Jack Vickers, Jacket Micro Devices

When it comes to developing a portable consumer WiMAX device, there are several significant issues to address. Advances in other technologies have conditioned the
Figure 1. Typical MLO module construction.
public to expect continual improvements in functionality and performance in ever-shrinking devices. As WiMAX becomes a commercial reality, designers and manufacturers who heed these demands will be better poised for this new marketplace.

For hardware developers, this means a need for smaller packaged front ends with significantly more embedded passive components. Historically, similar challenges related to size and functionality have been met through advances in integrated circuit (IC) and high-density interconnect methods. Unfortunately, these methods, which typically use packaging materials such as low-temperature co-fired ceramic (LTCC), silicon or gallium arsenide (GaAs), are reaching their size/performance ratio limits or suffer from compatibility issues with common printed circuit board materials. While they work well in simple mobile devices, they are not well suited for the complexity and sheer number of passive components required in highly integrated RF front end design for WiMAX MIMO applications.

Current 802.16e standard specifications for mobile WiMAX applications, coupled with ongoing consumer demand for smaller devices, are driving developments of a new class of RF front-end modules (FEMs) utilizing multi-layer organic (MLO) technology. These new FEMS will facilitate the implementation of highly integrated radio designs for WiMAX MIMO applications in the 2.5 GHz range.

What is Multi-Layer Organic Technology?
MLO represents a fundamental change in the way RF systems are designed. It is a new process for embedding passive components into a packaging substrate that enables System-on-Packaging (SoP)
Figure 2. Frame prototype of the FEM produced by JMD.
for RF and mixed signal applications. Based on research at the National Science Foundation at the Georgia Tech Microelectronic Packaging Research Center and commercialized by Jacket Micro Devices (JMD), MLO is already being used to produce highly integrated RF FEMs for consumer wireless applications. By considerably increasing the number of passives that can be packaged in a smaller area, MLO technology enables the development of new FEMs that meet current needs in WiMAX MIMO applications and is inherently versatile enough for significant improvements as the industry grows.

Figure 1 shows a typical MLO module construction. An MLO package consists of one or more RF dielectric layers embedded between other laminates to provide routing, shielding and bonding areas for SMT and die placement. The design incorporates very thin layers with low loss at common frequencies (including 2.5 GHz) while maintaining the high dielectric constant (Er) required for high capacitance density. This is accomplished through the use of organic substrate formulations such as liquid crystal polymer (LCP) and polytetrafluoroethylene (PTFE). Unlike traditional materials such as LTCC and silicon, these materials do not require lossy fillers to achieve the required Er.

These modules are compatible with standard printed circuit board manufacturing processes, including large-scale panelization producing more than 1000 modules per panel.

Designing a Single-Band 1X2 RF FEM for WiMAX Applications
The example explained here illustrates how MLO technology and SoP design can be used to produce a RF FEM that includes all of the necessary passive components in a mobile WiMAX solution, including RF decoupling, matching, bypass circuitry and an optional voltage supply path to the transceiver and the back end.

Figure 2 shows a frame prototype of the FEM produced by JMD. This FEM measures only 7X7.5X1.2 mm3 and integrates all RF functions between the transceiver and the antenna. Baluns and filters are designed using internal layers, while PA and SPDT dies are located on the top layer and are connected via wire bonding.

This unique design, coupled with the material properties provided by MLO technology, offers several significant advantages related to high Q rated passives and transmit/receive chain attributes.

High Q Embedded Passives
Figure 3. 3D layout of a filter balun.
Inductors and capacitors were chosen from within the component library that contains an array of one and two-part components designed using several different dielectric stackups and corresponding simulation data. The components were used to design all filters, baluns, and matching networks within the module. Finally, system level simulations were performed to verify module performance.

The inductors range in values from 0.5nH to 30nH; max Q-factors range from 40 to 180; and line widths and spacing range from 2 mils to 6 mils. The capacitors range in values from fractions of 1 pF to 15pF, with max Q-factors ranging from 300 to 400.

The passive library imparts several benefits, including:

•Optimization of Q ratings for operation in different bands •Availability of high Q passives with low DC and low RF resistance •Small size inductance densities of 10-20nH/mm2 •Current carrying capabilities of up to 2.0A •Variable self-resonant frequencies (SRF)

The library also enables quick and accurate synthesis of complex integrated passive devices including multiple receive chains and a transmit chain.

Receive Chains
The module has two receive chains — RX1 and RX2. RX2 consists of two second order inductively-coupled resonator filter networks, a narrow band balun circuit and a LC section. The LC section increases rejection at frequencies ranging from 3.3 to 4.5 GHz. Figure 3 shows the 3-D layout of a filter balun. RX1 has an identical lineup with an addition of a high linearity broadband GaAs PHEMT MMIC SPDT.

Transmit Chain
The transmit chain is made up of a narrowband balun, an inductively coupled band pass filter, a three stage amplifier, a high linearity, matching/bias/decoupling networks, harmonic traps and the SPDT switch. The module requires a low bias voltage of 3.3V and uses a 25 dB step attenuator for low gain operations. The TX lineup has an additional bias network for sourcing DC to the transceiver and system back end. This feature eliminates additional routing on the system board.

A critical specification across the band and drive level is the modulation accuracy, which — along with forward gain, current drain and harmonics — is controlled by optimizing the PA bias/matching networks.

Smaller, more functional FEMs are the key to producing high performance WiMAX MIMO mobile solutions that will meet current and future consumer demand. RF FEMs utilizing MLO construction and SoP design offer manufacturers a new way to meet those challenges and remain competitive and profitable as WiMAX mobile technology matures.

About the Author
Jack Vickers is product line manager for Jacket Micro Devices, 75 Fifth St. N.W., Atlanta, GA; (404) 961-7240;