Resonators as Microwave Devices

This blog post originated from Knowles Precision Devices, explores the basics of resonators as microwave devices and dive deeper into specifications for ceramic coaxial and dielectric resonators specifically.

Resonators are essential building blocks for bandpass filters as these components are designed to store frequency-dependent electric and magnetic energy.

A simple resonator, such as an LC resonator, can store the frequency-dependent electric energy in the capacitance (C) and magnetic energy in the inductance (L).

Resonant frequency of a resonator occurs when the energy stored in the electric field is equal to the energy stored in the magnetic field. In this post, we’re going to look more closely at two different types of resonators used in filters – coaxial ceramic and dielectric.

Figure 1. How electrical and magnetic energy travel in a coaxial transmission line; credit Knowles Precision Devices

Coaxial Ceramic Resonators

If you’ve ever had cable television service, then you have used a ceramic coaxial transmission line. The transmission line is constructed using a ceramic rectangular prism with a coaxial hole running through the center.

More specifically, the ceramic coaxial resonator is a transmission line resonator that operates in transverse electromagnetic (TEM) mode. TEM mode has both E- and H-field components at right angles to the z-direction and no signals in the direction of propagation as shown in Figure 1.

Each coaxial resonator transmission line is cut to a specific length based on the wavelength of the frequency of interest. To make the lines shorter, ceramics with specific dielectric constants are used to shrink the wavelength to the frequency of interest.

The dielectric used, length of the line, and metallization will dictate the performance at the frequency of interest. Coaxial resonators can be supplied as λ/4 resonators with one end fully metallized (i.e., shorted) and the other end open, fully metallized on both ends, or as λ/2 resonators with both ends open as shown in Figure 2.

Figure 2. How a signal will be impacted depending on the type of ceramic coaxial resonator added.; credit Knowles Precision Devices

Knowles Precision Devices design high Q ceramic resonators made with class I dielectrics to operate in UHF, VHF, L, S, and C bands and in microwave frequency ranges, covering a total frequency range of 300 MHz to 5 GHz. A sampling of Knowles ceramic coaxial resonators is shown in Figure 3.

Some of the other key features of our range of ceramic coaxial resonators that are built at our facilities in the US include the following:

Figure 3. A variety of Knowles tabbed and no-tab ceramic coaxial resonators.
Figure 4. The range of Knowles coaxial resonators.sizes

For surface mount devices (SMDs) tabs, tables, or slotted no-tab configurations are available, with our tabless version eliminating solder reflow issues and tab misalignments. Our resonators are pre-tuned to your specified frequency with a choice of tolerances and screened to ensure customer specifications are met. However, the self-resonant frequency (SRF) can be tuned by removing the silver metallization at either the open end or the shortened of the resonator to either increase or decrease the SRF.

For filtering applications specifically, our coaxial resonators made with modern, high-performance ceramic dielectric materials are an excellent option for bandpass, band stop, narrowband, delay, and EMI filters. You can also learn more about ceramic coaxial resonator technology in this related blog post.

Dielectric Resonators

Dielectric resonators are a kind of ceramic cavity resonator that work by confining frequencies inside the resonator material as a result of an abrupt change in permittivity at the surface, causing RF waves to bounce back and forth between the sides. The Knowles Precision Devices DLI brand includes a family of patented high-Q ceramic cavity resonators that offer an ideal solution for high-performance, low-cost microwave or mmWave oscillators with frequencies ranging from <1GHz to >67GHz (Figure 5).

Figure 5. Q vs frequency for various resonator types; credit: Knowles Precision Devices

As shown in this graph, these resonators can reach much higher frequencies than alternative technologies. These resonators are also fully shielded and designed on our temperature stable, high dielectric constant ceramics. Some of the key specifications that can be built into our range of DLI brand ceramic dielectric resonators include the following:

Dielectric resonator designs can be customized for either solder-surface mount or chip and wire applications as well.

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