EM.Picasso Tutorial Lesson 3: Analyzing a Planar Microstrip Filter

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Tutorial Project: Analyzing A Planar Microstrip Filter
Pmom lec3 3 sourceposition.png

Objective: In this project, you will build a planar microstrip filter and will analyze it as a two-port structure.

Concepts/Features:

  • CubeCAD
  • Hybrid Planar Mesh
  • De-embedded Source
  • Port Definition
  • Frequency Sweep
  • Adaptive Sweep

Minimum Version Required: All versions

'Download2x.png Download Link: [1]

Objective:

To construct a planar microstrip band-stop filter, explore its frequency response using uniform and adaptive frequency sweeps and examine the sensitivity of the filter response to some geometrical parameters.

What You Will Learn:

In this tutorial lesson, you will deal with a two-port structure. You will learn more about the subtleties of EM.Cube’s de-embedded sources and adaptive frequency sweep. You will also define variables to control the filter geometry with more complicated constraints on the object parameters.

Getting Started

Open the EM.Cube application and switch to Planar Module. Start a new project with the following attributes:

  • Name: EMPicasso_Lesson_3
  • Length Units: mils
  • Frequency Units: GHz
  • Center Frequency: 13GHz
  • Bandwidth: 8GHz
  • Planar Mesh Density: 30 cells/λeff
  • Number of Finite Substrate Layers: 1
  • Layer Stack-up:
    • Top Half-Space: Vacuum
    • Middle Layer: εr = 9, μr = 1, σ = σm = 0, thickness = 5mil
    • Bottom Half-Space: PEC

Drawing the Microstrip Line Components

Create a PEC group on the Navigation Tree and call it PEC_1. Draw 11 rectangle strip objects with dimensions and locations given in the table below:


The position of the microstrip components
Label Object Type Notes Function LCS Origin Length Width
L1 Rectangle Strip - Center Line Segment (0, 0, 5) 92 4.8
L2 Rectangle Strip - Joint (48.4, 0, 5) 4.8 4.8
L3 Rectangle Strip Copy of L2 Joint (-48.4, 0, 5) 4.8 4.8
L4 Rectangle Strip - Input Line 1 (65.8, 0, 5) 30 4.8
L5 Rectangle Strip Copy of L4 Input Line 2 (-65.8, 0, 5) 30 4.8
L6 Rectangle Strip - Upper Arm Connector (48.4, 4.4, 5) 4.8 4
L7 Rectangle Strip Copy of L6 Lower Arm Connector (-48.4, -4.4, 5) 4.8 4
L8 Rectangle Strip - Joint (48.4, 8.8, 5) 4.8 4.8
L9 Rectangle Strip Copy of L8 (Joint (-48.4, -8.8, 5) 4.8 4.8
L10 Rectangle Strip - Upper Arm (1, 8.8, 5) 90 4.8
L11 Rectangle Strip Copy of L10 Lower Arm (-1, -8.8, 5) 90 4.8

You can draw all the objects individually in any blank space in the project workspace and then move them to the their specified locations by entering the above LCS coordinates in their property dialogs. Or it may be more convenient and quicker to copy, paste and move (translate) objects using keyboard shortcut T. For example, objects L2, L3, L8 and L9 are all identical squares. Objects L6 and L7 can be copied from L2 with a slight width decrease, while objects L10 and L11 can be copied from L1 with a slight length decrease.

Defining Sources & Assigning Ports

You are going to define two de-embedded source for the two input lines of your filter structure. This type of sources is specifically intended for scattering parameter calculation of planar structures. Define a –X-directed de-embedded source on the object Rect_Strip_4 (L4) and a +X-directed de-embedded source on the object Rect_Strip_5 (L5).

Planar Module's De-embedded Source Dialog

Leave the offset value of both sources at zero so that the phase reference planes are established at the left and right ends of their host rectangle strip objects, respectively.

The position of the two ports
Planar Module's Port Definition Dialog

Next, insert a port definition and accept its default settings. Port 1 is assigned to the first source DS_1, and Port 2 is assigned to the second source DS_2. The dafault port impedances are 50Ω. Your project structure should look like the figure above at this point. You will not define any other observables for this project.

A Note on Mesh of Planar Structures

Before running a planar MoM simulation of your project, let’s examine its mesh at the mesh density of 30 cells/λeff. Your planar must look something like this:

The Planar Module's mesh of the Planar Microstrip Filter

There are, of course, many ways that you can draw and construct the same filter structure, possibly involving fewer CAD objects. For example, you can draw a single, long, through line like the figure below. However, in order to define de-embedded sources, you need rectangle strip objects with connected ends at the phase reference planes. That is why you split the long through line into three segments (L1, L4 and L5).

An alternative way of constructing the same filter structure

Another possibility is shown in the figure below, which tries to avoid the four small joints L2, L3, L8 and L9.

Another possible way of constructing the same filter structure

If you mesh the above structure, you will see a planar mesh like the one shown below. Note the size and shape of the small triangular cells sandwiched along the two vertical strips. It would take a mesh density of 50 cells/λeff to produce a good mesh of this configuration.

The Planar Module's mesh of the above filter structure
Attention icon.png When constructing microstrip structures, try to use joint objects to represent bends, tee junctions or cross junction. This will help you generate a smoother mesh with better consistency as well as higher resolution at discontinuities, if necessary.

Running a Uniform Frequency Sweep

Run a uniform frequency sweep of your filter with the following parameters:

  • Start Frequency: 9GHz
  • End Frequency: 17GHz
  • Number of Frequency Samples: 9

This will sample the frequency at every 1GHz. Plot the S11 parameter (return loss) and S21 parameter (insertion loss):

The plot of the S11 parameter as a function of frequency for 9 frequency samples
The plot of the S21 parameter as a function of frequency for 9 frequency samples

Next, increase the number of frequency samples to 17, i.e., sampling at every 0.5GHz. Run the planar MoM frequency sweep one more time and plot the the S11 and S21 parameters in EM.Grid:

The plot of the S11 parameter as a function of frequency for 17 frequency samples
The plot of the S21 parameter as a function of frequency for 17 frequency samples

As you can see from the graphs, there were more details in the frequency response of the filter’s stop band than your initial 9-sample uniform sweep could predict.

Running an Adaptive Frequency Sweep

Planar Module's Frequency Settings Dialog
In this part, you will run an adaptive frequency sweep of your band-stop filter. Use the following parameters:
  • Start Frequency: 9GHz
  • End Frequency: 17GHz
  • Min. Number of Frequency Samples: 5
  • Min. Number of Frequency Samples: 15
  • Convergence Criterion: 0.05


Run the sweep and wait until it is completed. Most likely, you will get a message that your specified maximum number of simulation engine runs has been reached but the convergence criterion has not yet been met. The program asks you whether you would still like to proceed. Answer yes and contiue the adaptive sweep process unitl it converges. At the end of the simulation, open EM.Cube’s Data Manager and plot the files “S11_RationalFit.CPX” and “S21_RationalFit.CPX” in EM.Grid. You will see two smooth graphs of the return loss and insertion loss. Note that Data Manager also contains the data files “S11.CPX” and “S21.CPX”. These files contain the actual planar MoM simulation data at the frequency samples where the engine was called on. The former data file contain the rational interpolation data computed and fitted based on the original full-wave data.

The plot of the S11 parameter as a function of frequency for Adaptive Frequency Sweep
The plot of the S21 parameter as a function of frequency for Adaptive Frequency Sweep

EM.Cube’s Planar Module allows you to further process the raw simulation data collected during a uniform or adaptive frequncy sweep. Your can fit your own rational function approximation on the available S parameter data. For this purpose, right click on the Port Definition item in the Navigation Tree and select Smart Fit from the contextual menu. The Smat Fit Dialog opens up. At the top of the dialog, there is a dropdown list labeled Interpolate, which has a default selection “All Available Parameters”. This list contains all the scattering parameters of your structure. In the case of your two-port filter, there are S11, S12, S21, and S22. The dialog also shows the Number o Available Samples. These correspond to the number of rows of your “Sij.CPX” data files. You can set the Interpolant Order anywhere from 0 to the “Max Interpolant Order”. Every time that you change the order, you must click the Update button to generate new interpolated data. The new data overwrite the old data in the ASCII files “Sij_RationalFit.CPX”. The Smat Fit Dialog is a modeless dialog. You can keep it open and go to the Data Manager to plot your updated data files. The figures below show the S21 results for Interpolant Order values of 1 to 4.

Planar Module's Smart Fit Dialog
The Smart Fit Dialog in the Planar Module's Navigation Tree
The plot of the S21 parameter as a function of frequency for Interpolant Order values of 1
The plot of the S21 parameter as a function of frequency for Interpolant Order values of 2
The plot of the S21 parameter as a function of frequency for Interpolant Order values of 3
The plot of the S21 parameter as a function of frequency for Interpolant Order values of 4



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