Microwave Journal

Choose Circuit Materials For Bandpass Filters

Bandpass Filters, Part 1

January 16, 2013

Bandpass filters are essential to many RF/microwave circuits and systems. They eliminate unwanted signals and noise, and can work with both receivers and transmitters. Bandpass filters can be assembled in a variety of ways, using lumped-element discrete inductors and capacitors at lower frequencies and semiconductor technologies for tiny monolithic filters at higher frequencies. Still, the most popular RF/microwave bandpass filters may be the ones based on microstrip transmission lines with distributed circuit elements on printed-circuit-board (PCB) substrates. With the right circuit material, microstrip bandpass filters can provide excellent performance in small circuits. This first of two blogs on RF/microwave bandpass filters will review some of their basic performance parameters and how they relate to PCB material characteristics, with a focus on one material in particular, RT/duroid® 6010.2LM circuit material from Rogers Corp. As a followup, the next blog will explore how bandpass filters perform on other circuit materials.

A bandpass filter is defined by a center frequency within a passband, channeling all signals within that passband with minimal loss while rejecting signals at frequencies above and below the passband with as much attenuation as possible. In contrast, a lowpass filter passes all signals below a given cutoff frequency, rejecting signals above that frequency, and a highpass filter passes signals above a cutoff frequency and attenuates signals below it. A band-reject filter suppresses signals within a designed bandwidth passing signals outside the rejection band with minimal loss. A bandpass filter can be described by various performance parameters, including center frequency, passband, passband insertion and return loss, upper stopband, lower stopband, and attenuation within the stopbands.

Transitions from a bandpass filter’s passband to its upper and lower stopbands can be extremely rapid or more gradual, typically described by different filter response types including Butterworth, Chebyshev, and Bessel filters. Each type of bandpass filter exhibits some form of tradeoff. For example, a Butterworth filter is typically characterized by flat amplitude response across its passband, sacrificing sharpness in the transitions from passband to stopbands. Chebyshev filters achieve sharp transitions, at the cost of higher amplitude ripple in the passband than a Butterworth filter. Bessel filters offer linear passband phase response, giving up some stopband attenuation compared to the other two filter types.

The performance of a PCB filter is highly dependent on the circuit substrate material. The choice of material can limit center frequency, passband loss, and other key filter parameters. For many filter designers, the choice of material starts with a laminate’s dielectric constant. For a distributed-element filter such as a microstrip bandpass filter, the size of the transmission lines and distributed filter elements is inversely proportional to the square root of the PCB material’s dielectric constant; in short, PCB materials with higher dielectric constants make it possible to design and fabricate smaller filters for a given frequency. RF/microwave filter designers have long favored PCB substrates with dielectric constants of 10 or higher to create filter circuits with relatively small dimensions for a given center frequency/wavelength.

Since the dimensions of microstrip and other PCB filters are determined by the dielectric constant of a circuit material, it is important that the value of dielectric constant used for a particular material is very accurate. The dielectric constant of any PCB material can vary, so it is critical that these variations remain within the dielectric-constant tolerance range cited for a particular material by its manufacturer, such as 10.2 ± 0.25. Whether a filter’s dimensions are calculated manually or with the help of a computer-aided-design (CAD) program, even small errors in the value of the dielectric constant used in the calculations will result in unwanted changes in designed wavelength/frequency and shifts in center frequency and passband.

Dissipation factor or dielectric loss is another important circuit material parameter for bandpass filters. Quite simply, low values of dissipation factor indicate materials capable of achieving low insertion loss. For a bandpass filter, low PCB dissipation factor also means high filter quality factor (Q), which translates into the potential for a filter with low passband insertion loss and sharper transitions from passband to stopbands.

When designing and fabricating RF/microwave PCB-based bandpass filters, variations in dielectric constant should be minimized whenever possible. A circuit material parameter known as moisture absorption can play a large role in the stability of the material’s dielectric constant under certain environmental conditions, notably under high humidity. Ideally, a PCB material’s moisture absorption should be as low as possible. A material with a high value of moisture absorption can suffer variations in dielectric constant and dissipation factor that far exceed the tolerance ranges specified by the manufacturer. The dielectric constant of the material will change with even a small amount of moisture absorption, resulting in unexpected performance variations in bandpass filter center frequency, passband, and passband insertion loss.

Filter designers choose PCB materials with high dielectric constants in order to minimize the dimensions of their RF/microwave filters. A popular dielectric-constant value for such materials is 10.2, typically for materials based on polytetrafluoroethylene (PTFE). Although a filled PTFE substrate has excellent electrical properties, it can be guilty of moisture absorption on the order of 0.25%. Although this is a relatively small value compared to most PCB materials, a PCB material with this value of moisture absorption can exhibit significant changes in dielectric constant and dissipation factor under conditions of high humidity, possible causing a filter to exceed its performance limits for passband loss or suffer a shift in center frequency and passband from expected values.

RT/duroid 6010.2LM microwave laminate from Rogers Corp. is a composite that blends ceramic filler with PTFE for stable performance and low moisture absorption. The material achieves small bandpass filter dimensions, by merit of its high dielectric constant of 10.2 in the z direction and 10 GHz, with tolerance of ±0.25, and features dissipation factor of only 0.0028 for low passband insertion loss. Its moisture absorption is a fraction of that for many filled PTFE substrates, at typically only 0.01% (compared to 0.25% for other filled PTFE substrates).

A bandpass filter fabricated on this material will have the same dimensions as a filter formed on filled PTFE with dielectric constant of 10.2. However, it will not suffer variations in dielectric constant and dissipation factor, with their resulting variations in filter performance, in environments in which humidity may change dramatically. In fact, the improvements possible with this material compared to PTFE for bandpass filters are detailed in a study available for free download from the Rogers’ web site, “The Benefits of Selecting RT/duroid 6010LM for Band Pass Filter Applications.”

RT/duroid 6010.2LM laminates can be specified with various thicknesses and cladding options and well suited for a wide range of RF/microwave bandpass filters. As the next blog will show, however, other circuit materials with lower dielectric constants and different parameters are available in support of repeatable, high-performance RF/microwave bandpass filters.

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.