NuHertz/HFSS: Planar LPF Chebyshev implementation using rectangular, radial and impedance stubs

Exploring Microstrip Lowpass Chebyshev Filter Design: A Practical Guide with NuHertz and HFSS

The design and optimization of filters are critical tasks in RF and microwave engineering, enabling efficient signal processing and interference management. Among the various filter types, the Chebyshev lowpass filter is known for its distinct passband ripples and smooth stopband response, making it ideal for applications where sharp transitions between passband and stopband are necessary.

This blog explores the step-by-step process of designing a Microstrip Lowpass Chebyshev filter using NuHertz and ANSYS HFSS. By leveraging these tools, engineers can compare different topologies, optimize designs, and achieve precise filter performance tailored to high-frequency applications.

What is a Microstrip Lowpass Chebyshev Filter?

Lowpass filters are traditionally implemented using lumped components like inductors (L) and capacitors (C). However, at high frequencies, these components become impractical due to parasitics and physical limitations. Instead, distributed elements such as RF transmission lines and stubs are used. A Chebyshev lowpass filter employs:

• Passband Ripples: To achieve sharper roll-off between the passband and stopband.
• Smooth Stopband: Minimizing undesired frequencies with high attenuation.

Key techniques, like Richard’s Transformation and Kuroda’s Identities, enable the replacement of inductors and capacitors with distributed elements, simplifying the implementation for high-frequency applications.

Why Use NuHertz and HFSS for Filter Design?

NuHertz is a powerful tool for synthesizing and designing RF filters. It offers a wide range of filter topologies and provides flexibility in defining design parameters such as passband and stopband characteristics. Once the design is complete, it can be exported to HFSS for 3D electromagnetic simulation, allowing for:

• Accurate Modeling: HFSS accounts for 3D effects like coupling and parasitics.
• Optimization: Refine designs for improved performance metrics like insertion and return loss.
• Comparison: Evaluate different topologies to identify the best design for specific requirements.

Key Steps in Designing a Chebyshev Filter

1. Filter Synthesis in NuHertz:

   • Define the filter type (Chebyshev Type II) and parameters like passband, stopband, and insertion loss.
   • Select distributed elements for high-frequency operation.
   • Compare various topologies such as split stubs, radial stubs, and stepped impedance for performance and geometry constraints.

2. Exporting the Design:

   • Use NuHertz to export the filter design to AEDT (Ansys Electronics Desktop).
   • Ensure the design is fully parameterized to facilitate optimization.

3. Simulation and Optimization in HFSS:

   • Solve the filter designs using HFSS to simulate real-world performance.
   • Use HFSS’s optimizer to refine parameters for maximum efficiency.

4. Performance Analysis:

   • Compare HFSS results against NuHertz predictions for metrics like:
     • Insertion Loss: How much signal is lost in the passband.
     • Return Loss: Measure of reflected signals indicating impedance mismatch.
     • Filter Dimensions: Physical feasibility for implementation.

What to Expect in This Blog

In the following sections, we’ll demonstrate:

• How to create and configure a Microstrip Chebyshev lowpass filter in NuHertz.
• The process of exporting the design to HFSS and simulating its performance.
• Detailed comparisons of various topologies, summarizing their strengths and trade-offs.

Whether you’re an RF engineer or a designer working with high-frequency filters, this guide will equip you with the tools and insights to create efficient and high-performing filters tailored to your needs. Let’s get started!

Our topic today is designing a Microstrip Lowpass Chebyshev filter using NuHertz and HFSS. A Chebyshev has ripples in the passband and smooth response in the stopband. We'll compare the different options in NuHertz.

Lowpass filters are made from lumped L and C components. Such an approach is hard to implement for high-frequency applications. Distributed elements are needed for high-frequency filters. This process uses RF lines and RF stubs.

It's possible to replace the capacitors with open lambda/4 stubs using Richard's transformation. Inductors have a problem, so they can only be replaced with lambda/4 stubs or lambda/2 stubs.

Figure 1. Richards Transformation

Lowpass can't be done with short stubs, and long stubs aren't practical either. Kuroda's identities let you replace the inductor with a short transmission line and a capacitor. Therefore, the filter is just a bunch of open stubs connected by short transmission lines.

Figure 2. Kuroda's identity

LPF can be implemented using smaller sections of transmission lines, each with a specific impedance, like low, high, low, high, etc.

Knowing what can be done, we go back to NuHertz. Lowpass and Chebyshev-type 2 are chosen. The requirements, the passband, the stopband, the stopband insertion, and others need to be specified. Use distributed elements. NuHertz gives you a lot of options by doing this. Which one should I pick? All of them will be implemented in HFSS and compared for performance, dimensions, sensitivity, etc. 

Figure 3. NuHertz setup for LPF Chebyshev Planar Filter

Select any one of the options and export to AEDT.

Figure 4. Exporting the design to AEDT

In AEDT, we solve all the options. You will notice that the filter is fully parameterized. Consequently, one can perform optimization.

Figure 5: All implementations in NuHertz

Here are the NuHertz predictions compared to the HFSS calculations. Because HFSS is a 3D EM solver, it's more accurate. You can use the optimizer to make the designs identical or to improve the response.

Figure 6: HFSS versus NuHertz Insertion (Green HFSS, Blue NuHertz)

Figure 7: HFSS versus NuHertz Return Loss (Green HFSS, Blue NuHertz)

Below is a table summarizing the differences between the topologies. Dimensions are in millimeters. All approaches are the same length, except for the spaced stubs, which are quite long. The rectangular stubs with a single side are narrower in width. The stepped impedance is the narrowest.

Table 1: LPF topologies

In Nuhertz, the following comments are added for each type:

With the exception of stepped impedance, all options have smooth curves. Even so, it has the highest average insertion and sharpest drop. After the passband, the stepped impedance is more stable and drops quickly.

Figure 8: Return loss

Figure 9: Return loss

Figure 10: Insertion Flat up to 1 GHz

Figure 11: Insertion