Jan 12, 2011

John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm

Isn’t designing a microwave filter as simple as loading parameters into a computer-aided-engineering (CAE) program? In truth, many modern CAE software tools are quite good, and can provide accurate predictions of performance when fed sufficient input data. However, most do not account for all variables influencing a high frequency filter, including the effects of anisotropic printed-circuit-board (PCB) materials. When designing RF and microwave filters, it helps to choose your PCB material wisely.

High frequency filters come in many forms, based on lowpass, highpass, bandpass, and band-reject responses. As their names suggest, they are designed to operate at specific frequencies or bands of frequencies, to allow some signals to pass with minimal loss and stop other signals with high attenuation. Modern cellular telephones, for example, rely on filters to separate different cellular frequency bands within a handheld transceiver. Because of the growing integration of multiple functions in electronic devices, such as Bluetooth receivers and Global Positioning System (GPS) receivers in cellular telephones, filter designers are being asked to develop improved performance but in smaller circuits. As filters are made smaller and packed more closely together on a PCB, the choice of PCB material becomes a critical step in achieving acceptable filter performance.

Any RF/microwave filter represents by its nature a compromise in performance, since an ideal filter response might be achieved in a computer program but not in reality. Some filters, such as Butterworth designs, aim for minimal amplitude ripple in the passband while sacrificing the sharpness of the transition from passband to stopband. Conversely, Chebyshev filters provide a sharp transition from passband to stopband, but with degraded passband amplitude ripple compared to a Butterworth filter. When linear passband phase is important, a Bessel filter can deliver it, but with some sacrifice in stopband attenuation.

 

All of these compromises are well known by filter designers, who work within the limitations of different filter configurations to achieve their required performance results. What may not be as well known is the impact that different PCB materials can have on filter performance. Microwave materials that may appear similar can often respond differently under different operating conditions, such as in high-humidity environments.

When selecting a microwave substrate material, most designers start with a material’s dielectric constant as a selection criterion. Materials with higher dielectric constants support the fabrication of more closely spaced conductors with finer geometries for a given impedance than materials having lower dielectric constants. Like filters, the choice of a PCB substrate also represents a compromise, since materials with high dielectric constants have typically suffered higher moisture absorption than materials with lower dielectric constants. While the dielectric constant of a PCB material may change very little over a wide temperature range, a PCB material’s loss tangent is sensitive to moisture. PCB material products from different suppliers (sometimes even from the same supplier) respond differently to even small amounts of moisture absorption. In bandpass filters, for example, the effects of PCB water absorption on loss tangent are typically seen as an increase in passband loss.

Studies at Rogers Corporation on materials such as RT/duroid® 6010LM material used for bandpass filters (as detailed in the company’s Properties 2.9.5 bulletin, “The Benefits of Selecting RT/duroid® 6010LM  for Band Pass Filter Applications”) have shown that materials susceptible to moisture absorption, even at a level as low as 0.25%, can effect a change on the relative dielectric constant as well as an increase in dielectric loss. The RT/duroid 6010LM material is PTFE with ceramic filler, engineered for extremely low moisture absorption compared to other PCB materials with similar electrical properties (relative dielectric constant of 10.2 and loss tangent of 0.0028 in the z-direction at 10 GHz).

Another factor to consider when choosing PCB materials for RF/microwave filters is that filter design programs typically consider PCB materials to have isotropic behavior, which is to say that they have the same dielectric constant regardless of the direction of the electric field. Ideally, that would be the case. But in the real world, dielectric PCB materials have anisotropic behavior, typically with three different relative dielectric constant values in the three directions of a planar dielectric material’s electric field. Although designers often choose dielectric materials by their relative permittivity or dielectric constant at 10 GHz, they are usually comparing a value in the z-direction (substrate thickness axis). The values of dielectric constant in the x- and y-directions tend to be considerably higher.

For accurate filter modeling, the PCB dielectric material must also be accurately modeled, with CAE tools that can account for a dielectric material’s anisotropic behavior. Until recently, even high-end planar electromagnetic (EM) modeling tools did not incorporate the capability of modeling anisotropic behavior in PCB materials, and have performed calculations of filter performance based on isotropic materials. But work recently presented at the Asia-Pacific Microwave Conference (December 7-10, 2010, Pacifico Yokohama, Yokohama, Japan) by researchers at EM software developer Sonnet Software (www.sonnetsoftware.com) and Rogers Corporation (www.rogerscorp.com) has revealed the errors of assuming isotropic behavior for anisotropic materials. Their work simplifies a dielectric material’s anisotropy to two dimensions, with values for the dielectric constant in the vertical direction of the electric field (perpendicular to the surface of the substrate) and in the horizontal direction (parallel to the surface of the substrate. Several of the same researchers had earlier presented details in Microwave Journal (see February, 2010 article) on how to quickly and simply measure the dielectric constants of a PCB material with such uniaxial anisotropy.

The Sonnet/Rogers study led to a model that provides predicted filter performance results more closely matched to measurements. It also includes the effects of PCB surface roughness, which is typically evidenced as excess inductance in a filter design. In their study (James Rautio, Brian Rautio, Serhend Arvas, Allen Horn, III, and John W. Reynolds, “The Effect of Dielectric Anisotropy and Metal Surface Roughness,” Proceedings of the Asia-Pacific Microwave Conference 2010, paper FR3C-4), they point out that an EM analysis of a bandpass filter using an isotropic dielectric substrate uses a single dielectric constant for both vertical and horizontal electric-field directions, and can lead to errors in the prediction of bandpass center frequency.

Their study is based on measurements of dielectric anisotropy using a multimode microstrip resonator fabricated on the material of interest. Such a resonator produces a large number of frequencies for analysis, with a value for dielectric constant extracted from each measured resonant frequency. Using an automated approach, a pair measured even/odd-mode resonant frequencies are converted to a pair of vertical/horizontal dielectric constants.

Of course, not all filter designs are equal, and some architectures, such as a stepped-impedance lowpass filter, may be less affected by the anisotropic properties of microwave substrates than an edge-coupled bandpass filter with even-mode propagation largely in the x-y plane of the substrate material. But understanding the anisotropic behavior of a microwave substrate and properly including that behavior in an EM model can make any design process more efficient. Those wishing to know more about the effects of microwave substrate dielectric anisotropy on the design of filters and other high frequency circuits, visit Rogers Corporation in Booth 2 at the upcoming 2011 Radio and Wireless Week (www.rawcon.org, Glendale, AZ, January 17-18, 2011).