AD9765AST datasheet(17/28 Pages) AD

17 / 28 page

REV. B

AD9765

–17–

FREQUENCY – MHz

4.85

4.90

4.95

5.00

5.05

5.10

5.15

–90

–80

–70

–60

–50

–40

–30

–100

–120

–110

Figure 40b. Notch in Missing Bin at 5 MHz Is Down

>60 dB (Peak Amplitude = 0 dBm)

Using the AD9765 for Quadrature Amplitude Modulation

QAM is one of the most widely used digital modulation schemes

in digital communications systems. This modulation technique

can be found in FDM as well as spread spectrum (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 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 modula-

tor is shown in Figure 41. The modulation is performed in the

analog domain in which two DACs are used to generate the

baseband I and Q components. Each component is then typi-

cally 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 inter-

symbol 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 typically

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 quadrature mixer

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

quadrature carrier frequency and then sums the two outputs to

provide the QAM signal.

DAC

CARRIER

FREQUENCY

MIXER

NYQUIST

FILTERS

QUADRATURE

MODULATOR

DAC

DSP

ASIC

Figure 41. 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 42 helps improve

upon the matching between the I and Q channels, as well as

showing a path for up-conversion using the AD8346 quadrature

modulator. The AD9765 provides both I and Q DACs as well

as a common reference that will improve the gain matching and

stability. RCAL can be used to compensate for any mismatch in

gain between the two channels. The mismatch may be attrib-

uted to the mismatch between RSET1 and RSET2, effective load

resistance of each channel, and/or the voltage offset of the con-

trol amplifier in each DAC. The differential voltage outputs of

both DACs in the AD9765 are fed into the respective differen-

tial inputs of the AD8346 via matching networks.

IOUTA

IOUTB

QOUTA

QOUTB

0.1 F

AVDD

AD976X

0 TO IOUTFS

VDAC

VMOD

AD8346

DCOM

FS ADJ I

TEKTRONICS

AWG2021

W/OPTION 4

0.1 F

DVDD

AVDD

RSET

3.9k

VPBF

BBIP

BBIN

BBQP

BBQN

AD8346

LOIP

LOIN

VOUT

PHASE

SPLITTER

CFILTER

IQSEL

SLEEP

“I” DAC

LATCH

ACOM

AD9765

IQCLK

IQWRT

“Q” DAC

LATCH

“I”

DAC

“Q”

DAC

RSET

3.9k

FS ADJ Q

REFIO

RHODE & SCHWARZ

SIGNAL GENERATOR

RHODE &

SCHWARZ

RFSEA30B

SPECTRUM

ANALYZER

DAC'S FULL-SCALE OUTPUT CURRENT = IOUTFS

NOTE: RA, RB, AND RL ARE THIN FILM RESISTOR NETWORKS

WITH 0.1% MATCHING, 1% ACCURACY AVAILABLE FROM

OHMTEK ORNXXXXD SERIES

RL = 200

RA = 2500

RB = 500

RP = 200

CA = 280pf

CB = 45pf

LA = 10 H

OUIFS = 11mA

AVDD = 5.0V

VCM = 1.2V

VDIFF = 1.82V p–p

DIFFERENTIAL

RLC FILTER

MODE

Figure 42. Baseband QAM Implementation Using an AD9765 and AD8346

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