Handset makes use of good design practices, especially in the critical RF subsystem - Embedded.com

Handset makes use of good design practices, especially in the critical RF subsystem

This design shows that it's possible to cut costs and board area without making compromises.

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Some Tears Downs are easier to do than others, and not simply because the product has more or less screws or because more glue was used in the design. Sometimes it's hard to take apart a product simply because it works so well you don't want to destroy it (and I do destroy every product I take apart).

The product that's the subject of this month's Tear Down was one that I really didn't want to destroy. It's a handset designed by LG Electronics, dubbed the VX8300. The reason I didn't want to take this handset apart (and I've taken dozens of handsets apart in the past) is because this one does just about everything right. In fact, I like it so much that I went out and got one for my personal use.

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The VX8300 “flip” phone has two nice bright displays, including a 65,000-color OLED (organic LED) for the external display; the keys aren't too close together; it offers Bluetooth 1.1 functionality; it has a slot for a microSD memory card; and the user interface is very intuitive. It's slightly larger than I would like (3.58 by 1.93 by 0.92 in.) and is designed with the dreaded external antenna stub, but those are tradeoffs I'm willing to make to get the quality that's offered by this handset. It weighs in at 3.88 oz.

While I don't take maximum advantage of the handset's multimedia capabilities, I know that they're available. These include Verizon Wireless' VCast music (using a pair of external speakers), video, polyphonic ring-tone support, and gaming experience. The GPS-enabled handset can access the Internet and contains a 1.3-Mpixel camcorder/camera with flash. The standard 1100-mAh Li-Ion battery provides up to 230 minutes of use time or 380 hours of standby time.

The VX8300 handset was designed internally at LG. The company prides itself on having a fast time to market. Hence, IC suppliers must potentially be ready to go to full production in just a few months.

The baseband functionality comes form the use of Qualcomm's MSM6500 chip set. Power management, including voltage regulation, is handled by a Maxim device.The RF quality comes thanks to the use of the Anadigics 6314R (the R stands for ROHS-compliant) power amplifier (PA). The 3- by 5-mm part operates over the 1.9-GHz CDMA PCS and 800-MHz CDMA (digital dual-band) frequencies. It actually integrates two separate PAs into one package. The result for LG is a reduced the external component count (compared to two separate packages) as well as a board-space savings.

Anadigics claims that the nearest competitor offers a 4- by 4-mm lead-frame module, thereby using one extra square millimeter. In addition, the 6314R results in fewer external components. Because you can't put any surface-mount components inside a lead-frame module, they must be outside.

“It's more cost effective to do it this way because there's less assembly costs involved,” says Jerry Miller, a product line director for CDMA and WCDMA products at Anadigics. “You're still putting down the equivalent of two PA die, but we only need about seven total components, compared with the competitor's eleven. Those would generally be capacitors, maybe an inductor. The cost savings would be somewhere between 5 and 10 cents.”

The alternative to using a dual-package module would obviously be to use two separate die, which are each generally around the same size as Anadigics' dual package. Many OEMs continue to use dual ICs, simply because they've gone that route in the past and don't want to re-layout the board, even if it results in a potential size and cost savings. The other reason to avoid the dual-package part is if you don't need to support dual bands, such as in the case of a similar phone that's designed for a Sprint PCS-only system, or in an emerging market like India, where they might only use a cellular band.

The best design practices put the 6314R PA near the transceiver, in this case a Qualcomm RFT6150 part. But there must put some space between the parts, as the SAW filters generally fit between the two. Each input to the PA must have a SAW filter between that input and the transceiver.

“From a layout perspective, designers must pay close attention to the PA subsystem. They must use good RF practices,” says 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, which many experienced RF engineers see as a “design crutch.” 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, which cuts down and contains the radiation. A potential problem is that 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 are used to circumvent that problem. They contain that radiation and make it conduct through the filter and not spray all over the phone board and radiate out.

To eliminate a potential routing nightmare, Anadigics offers a similar PA, the AWT6310, but with the pin-outs reversed. This part works with Qualcomm's RFT6100 transceiver, where the cellular and PCS outputs are reversed. The other difference is that the die for the cellular band on top of the package and PCS I/O is on the bottom.

On the output side of the PA lies the duplexers. Eventually, all those signals are routed to a single antenna through a triplexer, which combines the cellular, PCS, and GPS bands signals going to the antenna. It's a common practice to use one antenna for all three bands, which is the reason that the triplexer is popular.

Before the E911 mandates, 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 PA will integrate some of those switches as well as a voltage regulator.

Richard Nass is editor in chief of Embedded Systems Design magazine. You can reach him at .

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