AD9754ARURL7 datasheet(18/24 Pages) AD

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AD9754

–18–

REV. A

APPLICATIONS

VDSL Applications Using the AD9754

Very High Frequency Digital Subscriber Line (VDSL) technol-

ogy is growing rapidly in applications requiring data transfer

over relatively short distances. By using QAM modulation and

transmitting the data in multiple discrete tones, high data rates

can be achieved.

As with other multitone applications, each VDSL tone is ca-

pable of transmitting a given number of bits, depending on the

signal to noise ratio (SNR) in a narrow band around that tone.

The tones are evenly spaced over the range of several kHz to

10 MHz. At the high frequency end of this range, performance

is generally limited by cable characteristics and environmental

factors, such as external interferers. Performance at the lower

frequencies is much more dependent on the performance of the

components in the signal chain. In addition to in-band noise,

intermodulation from other tones can also potentially interfere

with the recovery of data for a given tone. The two graphs in

Figure 35 represent a 500 tone missing bin test vector, with

frequencies evenly spaced from 400 Hz to 10 MHz. This test is

very commonly done to determine if distortion will limit the

number of bits which can transmitted in a tone. The test vector

has a series of missing tones around 750 kHz, which is represented

in Figure 35a, and a series of missing tones around 5 MHz,

which is represented in Figure 35b. In both cases, the spurious

free range between the transmitted tones and the empty bins is

greater than 60 dB.

FREQUENCY – Hz

–30

–50

–110

–70

–90

600k

800k

1.0M

–40

–60

–80

–100

Figure 35a. Notch in missing bin at 750 kHz is down

>60 dB. Peak amplitude = 0 dBm.

FREQUENCY – MHz

–30

–50

–110

–70

–90

4.8

5.0

5.2

–40

–60

–80

–100

Figure 35b. Notch in missing bin at 5 MHz is down

>60 dB. Peak amplitude = 0 dBm.

CDMA

Carrier Division Multiple Access, or CDMA, is an air transmit/

receive scheme where the signal in the transmit path is modu-

lated with a pseudorandom digital code (sometimes referred to

as the spreading code). The effect of this is to spread the trans-

mitted signal across a wide spectrum. Similar to a DMT wave-

form, a CDMA waveform containing multiple subscribers can

be characterized as having a high peak to average ratio (i.e.,

crest factor), thus demanding highly linear components in the

transmit signal path. The bandwidth of the spectrum is defined

by the CDMA standard being used, and in operation is imple-

mented by using a spreading code with particular characteristics.

Distortion in the transmit path can lead to power being trans-

mitted out of the defined band. The ratio of power transmitted

in-band to out-of-band is often referred to as Adjacent Channel

Power (ACP). This is a regulatory issue due to the possibility of

interference with other signals being transmitted by air. Regula-

tory bodies define a spectral mask outside of the transmit band,

and the ACP must fall under this mask. If distortion in the

transmit path cause the ACP to be above the spectral mask,

then filtering, or different component selection is needed to

meet the mask requirements.

Figure 36 shows an example of the AD9754 used in a W-CDMA

transmitter application using the AD6122 CDMA 3 V transmit-

ter IF subsystem. The AD6122 has functions, such as external

gain control and low distortion characteristics, needed for the

superior Adjacent Channel Power (ACP) requirements of

WCDMA.

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