This blog describes how to design SIW filters using SynMatrix and HFSS. The SynMatrix software is a powerful tool for the design of cavity and planar filters, as well as diplexers and multiplexers. 

In this blog, we will design a single bandpass filter. Select Single:

Phase 1: Filter Synthesis

Start with the Synthesis process. It is the user's responsibility to specify the bandwidth, the number of orders, the requested return loss, and the unloaded Q requirement. In addition, the user can add zeros to force a fast drop after the band. Adding zeros allows the user to select a topology.

In this blog, we will select the parameters displayed in the following figure. Note how SynMatrix calculates the coupling required for each cavity. No matter what type of filter you use, these values are standard.

If the user changes anything, then click "Calculate ALL" to refresh all the calculations.

Phase 2: Filter Design

Move to the 3D Modeling and select Cavity. The following options are available: coaxial, rectangular waveguide, circular waveguide, SIW, or planar coupled resonator. Choose SIW. Confirm and begin a new design.

Step1: Cavity Dimensions

SynMatrix calculates the dimensions of the cavity in accordance with the specifications. Choose the shape of the cavity. It is possible to select a design that tilts to the left or right of the bandwidth. Modify the via diameter as well as the material. The diameter of the via should not exceed Lambda_g/5, and the spacing should not exceed twice the diameter of the via. This is very important. A design will be proposed by Synmatrix. To determine the right value for the SIW Side Length, the designer will have to go back and forth many times. This is the most critical dimension. 

Accept whatever values given by SynMatrix, and proceed to the next step. So far until now, analytical formulae are used. Time to activate HFSS. Select Single Cavity

Verify the numbers proposed from the previous stage. Activate or deactivate the tuning Vias. It is recommended no to activate them, but users who are expert in these types of filters can go ahead and activate this option. Press "Apply and Next Step"

Run the simulation without tuning vias. SynMatrix will access HFSS, build the model, do the setup, and then run the model. The HFSS calculates the cavity's resonant frequency. It is important that the calculated frequency falls within the middle of the band. Activates the tuning vias, and run over a range of Tuning Depth values. A wide band of frequencies should be displayed to the user, wider than the bandwidth selected. There is no requirement that it begin before the band's starting frequency. These calculations are intended to produce one number that can be used as a starting point in the design of all cavities. In the plot below, the recommended number to start with is 0.9mm. Press the "Previous Step", and enter 0.9 as the nominal tuning depth. Save.

In conclusion:

Step 2: Coupling Section

Select now the coupling scheme.

Select the type of coupling. Usually the second one is the most famous one. Click "Set as Main Coupling", and then "Apply and Next Step".

In the next page, run a parametric sweep. We would like to see a variation that covers all the required coupling possibilities. SynMtarix displays the required range in green.

Select the following numbers

Click the button seen below to enter the values.

Export the data to the 3D Model design.

Step 3: Input/Output Section

 Next, is the input/output section.

Again select the type of input to use. Select the first type. Press "Set Input & Output", then select "Apply and Next Step"

In the next window, run a parametric sweep to calculate the right insertion depth that leads to 1.12 coupling or 0.905 group delay. Notice how SynMatrix displays the target group delay in the graph.

We select the following value. Go back to the previous step and enter the new value as the nominal value. Save.

In fact there are 4 parameters that the designer can use to tune the input/output transition:

Step 4: Full Model

The final model is ready to be built. Switch to the Full 3D Modeling.

In the Main Body Design panel, click on any circle and check the properties. Check that all the numbers match the ones selected in the previous step.

Switch to the Modeling Panel, press Model Construction, then run simulation. Check the HFSS model and the results, once it is done. 

The first results are always bad. Things do not match at all. This is normal. Start by exporting the S-parameters, and make sure to select not to normalize the ports.

Phase 3: Manual Optimization

Back to SynMatrix. Activate the CAT Computer Aided Tuning option:

Click on "Load the data" and select the S-parameter file. 

Reverse engineering is performed by pressing Extract Matrix, which extracts the matrix obtained from the HFSS results. Select the Error Levels panel.

It is evident that the error is large, whereas it should be zero. It is therefore time to make the necessary corrections. Here are some guidelines:

1- In the case of cavities, increasing the tuning depth moves the bar to the left

2- In the case of coupling, increasing the iris width causes the bar to move to the left

3- A decrease in port depth moves the bar to the left for input/output

It becomes increasingly complex as you approach the optimum in terms of the interactions between the three types; the cavity Qs, the cavity-to-cavity coupling, and the input/output group delay.

Several manual trials have been conducted:

SIW Filter

In another blog, we will talk about the AI optimization capabilities of SynMatrix.