Filters perform a simple function in RF/microwave designs: to modify in a controlled manner a signal applied at its input. That function may be defined as passing signals in the middle of a band and rejecting all other signals (bandpass filter), attenuating a small portion of a bandwidth (band-reject filter), passing signals at the bottom of the band and rejecting other signals (lowpass filter), or passing signals at the high end of a band and rejecting the rest of the band (highpass filter). Filters can be fabricated with a wide range of technologies, from discrete lumped-element components to miniature surface-acoustic-wave (SAW) and low-temperature-cofired-ceramic (LTCC) designs.

Specifying an RF/microwave filter, as with any high-frequency component, involves setting desired values for a list of key specifications, both electrical and mechanical. Electrical specifications for a bandpass filter, for example, include total frequency range, passband insertion loss, out-of-band rejection, bandwidth for a given maximum insertion loss (such as 3 or 6 dB), return loss or VSWR, and power-handling capability. Mechanical specifications include physical size, package type, type of connectors and location on the package. Some of the electrical characteristics are related to the mechanical specifications. For example, although surface-mount filters can save a great deal of space in an RF/microwave system, such filters will be limited in power handling compared to a larger filter with connectors.

All RF/microwave filters impart some change to a signal passing through them, typically in amplitude but sometimes in phase as well. Depending upon a set of requirements, the choice in filter response can determine how the filter affects a signal at its output. For example, a Chebyshev filter exhibits a certain amount of passband amplitude ripple, with that sacrifice in amplitude flatness resulting in overall low passband insertion loss. A Butterworth filter, on the other band, provides a maximally constant amplitude response, with some trade-off in overall passband insertion loss. For pulsed applications, where the time-domain response of the filter is critical in such timing applications as radar systems, a Bessel filter response provides maximally constant time delay. When sharp edges in the filter's amplitude response are needed, as for pulsed signals, a Gaussian filter can provide those sharp edges to preserve pulsed signal characteristics through the filter. When signal phase must be preserved, an RF/microwave filter with linear phase response will exhibit a constant change in phase per unit frequency.

High-frequency filters have gotten smaller in recent years as manufacturers have fabricated filter circuits on such substrate materials as ceramics, such as low-temperature-cofired-ceramic (LTCC) materials, and even on glass substrates. Mini-Circuits offers extensive lines of standard and custom RF/microwave filters based on a number of different technologies, including LTCC. The firm, which offers a variety of coaxial and surface-mount filters through 13 GHz, supplies the four basic filter types along with diplexers, which are essentially a pair of filters in a common housing for such applications as connecting two receivers to a common antenna. Mini-Circuits also provides a "sampler kit" based on its LTCC filters, with five models each of any lowpass LFCN and highpass HFCN LTCC filters, from 80 MHz to 13 GHz.

StratEdge offers ceramic stripline filters based on patented ceramic processes developed for packaging. The firm provides custom lowpass, highpass, bandpass, and combination filters from 250 MHz to 18 GHz designed for small size and high performance. The thermally stable filters can be realized as interdigital or edge-coupled designs and provided as space-qualified products.

Integrated Microwave has developed a low-cost ceramic diplexer (model 930073 ceramic diplexer operating in the 2.4 GHz band. It features a channel bandwidth of 10 MHz and insertion loss of 2.5 dB or less. The 10-dB bandwidth is 75 MHz. The diplexer features transmit-receive isolation of 22 dB. Although it is only 1.1 x 0.47 x 0.31 in., it features two filter poles per channel and handles 5 W CW power. It can be supplied in custom frequencies. The company designs lumpedelement and distributed filters that can meet most linear-phase or pulsed signal requirements. The firm also supplies sets of filters that can be matched in phase, group delay, or amplitude.

Synergy Microwave Corp. also designs and manufactures compact surface-mount RF/microwave filters, such as the model FBS-200, which is supplied in a leadless surface-mount housing. It features 1-dB passband centered at 200 MHz. The maximum passband insertion loss is 6 dB. The filter achieves 40 dB rejection at 230 MHz and beyond and 50 dB rejection at 180 MHz and below. The firm also offers a line of bandpass filters in relay headers, including a unit with center frequency of 10.7 MHz and passband of 9.8 to 11.6 MHz and maximum insertion loss of 1.5 dB.

Bree Engineering has developed a series of lowprofile filters for use in multichannel avionics applications. Model 800825 measures just 1.50 x 0.50 x 0.23 in. and has a center frequency of 76 MHz with typical 3-dB bandwidth of 10 MHz. It offers the selectivity of a five-section Chebyshev filter with 4-ns time delay.

Wainwright Instruments GmbH, which offers all four basic filter types, recently developed a wideband tunable notch filter for laboratory applications. The model WRCT 700/1000-0.2/40-5-X filter can produce as many as five notches in the passband. The fundamental-frequency notch is determined by tuning each resonator. The notches are narrow, with attention of at least 40 dB within 100 kHz of the tuned-notch frequency. With a tuning range from 700 to 1000 MHz, the five-resonator cavity design exhibits 1 dB maximum passband loss from DC to 4 GHz (except at thirdharmonic notch frequencies) with minimum return loss of 14 dB.

Filters can provide either fixed or tunable passbands or rejection bands in order to isolate different frequencies across a band. Lorch Microwave, for example, offers tunable filters for test and communications applications, with bandpass and band-reject tunable filters covering 24 to 3000 MHz in octave bands. Micro Lambda Wireless offers tunable bandpass and band-reject filters based on its YIG technology. For example, the company's MLFL series of YIG-tuned bandpass filters are supplied in a 1-in. cube package for frequency coverage from 0.5 to 18.0 GHz. These are available with tunable 3-dB bandwidths from 15 to 40 MHz. A typical unit, model MLFM-42008, tunes from 2 to 8 GHz with 4 filter sections. It has a 30-MHz tunable bandwidth with maximum insertion loss of 5 dB. The isolation is typically 80 dB. The YIG filter exhibits limiting at input levels of +10 dBm or more. The company also offers tunable band-reject filters from 1 to 18 GHz in 1.4- and 1.7-in. cube-shaped housings.

Similarly, Omniyig offers full lines of tunable bandpass and band-reject filters. To aid YIG-tuned filter specifiers, the company provides a one-page application note, "Definition of YIG Filter passband Parameters," on its web site. The short note describes a number of parameters for a YIG filter, including isolation, passband ripple, selectivity, and limiting level.

Tunable filters from Lorch Microwave) are designed to provide high performance in a single package. While typically used in test and measurement applications, these products can also be ruggedized for mobile and remote applications. The firm's tunable bandpass and band-reject filters cover the frequency range of 24 to 3000 MHz in octave bands.

FINDING A FILTER
High-frequency filters come in many forms, including as high-power waveguide filters, such as the waveguide-band filters designed and manufactured by ARRA , dielectric-resonator (DR) filters, microstrip, cavity filters, interdigital filters, bulk-acousticwave (BAW) filters, surface-acousticwave (SAW) filters, and inductive-capacitive (LC) filters. Even finding a filter from a single company's site can be a challenge, unless armed with the proper search tools. On the Mini-Circuits' site, for example, the Yoni2 search engine can quickly guide a visitor to the choice of coaxial packaged of surface-mount filter based on required specifications.

On the K & L Microwave web site, the firm's Filter Wizard search tool helps find the right filter based on choice of operating temperature, filter type, passband and how it is defined (for example, by its 3-dB points for a bandpass filter), bandwidth at specified rejection points, and connector interface. Also, Lorch Microwave offers its Filter Select+Plus software that helps when specifying RF/ microwave filters. The software can be downloaded or used online. In addition, Guided Wave Technology features online waveguide bandpass filter designer for filters through 100 GHz.

Several filter companies provide useful guidance for selecting a filter. Anatech Electronics offers a free white paper on its site, "Getting It Right The First Time When Specifying Filters--What Electronic Design Engineers Need To Know." The company also features the white paper "Tradeoffs and Other Requirements When Specifying RF Filters." For those interested in a narrowband filter for communications applications, M/ACOM provides the application note "Narrowband Microwave Filter Design."