Whether you require thick materials for enclosures or thin materials for LCD gaskets, the **PORON® Urethanes Gap Filling Tool** helps you identify the right foam materials for your gap filling applications. The configurator is available for desktop and mobile viewing.

If this is your first time using the tool, we recommend that you ** watch the tutorial **(2:58) for an overview of how to use the tool.

At the bottom of the Gap Filling Tool page, you will find a technical question/feedback form. Please let us know what you think.

*This post authored by John Coonrod *originally appeared* on the **ROG Blog** hosted by **Microwave Journal**. *

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.

In general, consistency is important for any material intended for an LNA circuit, whether the consistency is in terms of dielectric constant, in dielectric thickness, or in conductor thickness. Tight impedance matching must be achieved with the circuits and devices within an LNA circuit to minimize noise. Excessive variations, for example, in the dielectric constant or substrate thickness of a PCB can result in inconsistent passive circuit elements and difficult impedance matching for low-noise transistors and ICs, yielding variations in noise figure with frequency.

What do LNA designers look for when selecting PCB materials? Creating a successful LNA circuit design involves a number of different factors, including the use of a PCB material with minimal conductor losses and dielectric losses, a circuit material that enables the fabrication of the passive circuit elements required to achieve the close impedance matching needed between the circuitry and active devices for excellent low-noise circuit conditions. Quite simply, circuit impedance mismatches will raise the noise figure of an LNA, so noise-figure performance can be optimized by minimizing those mismatches to the active devices. Circuitry that is not optimally impedance matched for a particular active device can also lead to unstable operating behavior, since low-noise active devices can oscillate at frequencies beyond their fundamental-frequency ranges when improperly matched.

Low-loss dielectric materials are usually the foundation for RF/microwave LNAs, to minimize transmission losses to and from the LNA’s active circuitry. In particular, it is critical to minimize transmission-line losses leading to the LNA input port, since such losses add directly to the amplifier’s noise figure.

One of the more popular circuit “starting points” for LNA designers **is ****RO4350B™ circuit laminate** from **Rogers Corp**. which is suitable for LNAs through millimeter-wave frequencies. This material exhibits the many characteristics that appeal to LNA designers, including tightly controlled dielectric constant, low dissipation factor, and high thermal conductivity. The RO4350B material is characterized by a dielectric constant of 3.48 at 10 GHz in the z-axis of the material, with dielectric-constant consistency across the board maintained within ±0.05. For an LNA designer, this characteristic helps translate into an LNA noise figure that remains consistent with frequency, input level, temperature, and other operating conditions.

The RO4350B material also exhibits low conductor and dielectric losses, both essential to achieving low LNA noise figures. In PCB materials, loss performance can usually be compared by means of dissipation factor, and in the RO4350B material, this parameter is typically 0.0037 in the z-axis at 10 GHz, dropping to 0.0031 in the z-axis at 2.5 GHz.

This material has many other characteristics that make it attractive to RF/microwave LNA designers and is even available in a version with low-profile copper, as **RO4350B LoPro™ materials**, for reduced conductor losses when searching for extremely low LNA noise figures. Both RO4350B and RO4350B LoPro circuit materials feature low dielectric loss and both circuit materials are formulated to allow circuit fabrication with standard FR-4 epoxy/glass circuit-material processing approaches. These standard circuit processing approaches include fabrication of reliable plated through holes (PTHs) for an LNA without special preparation steps as typically required with PTFE-based circuit materials.

In line with the FR-4 processing methods, the RO4350B and RO4350B LoPro circuit materials are often combined with low-cost FR-4 as a hybrid multilayer circuit. The hybrid PCB allows the use of the **RO4000® circuit materials** for the critical electrical performance layers and the FR-4 layers can be electrical layers which are less critical. The different circuit materials are often combined by means of a low-loss bondply material such as **RO4450F™ prepreg** from Rogers Corp. to preserve the low-loss conditions favorable for excellent LNA low-noise performance. The RO4450F prepreg features a dielectric constant of 3.52 in the z-axis at 10 GHz, with low dissipation factor of 0.004 in the z-axis at 10 GHz and outstanding dimensional stability to preserve the mechanical integrity of the joined circuit materials in an LNA design.

LNA designers in search of the optimum noise-figure performance from their devices will often seek circuit materials with stable dielectric constant and mechanical integrity and the lowest possible dissipation factor to minimize noise-figure contributions from the circuit-board material. One of the circuit laminates that serves as a building block for LNAs with the lowest possible noise figures is the **RO3035™ circuit laminates** from Rogers Corp. With almost negligible dissipation factor of 0.0017 in the z-axis at 10 GHz, these circuit materials contribute very little in terms of dielectric losses to LNA circuits, maintaining a dielectric constant that remains within ±0.05 of 3.50 in the z-axis at 10 GHz. These ceramic-filled PTFE composite materials are formulated with an in-plane expansion coefficient that is closely matched to that of copper, for outstanding mechanical stability in temperature-sensitive circuits such as LNAs. They exhibit good thermal conductivity of 0.50 W/m/K in support of stable, LNA circuitry through millimeter-wave frequencies.

These different circuit materials provide the stable performance characteristics with the low dielectric and conductor losses needed to achieve the low noise figures with frequency for LNAs based on different active devices. Some general circuit practices can help reach those lower LNA noise figures, such as using high-Q rather than low-Q capacitors in matching networks, and using thinner rather than thicker circuit materials when possible. But, in general, the choice of a circuit laminate with stable dielectric constant across frequency and low dissipation factor such as the several circuit materials noted above can only help in the quest for the lowest possible LNA noise figures at RF/microwave frequencies.

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.

The Forklift Test: Start with a bowling ball at 20 feet… then pit 2″ EVA common protective padding against PORON® XRD™ technology. On impact, the **PORON XRD** high performance molecules create a unique protective shield that absorbs up to 90% of the most intense force* – hit after hit.

PORON XRD technology is designed to meet the demand for a protective foam material that performs in the most intense conditions, while at the same time providing comfortable, lightweight, breathable, and soft/flexible qualities.

*This post authored by John Coonrod originally appeared on the **ROG Blog** hosted by **Microwave Journal**. *

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.

Ring resonators represent one method for determining the Dk of a PCB material. Resonance measurement techniques are narrowband and an alternative approach to more broadband transmission-reflection techniques for determining PCB material Dk. A major difference in the two approaches is that resonance measurement methods, which fabricate ring resonator circuit structures on a PCB, work at one frequency at a time, while transmission-reflection approaches can be used for swept-frequency measurements, to determine a circuit-material’s dielectric constant across a range of frequencies.

Especially as available higher frequencies are being used more and more for communications applications, there is a greater need for accurate characterization of the PCB materials used to fabricate those higher-frequency circuits. Ring resonators have often been used to measure the Dk of PCB materials, although they must be used with knowledge and with care. Ring resonators can be formed on a PCB material of interest using standard circuit-fabrication techniques. They can be constructed as single-port or two-port ring resonator structures. The simpler, single-port structure can be used to determine Dk but not Df. The two-port ring resonator, which is a thru-type circuit design with transmission lines leading to and from the resonant circuit, includes feed lines to and from the resonator structure, a closed microstrip transmission-line structure, and coupling gaps between the resonator and the feed lines. The method is well established and proven, and is typically the resonator structure of choice for measurements of PCB material Dk and Df.

This type of resonator structure typically suffers minimal radiation losses, rendering additional calculations or measurements of radiation losses unnecessary. For high-loss PCB materials, calculations of conductor losses can have minimal impact on estimations of Df. Conductor losses are more pertinent for low-loss materials, where those losses may play a more dominant role in the total loss behavior of the PCB material.

The frequency response of the two-port ring resonator structure can be measured with an RF/microwave vector network analyzer (VNA). The unwanted effects of connector interfaces of the ring resonator structure must be eliminated, which can be done through the use of a thru-reflect-line (TRL) calibration of the VNA with appropriate TRL calibration standards.

What are the PCB parameters of interest when using ring resonators to determine dielectric constant? Some pertain to the material itself, such as the thickness of the substrate material and the thickness of the conductor metal. For thinner circuit materials (less than 5 mils in thickness), a ring resonator may not represent the optimum method for measuring PCB material Dk, since it can be difficult to develop a clean resonant peak from the material and ring resonator for a measurement. Some of the PCB parameters of interest are related to the ring resonator structure, such as the fundamental resonant frequency, the line width of the ring resonator, the length of the feed lines, and the length of the coupling gaps.

A microstrip ring resonator is coupled through gaps to the microstrip feed lines for a two-port ring resonator (and single microstrip feed line for a single-port ring resonator). The operation of a two-port ring resonator is based on satisfying a simple condition for resonance defined by the equation:

2πR = nλ_{g} for n = 1, 2, 3…

where

R = the mean radius of the resonator’s ring;

n = the harmonic order of the resonance; and

λ_{g} = the wavelength of the resonance.

Based on the measured frequency response of the ring resonator, the PCB material’s Dk value can be calculated by determining the frequency-dependent value of the effective permittivity for that resonant frequency. The PCB material’s measured frequency response can also reveal other details about the material, including Df, dielectric material losses, and/or conductor losses. Of course, the surface roughness of PCB conductors can also contribute to conductor losses, with increased surface roughness resulting in higher conductor losses.

Rogers offers “**design Dk**” values of dielectric constant for its PCB materials, optimized for use in modern computer simulation programs. These dielectric constant values are determined by careful measurements, using ring resonator approaches as well as a microstrip differential phase-length technique. It is based on fabricating two microstrip transmission-line segments on a PCB material of interest. The transmission lines are identical in every way except for length. As characterized on an RF/microwave VNA, this difference in length will result in a difference in phase for the two transmission lines. The phase responses of the two transmission lines also depend on the Dk characteristics of the PCB material upon which the transmission lines have been fabricated.

The electrical contributions of the associated coaxial connectors and test fixtures, such as the reactances at the signal launches, must be minimized when using the microstrip differential phase-length approach, in order to produce results that reveal the PCB materials properties based on those two transmission lines. Further details on the **microstrip differential phase-length technique** can be found at the Rogers Corp. web site. The method can be used in combination with the ring resonator method when determining a PCB material’s Dk, with the combination helping to minimize issues with gap coupling for the ring resonator circuits.

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.

The **Rogers Innovation Center** in Burlington, Massachusetts is home to a new industry-academic partnership between Rogers Corporation and **Northeastern University**. The goal is for research in the areas of nanotechnology and nanomaterials to lead to commercially-viable innovations in advanced materials to address global challenges for clean energy, Internet connectivity, safety, and security. Researchers, engineers, faculty and students, and employees gathered to celebrate at the ribbon cutting on March 25, 2014.