Casio PH-250 Personal Handy Phone
In June 1993, the FCC designated a portion of the radio frequency spectrum for use by advanced communications services. The new spectrum allocations actually consisted of two separate sets:
- Narrowband services in the 900 MHz range, for use by interactive services such as two-way paging (see previous bulletin on the Motorola Tango two-way pager).
- Broadband services in the 1900 MHz range, for services now generally referred to as Personal Communications Services (PCS).
The auction of licenses for the PCS radio frequency spectrum in about 100 markets covering the entire United States netted the US Treasury over $7 billion. On average, licensees spent over $15 for each person living in the specific market area covered by the license.
As part of the license agreement, the licensees committed themselves to construct a PCS infrastructure that would be able to service one-third of the population in their respective markets within five years.
Sprint Spectrum was able to acquire the largest number of licenses, and its affiliate, American Personal Communications, was the first to offer PCS services in the United States. The Sprint Spectrum-branded service is now available in the Washington D.C./Baltimore area (covering 5 million people), and will be extended to other markets by the end of 1996.
Sprint Spectrum's PCS service is based on a digital base station network, whereas most cellular services in the US are based on analog systems. In comparison, a digital system improves the transmission quality considerably, thus offering a cleaner voice and higher transmission rates for data communications. Furthermore, digital systems facilitate the implementation of value-added services, such as those referenced below. (Note that cellular systems may also be digital. In fact, outside the US, digital cellular is increasingly popular.)
The base station networks of cellular and PCS systems are quite similar. However, whereas the former relies on few, very powerful base stations that may cover an area of 20 km in diameter, PCS base stations are very small with limited transmission power. They are deployed in greater numbers, however, so that the distance between a base station and a handset is typically no more than perhaps 1000 meters during operation. This in turn means that the handset itself can be less sophisticated and less powerful. Consequently, PCS handsets can be smaller and offer much improved battery life, i.e. talk time. PCS handsets are also cheaper, and service rates are very competitive. The Sprint Spectrum Service requires one of the following digital PCS phones:
- Motorola PCS Flare
- Ericsson CH337 PCS
- Nokia 2190 PCS
The system featured in this teardown bulletin is the Casio PH-250 Personal Handy Phone (priced at $350 in Japan). While it is a digital PCS phone, it will not work with the Sprint Spectrum service. Instead, it complies with the standards of Japan's PCS equivalent, called Personal Handy Phone System. The PHS system was rolled out in the Tokyo area in July 1995 and since has been extended to other parts of Japan's densely populated coastal area. It currently serves more than one million subscribers. Interestingly, the popularity of PHS has not come at the expense of Japan's cellular service: The cellular phone subscriber base grew by 130 percent in 1995 and now totals over 10 million subscribers.
The following discussion is an abbreviated description of the construction of the Casio PH-250 Personal Handy Phone. For a detailed discussion of these issues please contact the IEEC or Prismark Partners.
Casio PH-250 Personal Handy Phone
The Casio PH-250 is of a flip phone construction, with dimensions of 4.5" x 1.8" x 1.5" when the flip cover is closed. It weighs about 154 grams, with the battery pack alone accounting for 34% of the total weight. Interestingly, the second heaviest component is the housing, including the EMI/RF shielding covers at 31%.
The Casio contains three printed circuit boards of various sizes which are stacked on top of each other and connected via connectors or flexible circuits.
The main printed circuit board supports the logic circuitry, and is a thin, four layer glass epoxy construction. The board geometries are reasonably fine, and 245 mostly surface mount active and passive components are mounted on both sides.
The beeper printed circuit board is a small, thin, two-layer glass epoxy board with a simple design and sparsely populated. It is connected to the main board via two polyimide flexible circuits.
The RF printed circuit board supports all the RF circuitry, and is shielded by a metal cover that is soldered to the back of the board. It is slightly smaller than the main board to which it is connected with a 20-pin connector. The RF board contains four layers, and is of moderate complexity, except for a large number of filled blind vias of 10 mil diameter (see picture below). The front side is bare apart from a 20 pin connector, while the back side is densely populated.
RF Board Cross-SectionNote: The cross-section shows three blind vias in the four-layer RF board, and a second four-layer board soldered onto the RF board as a component. (See discussion below.)
Underneath the metal shield covering the entire RF board are two additional metal cans. The first shields a two-layer, 23 mil thick glass epoxy board which is populated on both sides. The second metal can hides yet another metal can that covers a four-layer, thin glass epoxy board with a number of passive components mounted on the top. (See cross section above).
The keypad is a membrane switch assembly that is mounted onto a metal frame which also supports the LCD module and provides EMI/RF shielding. The membrane keypad consists of four layers of partially circuitized polyester film, one of which is folded back on itself, so that the effective layercount is five. Circuitization is achieved with a conductive silver paste, and layer-to-layer interconnection is established by dimpling the material. In addition, SMT components are attached to the polyester film using polymer thick film adhesive and a polyester 'shrink wrap'.
Summary
FCC license agreements forcing a rapid infrastructure deployment guarantee a very favorable business environment for PCS base station suppliers. The popularity of PHS in Japan suggests that PCS handset suppliers will also experience rapid sales growth.
The requirements for PCS base stations and handsets are different from those for cellular equipment. The industry continues to search for novel approaches at the design, component and material level.
The Casio phone shows the need for further integration and miniaturization at the RF and digital level in portable communications products. Particularly, some of the RF components required complex and costly assembly processes. Even with the reduced power output of the PHS system, EMI/RF shielding was a major concern.
A previous bulletin featuring the teardown of the Ericsson DH338 digital cellular phone showed that, in a typical cellular phone, one of two rigid printed circuit boards is dedicated to provide a keypad and the connection to the LCD module. In the Casio, on the other hand, the keypad is constructed in the form of a low cost membrane switch, freeing up the board real estate for logic and RF circuitry.
To the end user, the Casio PH-250 is a compact and functional system, but it seems that the overall design could be perfected to simplify assembly and decrease component and material consumption.
Opportunities and challenges are abundant in the wireless communications market. New solutions are sought from system designers, and materials and components suppliers. Please contact Prismark Partners at (631) 367 - 9187 to discuss how your company can benefit as the electronics industry shifts from computing to communications.
Copyright 1996 Integrated Electronics Engineering Center and Prismark Partners LLC.
7/96
Warranty Disclaimer -- All information used in the preparation of this report was obtained from sources believed to be reliable at the time the information was collected. Prismark Partners LLC, its employees, its agents, and assignees have exercised their best efforts in preparing this report. Prismark Partners LLC extends no warranties with respect to this information and shall bear no liability whatsoever to the report recipient or to any other party as a result of the use of this report or the information contained herein.
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© 2000 [IEEC]. All rights reserved. Revised: February 28, 2002 .