LTCC Filter and Capacitor Component Selection for RF Designs

Integration started becoming a major trend in the embedded world long before the first smartphones, but even highly integrated systems need discretes and external passives to work properly. If you need to design filtration stages for an RF signal chain or to suppress unwanted reception in undesired bands, your choices are to use discretes or use an integrated solution. Designing with discretes is fine at low frequencies, but these circuits eventually become electrically long, even if the smallest possible component footprints are used during design.

Enter low-temperature co-fired ceramic (LTCC) components and hybrid circuits. LTCC filter circuits are an alternative to shielding for suppressing reception in unwanted bands. They are also a simple solution when filtering is needed, providing equivalent higher-order low-pass, bandpass, and high-pass filtration with high attenuation in stop bands. If you want to use an LTCC to provide high out-of-band attenuation and reduced system size, follow these guidelines.

Selecting an LTCC Filter

The primary reason for selecting an LTCC filter is its function as an impedance matched filter element without the need for external passives. The fabrication process for these components supports integration of multiple circuit elements into a small package with precisely engineered insertion/return loss in desired bands. These products are manufactured as monolithic ceramic structures with a standard rugged, low-profile SMD package.

These components integrate multiple circuits into a single multilayer structure on a ceramic substrate. The ceramic substrate in turn has high Dk value (can reach as high as 10). These circuits can include active or passive circuit elements. While the primary application for these components is for RF devices, other potential applications involve sensing, MEMS, and other analog applications.

Insertion and Return Loss

When selecting an LTCC filter, the most important specifications are the insertion loss and return loss spectra. These spectra are shown in graphs and have the same meaning as in transmission lines and other circuits. High quality LTCC filter circuits should have less than 1-2 dB insertion loss within the passband, and return loss should be better than approximately -20 dB.

Obviously, ideal filters should have negative infinity return loss and 0 dB insertion loss, so a 1 dB insertion loss value can limit the useful interconnect length. However, this should illustrate the advantage of using an LTCC filter. By routing through the LTCC, you reduce the total interconnect length by consolidating discretes into a single hybrid circuit. This could reduce total losses seen by your RF signal, particularly at very high frequencies.

Other Important Specifications

LTCC bandpass filters are generally designed to accommodate a specific protocol or frequency band, and product literature will normally state the intended band. This can help you get started comparing specifications for LTCC filter components. For broadband filtration, an LTCC filter could accommodate a range of bands as long as the insertion loss and return loss are not too large.

The other important specifications involved in selecting an LTCC filter include:

• Frequency range. Some component datasheets will just state an upper and/or lower cutoff frequency to define the passband. For bandpass filter circuits, the frequency range is sometimes specified using a lower cutoff and 3 dB bandwidth.

• Impedance matching. With LTCC filters being designed for use in RF circuits on a PCB, they are generally impedance matched to 50 Ohms at the input and output.

• Stopband attenuation. This is another way to communicate the insertion loss spectrum. This will sometimes just show insertion loss data over a broader range of frequencies.

• Product match. Some LTCC filter components are specifically engineered to match with specific connectors or other components. An example is the Johanson 0900FM15D0039E, which is designed to provide filtration and impedance matching to some Semtech transceivers.

• Package size. LTCC filters designed for RF applications can have 1206 (imperial) or smaller SMD package size. This puts them into the same footprint size as typical SMD capacitors.

• Power rating. If you need an LTCC filter for an RF power amplifier, take note of your power rating. These components have a higher power rating than discretes in the same package, but the power rating should still sit above the desired power of your RF signal.

2 Example LTCC Filters

As an example of what you can find on the market, take a look at the Taoglas LLP.5875.Y.A.30. This low-pass filter was designed for Wifi frequencies at 5.8 GHz with low passband ripple and high attenuation. Insertion loss is only ~0.6 dB in the passband with maximum 0.5 dB variation, which is good for a 1.25 mm long component.

Another option is the Johanson 5400HP05A0950T high-pass filter. This component also targets Wifi 5.8 GHz frequencies; it has higher insertion loss of ~1.5 dB, but it has higher out-of-band attenuation (-30 dB minimum) below ~4.9 GHz. This component is great for rejection of lower frequency signals in wireless devices, such as smart home devices and wearables. 

Pushing LTCC Filters Into mmWave Designs

LTCC filter components have a track record of success in supporting RF designs in common frequency bands (Wifi and Bluetooth), as well as for ensuring high precision reception and out-of-band rejection in ISM bands. For newer mmWave designs, companies are developing LTCC capacitors and filter circuits specifically to support higher frequencies. The push to these frequencies is being driven primarily by 5G rollouts and the need for ultra-compact high-frequency filter circuits.

Traditional mmWave filters are normally designed as passive elements directly on a PCB, by using SMD discretes with weak parasitics, or by taking advantage of cutoff frequencies in waveguides. Even though many SoCs and CPUs for mobile/IoT devices are highly integrated, SMD filter circuits operating up to 40 GHz are still desirable. LTCC filter circuits can provide a solution well beyond 6 GHz, with some components soon to be available in small package sizes (e.g., 1206 imperial).

In addition to LTCC filters and other components, RF designers need a range of components for mobile systems, edge systems, base stations, and other unique devices that lack integrated components. Some component requirements include:

• RF power amplifiers

• Antenna switches

• RF transceivers

• Beamforming ICs

There are many LTCC filter options on the market, and new LTCC circuits are being developed for commercialization. You can keep up with all the newest component developments when you use the complete set of advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to current distributor pricing data, parts inventory, and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our passive components page to find the components you need.

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