by Tim Regan
Introduction
Consumer desire for faster Internet access is driving the demand for
very high data rate modems. A digital subscriber line (DSL)
implementation speeds data to and from remote servers with data rates of 512Kb/s
to 8Mb/s, much faster than current 56Kb/s modem alternatives.
This speed of data communication is providing the Internet with the
capability to transfer information in new formats such as full-motion video,
while greatly improving the timeliness of conventional information access.
One very important feature of DSL technology is that the connection
is handled through a normal telephone line; therefore, no special high
speed cables or fiber optic links are required and every home and office is
most likely DSL ready. Another feature is that the data interface can
operate simultaneously with normal voice communication over the same
telephone line. This allows the modem to be connected at all times and
not interfere with the use of the same line for normal incoming and
outgoing phone calls or faxes.
The real "magic" of DSL technology stems from the application of
digital signal processing (DSP) algorithms and data coding schemes. The
implementations have built-in intelligence to accommodate the wide
variations of data transmission signal conditions encountered with each
connection through the telephone switching network. Sophisticated ASICs have
been developed to provide small modems for PCs and handheld devices and
the ability to compact many DSL lines on a single PCB card for telephone
central-office deployment.
However, as is the case with almost any system, DSL still requires
fundamental operational amplifier functions to put the signal on to the phone
line and to pick off the small signals received at the other end.
Although many system designers are competent and comfortable with DSP
and all things digital, they often find their understanding of analog issues to
be a bit rusty when it comes to implementing the physical connection
to and from the telephone line. This series of articles will provide an
overview of the requirements placed on the amplifiers and provide
guidelines to component selection and the implications on distortion performance
and power consumption and dissipation, the most important system
issues related to the analog components.
Figure 1 shows a complete central office DSL line driver/receiver. This
is the basic circuit topology that provides differential transmit signal
drive to the line and detection of the differential received signal. The
full requirements of DSL are easily met by using devices from Linear
Technology's broad line of high speed power amplifiers for the driver and
high speed, low noise dual amplifiers for the receiver. Using either current
feedback or voltage feedback topologies, the family of drivers consists
of amplifiers with bandwidths from 35MHz to 75MHz, slew rates in
excess of 200V/µs with output current capability from 125mA to over 1
Amp. The receiver family combines similar high speed performance with
low noise, less than 10nV/(Hz1/2), and low quiescent operating current, less
than 10mA. The devices shown in Figure 1 are the LT1795 500mA output
current, 50MHz bandwidth dual op amp and the LT1361 50MHz dual
amplifier with input noise voltage of 9nV/(Hz1/2) and total supply current
of only 10mA.
Although there are several variations of DSL technology (SDSL,
HDSL, HDSL2, VDSL and ADSL, to name a few) the requirements placed on
the amplifiers for these different standards are very similar. The
major difference between the approaches, as they affect the line driver, is
the amount of power actually put on to the phone line by the
line-driver amplifier. For simplicity, these
articles will focus on the most recently approved standard, ADSL
(asymmetric DSL), but the concepts discussed apply equally to any of the
other standards.
This first installment will provide an overview of the requirements
of ADSL and how it is done, as well as a discussion of the circuit topology
and the requirements for the components used for implementation.
The full specifications for ADSL are contained in two ITU
(International Telecommunications Union) documents called G.992.1, for
systems often referred to as Full-Rate ADSL or G.dmt, and G.992.2, a lower data
rate approach often called G.Lite. Both systems use a technique
called discrete multitone, or DMT, for transmitting data. With DMT, a
frequency band up to 1.2MHz is split up into 256 separate tones (also call
subcarriers) each spaced 4.3125kHz apart. With each tone carrying
separate data, the technique operates as if 256 separate modems were
running in parallel. To further increase the data transmission rate, each
individual tone is quadrature amplitude modulated (QAM). As shown in
Figure 2, the data to be transmitted is used to create a unique
amplitude and phase-shift characteristic for each carrier tone, through the
combination of I and Q data, called a symbol. The symbols represented by each
tone are updated at a 4kHz rate or 4000 symbols per second. Full Rate
ADSL uses up to 15 bits of data to create each symbol. This results in a
theoretical maximum of 60Kb/s for each tone. If all 256 tones are used
in parallel, the total theoretical data rate can be as fast as 15.36Mb/s.
For G.Lite, only 8 bits are used per symbol with only half of the carrier
tones used for a theoretical maximum data rate of 4.096Mb/s.
In an actual DSL application, the tones are allocated for use
depending on the direction of communication, as shown in Figure 3. Most of
the tones are used for communication from the central office (CO) to an
end user's PC modem (often referred to as the CPE or customer
premises equipment). This direction of communication is called
"downstream." The direction of communication
from a PC modem to the central office (and, ultimately, to an Internet server)
is called "upstream." The use of more tones for the downstream
direction makes sense from an Internet-access point of view, because most
users download more information than they upload. Most upstream
communication with a server is simply to request information to be sent quickly
downstream. This difference in data rates up- and downstream is the
reason ADSL is called asymmetric DSL.
Figure 1. Central office ADSL transceiver
The Requirements for ADSL
Figure 2. Quadrature amplitude modulation
