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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.

Taming Loss In Transmission Lines

Transmission lines are akin to electronic roadways, routing signals along different paths of a printed-circuit board (PCB). At RF/microwave frequencies, circuit designers often create PCBs based on three popular planar transmission line approaches: microstrip, stripline, or coplanar waveguide (CPW). Each uses circuit-board materials in a different way, with different results in terms of insertion-loss performance. By getting a grasp on the insertion-loss mechanisms for these different transmission-line formats, circuit designers can better match the mechanical and electrical characteristics of their circuit substrates to their intended applications and transmission lines when choosing PCB materials.
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Matching Materials To Bandpass Filters

Bandpass Filters, Part 2
Part 1 of this two-part series on bandpass filters—highlighted the versatility of one circuit material from Rogers Corporation, RT/duroid® 6010.2LM laminate, for fabricating RF/microwave bandpass filters. But not all circuit materials are the same and there may be some advantages to designing bandpass filters on other materials, such as Rogers RO4000® family of printed-circuit-board (PCB) materials. This blog will examine different grades of these and other circuit materials and the impact they have on the design and fabrication of high-frequency bandpass filters, especially compared to filters formed on filled-PTFE-based circuit materials.
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Choose Circuit Materials For Bandpass Filters

Bandpass Filters, Part 1

Bandpass filters are essential to many RF/microwave circuits and systems. They eliminate unwanted signals and noise, and can work with both receivers and transmitters. This first of two blogs on RF/microwave bandpass filters will review some of their basic performance parameters and how they relate to PCB material characteristics, with a focus on one material in particular, RT/duroid® 6010.2LM circuit material from Rogers Corp. As a followup, the next blog will explore how bandpass filters perform on other circuit materials.


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Prime Material Parameters For Impedance Matching

Impedance matching, Part 2
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.
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The Role of PCB Materials In Impedance Matching

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.
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Dielectric Concerns For Directional Couplers

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.
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Make Waveguide In Planar PCB Form

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.
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Creating Effective Cavity Resonators

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.
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Picking PCB Material For A Patch Antenna

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.
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Charting Conductor Profile Effects On Stripline

Microwave circuits are generally susceptible to the effects of copper conductor surface roughness, some less than others. As circuit designers learned in the last blog, microstrip designs that must minimize loss can benefit from the use of circuit materials with low-profile copper conductors, such as RO4000® LoPro™ series circuit laminates from Rogers Corp. As the last blog pointed out, CPW and conductor-backed CPW (CBCPW) circuits will not benefit at higher frequencies from the use of laminates with low-profile conductors to the same degree as microstrip circuits. But is this also the case with high-frequency or high-speed stripline circuits? Just how does conductor surface roughness influence high-frequency stripline circuit performance?


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