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

As dependence on wireless communications, radar, satellite, and other RF/microwave systems grows, bandpass filters become increasingly critical because they select desired frequencies and reject undesired ones within the spectrum of interest. While the classical lumped-element filter implementation based on discrete passive resistors, inductors, and capacitors (RLC) has served the industry well, designers need an option for gigahertz (GHz) filters to better meet the increasingly demanding requirements for performance, stability, losses, size, reliability, consistency, and cost.

This article explores the filtering challenges designers of high-frequency systems face. It then introduces low temperature co-fired ceramic (LTCC) technology and provides example filters from Mini-Circuits to show how it can be used to address these challenges.

The basics of filters

Passive-filter theory covers many filter topologies, their characteristics, and their roles, including low pass, high pass, stopband, and bandpass filters (Figure 1). These filters are analyzed using discrete RLC components.

Figure 1: Shown are the schematic diagram symbols (left) and gain versus frequency (right) for four basic filter functions. (Image sources: Learn About Electronics)

The four filter types provide these basic attenuation-versus-frequency transfer functions:

• Low pass filter (LPF): Passes frequencies below a specific cutoff frequency and attenuates frequencies above it
• High pass filter (HPF): Passes frequencies above a specific cutoff frequency and attenuates frequencies below it
• Bandpass filter (BPF): Allows a specific range of frequencies to pass while attenuating those outside this range
• Bandstop/notch filter (BSF): Attenuates a specific, narrow frequency band while allowing others to pass

Filter topologies are classified not only by their roles but also by their names, mathematical descriptors, and equations (some of which are linked to mathematicians who predate the electronics era). These classifications include first order, second order, pi-, maximally flat, T, Butterworth, Cauer, Chebyshev, and Bessel.

While the principles and mathematics that define these filters remain valid, their lumped-element construction becomes impractical as filter frequencies reach into the hundreds of megahertz (MHz) and GHz regions. Filters based on these discrete components are too large, costly, and inconsistent for high-volume production.

Unavoidable parasitics mean that filter theory and its physical implementation yield different results, often by a large margin. As frequencies increase, each physical embodiment of the filter may need to be trimmed (often by hand) to accommodate subtle variations in these parasitics and component tolerances.

Many parameters are used to formally characterize filter performance. Among these are:

• Center frequency: the central frequency of bandpass or notch (bandstop) filters
• Cutoff frequency: the frequency at which the output signal power drops by half (3 decibels (dB)) relative to the input, marking the boundary between the passband and stopband
• Passband: the range of frequencies that pass through the filter with minimal attenuation
• Stopband: the range of frequencies that are significantly attenuated or blocked
• Roll-off rate: the steepness of the transition between the passband and stopband, measured in decibels per decade or per octave
• Insertion loss: the loss of signal power resulting from the insertion of the filter into a transmission line, measured in dB
• Ripple: small variations in gain within the passband or stopband, common in higher-order filters
• Phase shift/group delay: the frequency dependent shift in phase of the signal passing through the filter relative to the input
• S-parameters (scattering parameters): used in RF/microwave filters, such as S₂₁ (gain/loss) and S₁₁ (return loss/reflection)

In addition, there are real-world issues such as temperature-induced drift, dissipation limits, and component aging.

Going beyond discrete elements

Bandpass filters are widely used in wired and wireless systems to select a relatively broad desired band of interest, with attenuation bands on either side, and to select a relatively narrowband signal corresponding to a single channel.

To overcome the inherent limitations of filters built from discrete elements, engineers have developed and refined a range of alternative filter technologies and fabrication techniques, each with unique attributes. Among these are planar transmission lines such as microstrip, coplanar waveguide (CPW), and stripline filters; cavity filters; LTCC filters; dielectric filters; and piezoelectric filters, including surface acoustic wave (SAW) and bulk acoustic wave (BAW) filters.

LTCC filters, in particular, are miniature, rugged, and high-performance RF components designed for high-volume, space-constrained applications that now include the Internet of Things (IoT), Wi-Fi 6E/7, and satellite systems. They operate from 400 MHz to over 40 GHz and feature low insertion loss, high selectivity, consistent performance, and compatibility with automated surface-mount technology (SMT) manufacturing processes.

LTCC devices are manufactured using an advanced multilayer process (Figure 2). 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 RLC components within a robust, miniaturized, and thermally stable package well-suited for high-frequency RF/microwave applications.

Figure 2: The LTCC process involves stacking and pressing layers of passive components on ceramic-glass substrates, which are then fired to form a unified, monolithic component. (Image source: Everything RF)

By combining advances in circuit topologies, materials, simulation capabilities, and fabrication processes, Mini-Circuits has developed a family of LTCC filters suitable for microwave and millimeter-wave systems, including 5G. These LTCC filters offer many advantages over earlier technologies, such as thin film on alumina, which has a larger footprint, is more costly, more sensitive to its surroundings, and more susceptible to detuning in dense layouts. In contrast, LTCC filters provide an excellent combination of performance, physical robustness, size, cost-effectiveness, and consistency.

LTCC technology enables the use of low electrical resistance metals, such as copper, as conductors, because it is co-fired at lower temperatures than other ceramic-based filters. As a result, it can offer lower insertion loss than conventional alumina ceramics and is used in high-frequency packaging and module integration applications.

Advances in LTCC approaches

LTCC filters have traditionally delivered stopband rejection of about 30 dB to more than 90 dB.

However, when using GHz-class components, their mounting and input/output signal paths are as critical as the components themselves; it is not just an issue of having the right parts on the bill of materials (BOM), as layout and production considerations also come into play.

The problem is that these high-rejection filters often require RF energy to be “launched” from a stripline, microstrip, or other controlled-impedance transmission line to achieve their full rejection performance. This constrains designers looking to incorporate LTCC filters onto their microstrip and CPW transmission lines. For example, some designs may feature coaxial RF launch structures on the bottom surface, requiring blind vias to the conductive layer of a stripline circuit board.

However, while many printed circuit board (pc board) manufacturers have developed the capability to reliably build SMT assemblies with blind vias, some designers still prefer CPW wiring boards, in which the contact between the conductive trace and the device ports is exposed on the top layer. In addition to addressing concerns about blind vias, CPW allows other SMT components to be soldered in shunt or series with the signal trace, and to tune the trace width and characteristic impedance for optimal matching.

Furthermore, the land pattern of a filter package may include conductive plating on the bottom surface. Consequently, simply soldering the unit to the exposed trace on a CPW board would cause functional issues due to shorting between the pc board metallization and the filter’s bottom surface. 

Pick-and-place LTCC filters

To address these issues, Mini-Circuits developed the BFHKI series of CPW-compatible filters for use with a pick-and-place SMT platform. These LTCC bandpass filters consist of a subassembly comprising the LTCC component and an interposer substrate that converts the LTCC’s coaxial launch to a CPW interface (Figure 3, top). The availability of this interposer enables the BFHKI series of LTCC bandpass filters to be mounted on microstrip and CPW traces (Figure 3, bottom), delivering significant performance benefits over traditional LTCC filters and other filter technologies.

Figure 3: The BFHKI series of LTCC filters features an interposer between the filter and the pc board (top), enabling easy use with top-layer transmission lines; the pc board layout guide for the series (bottom) shows that it supports device placement on microstrip and CPW traces. (Image source: Mini-Circuits)

Two examples show achievable performance

Two representative examples highlight the RF performance of the BFHKI LTCC filters. At the lower end of the spectrum, the BFHKI-5001+ (Figure 4) is a bandpass filter with a 4.5 to 5.3 GHz passband, supporting applications such as satellite communication links, aerospace and defense signal conditioning, and quantum computing. It features a high stopband rejection of 54 dB (typical) up to 13 GHz when mounted on CPW layouts, and has a 3.6 dB (typical) insertion loss over a wide band.

Figure 4: The BFHKI-5001+ 4.5 to 5.3 GHz bandpass filter features 54 dB (typical) stopband rejection up to 13 GHz. (Image source: Mini-Circuits)

The BFHKI-5001+ is housed in a compact 0.195" × 0.144" × 0.072" ceramic package, making it a good fit for dense pc board layouts. The integrated interposer board enables installation using automated manufacturing equipment.

The LTCC fabrication process ensures minimal RF performance variation and delivers a product suitable for extreme environmental conditions, including high humidity and temperature. The shielded construction minimizes interference, sensitivity to adjacent components, and detuning, all critical factors in dense RF layouts.

At the other end of the spectrum is the BFHKI-3142+, a miniature LTCC ultra-high stopband-rejection bandpass filter. With its 28 to 36 GHz passband, this model offers 30 dB (typical) stopband rejection up to 67 GHz when mounted on CPW layouts. It has a 2.8 dB (typical) insertion loss over a wide band.

As with the BFHKI-5001+, it is housed in a small 0.195" × 0.144" × 0.072" ceramic form-factor package with an interposer for design-in and production effectiveness. Other important performance indicators include broadband insertion and return loss (Figure 5, top) and passband insertion and return loss (Figure 5, bottom).

Figure 5: Users of the BFHKI-3142+ need to know the overall insertion loss and return loss specifications (top), as well as the same specifications within the passband (bottom). (Image source: Mini-Circuits)

Conclusion

To meet the performance, reliability, consistency, size, and cost requirements of modern RF/microwave systems, designers need to move beyond classic bandpass filters built from individual resistors, inductors, and capacitors. As shown, LTCC filters in the Mini-Circuits BFHKI family provide excellent performance and the necessary characteristics, and they feature an interposer that offers designers options for board-to-filter mounting and RF interfacing.