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K-Band Filter Technologies

Key Takeaways

  • When a filter is used for K-band frequency applications, it is called a K-band filter.  

  • Microstrip K-band filters can have low pass, high pass, band pass, or band reject characteristics.

  • The performance-to-size ratio and compatibility with other MMIC circuits makes MMIC technology-based K-band filters a key part of modern microwave communication systems.

  K-band filter

Modern microwave communication systems require filters

Filters are ubiquitous in electronics and communication engineering. They are two-port networks capable of passing or stopping certain frequencies of interest. The frequency responses of a filter can have low-pass, high-pass, band-pass, or band-reject characteristics.

There are different types of filters in microwave and radio frequency (RF) applications depending on their frequency range. In the microwave frequency range, the spectrum is classified into several bands, including the K-band, Ku-band, and Ka-band, to name a few. When a filter is used for a K-band frequency application, it is called a K-band filter. Likewise,  microwave filters can be Ka-band filters, Ku-band filters, etc. In this article, we will explore microwave filters, with a particular focus on K-band filters.

Microwave Filters

Filters possess the property of frequency selective transmission. Filters transmit energy in certain bands (called passbands) or attenuate energy in certain bands (called stopbands). In any microwave application, filters are inevitable. Filters can be low-pass, high-pass, band-pass, or band-stop filters. Usually, microwave filters are designed using a low-pass filter network. The required transfer function characteristics can be achieved by designing a low-pass filter prototype.

A ladder network (series inductors and parallel capacitors) is used in microwave systems for low-pass filtering. At infinite frequency, the series inductor becomes open-circuited with a ladder filter network, whereas shunt capacitors become short-circuited.

Like low-pass ladder networks, microwave filters can be realized using different prototype networks. By changing the component count and arrangement, the filter can be converted into high-pass, band-pass, band-stop, etc. Microwave filters can be developed using microstrip technology, substrate-integrated waveguide (SIW) technology, or monolithic microwave integrated circuit (MMIC) technology.

Let’s look at K-band filters and different microwave filter prototypes.

Microstrip K-Band Filters

In RF and microwave applications, circuits are typically designed in printed circuit boards. When the filter design is based on microstrip technology, the filter is considered a microstrip filter. Microstrip filters are commonly used in microwave and RF systems. Microstrip K-band filters used in applications can have low-pass, high-pass, band-pass, or band-reject characteristics. In communication systems, it is common to use microstrip K-band filters to reduce spurious emissions and harmonics from transmitters. They also improve the rejection efficiency in receivers. Microstrip K-band filters are designed as planar structures to achieve compactness and the easy integration of components.

K-Band Filters Employing Meta Material Configurations

As wireless technology grows rapidly, the demand for compact and multiband filters increases. Metamaterial (MTM)-inspired structures are used in microwave circuits as filters. Metamaterial filters are compact, follow homogeneity conditions, and showcase excellent electromagnetic characteristics. The two prototypes of filters available in metamaterial filters are T and ㄫ. By combining T and ㄫ-type MTM cell configurations, an increase in passband can be achieved in metamaterial K-band filters.

Substrate-Integrated, Waveguide-Based K-Band Filters

With technology advancements, filter requirements in K-band transceiver systems require parameters such as high selectivity, low cost, and wide stopband compactness. The substrate integrated waveguide (SIW) filter for K-band applications is popular due to its planar structure, similar to a microstrip filter. However, the quality factor of SIW filters is high, contrary to microstrip filters and similar to waveguide quality factors. The power efficiency and compactness of SIW K-band filters are significant compared to rectangular waveguide-based filters. The integration of SIW K-band filters with other active circuits is extremely good as well.

Monolithic Microwave Integrated Circuit (MMIC)-Based K-Band Filters

The performance-to-size ratio and compatibility with other MMIC circuits makes MMIC technology-based K-band filters a key part of modern microwave communication systems. This type of active K-band filter makes the integration of filter circuits easier. When a K-band filter is realized as an MMIC active filter, the bandwidth is further decreased or narrowed without compromising the insertion loss.

There are other technologies that can be used to realize microwave K-band filters. One such type of K-band filter is a micro-electromechanical system-based waveguide filter. As the field of tunable communication systems expands, tunable filters are becoming more necessary in electronic warfare, communication systems, radar systems, and sensing systems. Micro-electromechanical system-based waveguide filters are a reliable solution for K-band filter applications. 

PCB Tools Aid in Filter Design

To validate the claim of low costs, low power consumption, and low loss, microwave filter design software is available. Cadence’s suite of PCB design and analysis tools can help you design RF and microwave filters.

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