Baluns Shrink To Tiny 0404 Footprint

Nov. 17, 2005
These miniature baluns make the transition from balanced to unbalanced devices in a fraction of the real estate required by previously available surface-mount components.

Consumer and commercial electronic designs call for the smallest possible-components to save space and power. Balanced-unbalanced transformers or baluns have traditionally been based on transmission lines, such as coaxial wires, stripline, or microstrip, and often somewhat large as a result. By applying their multilayer techniques, Anaren Microwave had already succeeded in developing baluns in 0805 (0.08 × 0.05 in.) and 0603 (0.06 × 0.03 in.) package sizes. And to meet the ever-increasing demands for decreasing size, the firm has announced a 0404-packaged family of surface-mount baluns measuring just 0.04 × 0.04 × 0.026 in., with no sacrifice in performance compared to their larger counterparts.

The low height profile of these new baluns makes them ideal for module-designs such as system-in-package (SIP) solutions. In addition to low height, the baluns offer fine pin pitch, excellent electrical performance, and low price. The 0404 baluns feature a land-grid-array (LGA) interface to occupy only 0.0016 in. 2 when mounted on a printed-wire-board (PWB) substrate. In contrast to existing offerings, this SMT interface requires no additional PWB space for the solder land pads. In spite of their compact size and simple four-pin construction, these baluns are also suitable for DC biasing. The baluns, which operate past 5 GHz, are suitable for 2.4-GHz Bluetooth and IEEE 802.11b/g wireless local-area networks (WLANs), 3.5-GHz WiMAX, 5-GHz IEEE 802.11a WLANs and Home Cordless applications, and a wide range of consumer applications. Insertion loss is typically 0.6 dB at 2.4 GHz with better than 23 dB return loss at that frequency.

The firm's advanced passive-component technology allows for the integration of multiple 0404 balun building blocks into an integrated package. This integrated package can contain a mix of multiple frequencies (i.e., 2.4 and 5.0 GHz) and different balanced/unbalanced impedance combinations. Dual baluns (0804), triple baluns (1204), and quad baluns (1604) can be realized using this packaging concept.

Figure 1 represents a generic implementation demonstrating the standard "fan-out" configuration using standard 0805 balun and discrete devices connected to the input/output (I/O) pins of a typical transceiver chip. Significant PWB space is required in this realization.

Figure 2 clearly demonstrates the significant advantages of implementing a modular balun (dual-balun) approach to maximize on available space while not compromising performance or cost. This dual balun, measuring 0.080 × 0.040 in., contains the same functionality as two larger discrete baluns. The use of the dual balun allows for the mounting in close proximity to a semiconductor device since the pin pitch of the balun closely matches that of the IC. If additional matching is needed, 0201 discrete components can be used, complementing the component spacing allowed by the balun. Table 1 provides a list of current products available for sampling. All common impedances can be realized in these 0404 packages. The miniature 0404 baluns are very stable with temperature, with an insertion loss slope with temperature of about 1 mdB/°C for the BD2425N50100A00 part (Fig. 3) .

When measuring the balun or simulating performance using two single-ended ports rather than a differential port, the equations given below can be used to calculate the differential performance. The single-ended input is assumed to be port 1 and the differential output pair is assumed to be ports 2 and 3.

Insertion loss (IL) can be found from:

Balanced return loss can be found from:

The amplitude balance (AB) can be found from:

while the phase balance (PB) can be calculated from:

This phase balance is relative to the desired 180°.

Finally, the common-mode rejection ratio (CMRR) can be found from:

The 0404 parts have a pin configuration similar to an LGA integrated circuit (IC). A 0.5-mm pitch allows just four pins on these parts. With only four pins, there is no separate DC-bias pin as common for Marchand type baluns (Fig. 4) . However, by using a suitable capacitor to form an RF ground, these baluns can be used in applications requiring DC-bias through the part or for isolating a DC-bias present on the differential pair from ground. To provide the RF-ground, a capacitor must be selected that has a self-resonance above the frequency band of interest and a value sufficiently large as to not generate higher insertion loss and detune the performance. The baluns are wideband, except for the lower differential impedances and thus are generally forgiving when DC-biased. An inductor is needed for feeding the bias, resonance free in the band of interest andwith sufficient value to act as a choke.

The fundamental Marchand design equations are:

where: k = the voltage coupling ratio of the coupled sections and would be determined from the required bandwidth, etc.

If the internal ground reference is insufficient, the impedance of the balun will change, impacting performance. Equally important for good balun performance is a good RF ground on the balun's DC bias pin (pin 2). This provides the necessary reflections at the end of the coupled sections.

Since the small size of 0404 parts limits them to four pins, the functions of pins 2 and 5 have been brought together. When simulating the effects of having an RF ground as opposed to separate RF and DC grounds, a fourport S-parameter model representation should be used (Fig. 5) . This is no different from simulating the effects of a bias circuit on the traditional DC-bias pin on 0805 and 0603 baluns.

Simulations of performance have shown minimal improvement with capacitance values above 8 pF. This observation holds true for all other performance parameters, including balanced performance (not shown here). Simulation were based on 0201 series capacitors from Murata ( Smyrna, GA). Measurements were made using 0201 capacitors from AVX Corp. (Myrtle Beach, SC), although these were not used in the simulation and there is no direct correlation between the simulations and the measurements. For the conditions of Table 2, insertion loss ranged from 0.56 to 0.68 dB at 2.45 GHz (Fig. 6). Measurements showed that higher capacitance values were needed than those suggested by the simulations. Based on this comparison, balun users are advised to try capacitors from different vendors in the range shown in Table 2, and avoid using capacitance values much higher than needed since these can exhibit self-resonances that will degrade performance.

Simulated performance for the 5-GHz balun indicate that a 1.2-pF capacitor-does not provide enough capacitance for that balun, while a 3.2-pF capacitor provides too much capacitance. Similar to the BD2425N50100A00 part, the capacitor value for a preferred capacitor vendor will be slightly different than the simulation performed for the 5-GHz balun. In the above examples, 0201 capacitors were used. The performance of a DC-isolated configuration can be tested for both configurations by using test boards from Anaren.

Samples of the 0805 and 0603 devices are now available as well as various versions of the 0404 products. Custom impedances and package options are also available.Anaren, Inc., 6635 Kirkville Rd., East Syracuse, NY 13057; (800) 411-6596, (315) 432-8909, FAX: (315) 432-0189, Internet:

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