Finding Ways To Reduce Filter Size

June 17, 2009
Filtering functions to reject unwanted signals are being made smaller through a variety of circuit configurations, with packages that include drop-in and tiny surface-mount styles.

Smaller filters are a requirement for a growing number of commercial and military applications. With available spectrum becoming ever more crowded, filtering requirements are increasingly more severe, but filter suppliers are being asked to shrink their packages to fit more portable and compact applications, from cellular telephones with embedded Global Positioning System (GPS) receivers to tactical radios for miniature military unmanned aerial.

A variety of different technologies are used to achieve smaller microwave filters, including lumped-element, stripline, and microstrip circuit approaches and the use of select materials, such as ceramic substrates to reduce circuit dimensions. Merrimac Industries, for example, relies on its patented Multi-Mix multilayer fabrication technology to create fusionbonded stripline filter circuits with passbands as narrow as 1 percent. The firm's model FBGC-3-1.575G has a center frequency of 1.575 GHz with 80 MHz 3-dB bandwidth and measures just 0.60 x 0.70 x 0.105 in.

Mini-Circuits offers miniature surface-mount and drop-in filters based on lowtemperature- cofired-ceramic (LTCC) technology, such as the lowpass LFCN series and the highpass HFCN filters. Model LFCN-6400+, for example, is a RoHS-compliant lowpass filter with a passband of DC to 6400 MHz and nominal 3-dB cutoff of 7200 MHz. The stopband range is 7770 to 10200 MHz with 20 dB rejection at 8300 MHz and as much as 40 dB rejection from 7770 to 10200 MHz. The compact filter measures just 0.13 x 0.60 x 0.04 in. Synergy Microwave Corp. also offers compact drop-in and surface-mount bandpass filters past 500 MHz.

Surface-mount and drop-in bandpass filters from RLC Electronics can be supplied with as many as six sections for high rejection of unwanted signals. The SMBP series of filters, for example, measure 1.00 x 0.50 x 0.35 in. for filters with as many as five sections and 1.25 x 0.50 x 0.35 in. for filters with six sections. The filters can handle as much as 2 W input power for center frequencies from 4 to 6 GHz and 3-dB passbands of 5 to 50 MHz. In addition, surface-mount RF/ microwave bandpass filters as small as 0.5 x 0.5 in. are available from Bree Engineering.

K & L Microwave ( a unique approach to compact filter packaging with its Filter Adaptor family, essentially lowpass, highpass, bandpass, and band-reject filters built into coaxial connectors. The filters hold 3 to 5 sections, are 1.536 in. long, and operate from 2 to 10 GHz.

To aid designers in achieving smaller filters, Integrated Microwave offers coaxial transverseelectromagnetic (TEM) resonators that can be used for filters from 300 MHz to 10 GHz. The resonators range from 3 to 12 mm with dielectric constants of 8, 20, 36, and 98 with tolerances as tight as 0.25 percent available.

One of the limitations in making filters smaller is power-handling capability, since a great deal of heat must be dissipated at higher power levels. As a result, the physical size of a microwave filter is generally proportional to its powerhandling capabilities. As an example, model P/N 91122A-1 is a high-power bandpass filter from RS Microwave designed to handle 300 W peak power from 962 to 1213 MHz. Over its passband, it exhibits 0.8 dB maximum passband insertion loss. It achieves 8 dB minimum rejection at 944 MHz and 19 dB minimum rejection at 900 MHz. The minimum rejection is 50 dB from DC to 600 MHz and from 1484 MHz to 14 GHz. Even with its power-handling capability, the filter measures a mere 2.75 x 1.267 in.

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