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Mobile comms owe to chunky phones of the 80s

Posted: 07 Jun 2006 ?? ?Print Version ?Bookmark and Share

Keywords:David Carey? Motorola? DynaTac? analog? system?

"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-by-4.5-by-19.5cm (even absent the 14cm-long whip antenna) and weighing in at 670g, 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 FM versions of the spoken voice upconverted to the 824- to 850MHz 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 894MHz region.

Motorola DynaTAC 'brick' phone (Click image to view teardown diagram)

Within each of these transmit/receive bands, the cellular network assigns a narrow, 30kHz-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 PCBs. 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 245g NiCd 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.

Definitive component ID on the radio board was challenging for the same reasons, but inspection and trace-out suggest a single-chip transmit path and an intermediate-frequency (IF) demodulator. The MC145158 PLL frequency synthesizer used as the reference oscillator for mixing was one of the few parts still available as old stock through hobbyist and commercial hoards.

Front-end receiver amplification and RF-to-IF downconversion appear to be implemented in discrete components. Of particular note on the RF board are the RF power amplifier and duplex filter, the latter responsible for splitting transmit-and-receive sub-bands. Size and weight of both have dropped dramatically with the passage of time, with the duplex filter used here coming in at more than 100 times the weight and volume of modern equivalents.

Other noteworthy benchmarks include the use of socketed E2PROM from National Semiconductor, a company that left the often-brutal world of memory devices almost a decade ago. At 64KBytes, system code and user storage is a tiny fraction of that found in even the most basic of modern handsets.

To play the design forward, consider the impact of 15 years in phone design and evolution. Comparing a 2006 entry-level voice cellular terminal to the DynaTAC, we have one-tenth the weight and volume, five to 10 times the talk time and one-tenth the cost, all in an everyday phone. Continuing at this rate, the next 15 years will give us sugar-cube-size cellular communications with gigabytes of storage, weeks or months of practical use-time per charge and a cost of under $5. Of course, human interface limitations and some physics could get in the way of such a feat, but it's an interesting point to ponder.

While not too hip anymore, the brick launched an industry still growing by leaps and bounds today, with targets set for bringing on board "the next billion users." Interestingly, though, not everyone rejects the DynaTAC's somewhat gargantuan simplicity even now. Enterprising shops in Asia and elsewhere are actually hacking modern phone guts into the cases of their oversize forbears to make the MP3-playing, picture-taking, color-screen aspects of today's handsets available in brick form.

So, feel free to slip on your Ray-Bans, put on some Eurythmics and make that call in retro style. Just don't forget the significant and serious accomplishments of the cellular pioneers; we're standing today on the shoulders of some pretty impressive technology history.

- David Carey
President, Portelligent

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