FCBs Shrink Systems From 8 To 40 GHz

The unique concept of a ferrite circuit board (FCB) allows ferrite components from 8 through 40 GHz to be integrated with conventional thin-film microwave circuits.

Ferrite components, such as isolators and circulators, are essential to the operation of many microwave assemblies and systems, including power amplifiers. But these components tend to be bulky and have never lent themselves to integration on thin-film circuits. Dorado International (Seattle, WA) has taken a fresh look at these components, however, and developed integrated components and subassemblies called ferrite circuit boards (FCBs)—designs in which the traditional alumina dielectric substrate is replaced with a ferrite substrate. This allows thin-film assemblies to be produced with isolators and circulators fabricated on the same planar printed-circuit board as couplers, hybrids, attenuators, power dividers, and other high-frequency active and passive components. The technology is useful for designs requiring ferrite components and operating over the range from 8 to 40 GHz.

Compared to conventional approaches,FCBs yield components with low insertion loss. Lower losses stem from the elimination of microwelding, soldering, or other bonding methods used to interconnect conventional ferrite components to high-frequency circuits. The FCB approach allows a short interconnection path between the devices and components printed on the common FCB to further reduce losses (Fig. 1). And by reducing the small gaps between individual components, which become commonplace when melding conventional ferrite components to thin-film circuitry, the performance above 15 GHz can be dramatically improved. The manufacturing process for FCB circuits also lends itself to matching or tuning individual active and passive components.

All of these improvements result in a significant reduction in the insertion loss of the FCB when compared to circuits using discrete isolator and circulator components. Compared to discrete components, the FCB also offers a dramatic savings in size, due to the shorter line lengths possible between components. Using a single FCB will also provide the manufacturer with the flexibility to design a more effective layout to minimize its size. The FCB process is also less in cost than traditional ferrite fabrication methods, with significant savings in manufacturing due to a lesser number of total parts to assemble.

How well do the FCB components perform? As an example, junction devices fabricated for applications from 8 to 40 GHz provide high isolation between receivers and transmitters while cutting insertion losses to a minimum. The maximum measured insertion loss from antenna to receiver ranged from 0.5 dB at 9 GHz to 1 dB at 40 GHz. The maximum insertion loss from the transmitter to the antenna measured 1 dB at 9 GHz and 2 dB at 40 GHz. With maximum VSWR of 1.30:1 and power-handling capability of 0.5 W, the FCB multiple junction devices achieve minimum receiver-to-transmitter isolation of 20 dB and antenna-to-transmitter isolation of at least 33 dB. Such junction devices are suitable for use at the front end of microwave systems to provide isolation between different parts of a communications system without need for filters. Higher isolation is possible by using additional ferrite junctions.

In another design, four circulators were fabricated on an FCB for Ku-band applications (Fig. 2). The maximum insertion loss for each device is 0.5 dB while the isolation is 20 dB. With power-handling capability of 0.5 W, these four-circulator devices achieve maximum VSWR of 1.30:1 over operating temperatures of ?30 to +70 C.

Model DIH-15 is a pair of circulators wit 3-dB 90-deg. hybrids fabricated on a single FCB. Designed for Ku-band operation, the FCB device achieves 20 dB or more isolation while suffering total insertion loss of 3.9 dB or less. With maximum VSWR of 1.30:1, the dualcircular device handles 2 W maximum forward power and 0.5 W load power. The output amplitude unbalance is ±2.5 dB over an operating temperature range of ?30 to +65 C.

The FCB super-component is a new approach that offers the design engineer a solution to help solve some of the problems related to reducing space, minimizing costs and improving performance at the front-end of a phased-array antenna system. Dorado International Corp., 998 Industry Dr., Seattle, WA 98188; (206) 574-0900, FAX: (206) 574-0912, Internet: www.dorado-intl.com.

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