[Product Innovation]
Rate-Independent CDR Chip Locks In At Up To 2.7 Gbits/s
Using a patent-pending architecture, a monolithic, dual-channel device allows on-the-fly adjustment of data rate and eye positioning.
One of the key indicators of a fiber system's performance is how far a signal can be transmitted before it must be regenerated. A lot goes into determining this, from the quality and power of the original signal, to connectors, to noise within the fiber itself (dispersion). At the end of the day, however, everything rests on the clock-and-data-recovery (CDR) unit on the receiving end.
Given the unenviable task of having to regenerate a coherent data and clock output from what is more often than not a severely compromised signal, the CDR shoulders this responsibility in the face of multiple protocols and rapidly increasing data rates. All the while, it has to accommodate user demand for smaller footprints, lower power consumption, greater flexibility, and lower cost. This last factor, cost, acquires increasing significance as the rapid deployment of fiber-optic systems raises competition.
While CDR vendors to date have been able to answer subsets of these requirements, none has covered all the bases—until now. Using patent-pending design techniques, designers at Vitesse Semiconductor Corp. have overcome the problems. They have engineered a dual-channel, rate-agile, monolithic, CDR chip for broadband data streams that provides continuous coverage from 10-Mbit/s to OC-48+FEC (2.7-Gbit/s) rates, all in an 80-pin, thermally enhanced PQFP.
Dubbed the VSC8123, this device uses proprietary technology to provide telemetry on the condition of the incoming eye and the quality of the acquisition without taking the service channel offline. This intelligence is acquired using a microcontroller interface. The VSC8123 uses this interface to communicate with an external controller that provides the intelligence.
A key distinction is the ability to dynamically and automatically modify its acquisition point in both voltage and phase. This lets the VSC8123 acquire data in the presence of significant symmetry distortion in the data eye. Additional circuitry is provided to measure relative bit-error rates without affecting the integrity of the active data stream.
Other features include an integrated automatic-gain-control (AGC) front end with offset correction and on-die terminations, referenceless clock recovery, and customizable software control algorithms for acquisition, tracking, and error profiling.
The VSC8123 comes at a time of massive expansion in the fiber-optics arena. The Ciscos and Nortels of the world are deploying systems at unprecedented rates to accommodate the burgeoning demand for high-speed Internet access in every office, home, and soon, mobile laptops and third-generation telephones. According to Communications Industry Inc., Charlottesville, Va., the market for fiber-optic components alone will reach $6.1 billion by 2003. Also by that time, the market for dense, wavelength-division-multiplexing systems will total $6.3 billion, and the market for optical cross-connect systems will hit $1.3 billion. Heady figures, but the expansion rate certainly supports their validity.
The conversion from what was a slow, lumbering, high-end, elitist market into a dynamic, fast-moving, highly competitive field has had numerous side effects on both the business and technology. In the business, fiber-optic component manufacturers that once toiled in relative obscurity have become the darlings of Wall Street and novice investors alike. On the technology side, enormous innovation has had to take place to keep pace with increasing demand. Higher data rates have combined with multiple and varied protocols, leading to a need for higher tolerance of signal degradation, with enhanced flexibility.
Tolerance of signal degradation is crucial. As the distance a signal can go down a fiber before requiring regeneration increases, the cost of that system goes down. The time taken to deploy the system shrinks, and the reliability goes up dramatically. But some level of signal degradation is unavoidable, and its causes are numerous.
Just going down the fiber, the signal is subject to dispersive losses and jitter. This is due to the different times of flights of photons, which depend on the route they take—going down the center, or bouncing off the walls. There also are significant distortions and other losses incurred because of the electronics that are on either end of the fiber—the laser driver itself, the diode, the photodetector, and the post amplifiers that follow the photodiode. All of these tend to compromise the signal to some degree.
This degradation is measured using an eye diagram, which is essentially an oscilloscope's representation of a pseudorandom, non-return-to-zero data stream of ones and zeros. The oscilloscope triggers at a rate that isn't necessarily equal to the repetition rate of that pattern. In some cases, the pattern repeats once a second. The oscilloscope then will trigger significantly more frequently than that, perhaps 1/16th the clock rate, so a retrace is shown.
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