This is the best time to be a consumer of electronic products. Wireless mobile devices, especially cellular handsets, have more features, are smaller and have better battery life than ever before. The cellular handset manufacturing industry is continuously challenged to deliver high performance, cost-effective, small form function products to the market.
In order to meet the increasing expectations of the marketplace, component suppliers are developing electronic circuits that are smaller, more efficient and provide overall higher value to the wireless terminal manufacturer. Of particular interest are improvements in performance to products that typically consume the most power, such as power amplifiers (PA).
RF Micro Devices (RFMD) has developed and released the RF3146 - a 7.0 x 7.0 x 0.9 mm, quad-band GSM PA module with integrated power control, specifically designed to address key issues faced by handset development teams and manufacturers, including time to market, supply chain simplicity, cost of ownership and key system performance parameters. The RF3146 addresses these issues through RFMD's patent-pending integrated power control and patent-pending Lead Frame Module™ (LFM™) packaging technologies.
Powering the RF3146 is a proven AlGaAs/GaAs HBT engine, based on the prevalent RF3140 PowerStar™ PA. The RF3146 power amplifier is intended to capitalize on the PowerStar ease of implementation benefits. These benefits include industry-leading efficiency, burst timing and transient spectrum guaranteed by design and power ramp curve repeatability.
The key features that differentiate the RF3146 from other GSM power amplifiers for terminal applications are LFM packaging technology, the smallest form factor for a quad-band GSM power amplifier with integrated power control, and PowerStar performance featuring integrated power control.
LFM Packaging Technology
Lead frame module (LFM) packaging is a technology developed to address improvements in key areas related to performance and supply chain logistics for cellular components. A cross section of a typical LFM package is shown in Figure 1 . The use of LFM packaging technology with a solid copper slug and etched lead frame allows for improved thermal and electrical performance utilizing a simplified supply chain.
In addition to the improved electrical performance, LFM packaging technology has several practical advantages, including:
- Smaller footprint. The RF3146 is less than 50 mm2, which is, on average, less than half of the area currently required by the function in the market.
- Lower height. The RF3146 is typically 0.9 mm, which is a 30 percent reduction in the height of components for the same function.
- Simplified supply chain. LFM packaging technology does not require a laminate or LTCC substrate, nor does it require surface-mount components. This simplifies the production supply chain, removing at least two process and procurement steps, with relatively long lead times.
- Shorter lead-time for the product, resulting from the simplified supply chain. This fact allows for more flexibility in sourcing, and reduces inventory cost and exposure for both the manufacturer and the user.
- Improved moisture sensitivity levels, particularly when compared to similar laminate-based modules.
- Lead-free compatibility, an increasingly important requirement for electronic product manufacturers.
In addition, the RF3146 LFM has significantly reduced thermal resistance and ground return inductance. This reduction in thermal resistance leads to improved performance and long-term reliability. This is especially important with the transition in GSM systems to Class 10 and Class 12 GPRS multi-slot performance. With these systems, the power amplifier is required to operate at a duty cycle of between 25 and 50 percent. Traditional GSM handsets operate at a 12.5 percent duty cycle. This means the PA must be capable of dissipating heat much more effectively. With LFM technology, the thermal resistance is reduced due to the solid copper slug the PA is mounted on. In a conventional laminate module, the die is attached to a plated copper pad with thermal vias through to the backside ground. These thermal vias have a higher thermal resistance and lower thermal mass than a solid copper slug, meaning a laminate module has a higher overall thermal resistance than the equivalent LFM package. The thermal resistance of the LFM package is reduced by between 20 to 25 percent. This translates to an equivalent reduction in junction temperature.
Device junction temperature is exponentially related to the MTTF. Therefore, reducing the junction temperature by 20 to 25 percent will have the additional benefit of improving the long-term reliability of the power amplifier. Reducing the maximum junction temperature will also benefit the extreme output power capability of the power amplifier through reduced power sag as the power amplifier heats up, due to duty cycle increases. For the phone designer, this translates to less power margin required to meet the same extreme output power requirements. For the power amplifier designer, this translates into the ability to tune the power amplifier for higher efficiency at a lower room temperature set power point. Furthermore, the phone design engineer will see a practical end-user benefit in a reduction in heat generated by the power amplifier, transferred to the phone user, through the phone case.
Electrical isolation is another benefit of the LFM approach. The LFM solid copper slug has significantly lower ground return inductance that reduces the common impedance coupling within the module. Common impedance coupling is one of the key limitations on isolation in GSM power amplifiers. Reducing common impedance coupling typically improves isolation. With the LFM approach the common impedance coupling is no longer the limiting factor for isolation. This allows the power amplifier designer the flexibility to optimize isolation through tuning of the matching circuits. The RF3146 has improved isolation for the GSM band and the potential for improved isolation in the DCS/PCS bands, as shown in Figure 2 .
The RF3146 features integrated power control, which is a proprietary, patent-pending method of power control for GSM power amplifiers, designed to maximize transmitter yields and minimize transmitter design time. The integration of the power control function eliminates the need for directional couplers, detector diodes, power control ASICs and other power control circuitry, allowing the module to be driven directly from the DAC output. However, unlike other methods of power control, such as current sensing and power detect, integrated power control is designed to meet stringent European Telecommunications Standards Institute (ETSI) specifications for transient spectrum and burst timing. In addition, power amplifiers with integrated power control demonstrate minimum variation across temperature, frequency, voltage and input drive level, as shown in Figures 3 and 4 . As a result, the RF3146 PA provides handset designers with the most robust and consistent burst timing and transient spectrum performance (specifications required by ETSI for cell phone full type approval) along with the shortest development and end-of-line calibration time. Handset manufacturers using PowerStar PAs with integrated power control have typically demonstrated a two- to three-month reduction in the development and launch time of new wireless terminal platforms, and a 50 percent test-time reduction in end-of-line calibration.
As the performance graphs demonstrate, the output power and efficiency at room temperature for the RF3146 are greater than 34.2 dBm and 58 percent for GSM and greater than 32 dBm and 56 percent for DCS, respectively. These performance figures are improved over the industry-leading RF3140 power amplifier module. The improvement in efficiency due to the decrease in current consumption translates directly to improved talk time in a mobile application.
A key performance feature that allows for the shortest end-of-line phone calibration time is the consistency of the output power as a function of the Vramp input signal. The end-of-line phone calibration is an expensive, time consuming task that phone manufacturers must perform in order to ship ETSI-compliant handsets.
The output power versus Vramp data for the RF3146 shows very repeatable performance. The output power response for the sample of 18 PAs from different lots, shown in Figure 5 , are almost identical. On the secondary y-axis, the standard deviation of the measurement data is plotted. Based on ±3 sigma, it can be seen that the RF3146 will meet the GSM power level accuracy requirement imposed by ETSI for all power levels, with margin. The repeatability of this data means that one ramp profile can be used for all RF3146 power amplifier modules in a given handset production line, significantly reducing the end-of-line test development. This is one of the key benefits of all PowerStar PAs as it significantly reduces the amount of factory calibration to at least one point of measurement. In practice, most other methods require at least five points of calibration.
The RF3146 PowerStar solution, which is part of the Polaris™ Total Radio™ solution, represents an aggregate of today's most advanced design and packaging technology in GSM power amplifiers for wireless devices. The combination of integrated power control technology and LFM packaging technology results in a product that provides more value to the handset designer and manufacturer in the areas of time to market, supply chain simplicity and cost of ownership, particularly when compared to other power amplifiers currently available.
PowerStar™ and Polaris™ Total Radio™ are trademarks of RFMD LLC.
RF Micro Devices, Greensboro, NC (336) 664-1233, www.rfmd.com. Circle No. 302