*Authors include Adam Lapid, RF system development manager, Cable CommunicationsDick Hester, senior fellow, Broadband Silicon Technology Center
Michael Polley, DMTS and Manager of Wireless Broadband Architectures
Texas Instruments, Dallas and Tel Aviv, Israel
Cable modem, DSL and fixed wireless are the primary technologies driving new broadband applications and services in the home. While each technology utilizes a different medium for data delivery, all have one important thing in common. They provide the ultra high performance bandwidth delivery that is making the broadband dream an everyday reality.
Each broadband access technology uses the flexibility and power of the digital domain to achieve their ultra-high performance. Inevitably, however, any of these technologies must utilize the analog domain for the receive and transmit functionality needed traverse their respective medium.
The analog front end (AFE), which is made up of pure analog and mixed digital and analog circuitry, is responsible for several tasks. These include signal capture, analog domain filtering and handoffs to an analog to digital converter (ADC) in one direction as well as conversion from the digital to the analog domain by a digital to analog converter, analog filtering and power amplification in the other direction. With the complexities of the AFE and the multiple tasks it performs, a robust analog design is absolutely critical for proper modem operation.
This article will compare and contrast the analog front ends of the most common broadband access technologies: cable, DSL and fixed wireless. First, it will discuss general technical differences between the different access methods. Next, the article will provide an overview of a typical analog section for each technology, and will feature some innovative design strategies.
Despite the differences in how the broadband connection is delivered to the home, whether it's by copper wiring, coaxial cable or air, all broadband systems have certain elements in common. The chart below (Table1) provides an overview of the general broadband access technologies (cable, DSL and fixed wireless) including a look at the frequencies, modulations and duplexing schemes used with each technology.
Table 1.
Table 1: With different upstream and downstream data rates, broadband access systems are still dependent on analog.
While all of the broadband access technologies employ an AFE, the methods and critical requirements vary widely. This section of the article will provide a functional description of a typical analog section of each broadband technology -- cable, DSL and fixed wireless.
Pay heed to DOCSIS
The cable environment in general and compliance with DOCSIS cable modem specification in specific, create significant challenges for the analog and RF front end, including the tuner, ADC, DAC and the filtration scheme. Besides the technical challenges, which are presented below, there is an ongoing demand for the cable modem solution to be low cost, small and power efficient.
Figure 1.
DOCSIS cable modems utilize 256QAM decoding with an 800-MHz signal input.
Major challenges in cable modem design:
In the downstream section the front end must deal with a broadband input (800MHz), multi channel environment, high input power channels and high modulation level 256QAM. In addition DOCSIS stringent standard specification for: Es/No, ACP, image rejection, RSSI, must be met. The above environment and standard requirements create stringent requirements on the following impairments: phase noise, thermal noise, THD, IMD, CTB, CSO, XMOD, aliasing, images, multi paths and group delay variation.
In the upstream section DOCSIS Stringent standard specification for spectral emissions, during transmission and between transmissions, high output power +58dBmV for a QPSK signal, large Tx power range (50dB), impacts the DAC, PGA, filtration scheme, and board layout. The composite multiplexing scheme in the upstream include A-TDMA, S-CDMA and FDM configurations, resulting in complex testing scenarios and setups.
Downstream path
The tuner is the heart of any AFE for the more than 300 million broadband systems operating in the television band. In end equipment such as cable set top boxes, cable modems, cable telephony systems, WebTVs and PC/TVs, the core function of the tuner is to receive all available channels in the input bandwidth, select a desired channel and reject all others and to translate the desired channel to a standard intermediate frequency (IF).
Functions in the downstream path include a transformer that converts the single-ended coax input to a differential tuner input. The tuner then converts the selected frequency to a fixed intermediate frequency. The tuner can be of any topology, although double conversion has an added value in its image rejection, flat frequency response, and option to integrate into silicon. The tuner consists of the RF AGC stage, which seeks to control the power passed on to the rest of the RF chain while maintaining the optimum balance between linearity and noise figure. The first SAW shown in the diagram is a low-cost image rejection filter, while the second is a channel selection filter that rejects potentially high adjacent channels and allows optimization to the ADC input. The IF (intermediate frequency section) AGC is used to complement the RF AGC, it maintains a fixed-level at the ADC's input, while optimizing linearity and noise.
Upstream path
As shown in figure 1, the LPF performs reconstruction filtering of the DAC's output and is designed to reject the sampled signal's images. The programmable gain amplifier varies the Tx power by 50 dB minimum, while maintaining spectral purity requirements, and the transformer converts differential PGA output to the single-ended coax input.
As the cable industry moves from DOCSIS 1.1 to DOCSIS 2.0 there is a need for increased signal integrity within the transmitter to accommodate the higher order modulation methods employed in DOCSIS 2.0. Additionally, the spurious emissions requirements are more stringent.
Advanced strategies for DSL AFEs
The core of every DSL modem's AFE, whether it is in the central office or customer premises equipment, is made up of a transmitters and receivers (Fig. 2). A transmitter consists of a digital to analog converter (DAC), analog filters and line drivers. Receivers consist of preamp filters, a programmable gain amplifier and an analog to digital converter (ADC). Transmitters are designed to shape spectral density with two criteria in mind -- compliance with ITU limits and minimization of the corrupting effect of the transmitter echo on the receiver SNR. Receivers are designed to account for a wide range of signal and echo power encountered when connect to a range of subscriber loops.
Figure 2.
Though a hybrid line interface isolates AFE components, high voltage protection is still recommended.
In transmitters, the goal is to implement a design that minimizes the computational burden of the DSP by converting the minimum-sized IFFT output to the analog domain without significant pass-band droop. It also shapes the out-of-band psd to comply with either the ITU standard or the FDD-based modem receiver constraint when applicable. A typical approach to achieving this objective is by interpolating the sample rate of the minimum IFFT output. The benefits of this approach include low demand on the DSP, minimization of the I/O data transfer between the DSP and CODEC, and the opportunity to perform the psd shaping in the digital interpolation filter, which reduces the order of analog filter. This typical transmitter signal path consists of the digital interpolation filter, an oversampled DAC and a low-order continuous-time analog filter.
The receiver's job is to eliminate the echo as much as possible without distorting signal and amplify the signal to use as much of the ADC input signal range as possible without clipping. It does this by employing a hybrid circuit that subtracts a replica of the transmitted signal and by filtering the echo energy that is not in the receiver frequency band. Once the echo energy is minimized the programmable gain amplifier is set to amplifier the remainder and "fill" the ADC input range.
Partitioning
In most cases, the analog front end (AFE) of a central office ADSL modem is partitioned into two technologies. Data converters, analog filters, and (occasionally) receive amplifiers are fabricated on a 3.3V or 5V mixed-signal CMOS, while the rest of the design uses a higher voltage bipolar process. One alternative for advanced DSL AFE design is partitioning that integrates the analog filters and receiver amplifiers with the line driver in 15V dielectrically isolated bipolar technology.
The transmitter signal is capacitively coupled to the AFE, providing a first order, high-pass filter stage where corner frequency precision is determined by the precision of the external capacitor and the six percent accurate laser-trimmed inputs resistance of the 17.4 dB gain stage. Following the gain stage is an untrimmed, third-order Chebyshev low-pass filter with a nominal 1.3 MHz corner frequency and 0.25 dB pass-band ripple. Between the low-pass and the line driver, there is a laser trimmed second order high-pass. The collective transfer function of this and the first order high-pass input to the AFE is a third order Chebyshev high-pass with 155kHz corner frequency and 0.25dB pass-band ripple. The synthesized impedance line driver is the final element in the transmitter signal path. Driving the transformer-coupled subscriber loop through an external current sensing impedance, it is capable of sourcing roughly 400mA into a 30 ohm load.
The high-pass filter minimizes the DAC quantization noise that echos into the receiver passband. The low-pass filter eliminated the signal images produced by the DAC centered at the DAC sample rate for psd standards compliance. The synthesized impedance line driver creates the proper line termination without excessive transmitted signal attenuation, thus enabling low line driver power dissipation.
Integration
In addition to the partitioning example above, integration plays a key role in the success of the AFE, and ultimately the end system. One way Texas Instruments is working to impact integration, and thus density, is by combining high-speed line drivers and receiver amplifiers on the same chip and by integrating several gain resistors, filters and other passive components. Through this integration, the TI AFE cuts the number of discrete components typically required in half. The practical result in central office systems such as DSL access multiplexers (DSLAMs) or digital loop carriers (DLCs) is an increase in line card density from four or eight to 16 to 64 channels. This greatly saves floor space for DSL service providers and reduces overall power consumption by 20 percent.
Fixed Wireless
The fixed-wireless AFE translates between wireless spectrum used for over-the-air data transmission and the digital transceiver functions. Typical fixed-wireless installations employ one or more directional antennas that collect/radiate energy in a specified bandwidth, such as the 2.4 GHz ISM band or the 5.7 GHz U-NII band. The AFE is responsible for subchannel band selection, conversion to/from RF frequencies, filtering to prevent adjacent channel interference, and analog-to-digital and digital-to-analog conversion.
Figure 4 illustrates a typical fixed-wireless AFE design. The diplexor in this system separates transmitted and received data into different subchannel bandwidths in a frequency-division duplexed fashion.
Figure 3.
The diplexor in a fixed wireless access system separates transmitted and received data into different subchannel bandwidths in a frequency-division duplexed fashion.
Receive path
In this design, both transmit and receive signals are present at the same time on the antenna elements. However, they occupy different subchannel bandwidths according to a frequency-division duplex (FDD) band plan. In the receive path, the signal from the antenna is passed through the diplexor which eliminates the transmitted signal, leaving only the received signal. The two-stage downconversion tuner translates from RF frequencies to IF, selects the particular subchannel band of interest and performs SAW filtering, and then translates from IF frequencies down to low-IF. A second SAW filter further rejects adjacent channel interference. A VGA adjusts signal level to optimize signal levels, and then an oversampled differential analog-to-digital converter captures the passband (low-IF) signal. Digital filtering and downsampling is applied after the A/D converter to translate the low-IF signal to a complex baseband signal. Based on the received signal power level, the modem determines proper settings for the AGC2 setting in the RF section and AGC1 in the low-IF section to maximize receiver signal-to-noise ratio.
Transmit path
The complex baseband signal from the digital transmitter is passed through upsampling and digital filtering to produce a real signal at low-IF frequencies. The upsampled bandpass signal is passed through a differential digital-to-analog converter followed by a lowpass filter to reject digital images. A PGA adjusts the signal level before band selection and upconversion to RF frequencies. A PGA further adjusts transmit signal power, allowing for power control adjustments based on expected attenuation experienced in the path to the far-end receiver (this is a system level power control loop). A blanking circuit is employed to ensure that during quiet periods of no transmission, the transmitter does not leak interference. Finally, the diplexor places the transmit signal onto the antenna elements (in co-existence with the received signal).
Optional duplexing mode
Fixed-wireless communication can also be implemented with a time-division duplexed (TDD) mode of operation where a transceiver is either transmitting or receiving at any point in time, but never doing both simultaneously. Under these conditions, the system can use one subchannel bandwidth for both transmit and receive. For a TDD AFE, the diplexor and blanking circuit in the figure discussed above are essentially replaced with an RF switch to either connect the transmit path or receive path to the antenna elements. Determination of which path to connect when is determined by higher-level system protocols.
Throughout this article, we have shown that each broadband system has analog and mixed-signal components in different combinations and configurations. All three systems cited in this article use D/A and A/D converters, a crucial part of any broadband or communications application. Cable and fixed wireless have similarities in their architectures, each employing a tuner and diplexer. Fixed wireless, however, also includes additional components (an upconverter and blocking circuit) to connect to the antenna elements. In addition to the DACs and ADCs, DSL uses filters and line drivers within the transmitter to shape spectral density while receivers account for a wide range of signal and echo power encountered on the local loop.
As the broadband market continues to grow, the AFE is evolving to meet the new requirements, standards and features of the cable modem, DSL and fixed wireless industries. AFEs are crucial components of the overall broadband solution but require digital elements to create a robust, fully functioning system. TI integrates the digital and analog signal processing with software to provide complete platforms for current and next generation broadband products.
