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Analog Dialogue Current Issue


Innovative Mixed-Signal Chipset Targets Hybrid-Fiber Coaxial Cable Modems

Jim Surber and Curt Ventola
The world is on the brink of a new era of widespread access to the information super highway, and cable modems are poised to provide the high-speed "on-ramp". Cable modems enable a CATV system with bidirectional hybrid-fiber coax (HFC) capability to serve as a two-way high-speed data port, which can be utilized to provide telephony and Internet access service to the home. Though a relatively small percentage of the US population is presently connected to the Internet, clearly its reputation as a valuable advertising and information resource is quickly spreading; the Internet is well on its way to becoming the backbone of the Information Age.

However, a roadblock to widespread adoption of the Internet is its painfully slow access time to PCs via the telephone modem. The slow response, and consequent user frustration, has slowed market growth and prevented the Internet from becoming an indispensable information tool for the average home consumer. The cable network industry has seen this as an opportunity to generate additional revenue by utilizing their vast cable plant resources, and 1-GHz network bandwidth, to provide higher-speed interactive data services to homes, institutions, and businesses. The major cable industry multi-system operators (MSOs) have announced their intentions to have cable modem service fully deployed by 1998.

As originally designed, the typical CATV cable plant was intended for one-way delivery of high-quality television signals to the home. The prospect of offering cable modems and other interactive video services has required the system owners to upgrade their plants by providing bidirectional signal capability. This has entailed the installation of a bidirectional hybrid-fiber coax trunk and 2-way line amplifiers. It is estimated that approximately 20% of the existing CATV plants have already been upgraded to full bidirectional capability. This would mean that some 20 million US homes and businesses could take advantage of bidirectional cable service.

What are the winning advantages of Internet access via cable modems and the CATV network over the prevailing telephone modem connection? First, the cable modem operates in a burst mode; this means that, while it remains physically connected to the cable plant, it only uses network resources when it transmits a burst of data. This allows the cable modem to be effectively always "signed on" to the Internet and ready for instant two-way data transfer. To accomplish this with a telephone modem would require a dedicated phone line which leads to the next key advantage of cable modems: the cable modem does not tie up a phone line while the user is "surfing the 'net". With telephone modem access, unless there is a dedicated phone line, normal telephone service is suspended during Internet sessions.

Another advantage of cable modems is the dramatically increased speed of data delivery. Cable modems are capable of up to 36 Mb/s downstream data rates and 10 Mb/s upstream, compared to the standard telephone modem service of 28 kb/s up and downstream (56 kb/s max). This many-fold increase in data-transmission speed means that the Internet access speed will be generally limited by URL file servers rather than the modem baud rate. This is especially important when the user is downloading large graphic, video, or image files. A file that takes 8 minutes to download via a 28.8-kb/s telephone modem takes 8 seconds via cable modem. This increased access speed will unleash the true power of the Internet's imaging potential.

The cable industry would prefer that cable modems for Internet surfing become "off-the-shelf" items, purchased and maintained by the consumer, very much like telephone modems. To this end, cable modems would need to be interoperable, which means that a given cable modem will work in different cable systems, with different-vendors' head-end equipment. To achieve interoperability of cable modems, universal standards are required—and indeed, they are emerging. The Multimedia Cable Network Systems (MCNS) group has issued their "Data over cable services interface specifications" for interactive communications via the HFC network. The MCNS standards have been endorsed by many of the larger cable MSOs as their working standard. The IEEE 802.14 committee is also developing a set of standards for HFC cable networks, and the DAVIC and DVB standards have been released and are being deployed in Europe. For cable telephony, however, proprietary algorithms are employed for upstream/downstream transmissions, and interoperability is not a concern.

The basic cable modem consists of an RF receiver and transmitter physical layer, the PHY, that modulates/demodulates the data, and4 Analog Dialogue 31-3 (1997) a media access controller, the MAC, that performs the master system control function. When the standards are fully deployed, the downstream data delivery will take place in the 42-850 MHz band with existing 6-MHz CATV network channel spacing. The downstream digital modulation format will be 64-QAM (quadrature amplitude modulation), with a future migration to 256-QAM. The HFC data delivery system will be asymmetric; the data rate will be faster downstream than upstream. This is generally compatible with Internet surfing applications, since typical http navigation calls for much more data to be sent down to the computer than up to the network.

The upstream transmit path, required when using cable modems, is the major new requirement that has been placed on the CATV plant. The bandwidth that has been allocated for the return-path function by the cable industries is 5-42 MHz in the USA, and 5-65 MHz in Europe. This particular bandwidth is expected to contain substantial amounts of impulse noise, or &quto;ingress", which will make reverse path communication difficult. Initially, a relatively simple modulation format, quadrature phase-shift keying (QPSK), is being utilized by most cable modem vendors. In the future, as the cable plant environment is further upgraded and improved, there will be a movement to a 16-QAM upstream modulation format to increase the bits/Hz efficiency of the upstream data transmission.

Some of the technologically and market-driven requirements for the upstream transmitter (Tx) section of a cable modem are:

  • Output frequency agility with digital control
  • Full digital control of modulation and output power parameters
  • High spurious-free dynamic range (SFDR) on the modulated output carrier
  • Integrated digital signal processing with a high level of functionality
  • Low cost
  • Low power
Analog Devices is in a unique position to supply an optimum silicon solution for the upstream Tx requirement; it falls squarely in the domain of ADI's mixed-signal and linear core competencies. The AD9853/AD8320 upstream-Tx chipset*, available now, integrates the high-speed digital and analog blocks that provide a complete ASIC solution for the HFC upstream transmitter requirement. The AD9853 is a modulator function that has been specifically defined to meet the requirements of both interoperable and proprietary implementations of the HFC upstream function. The AD8320 is a companion cable driver amplifier, with a digitally programmable gain function; matched to the output of the AD9853 modulator, it directly drives the cable plant with the modulated carrier. Together, the AD9853 and AD8320 fully meet the HFC return-path requirement.

The AD9853 CMOS digital modulator combines high-speed conversion, direct digital synthesis, and digital signal processing technologies. The modulator architecture is digital throughout, which provides definite advantages in I/Q channel phase- and amplitude matching, and long-term modulator stability. The AD9853 is programmed and controlled via a serial control bus that is I 2 C compatible. The basic modulator block consists of an input channel encoder which formats the input data stream into the desired bit-mapped constellation and modulation format. The data stream is demultiplexed into I/Q channel data paths that are individually FIR-filtered to provide the desired pulse response characteristic for controlled output burst ramping. Then interpolating filter stages are used to match the effective output data rate of the FIR filters to the output sampling frequency of the direct digital synthesizer (DDS) for frequency upconversion. The AD9853 employs a state-of-the-art DDS function to generate precise sine and cosine digital waveforms to mix with the pulse-shaped data bitstream in a high-speed mixer stage, and create the 5-42 MHz modulated carrier. The DDS is also responsible for making the device highly frequency-agile; its 32-bit tuning word capability enables the modulated carrier at the output to be tuned with a resolution of 0.029 Hz.

A high-speed adder stage sums the upconverted digital I and Q data to create a single data path, which is ready to be converted into the analog domain by a high-speed 10-bit D/A converter. A SINC filter is utilized to "pre-compensate" the data stream for the sinx/x roll-off of a high-speed D/A converter's quantized output function. The patented architecture of the AD9853's CMOS D/A converter stage, with a 55-dB SFDR at 40 MHz Aout, rivals the performance afforded by expensive and power-hungry bipolar DACs.

A key system cost-saving feature in the AD9853 is its ´6 reference-clock multiplier circuitry, which essentially allows the AD9853 to generate the high-speed clock for the DDS synthesizer internally, saving the user the expense and system design difficulty of implementing an external 122-MHz reference clock (160-MHz clock for 65-MHz carrier applications). The SFDR specification is achieved with the low-jitter clock multiplier circuitry enabled. Additional programmable functions that support the requirements of HFC 2-way communication applications include forward error correction, data scrambling, and preamble word insertion. These are functions specified for successful burst packet data transmission in interoperable implementations of cable modems. The AD9853 also includes an output serial-data control function for interfacing directly to the AD8320 cable-driver amplifier. This control function allows the AD9853 to enable the AD8320 automatically at the appropriate time in a burst transmission sequence and allows the cable modem's MAC function to control the output power of the modem via the AD9853's control bus.

The AD9853 modulator output is connected to the input of the AD8320 programmable cable driver amplifier through an external low-pass filter, which is necessary to suppress the aliased images that are generated by the DAC's sampled output. The first aliased image occurs at Fsampling–Fout, which necessitates a fairly sharp-cutoff low-pass filter function. An inexpensive 7-pole elliptical low-pass passive 75-ohm LC filter can be implemented between the AD9853 and AD8320 to suppress the output aliases sufficiently for the HFC network application.

The AD8320 is a digitally-programmable cable driver amplifier (using a bipolar IC process) that directly interfaces to the 75-W cable plant. It provides 36 dB of programmable gain range with a maximum power output level >18 dBm (6.2 V) into a 75-W load. The gain of the AD8320 is controlled via an 8-bit SPI serial control word. The AD8320 accomplishes programmable gain control with a bank of 8 binary weighted transconductance (gm) stages, which are connected in parallel to their respective load resistors. The total attentuation of the core is determined by the combination of gm stages selected by the data latch. The eight gm stages, with their 256 levels of attenuation, provide a linear gain function with a dynamic range of 36 dB (@64 V/V full scale).

The AD8320's harmonic distortion is typically –57.2 dB for a 42-MHz output and –54 dB for 65-MHz output, at an output power level of 12 dBm into 75 W. This dynamic performance supports the requirements of cable telephony and data services over the HFC network. The AD8320's output stage has a dynamic output impedance of 75 W. This allows for direct single-ended connection of the device output to the CATV plant without back-termination, retaining the 6 dBm of load power that would be lost using the 75-W back-termination resistor required by the traditional low-output-Z driver amplifier. In fact, the AD8320 maintains 75-W impedance at its output during device power down to minimize glitches during transitions. This helps minimize line reflections and insures proper filter operation for any forward mode device sharing the cable connection. Another advantage of the dynamic 75-W output impedance is that it saves cost significantly by eliminating an expensive GaAs switch, which would otherwise be required to minimize transitional glitches.

The AD9853/AD8320 chipset combination offers the highest dynamic performance available from an integrated chipset for the HFC upstream Tx function. As Figure 5a shows, the chipset will typically deliver a signal to the cable plant's diplexer filter with <50 dB spur rejection for a 42-MHz 16-QAM-modulated carrier. Figures 5b & c show a typical eye diagram and constellation for a 16-QAM modulated carrier; the chipset delivers error-voltage magnitude (EVM) performance of <2%. I/Q phase imbalance is typically less than 10, due to the all-digital modulator scheme. Evaluation is facilitated by available board, the AD9853-45PCB, which includes AD9853, AD8320, and a 45-MHz LP filter.

To summarize, the upstream transmitting chipset, with its high level of functional integration and state-of-the-art mixed-signal technology, today offers an effective silicon solution for the two-way HFC network, to help usher in the next wave of information resources for the home consumer. Developments to look forward to include compact downstream tuners and demodulators, and— ultimately—a single-chip complete cable modem solution.