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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

Creating Effective Cavity Resonators

September 30, 2012
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Resonant circuits are critical for the generation and selection of desired RF/microwave frequencies. For any transmission line, including stripline, microstrip, or waveguide, a suitable length can be used as a resonator, with dimensions for the resonant structure that correspond to the desired wavelength. When that resonant structure is in the form of a cavity, it is simply called a cavity resonator. High-frequency cavity resonators, for example, serve as excellent starting points for RF/microwave oscillators capable of generating low-noise signals and for filters used to select signals at specific frequencies. For example, cavity resonators can be embedded within a multilayer circuit substrate, to achieve a high-quality resonance without a larger metal cavity or tuning screw. Excellent performance is available from such multilayer cavity resonators, given available high-frequency circuit laminates and the pre-impregnated glass fabric (prepreg) materials.

Cavity resonators are essentially hollow conductors or sections of a printed-circuit board (PCB) which can support electromagnetic (EM) energy at a specific frequency or group of frequencies. An EM wave entering the cavity that is resonant within the cavity will bounce back and forth within the cavity with extremely low loss. As more EM waves enter at that resonant frequency, they reinforce and strengthen the amplitude of the existing resonating EM waves.

The resonant frequency or frequencies of a cavity depend on several factors, including the dimensions of the cavity, the materials that form the cavity, and how energy is launched and/or extracted from the cavity. A resonant cavity is sometimes referred to as a form of in-circuit waveguide, short-circuited at both ends of the waveguide structure so that EM energy builds within the cavity at a designed frequency or band of frequencies. The size of a cavity resonator, for example, is a function of the desired resonant frequency and the characteristics of the PCB materials used for the resonator. PCB materials with higher dielectric constants will support smaller cavity resonators for a given frequency than circuit substrate materials with lower dielectric constants.

While there are many ways to create a cavity resonator in a PCB, most methods rely on either building up materials around an empty area on the PCB, or removing materials from a PCB structure to form an empty area, such as by means of laser ablation. In forming a window-type resonant cavity in a multilayer circuit assembly, the different layers that create the circuit assembly also form the walls of the resonant cavity. Such circuit-material layers often include a high-performance circuit material, such as RT/duroid® 5880, RO4003C™ LoPro™, or RO4350B™ LoPro laminates from Rogers Corp. (www.rogerscorp.com), and a compatible prepreg material, such as RO4450F™ prepreg, also from Rogers Corp., to bond the circuit layers together.

In the window-type approach to forming cavity resonators, windows are punched into some of the circuit layers used to assemble a multilayer circuit. As the laminate and bonding or prepreg layers are assembled, the layers forming the windows will create the walls of the soon-to-be resonant cavity. The size of this cavity, of course, determines the ultimate frequency or frequencies of the resonant cavity, so manufacturing efforts are usually focused on keeping the dimensions of the resonant cavity tightly controlled.

Ideally, prepreg materials used for bonding the multilayer structure have the flow characteristics required for a multilayer resonant cavity. For example, in a multilayer construction in which voids must be filled, such as in circuits with plated copper, prepregs with “high-flow” characteristics are desired. But when bonding of multilayers is needed, without flow into the resonant cavity formed by those multiple laminate layers, a “low-flow” prepreg is preferred, with a high glass transition temperature (Tg) for good reliability. Because the bonding materials in a multilayer circuit assembly will flow during lamination, designers must be wary of bonding materials that lack good flow control and might flow into the resonant window or cavity area, changing the dimensions of the resonant cavity (and its operating frequency or frequencies). An effective multilayer prepreg should exhibit low loss, good adhesion to commercial PCB laminates, stable dielectric constant with temperature and frequency, and the capability of supporting multiple or sequential laminations if needed.

Ideally, any prepreg in a multilayer circuit assembly with a resonant window should have not only low-flow characteristics, but predictable flow characteristics. The predictability allows for tight control of the circuit manufacturing process. In a circuit with a resonant cavity, a prepreg with predictable flow may alter the size of the cavity because of that flow, but it will be in a manner that can be predicted and even modeled in a commercial EM computer-aided-engineering (CAE) software program such as Ansoft HFSS. However, if the prepreg has low-flow characteristics without predictable flow, the final size of the resonant cavity will vary according to the flow characteristics, as will the resonant frequency or frequencies of the cavity.

As an example, RO4450F prepreg is a low-flow prepreg material with relatively well-controlled flow characteristics. It is compatible with RO4350B or RO4350B LoPro laminates and well suited for forming multilayer cavity resonators with consistent and predictable characteristics. In contrast, Rogers’ 2929 is also a durable prepreg material, but with greater flow than RO4450F material. Although both are candidates for a multilayer cavity resonator design, the fabrication and lamination conditions will dictate which prepreg provides a greater level of consistency in a final production run.

RO4450F and the RO4400™ family of prepreg materials are based on the RO4000® core materials and readily compatible with those laminates in multilayer constructions, such as in cavity resonator designs. The prepregs feature a number of key attributes that contribute to reliable performance in multilayer constructions, including a high post-cure glass transition temperature (Tg) of greater than +280°C, an indication that the prepregs are capable of handling multiple lamination cycles. The RO4400 prepregs also support FR-4-like bonding process conditions (+177°C), enabling the use of standard lamination equipment.

Optimum performance from any multilayer cavity-resonator-based design, whether for generating or filtering signals, requires careful consideration of the type of feed structure used with the cavity resonator, especially at higher frequencies. A number of approaches provide good results through millimeter-wave frequencies, including slot and probe excitation techniques. Using a slot is fairly straightforward and requires very simple fabrication while probe excitation, which can be somewhat more demanding in terms of fabrication, can yield extremely wideband results. Some cavity-resonator filters, for example, have used feed approaches as simple as a microstrip line through a coupled slot in the ground plane. In the case of either slot or probe feed in a multilayer cavity-resonator construction, high-quality prepreg materials help ensure minimal loss and stable performance.

The next blog will continue this discussion on PCB resonant cavities. But it will take a somewhat different point of view, focusing on buried waveguide structures and providing several examples.

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.   

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