Microwave filters come in a confusingly large range of shapes, sizes, and technologies. From the chiplike devices used in cellular telephones to the metal-housed cavities built to handle kilowatts of power, RF/microwave filters serve many purposes. Just knowing what is available helps when it is time to narrow the list of candidates for a particular application.

As with most electronic components, the trend in microwave filters is for miniaturization but with higher-power handling capabilities. Current designs are a fraction of the size of their predecessors, but without sacrificing overall performance and with outstanding power-handling capabilities. Along with the miniaturization, filter designers have also found different materials upon which to base their components. Traditional inductive-capacitive (LC) type filters are just one of many different types of RF and microwave filters based on resonant elements, such as ceramic resonators, crystal resonators, dielectric resonators, film-bulk-acoustic resonators (FBARs), surface-acoustic-wave (SAW) resonators, and even the exotic yttriumiron-garnet (YIG) resonators. Perhaps no component has invited more materials and structures (such as waveguide, combline, and slabline structures) as the microwave filter.

Opportunities in cellular communications base-station equipment and handsets are probably much to blame for the explosion of smaller, surface-mountable filters, since miniature filters were needed to isolate communications channels from interference and adjacent-channel signals in both handsets and the infrastructure equipment through about 2100 MHz. The trend will no doubt continue at higher frequencies with the growth of WiMAX equipment in the 3.5-GHz and higher bands.

Specifying a filter begins with understanding the type of filtering function required. Filters can be grouped into four basic responses: bandpass, band-reject, lowpass, and highpass filters. A lowpass filter helps remove higher-frequency noise and interference from a 60-Hz power line, for example. As the name suggests, it passes signals below a certain cutoff frequency and attenuates signals above that cutoff. A highpass filter does the opposite, passing high-frequency signals and removing lower-frequency distortion below a cutoff frequency. A bandpass filter passes a given band or channel of frequencies, rejecting signals on either side (below and above) the passband, while a band-reject or notch filter passes all signals except at a specific frequency and band-width around the frequency, providing attenuation of that specific interference or otherwise unwanted signal.

Ideally, each filter type would pass their desired signals without loss and attenuate undesired signals down to zero levels. But in reality, a long set of performance parameters is needed to understand how a filter deviates from ideal performance, using such parameters as VSWR, passband ripple, passband insertion loss, stopband rejection, filter shape, phase response, group delay, and quality factor (Q)—essentially a measure of a filter's resonant behavior. High-Q circuit elements provide high-performance narrowband filter responses while low-Q circuit elements yield higher passband insertion loss and lower stopband attenuation.

Simply put, a bandpass filter provides a clear look at a known signal or signals of interest, while a band-reject filter eliminates a known unwanted signal. RS Microwave Co. (www.rsmicro.com), for example, has developed the model 50822B-2 bandreject filter to resolve co-site interference problems with the SPS-49 Long Range Radar. The filter has a lower passband of DC to 800 MHz, upper passband of 1000 to 5000 MHz, and 45-dB rejection band of 850 to 945 MHz. It is designed to handle 10 W average power and 50 W peak power in the rejection band (the radar's operating range), while achieving low passband insertion loss of less than 1 dB. The filter features air-slabline construction for high unloaded Q and high stop-band rejection.

Late last year, the firm also reported on a new design approach for high-power band-reject filters. By employing parameter extraction using the HFSS electromagnetic (EM) simulation software from Ansoft (www.ansoft.com) and then circuit-level optimization using Ansoft Designer software, the company's designers have combined lumped and distributed circuit elements to create high-power notch filters ideal for suppressing military spread-spectrum radio signals such as from JTIDS systems. The approach was used in the design of a quasi-elliptic band-reject filter using parallel-coupled lines loaded with parallel-plate lumped capacitors (Fig. 1). The filter features lower passband of DC to 900 MHz and upper passband of 1286 to 5000 MHz, with maximum insertion loss of 1 dB in both bands. The 50-dB rejection band ranges from 969 to 1206 MHz. The filter handles 20 W average power and 80 W peak power.

Dielectric Labs (www.dilabs.com) is one of several Dover Technologies companies (www.dovercorporation.com) involved in the manufacture of microwave filters. The company has developed advanced resonator technology based on its ceramic materials to create miniature filters at frequencies to 67 GHz and beyond. Their bandpass filter types include a 2.14-GHz interdigitated filter as well as 3.5-, 4.2-, and 6.5-GHz symmetrical dual-mode resonator filters (SDMRFs). The firm has also fabricated a 37-GHz edge-coupled design. The compact ceramic filters are about 1/15th the size of printed-wire-board (PWB) filters with greatly improved repeatability and temperature stability. The 2.14-GHz seven-pole Chebyshev bandpass filter features 1.8-dB typical insertion loss in a design measuring 0.4 X 0.75 X 0.035 in.

K & L Microwave (www.klmicrowave.com), one of the better-established names in microwave filters (and another Dover Technologies company), offers a wide array of all filter types, from tiny surface-mount models to high-power cavity units. The firm's lumped-component filters are available in a wide variety of frequencies, topologies, and packages, including miniature packages from 0.5 to 200 MHz and microminiature packages from 30 MHz to 10 GHz in all four basic filter types as well as in multiplexer (multiple-filter) designs. The IB series, for example, covers the range from 30 MHz to 10 GHz with standard 3-dB bandwidths from 3 to 15 percent and custom 3-dB bandwidths as wide as 70 percent. Filters can be specified with two to ten sections for a wide range of rejection performance levels.

In contrast, the company also offers high-power waveguide filters for military and commercial radio markets. Rectangular-mode waveguide filters can be supplied in bands from 2.5 to 40 GHz in bandpass, lowpass, and diplexer (two-filter) configurations. Rectangular-mode waveguide filters can be specified with bandwidths from 1 to 20 percent and from 2 to 20 sections. Circular-mode waveguide filters can be specified with bandwidths from 0.1 to 1.8 percent and from 2 to 6 sections.

The firm also offers the K & L Microwave Filter Wizard software on its home page to aid filter specifiers in the selection process. Users simply enter their desired specifications and the software returns a list of products matching those requirements. For requirements that do not match existing products, the software opens a quote request page that can be completed and automatically e-mailed to K & L Microwave.