Clock generator for communication systems
18 March 2009
Electronics Technology
Analog Devices has introduced a clock generator that simplifies clock design and reduces the need for oscillators in multistandard networking and communications infrastructure systems.
The AD9551 features a new, simplified architecture that can generate and translate multiple precision network clock frequencies, which enables the replacement of as many as five oscillators. It supports forward error correction (FEC), holdover, switchover and precise frequency generation – functions that are essential in network switches, routers and line cards.
The device accepts one or two reference input signals and generates one or two output signals that are harmonically related by a programmable factor of one to 63. Precisely translating the reference frequency to the desired output frequency, it includes input receivers and output drivers that are capable of either single-ended or differential operation. On-chip reference monitoring and switchover circuitry internally synchronise the two references to prevent phase perturbations at the output in the event of a reference failure.
Should either or both references fail, the AD9551 maintains a steady output signal with no phase disturbance on the output. The device relies on an external 26 MHz crystal and the internal digitally-compensated crystal oscillator (DCXO) of the first of two cascaded fractional-N phase locked loops (PLLs) to provide a clean reference for the second PLL, and to hold the output frequency in case of reference failure. The second fractional-N PLL enables fine precision output frequency tuning with low phase noise.
The AD9551 provides a serial-peripheral interface (SPI) port and pin-selectable preset divider values that offer an assortment of frequency ratios, including all the standard rates for Gigabit Ethernet (644,53125 MHz), 10 Gigabit Ethernet (625 MHz), SONET/SDH (622,08 MHz), and Fibre Channel (657,421875 MHz) as well as the established FEC ratios (15/14, 239/237, 239/238, 255/237, 255/238).
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