Impedance transformers are one of the necessities of high-frequency design. Unfortunately, when miniaturization is an issue, the wire-wound ferrite components that represent traditional impedance transformers pose three major design challenges. For one thing, the dome-shaped ferrite core is not well suited for high-volume pick-and-place manufacturing equipment and must be used with a plastic cap. For another, the height constraints of modern commercial designs, such as mobile phones and other portable electronic devices, are not well supported by the relatively bulky size of wire-wound impedance transformers, especially when compared to the small size of typical surface-mount-technology (SMT) components. Finally, wire-wound baluns are hampered by the performance limitations inherent in working with nonlinear ferrites. Fortunately, impedance transformers have been improving with time and designers have some alternatives to the traditional wire-wound solution.

For narrowband applications, such as found in wireless consumer products, or medium-bandwidth applications such as satellite-based television, smaller non-ferrite-based balanced-unbalanced (balun) impedance transformers have been developed to save space compared to traditional wirewound transformers. Implemented using a lattice, Marchand, or Anaren-developed and patented Merrill structure, these solutions are practical where the task calls for no more than about 100 percent bandwidth, but they are not well-suited when a wider bandwidth is needed, such as terrestrial broadcast applications where the bandwidth exceeds 150 percent. Table 1 offers a comparison of typical bandwidths for several narrowband balun structures.

Wire-wound transformers have also gotten smaller over time, with some suppliers offering components as small as 4 X 4 mm with a height profile of 4 mm or less. Even with their recent size reductions, however, the shortcomings of traditional wire-wound baluns remain: they are not well suited for use in high-volume pick-and-place operations, the height of traditional wire-wound transformers is not well suited for low-profile electronic designs, and wire-wound baluns suffer from the material limitations of Ferrite construction with respect to repeatability, temperature stability, and intermodulation distortion (IMD).

To overcome these limitations, Anaren developed a new, non-ferrite transformer based on a patent-pending multilayer approach. The design incorporates the basic impedance-transformation principles first described by Guanella and Ruthroff and applied to wire-wound baluns. But the form factor of the new component is similar to that of ceramic-style SMT baluns.

The new design builds upon the Guanella and Ruthroff impedance-transformation structures, using a multilayer circuit fabrication approach to achieve unprecedented small size. Figure 1 shows configurations for a Guanella 1:1 balun (left) and the Ruthroff variation (right) while Fig. 2 shows the configuration for a Guanella 1:4 transformer. The relative performance profiles of the various impedance transformers have been collected in Table 1.

One of the advantages offered by the new multilayer balun is a reduced profile height and ‘true' SMT package. These improvements not only make these components much easier to incorporate into a fully automated, pick-and-place assembly environment—for the first time, the design allows lower-frequency baluns to be considered at the design stage for integration into ultraslim consumer handheld products such as modern handheld microcomputers and cellular telephones.

The size difference between a typical wire-wound 1:4 balun and one of Anaren's new 1:4 low-profile baluns for terrestrial broadcast applications is apparent (Fig. 3), hinting at the savings in printed-circuit-board (PCB) space that might be realized with the new component. The insertion loss (Fig. 4) and common-mode rejection ratio (CMRR, Fig. 5) of the two components have also been plotted for comparison. The curves show a significant advantage in insertion-loss performance for the smaller, multilayer balun compared to the wire-wound component, with comparable CMRR performance for the two components.

With its improved insertion loss and equivalent common-mode rejection ration performance, the Anaren multilayer balun can directly replace its wirewound counterpart for terrestrial broadcast applications. And, because of its more compact form factor—the PCB area taken up by the multilayer balun is less than 40 percent of that of the wire-wound part—it is possible to construct a layout that can fit either the wire-wound or the Anaren multilayer balun to facilitate easy, in–system performance comparisons and testing. A suggested layout for comparing the performance of a traditional wire-wound balun with that for the new multilayer balun is shown in Fig. 6.

Beyond its small form factor, the new balun benefits from the non-ferrite softboard circuit-board material used in its multilayer construction. The material, with a relative permeability of 1, is less susceptible to changes in temperature than high-permeability ferrite materials. This stable performance with temperature simplifies system and circuit design, since less performance margin is required over a wide temperature range.

The use of high-permeability materials traditionally used in wire-wound balun construction can also be a concern for the system designer since such materials are inherently nonlinear and, at high-enough power levels, can produce intermodulation distortion products that degrade system performance. Conversely, the new, low-permeability multilayer balun exhibits only intermodulation products caused by dissimilar metals, known as passive intermodulation (PIM), which are typically in the –100dBc range. Moreover, the intermodulation products for the Anaren multilayer balun are independent of any bias currents in the circuit.