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Spread spectrum confronts EMI

Posted: 01 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:spread-spectrum? clock generators? oscillators? Ethernet? HDTV?

The sleepy clock-generator chip market shook itself awake as two rival vendors rolled entries in a new class of programmable devices. Cypress Semiconductor Inc. and SpectraLinear Inc. have introduced clock chips based on spread-spectrum technology, which promises to tackle the growing problem of EMI in clock chip designs.

Cypress added a programmable clock generator and extended its current timing solutions. SpectraLinear announced its first productsa line of spread-spectrum clock generators said to reduce EMI in communications, consumer and computing applications.

According to SpectraLinear, the market for IC timing solutions totaled $1.1 billion in 2006. Other timing-solution players include Integrated Device Technology Inc., AKM Semiconductor, Fujitsu, Pericom, Realtek, Silego, Rohm and Texas Instruments.

EMI hurdles
Clock-generator chips combine analog and digital circuits that generate and send accurate signals in a system. In one form or another, the chips are used in PCs, communications systems and consumer items. Clock-generator device categories include crystal oscillators, PLL-based frequency synthesizers and standalone EMI-reduction chips.

OEMs have been shifting from oscillators to silicon-based PLLs to meet the growing demands in today's systems. Indeed, demand for faster systems is propelling the need for a new class of clock circuits to enable better timing performance, lower power, reduced EMI and more favorable cost, said Ilhan Refioglu, founder, president and CEO of SpectraLinear.

"The stakes are high," according to a white paper on the subject from IDT. "Rising clock frequencies, shrinking timing margins and tighter board layouts conspire to introduce new sources of skew, noise, crosstalk and other SI issues. If designers fail to efficiently design their clocking circuits, they can undermine the market success of their product."

The biggest challenges for clock circuits are jitter control and EMI. To address jitter, designers must define their clock-circuit strategies early in the development cycle, according to IDT.

Clock-generation and -distribution ICs are the primary sources of system EMI. "The well-known techniques of filtering and shielding via passive components, while effective, can cost money, board space and development time," according to a SpectraLinear technical material. IC vendors are moving toward spread-spectrum techniques, which "modulate the system clock with a much smaller frequency to reduce EMI emissions."

Intuitive chips
Last year, Cypress, the market's second-largest vendor, rolled out the first member of the InstaClock family of devices, programmable for 40 of the most common frequencies in a system.

In September, Cypress added the second family member, the CY22801. It features clocks that can be programmed in less than a minute, eliminating the need to wait days or weeks for custom-programmed parts or for a mask change, said Cypress.

Designers can also generate any desired frequency for the CY22801 clock with the CyClocksRT software. The chip supports frequencies used in consumer and communications applications such as Ethernet, PCI, USB, HDTV and audio.

EproClock chips tap 0.18?m technology and support 3-200MHz output from crystal or clock input.

In addition, Cypress has added more than 60 widely used clock configurations to those available for the original InstaClock chip, the CY22800. The chip can now be programmed for any configuration within the list of defined frequencies. The CY22800 features a programmable spread-spectrum range from

0.25 percent to

2 percent.

Both InstaClock devices have a single integrated PLL and deliver low jitter, said Wayne Gill, managing director for Cypress' general purpose clocks business unit.

SpectraLinear, meanwhile, is no stranger to competitor Cypress. In 1995, SpectraLinear's Refioglu became president of International Microcircuits Inc. (IMI). In 2000, Cypress acquired IMI.

After the acquisition, Refioglu ran the timing technology division for Cypress. He then left the company and started SpectraLinear in 2006. The startup acquired Cypress' PC clock-chip division in October of last year.

In its initial product announcement in September, SpectraLinear rolled the SL15100 and SL15101 for use in applications such as routers, switches, servers, HDTVs, STBs, base stations, PCs, games, digicams, printers and copiers. The 2.5V and 3.3V devices are fabricated in a 0.18?m process at Chartered Semiconductor Manufacturing and are based on the company's proprietary programmable and spread-spectrum technology, which it calls EProClock.

The spread-spectrum function can be turned off without performance degradation, letting system designers replace a range of crystal oscillators with a single programmable device, according to SpectraLinear.

The modulated clock can bring EMI into compliance with regulatory restrictions. The devices eliminate the need for expensive shielding and filtering via passive components, saving development time and manufacturing cost.

- Mark LaPedus
EE Times

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