EM Model and Symbol again. Be careful pins might
move in the circuit and cause a disconnected component!
Simulating a capacitor that is really “grounded”
The ground component is an ideal component that represents a zero-volt reference. In our microstrip
structure the ground plane at the bottom of the stack is the best ground approximation we have, but
we must reach it to “ground” a component. To do so, a via hole is used.
A via hole is a plated hole in the substrate that connects the top layer to the bottom layer, in our case
the bottom layer is ground.
In the design kit there is a predefined via-hole component that we can use. Let’s Save As the
CapacitorSimulator_CompareEM schematic in CapacitorSimulatorViaHole_CompareEM. Let’s place
the via-hole from the design kit instead of the ground component:
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
And simulate, calculating the equivalent capacitance:
As we can see, the value of the capacitance changed quite a lot. This is due to the effect of the via-
hole; the via-hole has physical dimensions that in first approximation can be modelled as an
inductance. This means that the series resonance that we could already see due to the real MIM
capacitor is brought down to even lower frequency.
Try to substitute the via hole with an inductance and estimate its value.
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Let’s now simulate the via-hole in the EM simulation. Save As the CapacitorEM schematic into
CapacitorViaHoleEM. When prompted, tick the box Save Entire Cell. This will save not only the
Schematic view, but also all others associated with the cell.
Add the via hole: we have to options:
- 1. Connect the via hole on the bottom metal side of the capacitor
- 2. Connect the via hole on the top metal/air bridge side of the capacitor
Let’s start with option 1. The bottom metal side of the capacitor is indicated by the MET1 pin.
Let’s create the layout.
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Unfortunately, as you can see, the layouts of the capacitor and the layout of the via-hole are
overlapping. This might create problems since via holes are usually topped by a stack that includes
both bottom and top metal, meaning that a capacitor cannot be built on top of them. We can confirm
this by looking into the 3D visualization:
How can we solve this? We need to add a piece of line to distance the capacitor from the via. Let’s
take a measurement in the layout (CTRL+M)
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
There are 30.5um from the centre to the edge of the via hole, so we need some more to be sure we
disconnect the top metals of the capacitor and the via holes.
Let’s go back to the schematic, and add a 35um piece of line between the capacitor and the via hole.
Select MET1 for the layer, 35um for the length, and around 30um for the width (considering that our
capacitor has a width around 35um)
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
Generate the layout: you can see that now the two components are separated; verify that in the 3D
view.
If you Saved As, the settings you did last time (ports, frequency, mesh) should still be valid. Since we
are going to simulate a prominent rounded shape (via hole) let’s set in the EM Setup also the
Options→Preprocessor with an arc resolution of 15 degrees.
ENT672 Introduction to ADS – Lecture 5 Roberto Quaglia
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Cardiff University – School of Engineering – Centre for High Frequency Engineering
And then Simulate. If prompted, click Yes or OK until simulation starts. Being a more complex
structure, it will take time to simulate.
When finished, compare the simulation result with the circuit simulation you made.
Retune the capacitor dimensions to achieve the target 0.4 pF at 32 GHz.