Cordless phone enters next phase

The high-volume, low-status cordless phone is a fixture in most households, garnering little mindshare in the milieu of modern gadgetry. But the product category continues to present a picture of selective integration, design-for-cost and standards advancement that is worth examining.

Consider the VTech 6032, a handset/base combo built around the newly minted DECT 6.0 cordless standard. While U.S. cordless products have historically used the unlicensed industrial, scientific and medical (ISM) bands (900MHz/2.4GHz/5.8GHz), along with varied proprietary air interface protocols, DECT 6.0 in the United States is based on the 1,920-1,930MHz unlicensed PCS (UPCS) spectrum released by the FCC in 2004. European variants operate in a close, 1,880-1,900MHz bandwidth. The key advantage in either case is avoidance of the increasingly crowded ISM frequencies, to combat interference.

Quick review: DECT
The UPCS band opened by the U.S. FCC does not require spectrum license fees per se, but there is a modest (and declining) cost to the equipment manufacturer's implemented DECT 6.0 products. An up-front fee of $50k and a "per-radiating-device charge" of about a nickel in 2008 help offset the costs of clearing straggler microwave towers that used the UPCS spectrum.

Within the DECT standard, radio traffic is both time- and frequency-division multiplexed: Multiple timeslots for downlink/uplink combine with multiple RF channels per timeslot to maximize spectral efficiency and security. The quarter-watt maximum RF output power is far lower, on average, given that only a single channel slot of the 24 available gets used during time-division multiplexing.

When duty cycle is accounted for, a more miserly 10mW average output RF power is seen in practice.

On the security front, 64bit encryption and "challenge and response" authentication during call setup deter would-be eavesdroppers.

Overall, DECT's advantages in sound quality (digital processing), battery life, security and interference resistance seem to have brought on a tipping point for widespread adoption for the U.S market. In short, if you don't have one already, your next cordless setup may very well be DECT-based.

Handsome phone
The VTech 6032 comprises a single handset and basestation with a built-in digital answering machine capable of storing up to 15mins of messages. Fifty-caller ID storage and speakerphone join with selectable ring tones for the 6032, and step-up models add handsets to the shared base.

The two case halves of the base unit open to reveal a pair of circuit boards, one containing the wireless meat of the system, the other the keypad and LED readout. Along with copious use of screw fasteners, other construction attributes are indicative of the 6032's China-based final manufacturing. Extensive hand-soldering and even hot-glue strain relief of the board-to-board cable connector (see inset photo on teardown image) speak to a reliance on human capital over more-expensive components conducive to automated assembly.

An off-catalog NXP processor chip—actually composed of two separate chips in a single surface-mount package—forms the digital portion of the system. The ARM marking on the package is indicative of the controller core, and the two chips probably partition functions as one device for the DECT baseband and the other for control/DSP. An Atmel 1Mbyte serial flash is joined to the processor, holding what is likely the stored message data. An ISSI 8Kbyte EEPROM hosts system (boot?) code, and an audio amp from UTC is used to drive the 6032's speakerphone transducer.

The radio portion of the basestation relies on a separately mounted, stamp-sized circuit board module. De-lidding reveals two chips, both from NXP as well. One die is wirebonded directly to the radio module and glob-topped, serving as the DECT transceiver. The second packaged component provides the RF front end, likely a mix of transmit power amplification, transmit-receive switching, antenna switching and, possibly, a receive low-noise amp. As with the dual-chip processor, none of the devices could be found in NXP's online literature.

The VTech 6032 is a handset/base combo built around the DECT 6.0 cordless standard. (Click to view teardown.)

Intuitive design
Substantial circuit commonality is visible in the handset, where the same radio module is mounted alongside a modified two-chip package (NXP) with baseband and control functions. Since its duties are simpler, the handset swaps out the ARM-based controller used in the basestation for another ARM-based device. On Semiconductor provides the handset audio amp, and an EEPROM identical to the one in the basestation is married to the NXP processor.

The NXP/Philips heritage in the cordless components is evident, but the recent sale of NXP's cordless-phone chipset business to the DSP Group (DSPG) may explain the lack of traceable parts. As of September 2007, DSPG is perhaps the more correct branding to assign to the VTech phone's core components.

The basestation antennas are simple, essentially comprising orthogonally arranged, stamped metal pieces implementing an inverted F antenna topology. The handset antenna is simpler still: a single, monopole antenna wire soldered to the handset pc board.

While the passive-component count is still notable, silicon integration keeps chip count fairly low. Nonetheless, there is more partitioning than might be expected based on recent progress in mixed-signal integration. Given the relative newness of the DECT 6.0 standard, the next turn of the crank from DSPG and others will likely wait until market volumes build sufficiently to justify further integration.

In the meantime, mix-and-match packaging of individual reusable silicon blocks combines with standardized radio modules for economy of scale and simplified RF testing to keep costs low. With a $60 retail price for the product (and implied manufacturing BOM cost of probably half that or less), VTech and its suppliers must carefully balance the pace of integration to keep large development costs from hitting before the market volumes build.

Of course, cost engineering has many facets. The keypad is inspired by Motorola's Razr; indeed, the first promotional photos of the VTech 6032 show a design with the "fly cut" metal-finish keypad introduced in the iconic Motorola phone. Such touches come at a premium, however, and this one didn't quite make it into the final production product bought for this analysis.

- David Carey
EE Times