RF DACs simplify power and space in downstream cable transmitter systems

Daniel E. Fague and Sara Nadeau -December 15, 2012


The amount of broadband data used over cable systems has grown tremendously over the last ten years. Since 2003, the number of subscribers to broadband data over cable services has increased at a compound annual growth rate approaching 14% [1]. The annual growth rate has slowed to single digits, but a recent trend is the double digit growth in premium or so-called “wideband” broadband services.

These services provide higher downstream and upstream bandwidths for heavy data users. The trend of increased data use shows no signs of slowing, as more and more consumers utilize Web-based services for video streaming, audio-streaming, and gaming.

Cable service providers are upgrading their distribution systems to stay ahead of the increased consumer demand for data. The nature of the transmission systems is evolving from a multi-cast system, where all subscribers are sent the same signal, to a combined multi-cast and narrowcast system, where some content is shared by all subscribers, and some content is directed to a particular subscriber. It is the increase in narrowcast services, which includes data but also can include on-demand video, pay-per-view, and other services, that is requiring continued network upgrades.

Cable System Downstream Transmitters

Digital cable transmitters have advanced from a conventional transmitter, where a pair of baseband DACs are used to drive a quadrature modulator, whose local oscillator is used to choose the correct RF frequency, to direct modulation techniques.

In the direct modulation transmitter, an RF DAC is used and the cable channel is created entirely in the digital domain, typically an FPGA. The digital signal is sent from the FPGA to an RF DAC, where it is converted to an analog signal and sent to the power amplifier. A simplified block diagram of a typical cable transmitter is shown in Figure 1.

Figure 1. Block diagram of (a) typical cable transmitter using multiple RF DACs and an RF combiner to achieve a full cable spectrum, and (b) a new cable transmitter using the new AD9129 RF DAC.

In Figure 1(a), the transmitter is composed of several RF DACs being driven by several FPGAs, and then the output of each of the RF DACs is sent to a pre-amplifier. The outputs of the pre-amplifiers are combined to feed into a single power amplifier which drives the cable plant.

This architecture is used because the gate count and capacity of the FPGAs to synthesize large numbers of digital signals with a reasonable current consumption was limited, and each RF chain could be optimized for a particular frequency band.

The RF DACs often have signal processing on them that limit the total RF bandwidth that can be generated, but ease the interface requirements for the FPGAs. Previous generation RF DACs had good performance, but harmonic performance did not meet the demanding DOCSIS specifications, so detailed frequency planning and RF filter design was necessary to achieve acceptable performance.

The architecture, which may have two, four, or eight 256-QAM channels per RF DAC, allows for scalability, albeit at a cost of extra hardware. There are several disadvantages to it. The RF combiner gets more complicated with a higher number of channels desired, and with each added DAC channel, the combiner’s losses also increase.

Each of the FPGA+RF DAC+pre-amplifier chains draws a significant amount of power, maybe 10 Watts per channel. The multiple RF chains needed could require multiple cards to achieve a 158-channel, full cable spectrum, with each card drawing as much as a kilowatt or more. Housing multiple cards in a single facility is necessary to service an example 1000 household group.

The system becomes big, with many cards needed to serve each 1000 household group. The result is a need for a large facility or building to house all of these cards in large racks or chassis, with significant attention paid to cooling systems for the racks, and expenses are high to keep the building at a reasonable operating temperature.

Today, with the higher gate counts and finer-line CMOS processes available, it is possible for FPGAs to have high enough density to enable creation of the entire 158 6 MHz-wide cable channels on one FPGA, driving one RF DAC. When combined with the new AD9129 RF DAC from Analog Devices, a much simplified cable transmitter can be designed.

Figure 1(b) shows a block diagram of a new transmitter that is capable of synthesizing the entire downstream cable spectrum from 50 MHz to 1 GHz. The digital modulator in the FPGA drives the AD9129 RF DAC with a high sample rate of up to 2.8 GSPS.

The DAC has an optional 2x interpolator filter that can be enabled to implement on-chip digital filtering for out-of-band components, which increases the effective sampling rate to up to 5.6 GSPS. The DAC output is filtered with a low pass filter and sent to a new, highly integrated variable gain amplifier and driver amplifier from Triquint, the TAT2814.

The amplifier achieves new levels of integration by integrating a pre-amplifier, a variable attenuator, and a driver amplifier all in a single module. This enables a compact layout for the radio section and reduces the physical size of each radio port.

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