Leaping ahead in the race to deliver more and more functionality and value to the handset maker, Skyworks Solutions has introduced the industry’s best transmit front-end solution (SKY77506) for GSM/GPRS segment. Two InGaP HBT power amplifiers (PA), a BiCMOS power controller and two PHEMT-based RF switches have been integrated into a compact 8 × 8 × 1.2 mm module. All RF output and input ports are DC blocked, ESD protected and matched to 50 ?. Figure 1 shows the block diagram of the device.

Key Features

The SKY77506 module features direct antenna connection without additional harmonic filtering or ESD protection components. It represents the lowest power consumption transmit solution and offers 48 percent GSM power-added efficiency (1200 mA @ 33 dBm) and 42 percent DCS/PCS power-added efficiency (680 mA @ 30 dBm), resulting in the longest talk-time transmit solution. Its power flatness allows calibration at only one frequency per band. It also features a wide input power range (0 to 6 dBm) and there are no oscillations greater than –36 dBm at 12:1 VSWR.

Fig. 1 Block diagram of the SKY77506 front-end module.

Receive port selectivity is 25 dB and only four external passive components are required. The new SKY77506 module is extremely rugged with no risk of damage transmitting into any load impedance.

Comparative Advantage

SKY77506 provides a significant advantage over the PA + ASM approach to design front-end solutions. In summary, the front-end module (FEM):

• Eliminates the need for a PA-to-switch design effort while providing optimal matching and all harmonic filtering.
• Offers significantly lower current than a PA + ASM combination, which directly translates to longer talk-time (see Figure 2).
• Post-PA losses are minimized, enabling higher efficiency and longer talk-time.
• Reduces BOM part count, easing supply chain, purchasing and inventory management.
• Provides tested, guaranteed performance at the highest assembly level; no yield loss due to tolerances stack-up or mismatch between PA and an ASM on the phone board.
• Is compatible with 1.2, 1.5, 1.8, 2.5, or 3.3 V control logic.

Fig. 2 Battery current and talk time.

Switch Diplexer Front End

The SKY77506 is configured with separate 1P3T T/R switches for high and low bands. A diplexer, positioned between the switches and the antenna, combines the two bands but also provides additional value to the designer. It provides a DC path between the antenna pin and ground, which provides a high degree of ESD protection and eliminates the need for an external shunt inductor or spark gap. It also provides DC blocking between the antenna pin and active internal circuits. The antenna pin can be directly connected to the antenna or a coax connector. The diplexer provides a significant amount of frequency selectivity between the PHEMT switches and the antenna (see Figure 3). This greatly reduces the handsets’ vulnerability to strong out-of-band interferers or “blockers,” such as from broadcast television or radio transmitters that can intermodulate with unprotected switches and generate unwanted interference.

Fig. 3 GSM/DCS switch insertion loss.

Current-regulation Power Control Advantages

The SKY77506 is based on the innovative Skyworks integrated power control (iPAC) current-regulation power control approach, which has proven to result in the lowest current and longest handset talk-times. Current regulation is accomplished by measuring the differential voltage across an internal precision low loss sense resistor. An internal control loop automatically adjusts the PA bias to maintain a constant ratio between ICC and VRAMP. Unique design approaches were used to provide immunity to temperature and supply voltage variations. The iPAC function also eliminates the need for directional couplers, detector diodes, power control ASICs and other power control circuitry. The power-added efficiency (PAE) versus POUT characteristic of a current controlled PA is distinctly different than that of a voltage-controlled PA. Most importantly, the peak efficiency occurs at a power level that is approximately 0.5 dB lower than the maximum output power. This results in a PAE that is 5 to 8 percent higher over the critical output power range (see Figure 4). This translates to a current that is more than 15 percent lower at the highest power levels. The current-regulation method of power control also maintains a predictable supply current that is independent of antenna impedance changes commonly caused by hand/body proximity. This alleviates any need for concern about call-drops due to drooping battery voltage during high current surges, and allows more of the battery discharge cycle to be used.

Fig. 4 Power-added efficiency of a current-controlled PA versus a voltagecontrolled PA.

Power Control Accuracy

Integrating the power amplifiers, matching networks and harmonic filters into a single module enables predictable impedances to be optimally tuned by design. This results in a very flat output power versus frequency. This allows accurate power control to be achieved by calibrating at only one frequency per band (see Figure 5). Additionally, Skyworks has developed a quick calibration algorithm based on an algebraic curve fitting that has enabled customers to achieve complete power calibration of a quad-band handset in less than six seconds. This greatly enhances the production test capacity and throughput by reducing the amount of test stations required. Contact Skyworks application engineering for calibration details and S/W tools.

Fig. 5 Power-controlled accuracy in GSM and DCS/PCS bands.

Easy Design-in for Rapid Development

The result is a compact, high performance module that greatly simplifies transmitter design. The repetitive layout turns, previously required to match a discrete PA to a discrete antenna switch module, are no longer necessary. With these functions integrated into a single module, optimal matching and harmonic filtering can be easily achieved in a single design pass.

Conclusion

Skyworks’ industry-leading RF integration expertise has been combined with its best-in-class power amplifier performance to yield a high performance, highly integrated, low cost, easy to use transmit solution for GSM/GPRS handsets. The SKY77506 is a third generation transmit plus front-end module (Tx-FEM) that is a significantly evolved successor to the SKY77500 and SKY77501, Tx-FEMs that are presently shipping at a millions-per-week rate.

Skyworks Solutions Inc.
(781) 376-3000
www.skyworksinc.com