Fiber to the premises/home (FTTP/FTTH) is growing rapidly, as telecommunications companies seek to provide the triple-play services of voice, video, and high-speed data. Part of the last mile in this communications link includes the final 100 feet, where optical signals are converted back to electrical signals and distributed throughout the home or office. For video, the critical point in the signal path is the RF video amplifier, and picking the right one can ensure the quality of an FTTP/FTTH installation.

Recent regulatory changes have freed telecommunications service providers to compete with services other that their traditional voice lines, adding data and video to their networks for true triple-play functionality. These multiple-service offers provide the means to recoup investments made on high-performance fiber-optic communications links, and companies such as Verizon Communications and AT&T are taking the lead with a rapid deployment and aggressive marketing campaigns.

The Fiber To The Home (FTTH) Council (www.ftthcouncil.org) reported in May 2006 that fiber was passing more than 4 million homes in the United States (sometimes hanging on utility poles, but available for hookups). ¹ That number represented a doubling in only six months. The phrase "homes passed" is a measure of how many residences have immediate availability to FTTP services. US telecommunications carriers have announced plans to pass 40 million homes with fiber-optic cable by the end of the decade.

As a result, optical-network-terminal (ONT) manufacturers are moving into second-and third-generation implementations, and they are looking to streamline their offerings and find competitive advantages that make them attractive in what is likely to quickly become a commodity market. Reducing the number of components and improving performance are two ways that device manufacturers can get the attention of the ONT manufacturers. The ACA2601 RF video amplifier from ANADIGICS (www.anadigics.com) offers just such an opportunity.

The ONT is essentially the component that connects a user's premises with the central office via the fiber-optic network. It also includes the components that perform the translation of optical signals to electrical signals (Fig. 1). Signals pass both upstream (from the home to the central office) and downstream (from the central office to the home) over a single optical fiber. Downstream traffic uses wavelengths of 1480 nm for data and 1550 nm for video. Upstream traffic in the return path is centered at 1310 nm. The design of the ONT is simplified by using an optical triplexer. The triplexer demultiplexes the incoming 1480 and 1550 wavelengths, each to its own photodiode for optical-to-electrical conversion and follow-on amplification and signal conditioning. The triplexer also contains a 1310-nm laser for communicating upstream with the central office.

Handling all of the analog and digital video, high-definition television (HDTV), and video-on-demand (VOD) signals, the RF video amplifier interfaces with the photodiode in the transceiver to provide the transition (Fig. 2) to a 75-Ω coaxial connector. As the second component in the RF video receiver, the RF amplifier must have low noise and low distortion in order to maintain exceptional signal integrity. The amplifier also should provide gain control for all video channels, including analog TV, digital TV, and VOD.

Service providers use one of two approaches to deliver broadband multimedia into the home. Some service providers are using the all-digital Internet Protocol (IP) video model. With IP video, the set-top box sends a request to the central office for a specific video stream, and then the central office sends the video stream to the set-top box. Unlike traditional cable-television (CATV) systems that broadcast most channels to the set-top box, this system must transmit a separate video stream for every active set-top box connected to the network. The bandwidth associated with these transmissions is the main disadvantage of IP video. Given that each ONT has a finite allocation of bandwidth dedicated to voice, data, and video, the total data throughput becomes limited by the number of simultaneous video streams or users. This challenge is exacerbated by the huge bandwidth required for IP delivery of HDTV content, which is approximately 15 Mb/s, versus approximately 3 Mb/s for standard definition content. It therefore becomes clear that having multiple set-top boxes receiving video may not only limit the computer data connection, but may also limit additional set-top boxes in the premises from receiving content.

In comparison, other service providers are using RF video transmission in an approach similar to current CATV systems. These systems use separate optical bands for data and video, enabling broad-cast-delivery of television channels. This approach can use an IP connection for two-way signaling and interactive services, such as VOD and pay-per-view. With RF video transmission, the number of active set-top boxes per premises is not limited by the throughput of the system.

With RF video delivery, the optical to electrical conversion delivers an 18-dBmV signal to the 75-Ω coaxial cable for whole-house distribution. The downstream RF video conversion circuit (Fig. 2) typically requires the photodiode, high linearity amplification, gain control, and any necessary impedance-matching circuitry. While the circuit could be built with discrete amplifiers and attenuators, a new generation of integrated FTTP RF video amplifiers simplifies design, lowers costs, and saves space by reducing on-board circuitry, reducing the number of components, and integrating multiple amplifiers and gain control into a single package. With these integrated circuits (ICs), the various stages of amplification and gain control are already optimized for FTTP applications, eliminating the need for ONT designers to match discrete components.

One amplifier that may come to mind for an FTTP implementation is the trans-impedance amplifier (TIA) for the 1480-nm downstream data stream. The purpose of the amplifier is to convert the photodiode output current to a signal voltage with enough gain so that subsequent circuits can perform clock and data recovery. A main characteristic of the TIA is the ability to handle a wide range of optical power levels. The dynamic range of the receiver is typically around 30 dB, enabling it to handle optical signals ranging from -30 to 0 dBm.