Filters may not be considered revolutionary, but they are quickly evolving to meet modern needs for miniaturization. To help designers shrink receivers or printed-circuit boards (PCBs), for example, more RF filter makers are now offering these components in surface-mount-technology (SMT) packages. On the military and wireless infrastructure side, ruggedness is a key requirement as well. Although the many wireless systems need filters to keep their signals separate, filters also are evolving to provide flexibility through tunability and meet the needs of testbench and other applications. In addition, designers are combining multiple filters and switches in switched filter banks to meet wide-bandwidth needs.

Some of the more interesting filter trends have been in the replacementor rather displacementof existing technologies. For example, EMI Filter Co.'s EMI Powerline "PL" series is designed to remove RF and microwave interference from power lines. These low-cost filters promise to outperform 1500-pF pi-type lowpass filters under load conditions. They will not saturate like conventional pi lowpass filters, and use multilayer ceramic capacitors to minimize dips across a wide range of frequencies.

According to Jeff Sloane, VP Operations at Integrated Microwave Corp., "Certain kinds of technology cannot be squeezed,' but instead must be replaced. Ceramic filtering product performance is related to the free space it occupies, and its performance is degraded as that space is reduced. SAW and BAW technologies are smaller, purported replacements for ceramic filtering technology, but they've fallen short for many applications. These technologies were supposed to encroach on ceramic filtering applications, but many ceramic filter applications cannot tolerate SAW or BAW filter performance."

Sloane points out that ceramic filters have been available in SMT packages for over 15 years. SMT filter packages use less space on the host board. They also are easy to install, and are associated with low NRE for custom designs. Even "small" ceramic filters handle far more power than SAW or BAW devices. But ceramic filters will never be as small as SAW or BAW filters.

As is true for many product segments in the RF and microwave industry, mobile communications have largely driven the need for smaller filters. According to Scott Klettke, Senior Group Product Manager for Murata Electronics North America, "As related to filters, miniaturization is being driven by the miniaturization of cellular telephones combined with the increasing number of radio systems and frequency bands included. To fit all the systems and radios into what consumers have recognized as an appropriate form factor, handset makers have been using modules and sub-modules. The requirements for small modules drive the need for small passives. Rather than taking space for additional packaging, the barest elements of passives are integrated into a single-packaged module."

For its part, Murata offers three basic RF filter technologies: multilayer, dielectric, and surface acoustic wave (SAW). Klettke notes that all three filter types have been in SMT packages for at least 15 years. "SMT allows for pick and place and reflow soldering, which translates into fast assembly time," he explains. "By using land-grid-array electrodes, you can reduce the overall space taken by not requiring additional space for solder fillets. The result is higher-density mounting."

In the mobile-phone arena, the usage of SAW components is set to increase with the onset of data-intensive applications. As an example, TDK-EPC now offers a band-rejection filter from EPCOS for mobile TV under the Japanese ISDB- T set of digital TV standards. Thanks to an 1411 SMD package measuring 1.4 x 1.1 x 0.4 mm3, the B8740 allows manufacturers to easily integrate the filter into compact mobile phones (Fig. 1).

For next-generation access technologies, such as Long Term Evolution (LTE), radio designers will need high-performance filters with low passband distortion to ensure high levels of signal integrity. Maintaining superior amplitude and phase and group delay linearity is becoming more challenging, however, as system bandwidths increase to match the rising data rates. Here, a SAW approach is being taken by Dover Corp. company Vectron International for a range of intermediate-frequency (IF) filters that are optimized to support transceiver and repeater designs in emerging LTE applications. These SAW IF filters address center frequencies ranging from 50 MHz to 1 GHz and bandwidths from 3.5 to 75 MHz with typical passband insertion loss of 5 dB.

Wireless applicationsalbeit of a different sortalso are the focus of a cavity bandpass filter (BPF) from Anatech Electronics. To handle outdoor 2.4-GHz WiFi installations, this filter has to be both ruggedized and weatherproof (Fig. 2). It also must provide high performance, such as handling RF input power to 40 W. The Wifi2437-6A has a center frequency of 2437 MHz and spans 2427 to 2447 MHz. It exhibits maximum insertion loss of less than 2 dB and passband ripple of less than 0.2 dB.

With the rapid increase in multimode, multiband systems, variable and tunable components are one potential way to reduce system complexity. Small size and tunability are provided in the MICROPOLE BPF series from cosite-mitigation specialist Pole Zero. These filters, which come in a surface-mount package, are varactor based in order to minimize size and power consumption. The series covers eight bands from 10 MHz to 2 GHz. From 90 to 200 MHz, the filter provides a 3-dB bandwidth of 5.0/5.7.

The surface-mount combline (SMC) BPFs from Lark Engineering Co. were developed to address the need for smaller-size, surfacemount devices at the X and Ku bands. Usually, BPF solutions for these frequencies were attained with large, machined, connectorized filters. In contrast, a new five-section version of the SMC line manages to cover 5 to 15 GHz while exhibiting insertion loss of 1 to 3 dBa.

Like Vectron and Pole Zero, filter providers K&L Microwave and Dielectric Laboratories (DLI) fall under Dover Corp.'s Ceramic & Microwave Products (CMP) umbrella. Compared to alumina and PWB materials, DLI's high-K ceramics are known for providing both size reduction and temperature stability. The firm has recently moved beyond microstrip bandpass designs to offer a range of custom options, such as notch, lowpass, and highpass filters.

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RISE OF SWITCHED FILTER BANKS
To satisfy military receiver designs, synthesizers, radar, and guidance systems, K&L Microwave's modular switched filter banks can be configured from two to seven channels (Fig. 3). In low-profile packaging, they manage to deliver channel-to-channel isolation of 60 dB minimum. Covering 100 MHz to 10 GHz, they deliver typical VSWR of 2.0:1 with bandwidth ranging from 10 to 100 percent, based on filter topologies. A range of standard switched-filter-bank models are available as well, boasting laser sealing to meet harsh military requirements.

In a similar vein, a six-channel switched filter bank from Lorch Microwave was designed to meet Mil-STD-202 environmental conditions. Dubbed the 6IFA-4000/80-2000-P, it promises to provide superior phase noise characteristics under vibration. The filter bank centers all channels at 4000 MHz. The 3-dB bandwidths for each channel vary, starting with 80 MHz on channel one and ending with 2000 MHz on channel six. This unit utilizes dielectric and LC filters featuring a very tight shape factor. It exhibits insertion loss to 8.0 dB.

The broadening of cellular spectrum also is inspiring many microwave engineers to look at switched filter banks as an option. According to Brian Hendren from Tektronix Component Solutions, "Market trends indicate that a large number of industries (radar, EW, spectrum managers, and communications) are moving toward wider bandwidth requirements. Switched filter banks allow the system to pass a wide swath of spectrum while rejecting unwanted out-ofband frequencies."

Hendren points out that a switched filter bank's wide bandwidth allow for wide frequency acquisition and generation. Compared to yttrium-iron-garnet (YIG)- tuned filters, he asserts that switched filter banks can achieve wider bandwidths while providing faster switching times with no hysteresis. Tektronix Component Solutions just debuted the TSFB- 900A switched filter bank, which covers 8 to 22 GHz (Fig. 4). It offers a minimum channel bandwidth of 1.5 GHz with 150 MHz of band-to-band overlap. Also pushing filter limits are the needs of test and laboratory equipment. Many firms are serving this segment. For example, Crystek Corp. recently introduced a line of highpass filters called the CHPFL series (Fig. 5). Encased in a rugged SMA housing, this line comprises five models covering DC to 100 MHz through 1 GHz.

RLC Electronics designs and manufactures tunable filters in bandpass and band-reject versions, which are well suited for precision test and measurement environments. Bandpass models in the CBPT series offer a center frequency of 2000 to 12000 MHz with 3-dB bandwidths to 3 percent of the center frequency and up to 10 section response for most models. In contrast, band-reject models in the firm's BRFT series offer a center frequency of 1000 to 6000 MHz with 3-dB bandwidths to 12 percent of the center frequency and nine section designs for most models (Fig. 6). Taking an assembly approach is a YIGbased offering from Micro-Lambda Wireless, Inc. The MLBF-Series Bench benchtop filter assemblies provide either bandpass or band-reject filters. Frequency coverage for bandpass models ranges from 500 MHz to 50 GHz while the band-reject models cover 500 MHz to 20 GHz.

The evolution of some filter types is challenged, as not all of them can be surface mounted or lend themselves to miniaturization. With cellular and broadband communications reaching more consumers in increasing ways while providing more data, however, the drive toward miniaturization is underway. Microwave designers will continue to wrestle between insertion loss and attenuation to achieve the performance they need. As Murata's Klettke notes, though, higher Q materials could keep high performance in smaller packages. Hopefully, the materials suppliers are already at work developing them.