"From a layout perspective, designers must pay close attention to the PA subsystem. They must use good RF practices," said Miller. "This board is designed with components of different technologies and characteristics that have to be mixed and matched. It makes the whole RF section a little more difficult and design-intensive. On the baseband side, it's the software issues that can kill you."
I found it interesting that there's almost no shielding on this design. The shielding on competitive designs tries to isolate the RF radiation from getting into other parts of the phone. LG makes use of vias in the board as one way to minimize the RF radiation and thereby avoid the use of costly shielding.
Between the PA and PCS duplexer (UA622FM) are double rows of vias that are tightly spaced to minimize and contain the radiation. If the PA output, which is a relatively large signal, gets around the filter and out to the antenna switch at the output, you could have an isolation problem. The vias circumvent that problem, containing the radiation and making it conduct through the filter, instead of spraying all over the phone and radiating outward.
To eliminate a potential routing nightmare, Anadigics offers a similar power amp, the AWT6310, but with the pinouts reversed. This device works with the Qualcomm RFT6100 trans- ceiver, where the cellular and PCS outputs are reversed. The other difference is that the die for the cellular band is on top of the package and PCS I/O is on the bottom.
On the output side of the power amp lie the duplexers. Eventually, all those signals are routed to one antenna through a triplexer, which combines the cellular, PCS and GPS band signals going to the antenna. It's a common practice to use one antenna for all three bands, which is why the triplexer is popular.
Before the E911 mandates for pinpoint locators, you could use a simple antenna switch there, rather than the triplexer. However, that doesn't allow the user to receive location information while on a call. The next generation of the Anadigics power amplifier will integrate some of those switches as well as a voltage regulator.
Richard Nass ([email protected]) is editor in chief of Embedded Systems Design.