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Successful high-frequency circuit design requires achieving an impedance match among a wide range of transmission-line features, circuit elements, and active and passive components. In the previous blog, some of the challenges in achieving good impedance match at RF/microwave frequencies were detailed, including the importance of a printed-circuit-board (PCB) material with stable and consistent effective dielectric constant. To further explore the impact of a circuit substrate on high-frequency impedance matching, two popular PCB materials from Rogers Corp. (www.rogerscorp.com), RO3010™ and RO3035™ circuit materials will serve as examples to show how circuit-material parameters can be translated into solutions for high-frequency impedance-matching issues.

Impedance matching is an aspect of RF/microwave design that has challenged even the best circuit designers from time to time. High-frequency circuit designers generally aim for a characteristic impedance of 50 Ω, unless they are working on cable-television (CATV) circuits, which typically operate at 75 Ω. For the lowest phase distortion and flat amplitude response, most RF/microwave circuit designers start with ensuring that all of the possible impedance mismatch points, such as transmission-line junctions, connections to components, and terminations with connectors, are as close to 50 Ω as possible.

Directional couplers are vital components for sampling signal power in an RF/microwave system without necessarily disturbing the signal path. Such couplers come in many forms, including in metal housings with coaxial connectors. A typical coaxial directional coupler has four connectors, for input, output, coupled, and isolated ports. The coupled port provides a small amount of power taken from the input port, defined by the coupling factor, such as a 20-dB coupler. This blog posts covers the advantages of using high quality PCB materials for directional couplers.

Forming resonant cavities on microwave printed-circuit boards (PCBs) is a good first step to the design of high-frequency oscillators and filters. Another approach is the use of substrate-integrated-waveguide (SIW) technology, which is not only suitable for oscillators and filters, but can be formed into extremely compact antennas, and can support signals well into the millimeter-wave range. SIW technology structures are versatile design elements for integrating active and passive circuits together with radiating elements, such as antennas, onto compact circuits using popular PCB laminate materials. As the last blog showed, creating resonant cavities in different circuit materials can lead to high-performance microwave oscillators and filters. As this blog will reveal, SIW structures can also serve as resonators in planar, multilayer PCBs, helping to create compact, high-performance filters, oscillators, and other resonator-based circuits.

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.

A patch antenna, which is also known as a microstrip antenna, can be fabricated with standard printed-circuit-board (PCB) processes using high-frequency laminate materials. An antenna can be as simple as a rectangular patch above a ground plane or as elegant as a complex array of patches, customized for a specific radiation pattern. As with other printed circuits, the choice of circuit material can greatly impact the performance possible from the final antenna design. That choice should be guided by a clear understanding of how a circuit material’s electrical and mechanical properties relate to the performance of a patch antenna.

Conductor surfaces on a printed-circuit board (PCB) can vary widely, from a “smooth-as-glass” finish to a more typically rough profile. As detailed in an earlier ROG Blog, a conductor surface profile can impact the performance of a sufficiently high frequency circuit, increasing insertion loss and dispersion and even causing propagation delays and changes in the effective dielectric constant of the circuit. But are all RF/microwave transmission-line technologies affected by conductor surface profiles in the same way? What about designs based on coplanar-waveguide (CPW) transmission lines, or stripline? Do they require circuit materials with low-profile conductors, or are they less affected than microstrip by conductor surface roughness?

The ROG Award Contest kicked off in November 2011. We invited customers to enter the contest by telling us about their applications using Rogers?materials. For the next several months, customers shared stories about their Best Digital Application, Most Challenging Board Build, Most Unique and Creative Use of Material, Most Innovative Design, Longest Product Life, and Most Extreme Conditions, all accomplished together with Rogers?materials. One winning application was chosen for each category.

Multilayer microwave PCBs can pack much circuitry into a small volume. But creating a multilayer microwave circuit is more than just stacking layers. It takes planning and knowledge of the multilayer fabrication process. In particular, an understanding of how prepreg materials are used in multilayer circuits can be helpful.