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 please see the ROG Blog: “Substrate Anisotropy Affects Filter Designs”(http://mwexpert.typepad.com/rog_blog/2011/01/substrate-anisotropy-affects-filter-designs.html), which also examines the effects of moisture absorption on circuit material Dk.