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issue: August 2008 APPLIANCE Magazine

Technology Report
Miniature Modems


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A new embedded modem offers engineers small size without compromising performance.

The XE5617G modem offers both small size and design flexibility. It can be placed as a surface-mount component or inserted into a 68-pin PLCC socket.

One of Cermetek Microelectronics’ (Milpitas, CA, U.S., www.cermetek.com) newest embedded modems, the XE5617G, utilizes a unique hybrid PLCC (HyPLCC) package to offer what is reportedly the world’s smallest modem solution. The FCC-registered device provides a complete modem in the same size footprint as a 68-pin PLCC chip, making it suited for space-constrained remote monitoring applications.

“The XE5617G is an ideal solution for home medical monitoring systems,” notes Steve Clary, vice president of marketing and project manager for the new modem. “These systems typically include a display and keypad to keep them simple to use. They also frequently include other communications options, including Ethernet and cellular. This doesn’t leave a lot of space for the modem. The XE5617G fits in that small space and still provides the option of transferring stored medical data over 56,000-bit-per-second dial-up connection or transmitting graphic data such as an ECG by fax.”

The modems also work well for residential security systems.  “Most alarm systems still rely on a low-speed dial-up modem connection to send the alarm,” Clary says. “Because the HyPLCC package permits the modem to be placed in a 68-pin PLCC socket, a modem failure, as can sometimes be caused by lightning strikes on the telephone lines, can be quickly replaced. This means minimum downtime for the system.”

According to Clary, small components were key to the modem’s compact design. Besides widespread use of 0402-sized passive components, the modem uses a modem chip in a very small TQFP48 package. Measuring just 9 mm2, the LSI CV92 chip serves as the foundation for the modem. The chip supports V.92, 56,000-bit-per-second data transfer, up to 300 bits per second. It also provides V.17 send-and-receive fax capability.

Another key design element is the modem’s circuit board. “We have developed a proprietary circuit board design technique that uses the PC board itself to provide most of the isolation barrier between the high-voltage telephone line-side circuitry and the low-voltage host-side circuitry,” Clary explains.

While many manufacturers have tried to offer similar solutions, Clary says modem performance has always been compromised. “One of the key factors is maintaining isolation between the telephone line and all other circuits in a package this small,” he says. “In addition to using the circuit board as an isolation barrier, we encapsulate these modules to provide additional isolation. Encapsulating such a small module requires a well-thought-out process to ensure that there is nowhere air can be trapped within the module. Any air trapped in the module would expand during solder reflow and damage the module.”

Engineers can incorporate the modem into new or existing designs by connecting it directly to the serial port on the host microcontroller.  “All modem control is done by issuing AT commands (serial ASCII data) from the microcontroller to the modem,” Clary says. “On the telephone line side, the only requirement is the RJ11 telephone jack. The design engineer will need to alter his board layout to accept the modem footprint and RJ11 jack.

“Because of the small size of these modems, it is not as difficult to incorporate these modems onto the circuit board as it would be to add a larger module,” Clary continues. “And it is certainly much simpler than adding the 30 or so individual components required for a modem design based on a modem chip only.”

 

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