AD9760 datasheet(18/23 Pages) AD

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AD9760

–18–

REV. B

APPLICATIONS

Using the AD9760 for QAM Modulation

QAM is one of the most widely used digital modulation schemes

in digital communication systems. This modulation technique

can be found in both FDM spreadspectrum (i.e., CDMA) based

systems. A QAM signal is a carrier frequency that is both

modulated in amplitude (i.e., AM modulation) and in phase

(i.e., PM modulation). It can be generated by independently

modulating two carriers of identical frequency but with a 90

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

nent 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 56. 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 sums the two out-

puts to provide the QAM signal.

AD9760

CARRIER

FREQUENCY

MIXER

DSP

ASIC

NYQUIST

FILTERS

QUADRATURE

MODULATOR

Figure 56. Typical Analog QAM Architecture

In this implementation, it is much more difficult to maintain

proper gain and phase matching between the I and Q channels.

The circuit implementation shown in Figure 57 helps improve

on the matching and temperature stability characteristics be-

tween the I and Q channels. Using a single voltage reference

derived from U1 to set the gain for both the I and Q channels

will improve the gain matching and stability. Further enhance-

ments in gain matching and stability are achieved by using sepa-

rate matching resistor networks for both RSET and RLOAD.

Additional trim capability via RCAL1 and RCAL2 can be added to

compensate for any initial mismatch in gain between the two

channels. This may be attributed to any mismatch between U1

and U2’s gain setting resistor (RSET), effective load resistance,

(RLOAD), and/or voltage offset of each DAC’s control amplifier.

The differential voltage outputs of U1 and U2 are fed into their

respective differential inputs of a quadrature mixer via matching

Ω filter networks.

It is also possible to generate a QAM signal completely in the

digital domain via a DSP or ASIC, in which case only a single

DAC of sufficient resolution and performance is required to

reconstruct the QAM signal. Also available from several vendors

REFIO

FS ADJ

IOUTA

IOUTB

CLOCK

RSET

2k *

RCAL1

CLOCK

I-CHANNEL

50 **

RLOAD

50 **

RLOAD

NYQUIST

FILTER

AND MIXER

REFIO

FS ADJ

IOUTA

IOUTB

CLOCK

RSET

2k *

RCAL2

100

Q-CHANNEL

50 **

RLOAD

50 **

RLOAD

NYQUIST

FILTER

AND MIXER

0.1 F

REFLO

AVDD

* OHMTEK ORNA1001F

** OHMTEK TOMC1603-50F

Figure 57. Baseband QAM Implementation Using Two

AD9760s

are Digital ASICs which implement other digital modulation

schemes such as PSK and FSK. This digital implementation has

the benefit of generating perfectly matched I and Q components

in terms of gain and phase, which is essential in maintaining

optimum performance in a communication system. In this

implementation, the reconstruction DAC must be operating at a

sufficiently high clock rate to accommodate the highest specified

QAM carrier frequency. Figure 58 shows a block diagram of

such an implementation using the AD9760.

AD9760

LPF

MIXER

STEL-1130

QAM

COS

SIN

I DATA

Q DATA

CARRIER

FREQUENCY

STEL-1177

NCO

CLOCK

Figure 58. Digital QAM Architecture

AD9760 EVALUATION BOARD

General Description

The AD9760-EB is an evaluation board for the AD9760 10-bit

D/A converter. Careful attention to layout and circuit design,

combined with a prototyping area, allow the user to easily and

effectively evaluate the AD9760 in any application where high

resolution, high speed conversion is required.

This board allows the user the flexibility to operate the AD9760

in various configurations. Possible output configurations include

transformer coupled, resistor terminated, inverting/noninverting

and differential amplifier outputs. The digital inputs are designed

to be driven directly from various word generators with the on-

board option to add a resistor network for proper load termina-

tion. Provisions are also made to operate the AD9760 with

either the internal or external reference or to exercise the power-

down feature.

Refer to the application note AN-420, “Using the AD9760/

AD9760/AD9764-EB Evaluation Board,” for a thorough

description and operating instructions for the AD9760 evalua-

tion board.

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