Spread-Spectrum Clocks Cut EMI

Dec. 12, 2008
Leveraging MEMS resonator technology and spread-spectrum techniques, these programmable clock oscillators handle rugged environments with minimal generation of EMI.

Electromagnetic interference (EMI) can emanate from the smallest of sources, even from a system clock source. For that reason, oscillator designers at SiTime Corp. turned to spread-spectrum technology as a way to literally spread the EMI generated by their oscillators over a wider segment of frequency spectrum than normal. The end result is that interference at any specific frequency becomes less, with less chance for causing interference with other systems or even with other components in the same system. And to ensure that these clock oscillators can stand up to high levels of shock and vibration, they are based not on quartz crystal resonators but on microelectromechanical-systems (MEMS) resonators. The new clock oscillators are the models SiT9001 and SiT9002, which are programmable at the factory for any frequency in the ranges of 1 to 200 MHz and 10 to 220 MHz, respectively. The clocks are available in packages as small as 2.5 x 2.0 x 0.85 mm. The SiT9001 offers a singleended output port while the SiT9002 is the industry's only programmable spread-spectrum clock oscillator with differential output port.

In order to minimize EMI, spreadspectrum technology is applied to these clock sources by spreading the generated energy across a wider swath of spectrum than in a conventional clock oscillator. The typical reduction in radiated EMI compared to a conventional clock is about 6 dB, although cases of slashing EMI by as much as 12 dB have been reported. The amount of EMI reduction to be expected is very much a function of the particular application and such factors as mounting practices, grounding, and surrounding components.

Although the timing stability might be suspect for an oscillator using such an approach, the SiT9001 and SiT9002 do not sacrifice in performance for the sake of lower EMI. In addition to being the industry's smallest programmable spread-spectrum clock oscillators, they also offer excellent cycle-to-cycle time jitter performance, which translates into low phase noise in the frequency domain. The SiT9001, for example, achieves almost negligible cycle-to-cycle jitter, at less than 30 ps for minimal impact on a system timing budget. And it can be programmed (at the factory) to any output frequency from 1 to 200 MHz with available frequency tolerance of 50 or 100 PPM.

The SiT9001's integrated MEMS resonator provides 10 times more robustness than conventional crystalbased clock oscillators; the programmable clock oscillator can withstand shock to 50,000 g's and vibration to 70 g's. The SiT9001 can operate with frequency spreading occurring around the specified center frequency (at spreading widths as a percentage of the center frequency of 1, 0.5, or 0.25 percent), or by using "downspreading" in which spreading takes place at frequencies below the specified center frequency (at spreading widths of -0.5, -1.0, or -2.0 percent). The SiT9001 can run on a number of different supply voltages: +1.8, +2.5, +2.8, and +3.3 VDC. The SiT9002, which is available at output frequencies from 10 to 220 MHz, can serve as a replacement for differential crystal oscillators supporting three common differential standards: LVPECL (2.25 to 3.63 V), LVDS (2.25 to 3.63 V), and CML (1.8 V or 2.25 to 3.63 V). Like the SiT9001, the SiT9002 features cycle-to-cycle jitter of less than 30 ps.

The programmable clock oscillators are designed as drop-in replacements for quartz-crystal clock oscillators. With their integral MEMS resonators, they are inherently rugged and do not require external components. The SiT9001 is available in a four-pad 2.5 x 2.0 mm package. The SiT9002 is available in six pad, 5.0 x 3.2 mm and 7.0 x 5.0 mm packs. SiTime Corp., 990 Almanor Ave., Suite 200, Sunnyvale, CA 94085, (408) 328-4400, Internet: www.sitime.com.

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