Clock-Recovery Module Sets Low-Jitter Standards

March 17, 2005
Working with a high-speed digital oscilloscope, this precise plug-in module can derive a clock signal from NRZ signals with rates from 50 Mb/s to 13.5 Gbs/s.

Jitter is one of the limiting factors in modern high-speed data-communications systems. Measuring jitter requires a reference source with almost ideal stability in order to detect the phase variations in the system or component under test. For that purpose, the model 83496A multirate optical/electrical clock recovery module has been developed for the model 86100C Infiniium digital communications analyzer (DCA-J) from Agilent Technologies (Palo Alto, CA). The module, which empowers the analyzer to perform clock extraction with from nonreturn-to-zero (NRZ) data signals from 50 Mb/s to 7.1 Gb/s in standard units (and 50 Mb/s to 13.5 Gb/s with Option 200), exhibits almost immeasurable root-mean-square (RMS) jitter of 300 fs.

The combination of analyzer and plug-in module (see figure) can measure the lowest levels of jitter in communications signals. It allows optical and electrical component and system engineers working in high-speed digital communications to achieve new levels of measurement accuracy. In addition, many of these digital communications systems, such as IEEE 802.3 Ethernet, and Fibre Channel, require the use of a "Golden phase-locked loop (PLL)" to control the spectrum of jitter that is being evaluated during the testing. With too narrow a PLL bandwidth, measurements can be obscured and readings inaccurate. With too wide a PLL bandwidth, too much jitter is removed from a measurement and results are again misleading.

The model 83496A clock-recovery module can function as a Golden PLL by setting the loop bandwidth to the exact value required for any data rate within its operating range, making it a key component for standards-compliant jitter tests. It supports tunable loop bandwidths from 30 kHz to 6 MHz to cover a wide range of standards.

The model 83496A clock-recovery module is available with a number of options, including Option 100 for electrical differential or single-ended clock recovery from 50 Mb/s to 7.1 Gb/s signals and Option 101 for clock recovery from 50 Mb/s to 7.1 Gb/s with optical single-mode fibers at wavelengths from 1250 to 1620 nm and multimode fibers at wavelengths from 780 to 1330 nm. Option 200 increases the operating range of either of the first two options to 13.5 Gb/s, while Option 300 adds the Golden PLL tunable-bandwidth capability (standard units allow the loop bandwidth to be adjusted to two discrete settings that depend upon the data rate). P&A: $18,000 and up (module) and $17,500 and up (Infiniium DCA-J oscilloscope mainframe); 30 days. Agilent Technologies, Test and Measurement Organization, 5301 Stevens Creek Blvd., MS 54LAK, Santa Clara, CA 95052; (800) 452-4844, Internet:

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