Quad Hybrids Serve 450-MHz CDMA Systems

Feb. 28, 2005
Available in a variety of form factors for numerous power combining and dividing applications, these miniature devices handle generous amounts of RF power for their size.

CDMA450 (at 450 MHz) as implemented in Europe and Asia shows great promise for low-cost voice and high-speed fixed and mobile Internet access. As always, competition among base station manufacturers is driving prices down, and RF component manufacturers are asked to respond with cost-effective solutions. Merrimac's (West Caldwell, NJ) response is the Multi-Mix QHD Series 3-dB, 90-deg. hybrid couplers, which provide cost-effective power combining and dividing in CDMA450 base stations.

All "quad hybrids" such as the QHD Series (see figure) are essentially 90-deg. power dividers and combiners that can be used to divide an input signal into two output signals, one of which is shifted in phase by 90 deg. This produces the "quadrature" relationship between the two output signals, since their phases differ by one quadrant of total phase (360 deg.). When used as a combiner, the device can combine two input signals with equal amplitude and quadrature phase into a single output signal. Typical applications include base-station power amplifiers, receive/antenna multicouplers, and essentially any RF signal-distribution situation.

CDMA450 offers significant advantages to wireless providers already operating systems at 450 MHz, as is the case in Europe and Asia. It is also a bit of an anomaly in the wireless world, operating in an area of the spectrum that is one of the lowest designated for personal-communications services. Nevertheless, CDMA450 is a good model for providing "3G" wireless telephony that includes fixed and mobile Internet access, and offers a clear path for future enhancements. It is essentially CDMA2000 deployed at 450 MHz.

The QHD Series devices have several characteristics that make them appealing in the CDMA450 environment. First, they deliver the requisite low loss, high isolation, phase stability, amplitude balance, and power-handling ability required in these systems. They also adhere to an extremely small "standard" form factor that allows designers to reduce circuit-board complexity by using a single package size, and they are very low in cost.

Base-station transceivers for CDMA450 (as well as any other wireless-communications band) can benefit from "commonality of form factor", i.e., using a single package size for a wide variety of functions, and OEMs have expressed an interest in achieving it. The QHD Series was designed with this in mind, and its package styles are the same as used for other Merrimac products based on Multi-Mixed Microtechnology, its patented multilayer design and manufacturing process.

The QHD family of quad hybrid couplers includes three models, each of which is enclosed in a different package size. All three devices are rated to operate from 380 MHz to 500 MHz, which in addition to the CDMA450 market, makes them useful for other UHF applications, such as in land mobile base-station amplifiers and radar systems.

The larger wavelengths at frequencies in the UHF region make achieving high performance in a very small device extremely difficult. The Multi-Mix process in which the QHD Series is designed removes many of these obstacles. Multi-Mix is based on fluoropolymer composite substrates that are bonded together into a multilayer stripline using a fusion bonding process. The fusion process provides a homogeneous dielectric medium that delivers superior electrical performance.

While its advantages are most significant at microwave and millimeter-wave frequencies, Multi-Mix also allows excellent results to be achieved at UHF frequencies. The multiple bonded layers can incorporate semiconductor devices, MMICs, etched resistors, circuit patterns and plated-through via holes. The result is a small, surface-mountable, self-contained module that requires no additional packaging and is compatible with microstrip or coplanar waveguide.

Even the largest of these, the QHD-2BH-0.45G measures only 1.0 × 0.5 × 0.07 in., making it and the other members of the family well suited to any new design optimized to provide the required functions in the least amount of space. Other members of the family include the QHD-2H-0.45G (which measures 0.56 × 0.35 × 0.155 in.), and the QHD-2N-0.45G (which measures 0.65 × 0.48 × 0.155 in.). The devices handle an RF input power up to 200 W, depending on the model. More detailed specifications for all three models are shown in the table. As with the company's other Multi-Mix products, the QHD series is available on tape and reel for automated manufacturing. Merrimac Industries, Inc., 41 Fairfield Pl., West Caldwell, NJ 07006, (973) 575-1300, e-mail: [email protected], Internet: www.multi-mix.com.

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