High-frequency filters are increasingly essential components within wireless products, especially as those wireless products continue to compete for limited frequency spectrum. Various types of RF/microwave filters help wireless radio transmitters and receivers operate with their proper signals while shielding them interference caused by out-of-band signals. Printed-circuit filters can be designed with various responses, including bandpass, bandstop, lowpass, or highpass filters, and from a number of different transmission-line technologies, including microstrip, stripline, or coplanar-waveguide (CPW) transmission lines. For the best results, filter designers should start with a printed-circuit-board (PCB) material having optimum characteristics for RF/microwave filters. The choice of circuit material can not only impact a filter’s performance, but even the size of a printed circuit filter.
The job of a filter is to shape part of the frequency spectrum, ideally stopping unwanted signals while passing desired signals with minimal loss or attenuation. Each filter type performs these functions by means of different spectral regions: stopbands, passbands, and transitions between a stopband and a passband. For example, a lowpass filter has one passband in the lower-frequency portion of its frequency range and one stopband in the upper-frequency part of its frequency range, with one transition region between them. A highpass filter is the opposite, with one passband in the upper-frequency part of its range and one stopband in the lower-frequency part of its range, and one transition region between them. A bandpass filter has a passband, lower and upper stopbands, and two transition regions. A band-reject filter can be thought of as the opposite, with a stopband with transition regions linking upper and lower passbands.
Different transfer functions describe a filter’s transition regions. A Chebyshev filter, for example, is characterized as having an abrupt transition from the passband to the stopband; i.e., very little spectrum is required to make the change from the lowest signal loss to the highest signal attenuation. A filter with a Butterworth or binomial function, on the other hand, makes a more gradual transition from the passband to the stopband. It requires a greater amount of frequency spectrum to make the transition from filter regions, but it can also achieve a passband with low loss and very little ripple compared to a Chebyshev filter with its shorter transitions.
A filter’s frequency response is really a composite of the responses of its different spectral regions, with the transfer function having a major influence on the loss characteristics of the passband and stopband regions. A Chebyshev filter is capable of a quick, clean transition from a passband to a stopband, but at the cost of some amplitude variations or ripple in the passband insertion-loss response. A Butterworth filter can achieve a much flatter passband insertion-loss response, but less attenuation of signals at frequencies closer to the passband than a Chebyshev filter.
A printed circuit filter designer is faced with achieving a set of responses for a desired frequency range but also with trying to minimize transmission and reflection losses at the filter’s input and output ports by means of impedance matched junctions. The input and output ports are often coaxial connectors and most typically at a characteristic impedance of 50 Ω. What difference can the choice of circuit material have on a particular filter design and why use one type of circuit material rather than another?
When sorting through PCB material options prior to a design, a filter designer usually starts with dielectric constant (Dk) as a key parameter. PCB filters are typically formed of resonant circuit structures, such as the quarter-wave or half-wavelength resonators used in edge-coupled microstrip bandpass filters. The Dk of the dielectric material will determine the dimensions of the transmission lines required for specific resonator characteristics and frequencies. Circuit materials with higher Dk values will yield smaller filter resonator structures for a given wavelength and frequency, when miniaturization of a filter design is an important goal. In any case, for predictable, repeatable filter and resonator performance, the Dk of a circuit material choice should be as consistent as possible, held to the tightest tolerance possible.
What many filter designers may not realize when choosing a circuit material, however, is the anisotropy of the material—that is, the Dk value is different in the x-y plane of the material than in the z-axis (the thickness) which is the material Dk value often used as a starting point for filter computer simulations. Due to such anisotropic behavior, for proper modeling and design of a microstrip edge-coupled bandpass filter, the coupled fields in the x-y plane should be calculated as a function of the x-y Dk value. Alternatively, a filter designer may select a circuit material with more isotropic behavior to simplify the design process.
In general, circuit materials with lower Dk values are more isotropic than circuit materials with higher Dk values. To compare two commercial circuit materials, RO3003™and RO3010™circuit materials from Rogers Corp. exhibit low and high Dk values, respectively, with different degrees of isotropy. RO3003 laminate has a z-axis Dk value of 3.00 (with a tolerance of ±0.04 in the z-axis) and is nearly a true isotropic material, with similarly low Dk value in the x-y plane. Designing filters with coupled resonant structures, such as microstrip edge-coupled bandpass filters, is straightforward often with first-pass design success when using commercial computer-aided-engineering (CAE) circuit simulators.
However, for designing much smaller filter circuits for a given frequency, RO3010 circuit material has a much higher z-axis Dk value of 10.2 (with tolerance of ±0.30 in the z-axis). It is much more anisotropic than RO3003 material, with Dk value in the x-y plane that is much closer to the 3.0 range of the RO3003 material. This means that filter design strategies and computer simulations must account for the significant difference of Dk values in the x-y plane and the z-axis of RO3010 material. But the higher Dk value of this material significantly increases the coupling between resonant structures, which can help improve the overall performance of a filter design while miniaturizing its dimensions.
Note: Those interested in learning more about how circuit material anisotropy can impact filter design see the ROG Blog, “Substrate Anisotropy Affects Filter Designs,” which also examines the effects of moisture absorption on circuit material Dk.
ROG Mobile App
Download the ROG Mobile app to access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.
Ask an Engineer
Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.
In this video, John Coonrod discusses why there are so many different dielectric constants (Dk) that are used in the microwave printed circuit board industry.
Send us questions/comments by tweeting us @Rogers_ACM!
There are a number of test methods to determine the dielectric constant of circuit materials used in the microwave or high frequency industry.
In this video, “Common Test Methods for Measuring Dielectric Constant,” you will learn about the most common test methods like Clamped Stripline Resonator Test, Split Post Dielectric Resonator, Full Sheet Resonance (FSR), and Microstrip Differential Phase Length Method.
For additional information and technical tools, join us at rogerscorp.com/techub
In this Coonrod’s CORNER video, “How to Interpret High Frequency Circuit Material Data Sheets,” John Coonrod explains the main categories within high frequency laminate data sheets: Circuit Design, Circuit Fabrication and Reliability Issues.
He also deciphers key concepts such as Dissipation Factor, Dielectric Constant, Dielectric Loss, Thermal Coefficient of Dielectric Constant, Peel Strength and Thermal Conductivity. Watch now: http://bit.ly/1llyJUR
Choosing the right circuit-board material can be a frustrating experience. After all, every manufacturer of printed-circuit-board (PCB) materials promises outstanding performance for their products, offering an array of dielectric formulations to achieve improved stability, or power-handling capability, or lower loss. Wouldn’t it be simpler to choose one material, such as FR-4, for all applications? In an ideal world, a single PCB material could serve all applications. But requirements for two applications may vary so widely, that no single PCB material can provide optimum performance for both.
Previous posts examined some of the key material parameters pertaining to high-frequency laminates, such as dielectric constant, thermal conductivity, coefficient of thermal expansion (CTE), and even flexibility when used in conformal circuits. But how does an engineer combine all this information about a material’s electrical and mechanical properties when trying to choose the perfect substrate for a particular application? It can be a complex process, but it may be possible to simplify that process.
Perhaps any search for suitable PCB material for a given application should start with the application itself. It is often a customer’s application, and its particular requirements, that drive particular material formulations for optimum performance in one type of application, such as a high-frequency, high-power amplifier. The better an engineer understands their target application, the easier the process is for selecting a circuit-board laminate for that application. Defining which performance parameters are the most critical for an application can help guide an engineer to the best choice of circuit-board laminate.
Defining laminate specifications
A high-frequency circuit design may have a list of required specifications that can fill a page or more, but typically a handful of those specifications are the critical ones that call for special design or fabrication approaches. For a PCB laminate, that can provide the foundation for meeting the most critical requirements. Where a bandpass filter is defined by such parameters as center frequency, percentage bandwidth, rejection, and passband insertion loss, a laminate is characterized by a completely different set of parameters, such as dielectric constant, thermal coefficient of dielectric constant, and CTE. Even the thickness of a PCB laminate can impact the high-frequency performance of a circuit. So, evaluating a particular application to help choose the right circuit board material is a matter of finding which of the application’s key performance parameters relate to which of the PCB laminate’s characteristics, and what possible tradeoffs may exist.
Defining the needs of an application can be as simple as a series of “filtering” processes, sorting by means of larger issues and working down to performance tradeoffs. For example, will a PCB laminate ultimately be used in a military system, for commercial use, for industrial use, in space, for a medical application, or in a combination of these application areas? Knowing where the circuit-board material is going, such as in a military electronic-warfare (EW) system, will eliminate some PCB materials from consideration, since they won’t meet the basic electrical and mechanical requirements for military use. Of course, if a designer is hoping to sell their circuit across commercial and military markets, the PCB material must be suitable for military environments. For circuit-board materials that will be used for products across multiple market areas, the market with the most rigorous set of requirements (usually military or aerospace) will set the requirements for the PCB material.
Reviewing the requirements
Reviewing the requirements of an application can also help to define necessary and unnecessary tradeoffs. One of the more obvious tradeoffs is cost versus performance. Compared to a high-performance PTFE-based laminate, FR-4 can save a bundle. But it may not provide the high-frequency performance needed for an application, and it may not provide much frequency or amplitude stability even if it does reach the right frequency. Even within such an obvious tradeoff are finer points for comparison: part of the overall price of using a given laminate includes processing costs—some materials are simpler and less expensive to process than others, depending upon the composition of the laminate. Circuit size can also contribute to lowering costs. Choosing a laminate with a higher dielectric constant can yield more circuits per laminate panel, provided that the electrical effects of the higher dielectric constant are acceptable for that application. These and other factors must be considered when making a “simple” tradeoff evaluation between costs versus performance for different laminate materials.
Most high-frequency laminate specifiers start with relative dielectric constant when comparing products from different suppliers, and then check other parameters, such as dissipation loss and CTE. Laminate manufacturers specify their products with a specific value and some amount of variation, such as 3.48 ± 0.05 in the z-direction at 10 GHz for our RO4350B™ laminate. But as noted in an earlier post about applying a dielectric constant, this may not be the best value to use in a computer simulation. Choosing the right laminate material requires confidence in how the material has been characterized, so that simulations will represent final results. A future post will detail some of the methods that laminate suppliers use to determine material parameters such as dielectric constant, typically by fabricating a circuit structure with known characteristics on the PCB material.
Coming up next
I will go into greater detail on how different PCB laminate specifications relate to the performance levels of different high-frequency circuits. For example, for a high-power microwave amplifier, a laminate’s thermal conductivity will certainly be one of the first parameters to compare among different substrates under consideration. But if a laminate has a high dissipation factor, it contributes to high circuit insertion loss. The higher loss results in more heat generated through the amplifier circuit, in turn requiring higher thermal conductivity. In this example, these two parameters (and possibly others) must be balanced and compared from laminate to laminate to make the best choice for a particular power amplifier circuit. To be continued ………
Just Ask an Engineer…
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.