Use Ceramic Bandpass Filters to Solve GHz-Range Design Challenges

Whether low pass, high pass, bandpass, or band reject, we recognize passive filters as an essential component in nearly all circuits and systems, ensuring that only the desired signal gets through while undesired ones are blocked or diverted. As operating frequencies move into the gigahertz (GHz) region and channel density rises, these filters' requirements also increase, pushing us to be discerning in our design choices.

Forget the classic lumped-element filter implementations using discrete inductors, capacitors, and resistors that professors loved to teach (and perhaps torment incoming students). Adding to the confusion were their bewildering names and identifiers, such as pi (often written as π solely because of its topological appearance), Cauer, Butterworth, Chebyshev, and Bessel, to cite a few.

In practice, achieving the performance and consistency required for today’s GHz designs with these handcrafted filters is difficult due to component parasitics, limited Q, manufacturing tolerances, and layout sensitivity. As a result, building our own from scratch is neither practical nor effective.

Instead, we can rely on filter OEMs that use a range of advanced materials and technologies to produce off-the-shelf, drop-in filters with the required electrical and mechanical attributes. Filter types and technologies include ceramic resonators, low-temperature cofired ceramic (LTCC), microstrip, thin-film on alumina, waveguides, dielectric, surface acoustic wave (SAW), and bulk acoustic wave (BAW).

How ceramic resonator filters work

Ceramic resonators and LTCC filters share base materials but differ significantly in construction. LTCC devices are manufactured using an advanced multilayer process in which ceramic-glass “green tapes” are printed with conductive materials to form the required passive filter elements, then stacked, pressed together, and fired at relatively low temperatures (below 900°C). The LTCC process enables high-density, three-dimensional integration of resistor-inductor-capacitor (RLC) components within a robust, miniaturized, thermally stable package suitable for high-frequency applications.

In contrast, ceramic filter design uses materials with a high dielectric constant and silver-plated transverse electromagnetic (TEM) coaxial forms to create a resonant structure. By leveraging the properties of suitable ceramic materials, this approach produces compact, high-Q bandpass filters for frequencies from 400 megahertz (MHz) up to 6 GHz and beyond.

In ceramic coaxial resonator filter construction (Figure 1, left), the resonators are soldered side by side to a ground plane. Each resonator is a quarter-wave shorted transmission line, and energy couples from one resonator to the next through coupling capacitors. These filters are compact surface-mount devices (SMDs) (Figure 1, right) with excellent RF performance.

Figure 1: The ceramic bandpass filter is a resonant-body, multistage device that can be modeled as a classic lumped-element filter (left) but uses a dramatically different physical structure; these filters are compact SMDs (right) with favorable RF characteristics. (Image source: Mini-Circuits)

Ceramic coaxial resonators excel with minimal tradeoffs in RF performance, temperature stability, and ruggedness. Their RF performance is largely due to their quality factor (Q) and dielectric constant (ϵ_r). While Q and ϵ_r both vary as a function of the ceramic material, Q ranges from the hundreds to as high as 1000, while ϵ_r spans from slightly less than 10 to nearly 100. High Q yields sharp resonant behavior (steep rolloff for filters), while high ϵ_r reduces the wavelength, and therefore the physical length required for a given electrical length, approximately in proportion to the square root of ϵr.

The result is a high-performance filter with a very small physical footprint, sharp passband “skirts,” narrow selectivity, and low insertion and return loss in the passband.

Temperature-induced drift is a major problem for many filter implementations and complicates the error budget. However, silver-plated ceramic materials are highly predictable and consistent in their temperature response. Their temperature coefficient of resonant frequency typically ranges from 0 to 10 parts per million per °C (ppm/°C). Even with a “high-end” drift of 10 ppm/°C, a filter centered at 1000 MHz would drift by only 0.6 MHz over a temperature range of 25 to 85°C.

Ceramic resonator filters are rugged. Fired at temperatures exceeding 1000°C and plated with silver, they can withstand very harsh operating environments. While they are affected by condensing humidity, once the water droplets evaporate, the filter returns to normal performance levels.

Two examples illustrate the capabilities

The CBP2 series of ceramic filters from Mini-Circuits shows what this filter technology can do for designers. The series targets applications requiring sharp selectivity and low insertion loss, including wireless infrastructure, satellite communications, aerospace and defense systems, test and measurement equipment, RF front-end modules, and interference mitigation and signal conditioning.

For example, the CBP2-1300BV+ is a 50 ohm (Ω), 1200 to 1400 MHz bandpass filter (200 MHz bandwidth). This SMD filter comes in a shielded 13.97 × 17.78 millimeter (mm) package with a height of 3.05 mm. It has sharp rolloff (“sharp skirts”) on either side of the passband (Figure 2).

Figure 2: The CBP2-1300BV+ 1200 to 1400 MHz bandpass filter has a 200 MHz bandwidth and sharp rolloff around the passband. (Image source: Mini-Circuits)

Insertion loss, an important parameter for determining how much “makeup” gain the analog signal chain needs, is low in the passband, typically just 2.1 decibels (dB) (Figure 3, left). It remains relatively flat and stable over temperature (Figure 3, right), two highly desirable attributes. Out-of-band (OOB) rejection is high: 65 dB from DC to 1000 MHz.

Figure 3: The insertion loss of the CBP2-1300BV+ out to 20 GHz shows a sharp drop in the passband (left); a zoom-in around the passband (right) shows the flatness of the insertion loss and its stability versus temperature, two highly coveted attributes. (Image source: Mini-Circuits)

Higher in the spectrum is the CBP2-3440CC+, which offers a 240 MHz bandpass centered at 3440 MHz (3320 to 3560 MHz). Insertion loss in the passband is extremely low at 1.5 dB, and OOB rejection is 65 dB. Like the lower-frequency filter, it uses an SMD package but has a smaller footprint and lower height, 11.43 × 12.70 mm and 3.1 mm, respectively.

Conclusion

Bandpass filters are critical building blocks in the analog RF signal chain. To meet the performance, consistency, and stability requirements for GHz signals, designers can use off-the-shelf filters with resonant structures and silver plating on ceramic substrates. The CBP2 series from Mini-Circuits exemplifies the capabilities of this filter technology.

Related Content

Understanding Ceramic Coaxial Resonator Filters

https://blog.minicircuits.com/understanding-ceramic-coaxial-resonator-filters/

Use LTCC Technology for High-Performance, Compact, and Stable RF Bandpass Filters

https://www.digikey.com/en/articles/use-ltcc-technology-for-high-performance-compact-stable-rf-bandpass-filters