'Brick' phone begat revolution

'The further backward you look," said Winston Churchill, "the further forward you can see." In that spirit, we've cracked open one of the original Motorola Analog Mobile Phone System (AMPS) "brick phones" to do a retro teardown.

It seems appropriate to pay tribute to the earlier days of mobile-phone development and--with a little extrapolation--use our findings as a benchmark for what the future may hold.

The late-1980s design--also dubbed DynaTAC--from the cellular leader of the day is certainly not small. Measuring 8.3 x 4.5 x 19.5 centimeters (even absent the 14-cm-long whip antenna) and weighing in at 670 grams, the handheld sold in earliest form for nearly $4,000 in mid-'80s money. Adjust for inflation, and those wanting to go mobile were staring at a price tag in excess of $7,000.

From a functional standpoint, the brick did pretty much one thing--make a call. While the design could store and recall selected phone numbers, there were no cameras, color screens, MP3 players or high-speed data capabilities to be found. Ironically, of course, voice remains the killer app for mobiles. Particularly for emerging markets, the voice-only handset remains the target device.

So what's going on inside this old-school hardware? As an analog phone, the architecture is quite a bit different from today's digital terminals. AMPS is based on frequency-division multiple access, which splits transmit-and-receive signals into different portions of the cellular band. Outbound signals are frequency-modulated (FM) versions of the spoken voice upconverted to the 824- to 850-MHz portion of the RF spectrum, where they are amplified and transmitted. Conversely, received FM signals are downconverted and demodulated from spectrum in the 869- to 894-MHz region.

Within each of these transmit/receive bands, the cellular network assigns a narrow, 30-kHz-wide channel for each caller, implemented in concert with the mobile device's tuner and the means by which multiple channels (callers) share a cell. Because the AMPS system uses purely analog modulation and transmission schemes, security in conversation is nonexistent. As with much of today's public-safety radio network, a readily available scanner allows call eavesdropping.

Digging deeper into the specific design--a DynaTAC whose exact model number wasn't clear--we find three primary printed-circuit boards. Function is partitioned onto a "human interface board" (supporting keyboard, display and audio I/O), a "baseband board" for nonradio electronics and the "RF board" containing all the air interface circuitry. A stout plastic two-piece enclosure with plated internal walls for shielding and a slide-on 245-gram nickel-cadmium battery, which runs the length of the handset, surround the three-board stack.

The human interface board implements a membrane keypad whose underlying technology is still employed in handsets today. Three small IC devices serve the needs of speaker interface, keypad control and display drive. The display itself is based on a seven-character, seven-segment LED module with integrated indicators for In Use, Roam and No Service (some things never change).

Baseband functions are contained on the second board, whose primary devices are Motorola (now Freescale) chips. As with most devices in the phone, parts are of a custom nature. That, combined with the age of the components, made research on IC function next to impossible; so the designation of microprocessor and audio processors is speculative, but consistent with expected architecture. A dedicated analog chip was used for battery interface and system power regulation.