technology microelectronics
of information through the network. If this distortion were not reduced, it would impair the effectiveness of lightwave systems for cable applications. Even early systems required extremely high linearity, and this demand became even more stringent over the years as bandwidth and channel counts increased.
TAT7467H: Edge QAM / DOCSIS 3.0 amplifier for CATV headend applications. A true differential amplifier for medium power applications with excellent 3rd order distortion performance. 350-380mA power usage that is up to 50% better than other solutions
Linearization of RF and optical systems over wide CATV bandwidths is far from trivial. When performed using external discrete circuits, this goal is complicated by parasitic effects that are difficult to compensate out of the linearizer. However, if these circuits can be integrated directly on-chip, adjacent to the amplifying device, then the parasitic problem is largely eliminated. Thanks in part to the use of techniques developed by TriAccess Technologies, a company that we acquired last year, we can now linearize amplifiers for DOCSIS 3.0 and cable infrastructure using on-chip techniques.
Substantial cost savings have already resulted from linearization, because it has enabled the construction of fiber optic modulators with the necessary distortion characteristics for transmitting analog content. Looking forward, linearization is poised once again to enable cost savings, this time to benefit RF amplification needs. Integrated linearization techniques are available to enable mature GaAs technologies to compete with more expensive semiconductor options without paying the penalty of a substantial sacrifice in performance. Linearization can also enhance the already excellent performance of newer device technologies, providing an even higher level of power efficiency to the operator.
GaAs, GaN, and cable It is safe to say that compound semiconductor technology along with advanced linearization techniques will be two
key enablers for allowing all types of future cable systems to fulfill the cost, service quality and competitive challenges lying around the corner. The RF, microwave and lightwave portions of the system are already relying extensively on compound semiconductor technologies, and will increasingly do so in the coming years.
On the RF side, the hybrid amplifier that has employed silicon bipolar transistors for decades to deliver the high RF output power and linearity required for Class A operation is on its way out. The performance of silicon- based hybrids has reached its limit by failing to keep up with increases in bandwidth made available to cable MSOs, which have grown from 300 MHz to 550, 870, and now 1000 MHz. In its place will be GaAs devices delivering greater performance in every key metric, combining greater bandwidth with much lower multi- carrier distortion, superior RF output power, lower noise and greater efficiency. Spurring the switch to the superior technology is a shrinking price gap between GaAs-based hybrids and their silicon rivals. The insurgent often benefits from a single RFIC that leads to improved push- pull amplifier matching and fewer distortion-induced problems such as composite carrier noise ratio.
One of the big questions hanging over cable hybrid amplifiers is this: how long will they be needed, as fiber continues its march deeper into the network? Industry prophets have long predicted the demise of this class of amplifier. However, to paraphrase Mark Twain, rumors of their death have been greatly exaggerated. The global market for cable hybrid amplifiers, while very cyclical, shows only small signs of diminishing. What’s more, it is rapidly being enhanced by widespread deployment of GaAs devices. While some regions of the world are significantly built-out with cable service networks, others are just beginning. In developing nations with growing economies citizens are just starting to enjoy the luxury of discretional income, and there’s no doubt that some of this will be used for entertainment, including cable TV.
Quality check on a wafer being fabricated at TriQuint’s Hillsboro, Oregon facility
18
www.compoundsemiconductor.net June 2010
In addition, suppliers of GaN-based RF power amplifiers have recently introduced their first products. Although in many respects GaN is still an emerging technology, it is still a very attractive contender, offering tremendous performance in several key figures of merit. Cost is a major concern – it is three to eight times that of GaAs devices – and this restricts GaN to use in situations demanding the highest possible performance. The higher price of GaN devices is mitigated to some degree by greater power output and other attributes, which can extend the reach from the fiber node to the customer (where the transition is made from fiber to coax) while maintaining low levels of distortion and power consumption. But it remains to be seen whether the market embraces GaN, and is willing to shell out for the greater performance that it offers.
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