Microwave and RF filters continue to provide invaluable signal-processing functions in receivers and transmitters, with trends toward smaller packages and higher power levels.
Filters remain one of the most difficult components to fabricate by means of standard semiconductor processes. For that reason, among others, the need for RF and microwave filters in commercial, industrial, and military-electronic systems remains steady if not strong, with filter designers constantly finding new ways to improve performance while cut costs and size. While it is fairly safe to say that there are probably more RF/microwave filter suppliers in the high-frequency industry than during any other time in history, it is also true that the large number of suppliers has resulted in strong competition and a true "buyer's" market even for filters based on custom requirements.
One of the longest running suppliers in the industry, ARRA, Inc, (www.arra.com), offers extensive lines of coaxial lowpass filters through 8 GHz and bandpass filters through 12.4 GHz. Model LPF5000A, for example, is a lowpass filter with SMA connectors with passband of DC to 5 GHz. The passband insertion loss is 0.5 dB or less while the passband VWR is 1.40:1 or less. The stopband rejection is at least 25 dB at 7 GHz and at least 60 dB at 9 GHz. The rugged filter handles 10 W average power. As an example of the firm's bandpass filters, model BPF 3700-4200 covers the satellite-communications band from 3.7 to 4.2 GHz with maximum in-band insertion loss of 0.7 dB and maximum in-band VSWR of 1.50:1. The lower 60-dB stopband is at 3.26 GHz while the upper 60-dB stopband is at 4.76 GHz.
To help engineers track down or customize a filter according to a specific set of requirements, K&L Microwave (www.klmicrowave) provides a useful little program on their website called the Filter Wizard. It allows visitors to define a filter by their own specifications, using one of four templates for the four filter types (bandpass, band-reject, highpass, and lowpass filters). For a bandpass filter, for example (Fig. 1), it is possible to even define the cutoff definition (0.5 dB, 1 dB, or 3 dB) and a temperature range in which to view the expected performance.
In standard product developments, K&L recently announced a notch combiner for Joint Tactical Radio System (JTRS) applications. The assembly combines a notch filter with power combiner, allowing a number of JTRS transceivers to be combined to a single radio while minimizing the effects of RF interference. The unit provides 45 dB rejection from 969 to 1210 MHz with insertion loss of only 1.2 dB from DC to 920 MHz and from 1270 to 2000 MHz. It is designed to handle 80 W average power and 160 W peak power in the passbands and 20 W average power and 160 W peak power in the rejection band. The low-loss JTRS notch combiner measures 9.75 X 6.0 2.0 in.
Mini-Circuits (www.minicircuits.com) provides similarly powerful search capabilities on its website, although it applies to all of the company's extensive product lines, including amplifiers, mixers, couplers, power dividers, and oscillators. The firm's patent-pending Yoni2 search engine sifts through millions of actual measured data points to find the best product fit for a set of requirements.
The recently announced VHF Series of highpass filters from Mini-Circuits employs a seven-section configuration to maintain extremely low in-band VSWR (typically 1.50:1) with sharp transition band and flat passband response. Using a 7-section highpass filter design, the VHF-SERIES filters provide excellent passband matching (typical VSWR is 1.5), flat passband response, and a sharp transition band. For example, model BPH-1000 has a nominal passband of 1000 to 3000 MHz with typical in-band VSWR of 1.90:1 and typical in-ban insertion loss of less than 1 dB. The stopband rejection is 40dB from DC to 550 MHz and better than 20 dB from 550 to 720 MHz.
Synergy Microwave (www.synergymwave.com) offers a collection of different filter types, including tiny surface-mount bandpass filters and diplexers. Model FBS-200, for example, is a high-suppression surface-mount-technology (SMT) bandpass filter with 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 fullband duplexer for Personal Communications Services (PCS) applications, channeling receive signals from 1850 to 1910 MHz with 1.4 dB typical insertion loss and transmit signals from 1930 to 1990 MHz with 1.6 dB maximum insertion loss. The antenna-to-receiver rejection is at least 62 dB from DC to 1770 MHz and at least 58 dB from 1930 to 5000 MHz. The antenna-to-transmitter rejection is at least 58 dB from DC to 1910 MHz and at least 62 dB from 2050 to 5000 MHz.
Wainwright Instruments GmbH (www.wainwright-filters.com), 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 a user by tuning each resonator. The notches are extremely 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 the third-harmonic notch frequencies) with minimum return loss of 14 dB. The maximum VSWR is 1.50:1. It measures 107 X 60 X 300 mm with SMA female connectors. For more on Wainwright Instruments and the firm's founder, Claire Wainwright, please see this month's Editorial.
StratEdge (www.stratedge.com) 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 for demanding applications.
Dielectric Laboratories (www.dilabs.com) also offers impressive ceramic materials processing technology that it has traditionally applied to single-layer and multilayer capacitors as well as "built-to-print" custom designs. In recent years, the company has leveraged its expertise in ceramic resonators to develop lines of high-performance microwave and millimeter-wave filters in extremely small footprints and with excellent power-handling capabilities.
Another firm that has taken advantage of excellent materials processing capabilities to develop filters is Merrimac Industries (www.merrimacind.com). Using its proprietary Multi-Mix® multilayer circuit technology, the firm has developed a wide range of high-performance multilayer filters for commercial and military applications. For example, model FBGC-3-1.575G bandpass filter was developed for Global Positioning System (GPS) satellite applications at the GPS L1 frequency of 1575 MHz. It offers an 80-MHz 3dB bandwidth at that center frequency, with 1 dB nominal insertion loss and 1.50:1 VSWR. It achieves at least 45 dB rejection of unwanted signals at 1 GHz and at least 20 dB rejection at 1.35 GHz.
One of Merrimac's highest-frequency Multi-Mix filter designs is the model FBMM-42.0 bandpass filter with 3GHz passband centered at 42 GHz. The typical input/output return loss is 15 dB while the passband insertion loss is typically 3.5 dB. The tiny filter, which measures just 0.620 X 0.296 X 0.020 in. and weighs only 0.2 g, handles 1 W typical input power and provides 60 dB minimum rejection at 38.5 GHz and 30 dB minimum rejection at 46 GHz.
Trilithic, Inc. (www.trilithic.com) recently developed a low-loss bandpass filter for UMTS band receiver applications from 1899 to 1921 MHz. The rugged filter measures 3.42 X 2.20 X 1.38 in. with passband insertion loss of 1.15 dB and VSWR of 1.15:1. The input and output return loss is 15 dB. The filter achieves rejection of 50 dB from DC to 960 MHz, 40 dB from 960 to 1830 MHz, 45 dB from 1830 to 1850 MHz, 50 dB from 1850 to 1880 MHz, 25 dB from 1980 to 2660 MHz, and 60 dB from 2660 to 5800 MHz. The company also offers companion UMTS transmit-band filters with passband of ±2 MHz around a center frequency.
Lark Engineering Co. (www.larkengineering.com) has designed the SDP series of high-power surface-mount filters with center frequencies from 800 to 2000 MHz (650 to 2250 MHz in special designs) and 3-dB percent bandwidths from 3 to 7 percent. The filters can be constructed with 2 to 4 sections with maximum insertion loss from 0.5 to 1.25 dB, maximum VSWR of 1.50:1, and power-handling capability to 50 W average and 250 W peak power. The company also offers low-cost ceramic filters for cellular applications, diplexer filters for satellite-communications applications, and surface-mount combline bandpass filters for applications from 5 to 15 GHz.
Integrated Microwave (www.imcsd.com) offers a wide range of microwave filters and resonator products, including the model 930073 lowcost ceramic diplexer for transmitter/receiver separation in the popular 2.4-GHz band. The diplexer features a 3-dB channel bandwidth of 10 MHz with insertion loss of 2.5 dB. The 10-dB bandwidth is 75 MHz. The transmit/receive isolation of 22 dB. The diplexer, which measures just 1.1 X 0.470 X 0.310 in. in a standard SMT package, features two poles per channel and handles 5 W CW power.
RS Microwave (www.rsmicro.com) has been providing quality high-frequency filters for more than 25 years. One of the latest designs is the model 50703-4 switchable notch filter (Fig. 2). Developed for TACAN applications from 962 to 1213 MHz, the filter notch can be switched in and out, with low 1.4 dB insertion loss in the bypass state and less than 4 dB insertion loss in the filter state. The notch rejection is at least 46 dB from 1020 to 1040 MHz. The rugged filter, which handles 52 W average power and 500 W peak power, switches between states in 1 microsecond or less.
Lorch Microwave (www.lorch.com) recently developed several filter banks for military applications. Model 9IFA20/600-SR is a switched filter bank aimed at an airborne electronic-countermeasure (ECM) test application. It offers nine channels ranging from10 to 550 MHz with maximum VSWR of 2.0:1 and maximum insertion loss of 5 dB. The filter bank achieves 60 dB rejection to 10 GHz with 250-ns switching speed between channels. The firm also announced the model 6IFA-4000/80-2000-P is a six-channel switched filter bank, with all channels centered at 4 GHz. Each channel has a different 3-dB bandwidth allowing for programmable selectivity at 4 GHz. Based on dielectric-resonator and LC filters, the assembly measures a mere 2.7 X 2.25 X 0.4 in. excluding connectors and meets MIL-STD-202 environmental requirements.
Remec Defense & Space (www.remecrds.com) offers a variety of microwave filters, including lumped-element filters from DC to 26 GHz with standard bandwidths from 2 to 60 percent, combline cavity filters from 100 MHz to 26 GHz with bandwidths from 0.5 to 60 percent and power-handling capabilities to 10 kW peak power, and interdigital filters from 500 MHz to 26 GHz with peak power-handling capabilities to 10 kW.
In some cases, tunable filters are needed, for example for sensitive receivers or in spectrum analyzers, and YIG-based designs provide a solid combination of broad frequency coverage and high rejection with excellent linearity. One of the longest-running suppliers of YIG filters is Omniyig (www.omniyig.com) with diverse lines of multistage bandpass and band-reject filters. The company's bandpass filters tune from 0.5 to 18.0 GHz, such as the model S103, which is a three-stage bandpass filter with pass band of 20 to 25 MHz that can be tuned across 2 to 4 GHz. It provides at least 70 dB rejection of signals outside of the passband with 1.5 dB passband ripple.
Micro Lambda Wireless, Inc. (www.microlambdawireless.com) is another supplier of tunable YIG filters, also offering bandpass and band-reject filters. The firm's MLFM series of bandpass filters fit into a compact 1-in. cube but offer single-product coverage as wide as 2 to 18 GHz in the model MLFM42018. This four-stage filter features a 30-MHz passband with maximum insertion loss of 5 dB. The minimum isolation outside of the passband is 80 dB, while passband spurious and ripple fall within 2 dB.
The same firm's MLFF series of permanent-magnet YIG bandpass filters cover a total tuning range of 4 to 16 GHz, including the model MLFF-41012, a four-stage bandpass filter with 50-MHz bandwidth tunable from 10 to 12 GHz.
Additional RF/microwave suppliers include Aeroflex/KDI Integrated Products (www.aeroflex-kdi.com), Anatech Electronics (www.anatechelectronics.com), Bree Engineering (www.breeeng.com), BSC Filters Ltd. (www.bscfilters.com), Channel Microwave Corp. (www.channelmicrowave.com), Chelton Microwave (www.cheltonmicrowave.com),Coleman Microwave (www.colemanmw.com), DuCommun Technologies (www.ducommun.com), Eastern Wireless TeleComm (EWT, www.ewtfilters.com), Filtronic Components (www.filtronic.com), Flann Microwave (www.flann.com), KW Microwave (www.kwmicrowave.com), M/A-COM (www.macom.com), Microphase (www.microphase.com), Microwave Circuits, Inc. (www.micckts.com), Microwave Devices (www.mwdevices.com), Microwave Filter Co. (www.microwavefilter.com), Narda Microwave-West (www.nardamicrowave.com), Pacific Millimeter Products (www.pacificmillimeter.com), Planar Filter Co. (www.planarfilter.com), Pole/Zero Corp. (www.polezero.com), Radio Frequency Systems (www.rfsworld.com), Reactel (www.reactel.com), RFMW (www.rfmw.com), RLC Electronics (www.rlcelectronics.com), Spacek Labs (www.spaceklabs.com), Spectrum Microwave (www.spectrummicrowave.com), Teledyne KW Microwave (www.teledynekwmicrowave.com), TEMEX (www.temex.com), and TriQuint Semiconductor (www.triquint.com). For a complete listing of filter suppliers by product category, visit the online version of the Microwaves & RF Product Data Directory at www.mwrfpdd.com.