Power must often be combined from multiple transistors in an amplifier or antennas in a system, or divided among channels in a receiver. For that reason, the humble RF/microwave power dividers/combiners is among the most competitive product areas in this industry, with about 100 different suppliers. To make an educated decision when specifying a power divider/combiner, it may help to review the basics of these components along with their key performance parameters before reviewing some of the latest product offerings in this area.

The simplest power divider is a component that splits one input signal into two in-phase output signals. More complex units can be constructed from cascades of this binary configuration, with even-numbered output ports, although dividers with an odd number of output ports, such as a three-way divider, are not uncommon (and often referred to as N-way power dividers, with N equal to an odd number). Power dividers can be formed by internally terminating a 180-deg. hybrid or through the use of Wilkinson or tapered-line configurations. Wilkinson dividers can be realized in microstrip or stripline technologies as a series of cascaded quarter-wavelength transformers. The transformers are used to transform the input impedance, typically 50W, to an output impedance represented by the parallel combination of multiple outputs.

The simplest Wilkinson power divider is a single-section component with 50W input port, two 70.7W quarter-wavelength transformers, and a 100W resistor. This simple configuration is relatively limited in bandwidth; more transformer sections and isolation resistors are needed for increased bandwidth, although the increased complexity also leads to increased size and insertion loss. An excellent application note from M/A-COM (www.macom.com), entitled simply "Power Dividers/Combiners" (application note M561) offers a concise four-page summary of power divider/combiner technology and the importance of various performance characteristics.

In an ideal two-way power divider, a 2-W input signal would result in two 1-W output signals. But resistors and terminations are not without loss, and even the best power dividers suffer some amount of insertion loss from input to output. Insertion loss can be readily calculated for a component as equal to 10log(input power/output power). In comparing power divider/combiner specifications, it should be pointed out that the actual insertion loss in a two-way divider would be comprised of the 3-dB division loss as well as the dissipative losses but, in most cases, manufacturers assume these division losses and present the insertion loss as that value above the nominal division loss (such as 3 dB in a two-way divider and 6 dB in a four-way divider).

Ideally, that two-way power divider would feature two output ports that are infinitely isolated from each other so that the division of power from the input to the output occurs without unwanted signal leakage between ports. In the real world, however, the isolation is limited by variations in component values, manufacturing tolerances, and other factors. Still, high isolation is a measure of a high-quality power divider/combiner, typically exceeding 20 dB. As with insertion loss, the value of a power divider/combiner's isolation will vary with frequency, generally degrading with higher frequencies.

Two additional specifications that are important to power divider/combiner specifiers are amplitude balance (or unbalance) and phase balance (or unbalance). The two terms are also often denoted as amplitude tracking and phase tracking, although they represent simply the differences in amplitude and phase, respectively, between the output ports of a divider. Amplitude unbalance can be almost negligible in a two-way power divider/combiner, with performance levels of ±0.3 dB or less not unusual. As the complexity of a divider/combiner increases, however, this specification tends to increase. An eight-way power divider/combiner, for example, would more typically exhibit amplitude unbalance of ±10 dB between ports.

Phase unbalance will always be the larger number of the two parameters, since it is more difficult to control especially over wide frequency ranges. The small sizes of the latest drop-in and surface-mount power dividers/combiners, however, account for some impressive specifications. Traditional coaxial power dividers/combiners might show a phase unbalance specification of ±10 deg. depending on bandwidth and frequency, with the value increasing as a function of increasing frequency and bandwidth. Drop-in power dividers/combiners such as the Xinger™ line of miniature components from Anaren Microwave (www.anaren.com) achieve good amplitude and phase tracking between output ports through their small sizes and well-controlled manufacturing tolerances. The company's model 4A1305 is a two-way, in-phase power divider that measures 0.560 x 0.350 x 0.081 in. and operates from 1400 to 2600 MHz. It features broadband phase balance of 2.5 deg. with narrowband performance of 2.0 deg.

Such small power dividers/combiners are available from an increasing number of suppliers, including Merrimac Industries (www.merrimacind.com), Mini-Circuits (www.minicircuits.com), and Synergy Microwave (www.synergymwave.com). The newest power divider from Synergy Microwave, for example, is the model SPD-5-1000, a surface-mount four-way unit measuring just 0.8 x 0.3 x 0.2 in. It features an operating-frequency range of 5 to 1000 MHz with typical insertion loss of 0.7 dB from 5 to 50 MHz, 0.65 dB from 50 to 500 MHz, and 0.7 dB from 500 to 1000 MHz. Isolation is typically 23 dB at the higher frequencies and 27 dB at the low end. Maximum amplitude unbalance is 0.5 dB while maximum phase unbalance is 5 deg. The model SPD-5-1000 divider is designed to handle input power levels to 1 W. The company also offers power dividers/combiners in a variety of package types, including relay headers, leaded and leadless surface-mount packages, and large coaxial packages.