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.
Like K & L Microwave, RLC Electronics (www.rlcelectronics.com) offers filter selection software on its website. The firm's wide range of filter products includes the model F-17281 interdigital bandpass filter with wide passband of 20 to 40 GHz. Insertion loss is 1 dB or less at the center frequency while typical rejection is 40 db from DC to 14.5 GHz, and to 48.5 GHz on the high side, with ultimate rejection of 20 dB or more through 60 GHz. The wideband filter measures 0.91 X 0.38 X 0.38 in. and is supplied with 2.4-mm connectors.
Wainwright Instruments GmbH (www.wainwright-filters.com) offers extensive lines of microwave lowpass, highpass, and bandpass filters, including the K series of wideband bandpass filters. With passbands between 0.5 and 15.0 GHz, the filters offer stopbands through 21 GHz in packages measuring just 12.7 X 13.2 X 45 mm. One of the widest-band units features a 3-dB passband of 0.9 to 9.0 GHz with 2 dB or less passband insertion loss.
Trilithic (www.trilithic.com) also applied modeling software to its design of high-performance receive and transmit bandpass filters for UMTS base-station applications. The receiver filter covers 1899 to 1921 MHz with 15 dB input/output return loss and 1.15 dB passband insertion loss. It achieves 50 dB rejection from DC to 960 MHz and better than 40 dB rejection through 1880 MHz. Above the passband rejection is at least 25 dB from 1980 to 2680 MHz and 60 dB or more from 2660 to 5800 MHz. The transmit filter is available for ±2-MHz passbands at transmit frequencies of 1902.6, 1907.6, 1912.6, and 1917.5 MHz with insertion loss of 1.95 dB and out-of-band rejection of 60 dB or more.
One of the oldest types of filters for lower-frequency applications, such as intermediate-frequency (IF) channel filtering, is the crystal filter. Small in size and low in cost, the technology is applicable at frequencies to about 200 MHz before the dimensional tolerances required for higher frequency use become difficult to achieve. Network Sciences (www.networksciences.com), for example, offers the UM Series of custom crystal bandpass and band-reject filters from 40 to 200 MHz with fractional bandwidths from 0.015 to 0.6 percent in two-pole to ten-pole configurations. The number of crystal poles determines the amount of stopband rejection possible.
Murata (www.murata.com) has used dielectric resonators as the basis for the GIGAFIL line of filters. The DFCB bandpass filter series, for example, includes models with center frequencies from 836 to 959 MHz, such as the Industrial-Scientific-Medical (ISM) band model DFCB2915MLDJAA with bandwidth of 26 MHz centered at 915 MHz. It provides 27 dB attenuation of unwanted signals at 837.5 MHz with passband insertion loss of 2.5 dB. The filters measure 8.2 X 5.8 X 3 mm. At higher frequencies, the DFCB series covers applications from 1.5 to 5.0 GHz. The firm also manufactures filters based on ceramic and SAW resonators.
Along long associated with GaAs integrated circuits, with their acquisition of SAW filter/oscillator house Sawtek some years ago TriQuint Semiconductor (www.triquint.com) became a leading supplier of SAW filters. The company recently introduced a family of SAW filters measuring just 1.4 X 2.0 X 0.5 mm for next-generation GSM and CDMA handsets. According to product marketing manager Danny King, "The new CDMA and GSM SAWs are the smallest available, requiring far less phone board space than the previous generation while still offering exceptional performance."
Integrated Microwave (www.imcsd.com) recently introduced its model 925064 combline bandpass filter with 2-percent 1-dB bandwidth centered at 5.8 GHz (Fig. 2). The passband insertion loss is 1.5 dB. The filter, which has a 30-dB bandwidth of approximately 4.8 percent, is available with a variety of connector and mounting options, as well as other center frequencies. The company also offers the model 930073 low-cost ceramic diplexer for applications at 2.4 GHz. Designed to provide two 10-MHz channels for transmit and receive operations, the diplexer features low insertion loss (2.5 dB) and high transmit/receive isolation (22 dB). With two poles per channel, the diplexer handles 5 W CW power and measures just 1.1 X 0.470 X 0.310 in.
REMEC Defense and Space (www.remecrds.com) supplies microwave filters in a variety of configurations, including lumped-element filters, combline cavity filters, and interdigital filters. The lumped-element filters can be specified from DC to 26 GHz with typical bandwidths of 2 to 60 GHz and peak power-handling capabilities to 100 W. Combline filters can be specified from 100 MHz to 26 GHz with typical bandwidths of 0.5 to 60 percent and peak power-handling capabilities as high as 10 kW.
Lark Engineering Co. (www.larkengineering.com) has created an impressive line of surface-mount combline bandpass filters having bandwidths from 3 to 20 percent at frequencies from 5 to 15 GHz. The insertion loss of these filters is almost negligible, ranging from 0.5 to 1.5 dB depending upon frequency (Fig. 3), with typical return loss of 17 dB and minimum return loss of 14 dB. Designed to meet military environmental specifications, the combline filters measure as small as 0.5 X 0.5 X 0.75 in.
The firm also offers a line of high-power surface-mount filters for applications from 1800 to 2200 MHz. With typical passband of 60 MHz, the filters feature passband insertion loss of a mere 0.35 dB with passband return loss of 17 dB. Measuring just 0.62 X 1.09 X 0.75 in., these rugged filters can handle 50 W maximum power. They suffer a low 0.2 dB maximum peak-to-peak ripple while providing 20-dB minimum rejection at 430 MHz.
When receivers must be separated from transmitters, a duplexer provides more control than a traditional filter. One of the leading suppliers of miniature duplexers for cellular handsets, Avago Technologies (www.avagotech.com, formerly the semiconductor division of Agilent Technologies) recently announced tiny duplexers for US PCS and UMTS applications based on the firm's film-bulk-acoustic-resonator (FBAR) technology (Fig. 4). The models ACMD-7402 for US PCS transmit (1850.5 to 1909.5 MHz) and receive (1930.5 to 1989.5 MHz) and ACMD-7601 for UMTS Band I transmit (1920 to 1980 MHz) and receive (2110 to 2170 MHz) frequencies both measure 3.8 X 3.8 X 1.3 mm. The former handles 1 W (+30 dBm) transmit power with 42 dB minimum receiver noise blocking and 54 dB minimum transmit interference blocking. The latter handles 2 W (+33 dBm) transmit power with 45 dB minimum receiver noise blocking and 51 dB minimum transmit interference blocking.
Filters at Merrimac Industries (www.merrimacinds.com) are manufactured using the company's patented Multi-Mix multilayer fabrication process. By fusion-bonding multiple circuit and substrate layers, the company's engineers can achieve miniature, repeatable bandpass filters with passbands as narrow as 1 percent at center frequencies through 40 GHz. As an example, the company fabricated a bandpass filter with passband of 29.5 to 30.0 GHz measuring 0.413 X 0.296 X 0.02 in. Weighing just 0.1 oz., the filter handles 1 W power with 3-dB maximum insertion loss and 12-dB return loss. Rejection is better than 43 dB at 27 GHz and at least 30 dB from 32 to 33 GHz.
KW Microwave (www.kwmicrowave.com) offers a wide range of microwave filters including combline and interdigital types from 30 MHz to 18 GHz. The firm also manufactures the somewhat less-common tubular type filters from 20 MHz to 12 GHz. Essentially resonators capacitively coupled and surrounded by a tubular housing, these filters are available with 2 to 15 sections and can be specified with lowpass and bandpass responses.
For those willing to experiment, Pulse Research Lab (www.pulseresearchlab.com) offers a Signal Conditional Kit for the quick and easy fabrication of signal-conditioning circuits, including attenuators and filters, at frequencies through 3 GHz. The kit is available with male or female BNC connector styles as well as a low-profile design for inline insertion into a transmission line, with or without cables.
The list of additional filter suppliers is extensive and includes such trusted names as Mini-Circuits (www.minicircuits.com) and Synergy Microwave (www.synergymwave.com). For a complete listing, please refer to filter listings in the Microwaves & RF Product Data Directory at www.m-rf.com.