Choosing To Combine Or Divide Power

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.

Page Title

Merrimac's Multi-Mix microtechnology has been applied to the development of a series of miniature power dividers/combiners known as the Pico™ line. The company's Pico Z-series power dividers/combiners are fusion-bonded multilayer stripline assemblies that can be supplied in tape-and-reel format for surface-mount applications. In particular, the model PDD-2Z-1.7G is a two-way power divider/combiner that operates from 1.5 to 1.8 GHz with low loss and reasonable isolation.

Mini-Circuits, which offers one of the most extensive component lines in the industry, also supplies a tremendous variety of power dividers/combiners, including surface-mount and drop-in 50- and 75W components and larger, higher-power coaxial units. The model SBB-2-23, for example, is a two-way surface-mount power divider/combiner that covers 2000 to 2300 MHz and measures just 0.2 x 0.275 x 0.070 in. In spite of the small size, it achieves better than 20 dB isolation across its operating range with phase unbalance of better than 1 deg. When used as a power divider, the tiny component can handle 10 W power, although it is rated for internal (resistive) dissipation of 0.25 W. In contrast, the firm's model ZB4CS-700-10W power divider/combiner is a four-way coaxial unit that is rated for maximum internal power dissipation of 8 W. It operates from 400 to 700 MHz with impressive typical isolation of 25 dB and typical insertion loss of 0.35 dB. (The company's date sheets are among the few in the industry that show actual S-parameters for insertion loss, indicating the insertion loss with division losses included.)

It should be noted that manufacturers often present both amplitude unbalance and phase unbalance in several formats, including as a simple value representing a range, such as 0.5 dB or 6 deg., or as a deviation, such as ±0.4 dB or ±5 deg. It is important to recognize the format used by a given manufacturer and to "normalize" the specifications when performing a side-by-side comparison.

Additional specifications to compare when specifying power dividers/combiners include VSWR and power-handling capability. VSWR, or voltage standing wave ratio, is a measure of how well a component's input and output ports are matched to each other and to the outside world which, in high-frequency systems, is usually at a characteristic impedance of 50W. W.> Simply put, the best-matched components will exhibit the lowest VSWR. As with many specifications, manufacturers often list both maximum and typical numbers, with values that vary as a function of frequency. In general, maximum VSWR numbers that are below 2.0:1 indicate good-quality materials and manufacturing processes, especially for broadband components. For example, four-way power dividers/combiners from ARRA (www.arra.com) with SMA connectors are available in frequency bands from 0.005 to 18.0 GHz. One of the company's broadest-frequency components in this line covers a range of 8 to 18 GHz with maximum insertion loss of 1 dB and minimum isolation of 17 dB. In spite of the wide frequency range, the maximum VSWR is a respectable 1.60:1.

Power-handling capability is a function of several factors, including the physical size of the component (and its ability to dissipate heat), the type of connectors, and how well the ports are matched. Obviously, a rugged coaxial power divider/combiner with machined aluminum housing will be able to dissipate more heat than a chip-sized drop-in or surface-mount power divider/combiner.

In terms of power, most manufacturers provide two levels of capability, average (continuous) power and peak (short-duration) power. The SMA four-way power dividers/combiners from ARRA, for example, are rated for 20 W average power but 1 kW peak power. ARRA, as with most manufacturers, presents these values based on the assumption that all ports are matched (output ports terminated in 50W for a precisely balanced load). Under unmatched conditions, the limiting factor on a power divider/combiner's power-handling capability will be the ratings of the internal resistors (which are rated for about 0.5 W in the case of the ARRA four-way SMA units).

For nearly 40 years, Werlatone (Brewster, NY, www.werlatone.com) has been a company synonomous with high-power passive components. The company offers an extensive line of very high-power dividers/combiners with continuous power ratings in the kW range. The company's model D3897, for example, is a two-way divider/combiner that operates from 500 to 1000 MHz with a power rating of 1 kW CW. To achieve this, the rugged design employs Type N connectors and achieves maximum insertion loss of 0.2 dB. The component also boasts impressive numbers for amplitude and phase unbalance, at 0.3 dB and 3 deg. maximum, respectively.

In terms of sheer bandwidth, another New York company, RLC Electronics (Mount Kisco, NY, www.rlcelectronics.com) offers some of the widest-range power dividers/combiners in the industry. The new model D-0640-W two-way power divider/combiner, for example, is a multisection Wilkinson design that ranges from 6 to 40 GHz with 15-dB minimum isolation and 1-dB maximum insertion loss. Over the broad range, the amplitude unbalance is ±0.5 dB while the phase unbalance is ±15 deg. The component is supplied with 2.92-mm K connectors to continuously cover the broad frequency range.

Using the same type of connectors, the model 4318-4 from Narda Microwave (www.nardamicrowave.com) is a four-way power divider/combiner with frequency range of 18 to 40 GHz. With maximum input/output VSWR of 1.90:1 and minimum isolation of 14 dB, the rugged component is a Wilkinson-type divider/combiner that is suitable for outdoor commercial and military applications. The 50-year-old high-frequency firm (see Microwaves & RF, June 2004, p. 33) offers a wide range of power dividers/combiners, from simple two-way models to more complex 12- and 16-way units. The company has supplied higher-order dividers through 48-way and 64-way dividers with every type of connector from tiny 1.85-mm connectors through rugged EIA 7/16 high-power connectors.

The scope of this article prohibits listing all hundred or so suppliers of RF/microwave power dividers/combiners. Those wishing a representative listing are advised to visit the Microwaves & RF Product Data Directory website at www.m-rf.com, click on the Visitors button, log in (or skip log in for quicker access to the site), click on the "Search Manufacturers by Product Category" button, click on the "Components" menu selection, then select either the "Power Combiners" or "Power Dividers" category for an extensive listing of suppliers, many with active links to their websites.