BP Filter Design Using Synmatrix and HFSS Integrated Workflow: Part 1

Hi, I'm Aziz from Ozen Engineering. In this video, I will demonstrate designing a Band Pass (BP) filter using the Synmatrix HFSS integrated workflow. An integrated Synmatrix HFSS workflow consists of three steps:

1. Filter Design: Starting from the filter specification, create a filter design using powerful Synmatrix synthesis tools and built-in algorithms.
2. 3D Model Construction: Build a fully parameterized 3D model of the filter in HFSS using Synmatrix automated layout tools.
3. Simulation and Optimization: Import the simulated result in HFSS back into Synmatrix to start the tuning and optimization process.

For this demonstration, we're going to use the Synmatrix desktop version, which is needed to link to HFSS. We will follow the same process for a full 3D simulation, considering a BP filter with the following specifications:

• Start Frequency: 3.55 GHz
• Stop Frequency: 3.7 GHz
• Return Loss: 20 dB
• Lossless Case with Unloaded Q: Infinity
• Filter Order: 7

We can calculate the filter performance from 3.2 to 4 GHz and add a couple of transmission zeros at 3.5 GHz and 3.75 GHz. We define a physical topology using the "Edit Topology" button, with options for user-defined topology or selecting from the existing library. For this demonstration, we'll use the default Synmatrix topology, which is a Chebyshev type with a folded canonical matrix.

Filter Specification and Calculation

After entering all the filter specs, we hit "Calculate All" to update the filter performance. The ideal filter performance exhibits transmission zeros at 3.5 GHz and 3.75 GHz with a return loss of 20 dB over the bandwidth from 3.5 to 3.75 GHz. Synmatrix also calculates the group delay and power analysis, showing maximum power handling at resonator number four and displaying the air breakdown voltage.

The coupling matrix is displayed for the selected filter, with formats available in bandwidth format and normalized format. The next step is to save the file specification and proceed to 3D modeling using HFSS.

3D Model Construction

We create the HFSS file and select a coaxial cavity for the current design. The cavity tuning mechanism options include:

• Flat on Top
• Ball on Top
• Disc on Top

We select the "Ball on Top" option and choose a square resonator shape. The cavity dimension and tuning screw dimension are calculated by Synmatrix based on the center frequency of the filter. We can adjust the tuning screw radius using different screw sizes, such as M5.

Tuning Scheme and Material Selection

We define the tuning scheme with options for non-partial or partial filling. For temperature compensation, applicable to partial structures, we define materials for the top partial, bottom partial, and tuning screw. Default or calculated values from Synmatrix can be modified at this stage.

We set up HFSS for an eigenmode analysis with a minimum frequency of 2.9 GHz, a maximum frequency of 7 GHz, and five modes for the cavity. We use auto meshing and can select curvilinear meshing for curved surfaces. A peak power analysis is also possible at this stage.

Simulation and Analysis

We construct the model and launch HFSS for eigenmode simulation of the single cavity, which is fully parameterized. At this stage, we analyze the cavity for resonance, power handling, and losses. A parametric study can be conducted to see the effect of the tuning screw on frequency and unloaded Q.

Coupling Scheme Definition

We define the coupling scheme with options including:

• Top Window
• True Window
• Loop
• Probe

We select the "Top Window" option and proceed to define the eigenmode analysis for the coupling scheme. The connector type is an SMA connector, selected for both input and output. We construct the model and launch HFSS to simulate the input and output cavity.

Full 3D Modeling

We choose full 3D modeling to design the filter layout. The topology can be changed arbitrarily by rotating resonators or changing their orientation. Input and output can be adjusted by rotating the probe around the resonator.

We define the cross-couple structure, selecting from five options for different cross-coupling applications. Finally, we construct the model and define simulation parameters to be passed to HFSS. Once the model construction is complete, we run the simulation by invoking HFSS.

Simulation Results

The simulation of the complete filter is conducted, and we examine the rectangular plot of scattering parameters S11 and S21. As observed, S11 and S21 are not meeting the specs, prompting the next step of computer-aided tuning in Synmatrix.