Introduction

Filters are key components in radio frequency (RF) and microwave systems that can selectively process signals of specific frequencies. By designing specific circuits or structures, they allow signals within the target frequency range to pass through smoothly while significantly attenuating or suppressing other unwanted frequency components. Their core function is to precisely separate useful signals from interference signals. For example, in communication systems, they filter out spurious interference from adjacent channels; in radar systems, they suppress noise to improve target detection sensitivity; and in test and measurement, they ensure signal purity to guarantee data accuracy. The importance of filters lies in their role to maintain the performance of RF and microwave systems. Without high-performance filters, systems may suffer from a decrease in signal-to-noise ratio, an increase in communication bit error rate, functional failure, or even complete paralysis due to various interferences. Filters directly determine the signal quality, operational stability, and application reliability of the entire system, and are an indispensable core component in everything from basic electronic devices to complex aerospace systems.