Comparing the AX508 to the AX502

Andy Wang

The Axiom AX502 was the first fully integrated CMOS Power Amplifier for mobile handsets and had been considered reliable and adopted by many handset manufacturers. To explain it's popularity, as of December 2008, 25 million units of the device have been shipped to their customers.

Recently, Axiom Microdevices, Inc. issued their newest power amplifier, the AX508 power amplifier and with it, expanded their family of GSM/GPRS power amp. In their documentation, Axiom claims the newest product is more powerful and more competitive than its predecessors. The AX508 integrates full quad-band GSM/GPRS functionality on a single integrated circuit (IC), using mainstream Silicon CMOS process technology. Key benefits of the AX508 include improved performance (compared with the earlier AX502), which translates to a longer talk time in handset designs. Axiom also offers a smaller footprint on AX508, by housing the PA in an ultra-low profile micro lead frame package of 5×3.5×0.9 millimeters (mm).

The AX508 is known for its power&#45added efficiency (PAE) E&#45GMS of up to 50% at +35dBM, ,according to the datasheet, while the PAE for the AX502 is only 47%. What that means is more power saved by the system and less heat dissipated by the device. The AX508 also features a micro lead frame package with a smaller footprint of 5×3.5×0.9 millimeters (mm), compared with the AX502's package of 5x5x0.9 millimeters (mm).

Package Comparison of the AX508 vs AX502

These particular differences between models seem somewhat small, which brings us to the big question; what are the key differences between AX508 and the AX502? To answer that question, the engineers at Sanguine decided to do decaps on both the AX508 and the AX502 to form a comparison and identify some of the many things you can learn when you look beyond the packaging.

The first difference you notice is the appearance of an additional pad in the AX508 versus the AX502, as seen in the X-ray images shown below.

X-Rays of both Devices – Click on image to enlarge.

Next we take a look at the circuit layout of the two devices. Examining the two images reveals how the AX508 features an additional two test pads and a new, unknown pad.

X-Rays of both Devices – Click on image to enlarge.

The die-mark on AX508 is almost identical with that of the AX502. This includes the same distinguishing marks seen on the AX502 that are seen when taking a much closer look at the circuit schematic. Markings become visible at this close range; like writing of their claim of “the World's First Fully Integrated Cellular Phone Power Amplifier “, the names of all their designers, their development team and their technology, and identification of its origin at California Institute of Technology.

Designers get their credit! – Click on image to enlarge.

Inside the process design

Both the AX508 and the AX502 adopt a 7-metal single poly&#45silicon 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&#45Band output (HBOUT) and Low&#45band output (LBOUT) frequencies via the RF coupler, and feeds the detected RF signal to the power&#45control 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&#45band RF coupler is unchanged between the two devices. However, the High&#45band 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.

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