ROG Blog

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 Discontinuities in Microwave Circuits

 Designing high-frequency microwave circuits and, with increasing frequency, millimeter-wave frequencies require for the most part laying out carefully conceived transmission lines to carry those high-frequency signals across a printed-circuit board (PCB). Of course, if the task of fabricating the PCB was simply a matter of adding circuit elements, such as resistors, capacitors, and inductors, to create the necessary frequency-domain/time-domain response for the PCB, it might go somewhat easier. But every PCB with high-frequency transmission lines must also manage any number of circuit discontinuities and junctions. Since these and similar discontinuities can be found on all but the simplest of RF/microwave circuits, the question is “How can the effects of these discontinuities be minimized through the thoughtful choice of PCB material?” 

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Probing Microwave PCB Heating Patterns

Learn more about this topic at IMS 2014

RF/microwave power applied to a printed-circuit board (PCB) will generate heat. A key to designing a practical circuit on a given PCB material is to understand how different circuit material properties can impact the heating patterns on an RF/microwave PCB, and to work within the limits of a high-frequency circuit material.

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Sorting Circuit Materials For Low-Noise Amplifiers

Low-noise amplifiers (LNAs) are essential in many high-frequency receivers, delivering gain where needed while keeping noise levels to a minimum. Designing an effective LNA circuit often comes down to a critical choice of active device, such as a transistor or integrated circuit (IC). But selecting the right printed-circuit-board (PCB) material can also have a lot to do with achieving LNA performance goals, since circuit laminates can contribute a great deal to final amplifier noise-figure performance. It can help to know what to look for when selecting PCB materials for RF/microwave LNAs.

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Comparing Differential and Single-Ended Transmission Lines

Circuit designers often channel RF/microwave signals by means of a variety of different transmission-line technologies, such as microstrip or stripline transmission lines. They may also choose between single-ended and differential (also known as balanced) circuit configurations for certain applications, such as when the benefit of differential transmission lines may be necessary to suppress the influence of outside noise or signal sources. How do these types of transmission lines and circuit configurations differ, and how do they impact the choice of circuit material? Each circuit approach has positive and negative aspects, and the choice of printed-circuit-board (PCB) materials can play a role in the level of performance possible with each circuit configuration.
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Ring Resonators Help Characterize PCB Materials

Circuit designers often reach for a particular printed-circuit-board (PCB) material based on what they know of its essential material properties, such as dielectric constant (Dk) and dissipation factor (Df). At lower frequencies, having accurate material parameters may be helpful but not crucial whereas, at higher frequencies, knowing such circuit-material parameters as Dk and Df can be critical to the success of a circuit design. For those who need to know, fortunately, a number of different methods have been developed over the years for measuring and calculating a PCB material’s Dk at different frequencies, perhaps other than what is provided on the data sheets by a circuit-material’s manufacturer. One of the most reliable methods for determining a PCB material’s Dk and Df values is through the use of microstrip ring resonator circuit elements, due to the relationship of the resonant frequency of these circuit elements to the permittivity of the PCB material.
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Sending Circuit Materials Into Space

Satellites in outer space are vital and increasingly important parts of modern communications, as signals are constantly being bounced off orbiting spacecraft to enable everything from telephone calls to video entertainment. Printed-circuit boards (PCBs) in space and satellite-communications (satcom) applications are often taken for granted, but they are special, since they must function reliably in extremely hostile environments. The choice of PCB material for these applications should be carefully considered, since these circuits must literally last a lifetime, or at least through the life of the satellite.
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Calculators Enhance Circuit Materials

Choice of printed-circuit-board (PCB) material can be critical to the performance of an RF/microwave circuit design, since so many different circuit-material parameters must be considered. As those who follow this Blog series know, many parameters can impact the selection of a PCB material for a particular design, including dielectric constant (Dk), dissipation factor (Df), and a variety of thermal traits. To ease the process of reviewing these many circuit-material parameters, Rogers Corp. offers a number of practical calculators on their web site’s Technology Support Hub. One of the more useful of these personal-computer-based calculators is the ROG Calculator Online.
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Perusing PCBs For Low PIM Levels

Passive intermodulation (PIM) is the unwanted mixing of two or more signals in a passive circuit or component, resulting in unwanted spurious or harmonic signals. These additional signals can clutter a system’s operating passband and cause interference in a system’s receive band. Although PIM is often associated with certain high-frequency passive components within a system, such as connectors, cables, filters, and couplers, it can start with the printed-circuit-board (PCB) materials, in particular when those materials are used for components critical to communications systems, such as PCB-based antennas and filters.
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Remember the Copper When Choosing PCBs

Achieving reliability and high performance from a PCB material is not simply about selecting the optimum dielectric material for an application—the copper on that material also plays a critical role. The quality of the copper on a PCB, including its surface roughness, and how the copper is joined with the dielectric material, can go a long way towards determining the performance possible with that PCB material, particularly at RF and microwave frequencies.
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Teaming Software Tools With PCB Materials

Choosing a printed-circuit-board (PCB) material is a critical step in the design of any new RF/microwave circuit, since the material’s characteristics will ultimately determine the performance and capabilities of the circuit. Picking an electronic-design-automation (EDA) software package to model any new design can go a long way towards improving the design experience (and the levels of performance possible) since the right software can help predict the performance of circuits fabricated on different PCB materials. Understanding how these EM simulation software tools differ and how they can be applied to model different PCB materials can greatly benefit the design process.

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