The Ultimate RF Filter Guide: How to Maximize Signal Integrity, by Colin Field

Navigating the complex realm of electronic design requires a thorough understanding of RF filter technology. Designed to eliminate unwanted interference, improve signal integrity, and prevent signal distortion, RF filters are a key consideration in the development of wireless communication systems. Spectrum usage is closely controlled by the ITU (International Telecommunications Union) and achieving the emissions limits of most communications protocols will require some form of filtering technology.

As filter technology rapidly evolves and spectrum usage expands towards higher frequencies, system engineers are faced with an ever more complex selection process when identifying an ideal RF filter that matches their specific design requirements. Our comprehensive RF Filter Guide offers expert insight into design considerations, trending filter technologies, and featured products to empower you in making informed decisions for seamless integration and optimal performance in your designs.

Why are RF Filters Used in Circuits?

In the simplest terms, RF filters eliminate undesired elements from the electromagnetic spectrum to allow a circuit to focus on its intended function within the desired range of the spectrum. For example, consider a designer looking to create a receiver for a GPS L1 signal (1575.42MHz) originating from a satellite in Earth’s orbit. The desired signal, significantly weakened as it traverses the atmosphere and reaches the receiver, will need to be amplified before processing. While an amplifier circuit for this application will enhance the desired signal’s strength, it will also amplify unwanted signals below and above 1575.42 MHz that may interfere with the desired 1575.42MHz signal and cause the receiver to process information incorrectly. To address this issue, incorporating a bandpass filter in front of the amplifier will isolate the wanted signal, enabling the receiver to operate its task more efficiently.

What Types of Filter Technologies are Being Adopted?

The types of RF filters that can be designed and technologies that can be used to create those filters is an exceedingly broad subject. Encyclopedias can, and have, been written on this subject since the first experiments in wireless communication in the mid-19th century. However, RF filters can generally be classified into four basic categories:

Source: All About Circuits

Bandpass Filters

Bandpass filters allow the transmission of desired signals at a specific frequency, or range of frequencies, while rejecting signals that are both higher and lower in frequency than the desired signal

Band Stop Filters and Notch Filters

Band stop filters, also known as Notch Filters, reject unwanted signals at a specific frequency or range of frequencies while allowing the passage of signals both higher and lower than the unwanted signal.

Low Pass Filters

Low pass filters allow the transmission of signals below a defined frequency and reject signals above it.  

High Pass Filters

High pass filters Reject signals below a specified frequency and allow signals above it.

While the four basic filter types remain consistent, RF filters in wireless communication have practical applications across an extremely broad spectrum, ranging from 1MHz to 100GHz. To grasp the significance of this range, a comparison to audio can be enlightening. Consider a concert grand piano that uses wires as resonators and hammers to generate resonance (audible sound). The piano’s design accommodates the need for large resonators for low frequencies, with the resonator length halving in length for each octave increase (doubling) in audible tone. The result is that the piano will be in excess of 2.5 Meters on the left to accommodate the lower frequencies whereas on the right it could be as small as 50 cm. A concert grand piano covers seven octaves. Now, compare that to the practical RF spectrum (1MHz to 100GHz), spanning approximately 18 octaves! Resonators used in filters designed for the lower frequencies will be totally impractical at higher frequencies.

RF filters use a plethora of different materials to create the “resonators” used to create filters needed in RF design applications. The physical properties of those materials will limit or enhance the ability of a particular material to create a “useful” filter for a specific application.

 

Top Applications of RF Filters in Electronics

RF filter solutions ensure optimal signal precision and management in a wide range of applications. Their versatility contributes to the efficiency, reliability, and performance of electronic systems in various industries. Top applications include:

• Space – Filters designed for space applications must have the highest performance in terms of reliability since maintenance is impractical in outer space. No one is sending a maintenance engineer out there to fix something, so it better not fail. Given the substantial investments associated with space applications, any filter technology that contributes to a favorable total cost of ownership for the entire application merits careful consideration. For instance, a Ka band filter used in a satellite downlink with a 0.5dB improvement in insertion loss can lead to a corresponding reduction of 0.5dB in amplifier output power. This reduction in power requirements can substantially lower the satellite’s DC power budget, minimizing the need for an extensive solar array, and ultimately lowering launch costs due to the reduced mass of the payload.
• GNSS – Commercial GNSS filters are widely accessible in the market, driven by the demand for cost-effective and compact GNSS capability in smartphones. Although they deliver excellent performance at very low Average Selling Prices (ASPs) for most adjacent applications, situations arise when GNSS availability becomes “mission critical”, such as in guided weapons systems. In such scenarios, the use of filters capable of better resisting jamming emerges as a crucial consideration.
• Mobile Phones – In the 1980s, first-generation mobile phones operated within a single frequency band in one region, necessitating a single RF filter. However, by the end of the decade, demand for roaming and the introduction of new spectrum required phones to incorporate 6 or 8 RF filters. Today the typical smartphone will contain over 60 different RF filters to manage the various cellular network frequencies worldwide, along with multiple GNSS bands, Bluetooth, Wi-Fi, and satellite communications applications.

Deciding Factors When Picking an RF Filter

Choosing an ideal filter is not just a question of the technical needs and physical characteristics of a solution. With such a wide variety of solutions available, it’s critical to have an understanding application and use case in order to determine the “right solution” for the application.

When selecting an ideal filter solution, RF designers should consider:

Cost

Beyond the technical suitability based on material properties, practical consideration must be extended to the cost and reproducibility of a particular filter design. If a filter is to be used in a cellular handset, for instance, the consideration of cost and reproducibility significantly influence material selection. While a “better” technical solution may be available, if it fails to meet the cost and reproducibility criteria, then opting for the “better” technical solution may not align with overall objectives.

Power and Size

Additionally, it is important to get into some detail of the minimum technical requirements essential for the filter’s viability in its intended application. Aside from basic frequency and bandwidth requirements, engineers must consider other factors like:

• How much RF power must the device withstand?
• The practical size limit of the filter needed and the interface to the outside world. Will it use connectors or will it mount to a PCB?

Rejection

Finally, some consideration to practical upper electrical limits of the filter are worth discussing. For example, a bandpass filter operating at 1500MHz, may need to reject an unwanted signal at 1800MHz. An often-overlooked question is, “What happens after 1800MHz?” It cannot be presumed that the filter will indefinitely provide rejection beyond this point. Understanding the practical upper limit where rejection no longer matters is crucial.  

Discovering Filter Innovations

Luckily at RFMW, our line card features an extensive range of suppliers that provide the latest in RF filter technology. Many of our suppliers feature unique solutions in the overarching categories of filters. Moreover, for developers seeking precise solutions, it is essential to be informed about the specific supplier to explore within our comprehensive network.

Qorvo

For over 30 years, Qorvo has continuously supplied state-of-the-art, high-performance diplexers, triplexers, and discrete RF SAW and BAW filters, giving designers the flexibility to choose between discrete single-band building blocks and integrated multiband filtering. Furthermore, their ability to integrate passive technology in their active portfolio has resulted in a comprehensive range of Front-End Modules (FEM) that streamline customer designs and accelerate time to market.

Qorvo addresses market needs with intelligent solutions, in particular in aerospace and defence markets where the focus is on high performance low loss solutions. For example, their QM28014 L1 RF SAW filter uses Temperature Compensating technology of give the customer excellent out of band rejection performance reducing the risk of unwanted signals interfering with their GPS receiver.

Knowles Precision Devices

With a rich heritage in planar microwave filters based on proprietary ceramic technology and high-volume manufacturing processes, Knowles Precision Devices portfolio features high-performance, repeatable filter solutions. Ideal for applications from C band to Ka Band, Knowles wideband high rejection planar solutions are known for their small form factor and stable temperature characteristics. The B291MB0S, for example, is a temperature stable, SMD full band Ka Band uplink filter with high power capability that is suitable for ground station applications. Fractional bandwidths range for 10% to 60%.

Knowles has additionally expanded its portfolio to offer customers more traditional filter technologies for high reliability applications, such as Cavity, Waveguide, and Coaxial Resonator filters down to VHF. With in-house reliability and test laboratories, Knowles enables a wide range of evaluation and qualification work to be conducted on-site, improving response times to customer requests.

Marki Microwave

Marki Microwave uses a planar approach on GaAs substrates in the design of their RF filter solutions. Utilizing semiconductor fabrication techniques, they achieve narrow line widths and highly repeatable processing, resulting in small form factor passive devices for high frequency (mm wave) ultra-wideband solutions. For example, their is a bare die (2.4mm2), Ka Band Filter with less than 2dB insertion loss and over 8GHz of Bandwidth.

Sangshin Elecom

With a 25-years of success in RF filter design and fabrication based on ceramic technology, Sangshin Elecom stands as a pioneer of Monoblock and Coaxial Resonator devices up to 10 GHz. Operating from three locations and providing high-volume solutions for consumer applications, Sangshin remains flexible and engineering-focused. They deliver fast turnarounds for quotes and specification sheets, even for quantities as small as one hundred pieces.

The company developed Ceramic Waveguide Technology capable of withstanding 100W of CW power in a compact form factor, specifically designed for applications from cellular to C band. Although having greater Insertion loss than a cavity filter, these devices present a viable compromise to traditional cavities due to their smaller and lighter build, impacting the feasibility within a phased array. Ideally suited for use in n77 C band applications, their SEWB3500SP200K is a good example of this technology as it covers 200 MHz of C band, delivers power handling of 12 Watts average with a footprint of just 20mm x 30 mm.

Smiths Interconnect

Smiths Interconnect operates RF Filter design and manufacturing locations in both the US and the UK, offering a localized source for more discerning customers. Their commitment is to provide their customers with the best value in total cost of ownership. With a large team of experienced design and production engineers, Smiths specializes in designing custom devices and integrated assemblies for defense and aerospace applications globally. Smiths consistently delivers a diverse range of filter-based solutions for High REL and space applications.

Akoustis

Akoustis is a global leader in advancing BAW technology through their proprietary X-BAW® process. Akoustis offers the widest portfolio of Wi-Fi 6, 6E, and 7 filters available in the market today. Their current BAW filter portfolio features standard products ranging from 2.4GHz to 6.5GHz, including solutions for V2X automotive and 5G C band applications. The AKF-1336 from Akoustis is a CBRS band filter which is unique in the market in terms of performance and form factor as it offers 150 MHz bandwidth, low insertion loss at 3.6 GHz, and high out of band attenuation all at the ultra-small form factor of 2.5mm x 2.0mm x 0.8mm.

Beyond their high-volume standard products, Akoustis has developed new advanced technologies that increase the technical reach of BAW design and manufacturing to 18GHz. They have recently launched foundry service capability up to X band, enabling the designers to create their own custom BAW solutions.

Cubic Nuvotronics, Inc.  

Cubic Nuvotronics employs an innovative Additive Manufacturing Technology to produce copper structures with precision surpassing that of comparable methods like 3D printing. Their patented Polystrata technology features micron level accuracy and enables the fabrication of low loss, broadband passive devices with exceptionally compact form factors. This advancement enhances RF performance and reduces Size, Weight, and Power (SWaP) when compared to alternative market solutions. Cubic Nuvotronics’ solutions encompass V Band Diplexers and ultra broadband filters to 50GHz.

TST Tai-Saw Technology Co. LTD.

Tai-Saw Technology Co. LTD. (TST) is the leading independent provider of SAW technology solutions in Asia. Emphasizing high performance design for cost-effective solutions, TST has a vertically integrated manufacturing approach, offering a diverse portfolio of components across various markets. With a significant presence in the automotive sector, TST offers solutions for LTE, GNSS, SDARS, and V2X applications. Headquartered in Taiwan, TST has the ability to cater to global markets from consumer IoT to space flight applications.

Conclusion

In an industry where technology is constantly developing and capability is rapidly evolving, the significance of new developments and choices of RF filter technology is apparent. Given the complexity of the choices open to an engineer, RFMW has developed an industry leading, comprehensive portfolio of RF filter solutions that address the diverse needs of engineers. As a specialist in RF technology, we have invested in our own, independent, technical resources to help our customers identify the best filter solutions for their specific applications, considering all of the variables outlined in this guide.

Our commitment to providing best-in-class technology through a curated line card of RF and power suppliers allows developers to conveniently select filter solutions that are ideal for their design’s specific requirements. By positioning RFMW as your trusted source for cutting-edge filter solutions, you can confidently discover the RF filters you need for optimal performance and seamless integration.