Inside the process design
Both the AX508 and the AX502 adopt a 7-metal single poly-silicon 0.13-micrometer CMOS process technology. The top two layers of both devices are unusually thick, which helps to increase the inductor Q-factor and allows this power amplifier to meet its specifications. The M7 layer of AX508 is about 3.35 microns and the M6 layer is about 3.1 microns. The M7 layer is thicker than the M6 layer on the AX508, while it's the opposite case for the AX502. This reversal indicates to us that the inductor for the AX508 has changed and improved upon its predecessor.
AX508 Cross-Section - Click on image to enlarge.
AX502 Cross-Section - Click on image to enlarge.
The RF Coupler
Another difference that's revealed through circuit analysis is the RF coupler in the AX508 versus the AX502. The RF detector detects the High-Band output (HBOUT) and Low-band output (LBOUT) frequencies via the RF coupler, and feeds the detected RF signal to the power-control block, forming a closed-loop power controller, thus eliminating the need for external couplers, detectors, and error amplifiers. The RF detector contains an internal differential amplifier for each HBOUT and LBOUT frequency thus covering the GSM, DCS, and PCS bands. The location of the RF couplers is shown in cross-section picture seen below. A closer view of the LBOUT RF coupler on both the M7 and M6 layers is shown in the circuit image. The Low-band RF coupler is unchanged between the two devices. However, the High-band RF coupler has been modified in comparison to the AX502, which which improves the Q factor of the coupler and provides better performance (as shown below).
RF Coupler HBOUT Comparison - Click on image to enlarge.
Through reverse engineering, Sanguine was able to investigate all the schematics of the AX508 and AX502 devices. Through this analysis, we were able to find many differences between the AX508 and the AX502 that we couldn't cover in this space. For more information about our work and any questions on this report, please contact us at Sanguine Insight Technology.
Click on image to enlarge.
Andy Wang, currently the Senior Engineer and manager of Circuit Analysis Team, has worked at Sanguine (Shanghai) since 2002.
He holds the Bachelor of Science electronics engineering from University of Liaoning, specializing in power management and radio frequency researches.