Narrowing Choices For RF/MW Filters

Filters play key roles in RF and microwave systems although the number of different filter types and technologies can make the selection process somewhat challenging.

Microwave filters can be simple to build, but complex to unders tand. They perform a basic function in a system: stop some signals and pass others. But they can achieve this function in many different ways, and with many different side effects, such as distortion to the amplitude and phase responses of the system. Before making filter choices, it helps to know how the choices differ.

Filters come in a variety of configurations: lowpass, highpass, bandpass, and band-stop or band-reject filters. As the name implies, a lowpass filter has minimal attenuation below a cutoff point while suppressing signals above the cutoff point. The highpass filter works in reverse. A bandpass filter has minimal attenuation in a passband around a center frequency, with high rejection above and below the passband. A band-reject filter works in reverse, stopping signals within a narrow bandwidth around a center frequency and allowing all other signals to pass. In addition, a pair of bandpass filters with different frequency ranges can be combined to form a diplexer (or three bandpass filters into a triplexer), while a lowpass and a highpass filter can be joined to create a duplexer.

Ideally, a filter would suffer 0 dB attenuation for the signals it was designed to pass, and infinite attenuation for signals it was designed to reject. In real life, dielectric substrate materials, conductors, passive components, and connectors contribute to losses and less-than-ideal filter behavior. Those specifying filters must sort through a number of tradeoffs to decide which filter is best for a given application.

Filter response types include Butterworth, Chebyshev, Bessel, and elliptic filters, each with a different response shape that is suited to a specific set of applications. For example, Butterworth filters sacrifice the sharpness of the transition from passband to stopband for the sake of minimal amplitude ripple in the passband. Chebyshev filters feature a sharp transition between the passband and the stopband, characterized as a filter with high quality factor (high Q), with some compromise in amplitude flatness and insertion loss in the passband. A Bessel filter has good amplitude and transient response but sacrifices in stopband attenuation. It is known for its linear phase characteristics, resulting in flat group delay across the passband. An elliptic filter is capable of sharp transitions from the passband to the stopband, at the expense of passband amplitude ripple and excessive passband group-delay variations.

The performance levels of different types of filters can best be compared by a common group of specifications that includes insertion loss, rejection, VSWR, and power-handling capability. Insertion loss, which refers to signals within a filter's passband, is the difference (in dB) in signal amplitude between the output and the input. As mentioned previously, ideal passband insertion loss would be 0 dB, but realistic numbers are higher and can often be above 1 dB depending upon frequency and filter type.

A filter's stop band is the frequency range over which it is specified to attenuate signals by a certain level. That level may be 20 dB or more, depending upon how a manufacturer characterizes their filters. For a given application, the amount of rejection should be at least enough to decrease the amplitude of an undesired signal below, for example, the sensitivity of a receiver front end in the same system. In some filter types, stopband rejection may be 80 dB or more.

A filter's cut-off frequency essentially separates its passband from its stop band. The cut-off frequency is defined as the point where the insertion loss (or rejection) is equal to the 3-dB or halfpower point. For a lowpass or highpass filter, there is one cut-off frequency. For a bandpass or band-stop filter, there are two cutoff frequencies, above and below the passband in a bandpass filter and the rejection band or notch in a band-stop filter. In addition, for a bandpass filter, the center frequency is typically the geometrical mean between the upper and lower cut-off frequencies. For example, a bandpass filter with lower cut-off frequency of 2400 MHz and upper cut-off frequency of 2500 MHz has a center frequency of 2450 MHz and a 3-dB bandwidth of 100 MHz.

A filter's voltage standing wave ratio (VSWR) is a measure of how it is impedance matched to the characteristic impedance of the system in which it will be used. The VSWR of one port is the impedance looking into that port while the other filter port is precisely matched to the characteristic impedance of the system (typically 50 Ohms in high-frequency systems). As a result, a filter's specifications usually include typical and maximum values for both input and output VSWR, representing how well the filter is impedance matched to the source and load, respectively, to which it will be connected. VSWR is denoted as a value in a ratio to unity, such as 1.50:1, but can also be expressed as the filter's return loss (in dB). A filter is considered absorptive if it is impedance matched across both its passband and its stopband, and reflective if it is matched only in its passband. In the latter case, a filter will exhibit high VSWR in the stopband, such as 20.0:1 or higher. The power-handling capability of a filter is often a function of the physical size of the filter and its operating frequency range, as well as the filter technology and substrate material type, the type of package, and the thermal dissipation limits of those materials. Maximum power limits are also a function of signal type, such as continuous-wave (CW) or pulsed signal and the type of modulation used with the signal.

Several companies offer downloadable white papers or application notes for those wishing succinct reference materials on filter specifications, including Anatech Electronics with "How to Specify RF and Microwave Filters" with an excellent overview of different filter types, the same company's "Lumped Element (LC) Filters" with a review of these popular RF filters, and Mini-Circuits with its application note, "Filters: Introduction, Definition of Terms, Q & A's," which reviews the meanings of filter specifications and provides a few application examples based on the company's compact filters.

The types of filters are many, and include fixed and tunable types, at low and high frequencies, such as lumpedelement filters based on discrete inductors (Ls) and capacitors (Cs), crystal filters, ceramic filters, cavity filters, surface-acoustic-wave (SAW) filters, bulk-acoustic-wave (BAW) filters, filmbulk- acoustic-resonator (FBAR) filters, microelectromechanical-systems (MEMS) filters, and even active, semiconductor-based tunable filters. Lumped-element or LC filters are commonly used below about 3 GHz. The size of these filters is dictated by the operating frequencies and the required sizes of the L and C components.

Helical filters, composed of a series of magnetically coupled cavities, are also LC filters and similarly limited to about 3 GHz in bandpass formats only. Although capable of sharper responses than conventional LC filters, they are limited to about 5 W input power.

Ceramic filters are fabricated on thin ceramic substrates using discrete or integrated components to form resonant circuits. Depending upon the dielectric constant of the ceramic substrate, ceramic filters can be made extremely small, with higher-dielectric-constant materials yielding smaller filters. They can be made low in cost by using volume production methods and can also be made small in size. They are limited to about 6 GHz and to power levels of about 5 W, but are suitable for bandpass and band-reject filters when small size is important. Cavity filters can handle power levels to about 500 W, with outstanding insertion-loss performance. Cavity filters can be designed to about 30 GHz. They are large compared to LC and ceramic filters (see figure) and expensive since they are typically machined from blocks of aluminum.

SAW, BAW, and FBAR filters are made by semiconductor fabrication techniques, using photolithography to pattern fine features, while MEMS employ these processes to form threedimensional structures. All can be made as small as 2 x 2 mm, although limited in power-handling capability. SAW, BAW, and FBAR filters are typically used in cellular communications handsets to about 3 GHz. MEMS filters offer the potential to 18 GHz and beyond.

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