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AD9752

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maintain optimum performance. Care should be taken to ensure

that the ground plane is uninterrupted over crucial signal paths.

On the digital side, this includes the digital input lines running

to the DAC as well as any clock signals. On the analog side, this

includes the DAC output signal, reference signal and the supply

feeders.

The use of wide runs or planes in the routing of power lines is

also recommended. This serves the dual role of providing a low

series impedance power supply to the part, as well as providing

some “free” capacitive decoupling to the appropriate ground

plane. It is essential that care be taken in the layout of signal and

power ground interconnects to avoid inducing extraneous volt-

age drops in the signal ground paths. It is recommended that all

connections be short, direct and as physically close to the pack-

age as possible in order to minimize the sharing of conduction

paths between different currents. When runs exceed an inch in

length, strip line techniques with proper termination resistor

should be considered. The necessity and value of this resistor

will be dependent upon the logic family used.

For a more detailed discussion of the implementation and

construction of high speed, mixed signal printed circuit boards,

refer to Analog Devices’ application notes AN-280 and

AN-333.

FREQUENCY – Hz

–30

–40

–100

600k

800k

–50

–60

–70

–80

–90

Figure 35a. Notch in Missing Bin at 750 kHz is Down

>60 dB. (Peak Amplitude + 0 dBm).

FREQUENCY – MHz

–30

–40

–100

5.15

4.85

–50

–60

–70

–80

–90

–110

Figure 35b. Notch in Missing Bin at 5 MHz is Down

>60 dB. (Peak Amplitude + 0 dBm).

APPLICATIONS

VDSL Applications Using the AD9752

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 be 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.

Using the AD9752 for Quadrature Amplitude Modulation

(QAM)

QAM is one of the most widely used digital modulation

schemes in digital communication systems. This modulation

technique can be found in FDM as well as spreadspectrum (i.e.,

CDMA) based systems. A QAM signal is a carrier frequency

that is modulated in both amplitude (i.e., AM modulation) and

phase (i.e., PM modulation). It can be generated by indepen-

dently modulating two carriers of identical frequency but with a

° phase difference. This results in an in-phase (I) carrier com-

ponent and a quadrature (Q) carrier component at a 90

° phase

shift with respect to the I component. The I and Q components

are then summed to provide a QAM signal at the specified car-

rier frequency.

A common and traditional implementation of a QAM modu-

lator is shown in Figure 36. The modulation is performed in the

analog domain in which two DACs are used to generate the

baseband I and Q components, respectively. Each component is

then typically applied to a Nyquist filter before being applied to

a quadrature mixer. The matching Nyquist filters shape and

limit each component’s spectral envelope while minimizing

intersymbol interference. The DAC is typically updated at the

QAM symbol rate or possibly a multiple of it if an interpolating

filter precedes the DAC. The use of an interpolating filter typi-

cally eases the implementation and complexity of the analog

filter, which can be a significant contributor to mismatches in

gain and phase between the two baseband channels. A quadra-

ture mixer modulates the I and Q components with in-phase

and quadrature phase carrier frequency and then sums the two

outputs to provide the QAM signal.

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