Foreword: This paper presents a straightforward design methodology for dual-mode dielectric waveguide filters, aimed at assisting filter engineers in developing base station filters. The approach is particularly beneficial for those who are new to the field and looking to understand the fundamentals of dielectric waveguide filter design.
1. Introduction
With the continuous advancement of communication systems that demand smaller, lighter, and higher-performance equipment, there is an increasing need for more efficient front-end frequency-selective components. As 5G standards become more established, high-performance, compact, and lightweight dielectric waveguide filters have emerged as a top choice for modern communication infrastructure.
This paper introduces a practical method for designing dual-mode dielectric waveguide filters, providing a step-by-step guide for less experienced engineers to follow and gain hands-on experience in this specialized area.
2. Introduction to Dielectric Waveguide Filters
The dielectric waveguide filter discussed in this paper operates based on the TE mode and offers advantages such as a high Q factor, low loss, and high power handling capability. One of the key challenges in designing these dual-mode filters is determining whether traditional multi-port computation methods can be effectively applied. This paper explores this issue in detail.
The following figure illustrates the four modes present within a single resonator:
From the electric field distribution diagram above, it's clear that both the dual-mode and triple-mode configurations can be selected depending on the design requirements. At frequencies where higher-order modes appear, careful selection of the dielectric constant and cavity size can help suppress these modes, minimizing their impact on the desired passband.
3. Design Process
3.1 Multimode Filter Design
The evaluation of pass-through performance for a multimode filter is similar to that of a single-mode filter. However, when evaluating the Q value, it is observed that multimode filters generally exhibit a higher Q value compared to single-mode ones, which can be verified through simulation. The following figures show the simulation results of a two-cavity, four-mode filter:
3.2 Coupling Bandwidth and Input/Output QL
3.3 Input-Output Coupling in HFSS
Input-output coupling is calculated using HFSS, and the magnitude of the alignment delay helps determine the coupling strength initially. During simulation, the direction of the connector probe must align with the electric field of a specific mode, as shown below:
When setting up the input-output coupling, it's important to isolate other modes due to the influence of the second or third mode. In cases where coupling between the first and second modes occurs, the delay waveform becomes bimodal. Frequency separation techniques can be used to reduce interference from the second mode on the first.
3.4 Coupling Bandwidth and Frequency Calculation
In this stage, the multi-port extraction method is used to extract the resonant frequency and store the coupled bandwidth. During simulation, special attention must be paid to the port’s mode, which should follow the direction of the electric field. While calculating the coupling bandwidth, the frequency points where harmonics appear in the passband can also be observed. By carefully selecting the size and orientation of the coupling window, the amplitude of high-order mode harmonics can be minimized.
3.5 Passband Calculation and Optimization
For optimizing high-order multimode filters, the two-port S-parameter extraction method is commonly used, along with spatial mapping and genetic algorithms. Choosing an appropriate optimization technique can significantly speed up the design process, save time, and shorten the development cycle of the filter.
Summary: This paper has provided a comprehensive overview of the design process for multimode dielectric waveguide filters. These filters offer advantages such as compact size, light weight, and excellent performance, making them ideal for 5G communication systems. It is expected that their application will continue to expand in the coming years, especially in next-generation wireless infrastructure.
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