REV. B

–14–

AD768

AD768 IN MULTITONE TRANSMITTERS (FOR ADSL)

Communications applications frequently require aspects of

component performance that differ significantly from the

simple, single tone signals used in typical SNR and THD tests.

This is particularly true for spread-spectrum and frequency divi-

sion multiplexed (FDM) type signals, where information con-

tent is held in a number of small signal components spread

across the frequency band. In these applications, a combination

of wide dynamic range, good fine-scale linearity, and low inter-

modulation distortion is required. Unfortunately, a part’s full

scale SNR and THD performance may not be a reliable indica-

tor of how it will perform in these multitone applications.

One example of an FDM communications system is the DMT

(discrete multitone) ADSL (Asymmetrical Digital Subscriber

Line) standard currently being considered by ANSI. Figure 33

shows a block diagram of a transmitter function.

The digital bits are used to QAM modulate each of approxi-

mately 200 discrete tones. An inverse FFT turns this modu-

lated frequency domain information into 512 time points at a

2.2 MSPS sample rate. These time points are then put through

an FIR interpolation filter to upsample (in this case to 4.4 MSPS).

The bit stream is run through the AD768, which is followed by

a 4th order analog smoothing filter, then run to the line-driving

circuitry

TO

TRANSMITTER

BIT

STREAM

QAM

ENCODER

256

MODULATED

FREQUENCY

BINS

512 TIME

POINTS

@ 2.2MSPS

1024 TIME

POINTS

@ 4.4MSPS

4TH ORDER

SMOOTHING

FILTER

AD768

+BUFFER

2X

INTERPOLATOR

FIR

INVERSE

FFT

Figure 33. Typical DMT ADSL Transmit Chain

Figure 34a shows a frequency domain representation of a test

vector run through this system, while 34b shows the time do-

main representation. (Clearly the frequency domain picture is

more informative.) We wish to optimize the SINAD of each

4 kHz frequency band: this is a function of both noise

(wideband and quantization) and distortion (simple harmonic

and intermod).

FREQUENCY – Hz

0

–20

–40

–60

0

1.1M

–80

Figure 34a. Output Spectrum of ADSL Test Vector

TIME – 25µs/DIV

1.5

1.0

0.5

0

–0.5

–1.0

–1.5

Figure 34b. Time Domain Output Signal of ADSL Test

Vector

Table I and II show the available SNR and THD at the output

of the filter vs. frequency bin for the ADSL application. The

AD768’s combination of 16-bit dynamic range and 14-bit lin-

earity provides excellent performance for the DMT signal. Its

fast input rate would support even faster rates of oversampling,

if one were interested in trading off digital filter complexity in

the interpolator for a simplified analog filter.

Table I. SNR vs. Frequency

Frequency

SNR

151 kHz

70.1 dB

349 kHz

69.7 dB

500 kHz

69.4 dB

1 MHz

69.8 dB

Table II. THD vs. Frequency

Frequency

THD

160 kHz

–68.9 dBc

418 kHz

–64.0 dBc

640 kHz

–64.3 dBc

893 kHz

–63.8 dBc