RF Video Amplifiers Enable FTTP's Last Mile

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.

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For RF video amplification, however, high linearity and good low-noise performance are much more critical than with data. The amplifier must provide relatively flat and consistent response over the entire RF video bandwidth from 50 MHz to 870 MHz and be able to accommodate variations in loss that the signal will experience during transmission. The RF video amplifier must provide good linearity and sufficient gain control range, and contribute minimal noise in order to maintain signal integrity with a good carrier-to-noise ratio (CNR) for sharp, clear images in a densely packed spectrum.

In CATV applications, linearity is measured using composite-second-order (CSO) and composite-triple-beat (CTB) ratios instead of second-and third-order intercept points as in wireless communications systems. For nominal operation in RF video systems, the CTB is typically -65 dBc and CSO is usually less than -60 dBc in order to minimize distortion products, which can degrade signal integrity. For instance, the ACA2601 video amplifier from ANADIGICS maintains CSO/CTB linearity of -65 dBc or better across 50 to 870 MHz (Fig.3). When selecting an RF video amplifier, it is important to specify that the device maintain low CTB and CSO levels in full-bandwidth (132-channel) systems, and across a wide gain adjustment range.

As the second RF component in the receive chain, the RF video amplifier has a great effect on noise figure. In order to boost signal levels while maintaining low system noise levels, the first amplifier stage is usually a low-noise amplifier (LNA). The equivalent input noise (EIN) of an RF video amplifier is a key parameter for this application. Some of the latest amplifier designs, such as the model ACA2601 from ANADIGICS, are achieving EIN performance of 4.5 pA/(Hz)^0.5 which helps to ensure a good CNR that will produce a better picture quality at low signal levels.

In order to deliver signals that are received across a broad range of optical input power levels, typically from -8 to +2 dBm, a video amplifier for FTTP/FTTH applications needs a robust gain control range. The 20-dB gain control range of the ACA2601, for example, ensures that this amplifier can maintain +18 dBmV per channel over this broad optical input power range. Excellent linearity and low noise must be maintained across the entire gain control range in order to minimize the effect of system impairments and deliver the highest quality signal for crystal clear video.

A RF amplifier optimized for FTTP/FTTH applications should integrate all three of the above features: a low-noise amplifier, a voltage controlled attenuator, and a high linearity output amplifier (Fig. 2). A single integrated solution simplifies the design process by eliminating the need to evaluate and specify different individual components. Each stage of the amplifier is designed to work seamlessly with the other stages and to interface with the input and output circuits. To simplify design and save board space, it helps if the device operates from a single supply. Some manufacturers, such as ANADIGICS, also offer a high-impedance 400-Ω input, which eliminates the need for a matching transformer and allows a direct interface to the photodiode with an inexpensive LC network.

By eliminating external components, such as a transformer and gain control circuitry, an integrated RF video amplifier enables smaller, higher-density designs and simplifies procurement by requiring a smaller bill of materials (BOM). As FTTP implementations continue on their impressive path of growth, they will undoubtedly require greater integration and more cost-effective, reliable designs. Choosing an integrated RF video amplifier helps designers stay on course.

1. The May 2006 Newsletter for the Fiber to the Home Council (www.ftthcouncil.org) reports 4 million homes passed by fiber in the US, which was up 2 million in two months. 621,000 homes were connected as of May, which was up 71,000 from the previous month.