Power Dividers/Combiners Rise In Frequency, Shrink In Size

April 6, 2009
Power dividers and combiners are often needed to divide or combine signals in a variety of commercial and military systems. They are still available in traditional metal housings with coaxial connectors, but surface-mount versions are ...

Power dividers and combiners are often needed to divide or combine signals in a variety of commercial and military systems. They are still available in traditional metal housings with coaxial connectors, but surface-mount versions are gaining ground, along with increasing requirements for higher frequency and wider bandwidth. Modern power dividers and combiners offer ever-improving performance in terms of insertion loss, linearity, power-handling capability, isolation, and voltage- standing-wave-ratio (VSWR) characteristics.

Some key suppliers driving improvements in this product area include Anaren, JFW Industries, Krytar, MECA Electronics, Microlab/FXR, Mini-Circuits, Renaissance Electronics Corp., RLC Electronics, and Technical Research and Manufacturing (TRM). Targeting broadband electronic-warfare (EW) systems and complex switch-matrix applications, for example, RLC Electronics offers singlepackage, two- and four-way broadband in-phase power dividers/combiners. Designed to cover 6 to 40 GHz, the D-0640 series of dividers/combiners boast average power-handling capability of 10 W.

While the twoway model exhibits insertion loss to 1.0 dB, the four-way version is rated for 1.6 dB. The units feature a VSWR of 2.0:1 with at least 15 dB of isolation. To attain this performance over the frequency range, the power dividers/combiners use a multisection Wilkinson-type design with precision etching of a single microstrip board, states Peter Jeffery, RLC's Director of Marketing. The D-0640 dividers/combiners come with 50-Ohm, 2.92-mm connectors (Fig. 1). The maker also offers four- and eight-way dividers for 0.5 to 18 GHz. Labeled D-05180-4/-8, these units come with SMA connectors.

Also in the race to offer high-performance broadband power dividers/ combiners is MECA Electronics . By combining simulation skills with clever stripline techniques and improved materials, MECA has crafted broadband two- and four-way dividers/combiners to effectively serve military receivers, radar systems, and telemetry applications. From 2 to 18 GHz, for example, the two-way model 802-2-10.000 and the four-way version display similar performance in insertion loss, isolation, and phase/amplitude tracking (Table 1).

Unlike Wilkinsontype, terminated quarter-wave sections, Krytar employs a patented matchedline configuration to achieve a high-directivity, broadband-power divider/combiner (Fig. 2). According to President Doug Hagan, the matched-line directional divider (MLDD) with parallel tapered transmission lines offers high-pass frequency response to attain low insertion loss and high isolation over a broad frequency range. Thus, Krytar has extended the frequency range of its twoand four-way units to 26.5 GHz with 19 dB isolation and 0.5 dB amplitude unbalance. The two-way model exhibits insertion loss to 1.5 dB across the frequency range. Rated for 10 W input power, the models 6005265 and 7005265 come with 3.5-mm coaxial connectors. The company also is extending this technique to lower frequencies over narrow bandwidths.

To achieve a multi-octave bandwidth with moderate power-handling capability and superior phase and amplitude balance, TRM utilizes a cascaded Wilkinson design implemented with microstrip construction for its power dividers/combiners. The units, dubbed the DMS265 and DMS485, employ SMA female connectors. While the maximum input-power capability for the two-way DMS265 is 10 W, the four-way DMS485 can handle 30 W at the input. Similarly, JFW Industries recently added broadband two- and four-way power dividers/combiners to its catalog of Wilkinson-based offerings. With average input power rated at 5 W, the twoway 50PD-645 and four-way 50PD650 are designed to cover a frequency band of 500 MHz to 6 GHz.

Using Wilkinson topology and microstrip construction on a low-loss substrate, Mini-Circuit has crafted two- and four-way dividers covering a bandwidth of 0.5 to 5.0 GHz. According to the company's application note, AN10-003, the dividers/combiners employ computer-aided circuit analysis to deliver superior performance. The two-way wideband model ZN2PD2-50 typically exhibits 0.8 dB of insertion loss with 23 dB isolation. The divider displays insertion loss to 1.4 dB and at least 15 dB of isolation. Similarly, the fourway ZN4PD1-50 exhibits typical insertion loss of 1 dB with 23 dB isolation. It displays insertion loss to 1.8 dB and minimum isolation of 13 dB. The dividers/combiners are housed in a metal case with SMA connectors.

Handling High Power
For high-power applications above 50 W (CW), RLC Electronics taps resistive structures to provide custom solutions for two-way, in-phase divider/combiners covering 750 to 2400 MHz. Unlike the Wilkinson, however, the bandwidth of these devices is limited to 12 percent. As a divider, the device can handle power to 200 W, assuming that the load VSWR is better than 1.20:1.

Power dissipation has been a limiting factor for many Wilkinson power dividers used as combinersespecially when input signals are out of phase, noncoherent, or have amplitude unbalance. A cancellation is thus created across the isolation resistors, resulting in high power dissipation. Typically, such devices utilize low-power, alumina surface-mount resistor chips on a thermally insulative circuit board, notes MECA. The surface-mount chip resistors heat up and degrade if the rated power is exceeded, resulting in loss of port-toport isolation and VSWR.

According to MECA, its 50-W M-series power dividers/combiners overcome these limits by employing higher-power resistors mounted on a high-thermal-conductivity substrate. This substrate boasts three times the heat-transfer capability of ordinary circuit-board materials. As a result, it increases maximum input-power capability for combining non-coherent signals on adjacent ports. Designed for system applications from 0.8 to 2.2 GHz, the M-series offers standard models in twoway through eight-way configurations. They are available in both N-female and SMA-female connectors with gold-plated contact pins and a rugged aluminum housing to minimize RF electromagnetic interference (EMI).

For applications requiring more than 50 W input power, MECA has crafted the H-series 80- and 100-W dividers and combiners. Targeting wireless applications between 400 MHz and 2.2 GHz, the 100-W H series exhibits roughly 25 dB of isolation and under 0.3 dB insertion loss with a VSWR of 1.15:1. This series, which is constructed to combine unbalanced inputs, comes in two-, three-, and fourway configurations. According to the maker, these specifications vary somewhat depending on the type of connector used. Connector choices include 7/16 DIN, N, and SMA.

Known for tackling high power, Werlatone has developed a tiny 20-to-500-MHz part aimed at VHF/UHF applications. This component is capable of accepting continuous- wave (CW) power from 25 to 1500 W with extremely low insertion loss. For instance, the 1500-W-rated D5576 exhibits 0.5 dB or less insertion loss and at least 15 dB isolation. Phase balance ranges to 5 deg while VSWR is specified at 1.35:1.

For cellular, PCS, and DCS applications in the 600-to-2300-MHz range, Mini-Circuits has produced a coaxial four-way power combiner labeled ZB4PD-232-50W+. This in-phase combiner, which also can be used as a power splitter, handles power levels at each of the four ports to 50 W. Above the nominal 6-dB combining losses, it typically exhibits 1.1 dB insertion loss across the full frequency range. Between the ports, it usually displays 19 dB of isolation across the full frequency range. The combiner exhibits insertion loss of 0.9 dB from 1.8 to 2.0 GHz with typical isolation of 28 dB. Amplitude unbalance is usually 0.05 dB from 600 to 2300 MHz while phase unbalance is 0.9 deg. across that same frequency range. The ZB4PD- 232-50W+ exhibits good input and output impedance matching with typical combiner input VSWR of 1.20:1 or less on all four ports and VSWR of 1.10:1 on the summed (combiner output) port.

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Surface-Mount Packaging
Many cellular and emerging wireless applications are demanding RF and microwave components in tiny surfacemount packages. To answer this need, Anaren Microwave has developed miniature power dividers/ combiners covering 400 MHz to 8 GHz. The PD0922J7575D2 two-section, Wilkinson-design-based, two-way divider, for example, has a height of 0.8 mm while exhibiting typical insertion loss of 0.8 dB. From 950 MHz to 2.150 GHz, isolation is 16 dB. Matched to 75 Ohms input and output, the divider is rated for 2 W CW power. The unit is constructed from ceramic filled polytetrafluoroethylene (PTFE) composite material for good thermal stability.

To add up to three different signals without any mutual interaction, Microlab/FXR, a wholly owned subsidiary of Wireless Telecom Group, has released a broadband 3-x-3 hybrid combiner spanning the 700-to-2700-MHz frequency band. Designed for combining three signals in radio base stations, the CM-81 flaunts input isolation above 25 dB while passive intermodulation (PIM) is rated at -150 dBc. The average power for each input is 60 W with 3 kW peak power capability. The hybrid combiner comes with either N or 7-16 mm connectors.

For wireless applications that need in-phase and out-of-phase signals from the same unit, MECA has developed a two-way hybrid divider/combiner that utilizes substrate-based microstrip design (Fig. 3). Offering 10 to 12 percent bandwidth in the 810-MHz-to-6-GHz range, the 707S-X 3-dB hybrid divider/ combiner family is band-specified. To deliver 180-deg. out-of-phase signals, the two-way divider/combiner incorporates hybrid branchline technology, states Tom Hickey, the company's Vice President of Engineering. Table 2 illustrates typical port configurations to achieve in-phase and out-of-phase signals at the output ports. While minimum isolation for the 50-Ohm 707S-X family is rated at 22 dB, the average power handling ability is given as 100 W with maximum port VSWR of 1.25:1.

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