Part a - The Momentum 3D Planar EM Simulator in Advanced Design System

In this short video on ADS Momentum, I’ll cover a quick overview including the steps
for using Momentum. And I’ll show you a setup and layout to demonstrate how Momentum
works and its value, and that will be seen when we look at the results in the Data Display.
Finally, a built in example of a spiral inductor.
This list is an overview of ADS Momentum. It describes what it is and what it can do
for you. So, basically it’s an electromagnetic solver. It runs out of ADS layout and you
can get S-parameter results so you can have a model for any passive structure that you
can draw and layout and set up correctly with Momentum.
It does use, as you can see here in this list, Method of Moments. It solves passive structures
that are three dimensional, with some restrictions, it does vias easily. It uses precomputed greens
functions, so it can go quickly if you have everything done in advance. A mesh pattern
or grid of cells is set up over each structure, and you’ll see that, we call it a mesh,
and then each cell is solved with relationship to all the other cells, both in the X and
Y direction. So it can do coupling extremely well.
You can visualize current, look at Far Field patterns. You can create lookalike components,
and basically this means that if you draw, for example, a spiral inductor and solve it
in Momentum, you can then have that drawing put back in schematic and it looks exactly
like this spiral you drew. So it’s basically a symbol for your Momentum-solved spiral inductor,
or anything else you draw. And we can do co simulation.
Now, we’re not going to focus on some of these details, we’re going to cover the
basics of using Momentum. So, let’s look at the steps next that you’d take to do
a Momentum simulation.
Here are the basic steps for using ADS Momentum. Now I want to point out that we have a Momentum
course that covers all the details, all of the different types of structure, substrates,
vias, 3-D, coplanar waveguide; that’s all covered in the Momentum course. But for this
video we’ll use a very simple example.
Now you can start in schematic in ADS, and if you do all the components require footprints
or artwork, and that’s automatically generated into ADS layout. So beginning in ADS layout,
and you can even import files from other drawing formats, you make sure that you have ports
connected to your structure and then you define the layers. The layers include both the substrate
material where we define that material and you can have multiple layers, and then the
mapping of all the individual drawing layers to the substrate layers.
Once that’s done the Momentum engine will then create the mesh pattern, that’s what
you see here. A series of individual cells or grid pattern where all the L, R, and C
values of each one of these is calculated with respect to all the other cells in both
the X and the Y direction, and that includes coupling that could exist between closely
spaced lines. And the result after the analysis is run is an S-parameter model for your passive
layout.
Now let me show you how this is actually done in ADS. I’m beginning in schematic, and
I’ll create the layout from the schematic design. I’ve taken elements from the microstrip
palette; these include microstrip lines or MLIN’s, and microstrip corners. There are
two ports grounded here for the S-parameter simulation so we’ll get the results of this.
Now, creating the layout is simple because the command layout/generate/update layout
automatically creates the layout and that’s because we’re using artwork generated from
ADS with these microstrip lines. Here’s the result. As you can see, we have the S
meander line and the width and the spacing is the same. So I want to go back to schematic
for a moment and show you how I set this up, and then we’ll look at the results that
the circuit simulator gives for this design.
To create the spacing this MLIN and this MLIN use a variable for the length, as you can
see here. L equals S mils, and then I simply set the variable here, S equals 10. That results
in 10 mils here and 10 mils here for our spacing.
X 10 is the width, and you can see X mils, X mils, and that’s for the entire meander
line. So by having this set to a variable I can go back and change the spacing easily
and that’s how I will see the value of Momentum for this design where there can be coupling
between closely spaced lines. Now I’m going to run the simulation and plot the results.
Here in the Data Display I’ve plotted the results in a rectangular plot, dB of S21.
So this is the transmission measurement through our S line and you can see there is 1 dB or
less of loss, and it varies across our frequency sweep from 1 to 10 GHz. Now, let’s go back
to layout and set up a Momentum simulation and compare the results from the circuit simulator.
So I’ll be plotting both traces on the same plot and we’ll see if they match.
So let’s begin here in layout where you see the Momentum command. If you click on
it the first menu choice is to enable the RF mode. If you have a circuit that’s lower
frequency, electrically small, then you want to use the RF mode. It’s a little different
solution process, but it works very fast. Because we have a higher frequency circuit
we won’t enable that. So keep in mind this is just a toggle, and again all of this is
covered in our ADS Momentum course.
So the first thing we’re going to do is set up our substrate, and to do that we’ll
simply update it from schematic. And then we can look at by going to this command, create/modify,
and here is the substrate menu. It shows both the substrate layers and the layout layers.
If you recall, our thickness was 5 mils, here’s the ER of 9.6, and that was simply transferred
from schematic into Momentum, and you can see the values here. Here in the layout layers
tab, the drawing layer, which was COND, and they’re all listed here, that microstrip
drawing that we have is on this layer and it’s mapped, and the mapping is done down
here, as a strip conductor and it’s between the interface of our dielectric material,
that’s the Msub, and free space.
So, just in review, although we copied from schematic, this is where you would set up
your own substrate definitions. You could add, cut, and pasted layers, define all their
properties, give them a name, and then in the layout layers tab you would then assign
any drawing layers that are applicable, define their properties, and then map them as either
strips, slots, or vias depending upon what you’re doing. So this is our substrate definition
in Momentum, the dielectric material, and the drawing or layout layers.
Now the next step will be to have our ports defined using this port editor, and I’ve
already used this icon here to insert ports one and port two. Here, in the port editor,
you can select the port and then it will allow you to define that type. Internal, which would
be if it was internal to the structure, or these other modes. We’re going to use single
mode, which happens to be the default so you don’t really have to set it. You can change
the impedance as you see here. Let’s cancel this and let me zoom in and show you something
important about ports for Momentum.
They need to be snapped to the edge of the metal to be calibrated. That means at every
frequency Momentum will go back one half wavelength so that you have zero dB and zero phase at
this point, just like the calibrated ends of a network analyzer.
Now, before we set up the simulation, let me show you the mesh set up.