Multilayer baluns conflict size barriers

Impedance transformers are an essential part of high-frequency designs. However, the wire-wound ferrite portion of the traditional impedance transformer presents three major design challenges with regard to miniaturization: First, the dome-shaped ferrite core is not well suited for high-volume assembly and must be used with a plastic cover. Second, the relatively large size of the wire-wound impedance transformer, especially compared to the small size of typical surface mount (SMT) components, often does not meet the height constraints of modern commercial designs, such as mobile phones and other portable electronic devices. Finally, the wire-wound balun is limited by its inherent performance when combined with nonlinear ferrites. Fortunately, impedance transformers are constantly improving, and designers can adopt some alternatives to the traditional winding method.

For narrowband applications, such as in wireless consumer products, or medium bandwidth applications, such as satellite television, nonferrite balanced-unbalanced (balun) impedance transformers that are smaller than traditional wound transformers have been developed to save space. Implementations can use LATTICE, Marchand, or the Merrill structure developed and patented by Anaren, which is practical when the design task requires no more than 100% bandwidth, but is no longer suitable when wider bandwidth is required, such as terrestrial broadcast applications with bandwidths exceeding 150%. Table 1 provides a comparison of the typical bandwidths of several narrowband balun transformers.

Wirewound converters are also becoming smaller, with some manufacturers offering 4×4mm parts that are no more than 4mm high. Even with the current reduction in size, traditional wirewound baluns still have some disadvantages: they are not very suitable for high-volume assembly operations, the height of traditional wirewound converters is not suitable for small electronic products, and wirewound baluns are limited by the material limitations of ferrite structures for reusability, temperature stability and intermodulation distortion.

To overcome these limitations, Anaren has developed a new non-ferrite transformer based on a multi-layer approach (patent pending). The design incorporates the basic impedance transformation principles first described by Guanella and Ruthroff for wirewound baluns. However, the new product has a form factor similar to a ceramic SMT balun.

This new design is based on the Guanella and Ruthroff impedance transformer structures, using a multi-layer circuit construction method to achieve unprecedented small size. Figure 1 shows the Guanella 1:1 unbalanced transformer (left) and Ruthroff transformation (right), and Figure 2 shows the Guanella 1:4 transformer.

The advantages of this new multilayer balun are its reduced profile height and "true" SMT packaging. Not only do these improvements make it easier for these parts to fit into a fully automated pick-and-place assembly environment for the first time, but the design also makes it possible to consider the use of low-frequency baluns during the design phase of ultra-thin consumer handheld products, such as the latest handheld microcomputers and mobile phones.

一个典型绕线1:4不平衡变压器和一个用于地面广播的Anaren最新1:4小型不平衡变压器之间的显著外形差异见图3，可见这种新产品可以节省PCB空间。这2个部件的插入损耗和共模抑制比(CMRR)见图5，以供比较。曲线表明：相对于绕线部件，这种小型多层不平衡变压器插入损耗性能的显著优势，两者的CMRR性能接近。

Due to its improved insertion loss and comparable CMRR performance, the Anaren multilayer balun can directly replace wire-wound components in terrestrial broadcast systems. Furthermore, due to its more compact form factor (a multilayer balun uses 40% less PCB area than a wire-wound component), a design can be constructed that meets the requirements of both a wire-wound or Anaren multilayer balun to facilitate simple in-system performance comparison and testing. A reference design comparing the performance of a conventional wire-wound balun and the new multilayer balun is shown in Figure 6.

In addition to its tiny form factor, the new multilayer balun benefits from the use of non-ferrite flexible circuit board materials for the multilayer structure. This material, with a relative permeability of 1, is less temperature sensitive than highly permeable ferrite materials. The stability over temperature simplifies system and circuit design because tighter performance tolerances are required over a wide temperature range.

The highly permeable materials traditionally used in wire-wound balun construction can also be a concern for system designers because such materials are inherently nonlinear and can generate intermodulation interference that degrades system performance at sufficiently high powers. In contrast, the new low-permeability multilayer balun only causes intermodulation interference caused by dissimilar metals, known as passive intermodulation (PIM), which is typically in the -100dBc range. Furthermore, the intermodulation interference of the Anaren multilayer balun is independent of any bias current in the circuit.

Another benefit of this multilayer balun soft board construction is compatibility with the typical coefficient of thermal expansion (CTE) in PCB materials, so the PCB and balun materials tend to expand and contract at the same rate (a function of temperature). Of course, since this new balun uses low permeability materials, there is a lower frequency application limit to the design, usually around 50MHz. For frequencies below 50MHz, the wound ferrite balun is still the best solution.

Using a simple discrete network, this new multilayer balun can be tailored to meet the needs of specific frequency band applications. A recommended capacitor network (Figure 7) improves insertion loss at the lower end of the frequency band. An inductive network (Figure 8) improves balance performance at the higher end. If desired, both networks can be used simultaneously. However, as with any performance adjustment, this is a tradeoff that needs to be implemented based on the actual application. Figure 9 compares the performance of the balun with and without passive circuit components. This new 1:4 multilayer balun can also be used as a single-ended to single-ended resistor converter (unun) as shown in Figure 10.

Together, these performance and form factor improvements are inspiring engineers in markets such as terrestrial broadcast system design, which have traditionally been limited by the limitations of wire-wrap technology. Facing the same top-level design challenges common in other electronics markets (e.g., miniaturization, trends toward lower PCB costs, etc.), today’s terrestrial broadcast system designers face specific market demands and functional requirements from content and service providers. These include Internet Protocol Television (IPTV), additional hardware drivers , and the integration of multiple tuners to provide viewing/recording, picture-in-picture, and other valuable features. With this new family of true SMT Anaren miniature non-ferrite broadband baluns, designers of terrestrial broadcast electronics solutions no longer need to consider the converter a limiting factor in a tight design.

In a design environment that has almost completely migrated to surface mount and miniaturized components, wire wound baluns present system designers with several limitations, especially in the terrestrial broadcast arena. Anaren's new multilayer balun eliminates many of these limitations, giving design engineers a new option for developing compact designs suitable for today's real-world applications.

This new series of multilayer baluns are a logical replacement for wirewound ferrite transformers for the broadband communications market. The new products offer 20:1 bandwidth, excellent temperature stability, low insertion loss over the entire frequency range, good amplitude and phase balance, and form factors that support the latest mobile phones and other thin and light electronic designs, including digital TV set-top boxes and LCD display TVs. Samples of the multilayer baluns are available online or by contacting Richardson Electronics, Anaren's exclusive distribution partner for consumer product components.