AD9709 datasheet(16/27 Pages) AD

16 / 27 page

REV. 0

AD9709

–16–

APPLICATIONS

Using the AD9709 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.

DAC

CARRIER

FREQUENCY

MIXER

NYQUIST

FILTERS

QUADRATURE

MODULATOR

DAC

DSP

ASIC

Figure 40. Typical Analog QAM Architecture

A common and traditional implementation of a QAM modula-

tor is shown in Figure 40. 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 typically

applied to a Nyquist ﬁlter before being applied to a quadrature

mixer. The matching Nyquist ﬁlters shape and limit each com-

ponents spectral envelope while minimizing intersymbol inter-

ference. 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 modu-

lates the I and Q components with the in-phase and quadrature

carrier frequency and then sums the two outputs to provide the

QAM signal.

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 41 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 AD9709 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 attributed

to the mismatch between RSET1 and RSET2, effective load resis-

tance of each channel, and/or the voltage offset of the control

ampliﬁer in each DAC. The differential voltage outputs of both

DACs in the AD9709 are fed into the respective differential

inputs of the AD8346 via matching networks.

I and Q digital data can be fed into the AD9709 in two different

ways. In dual port mode, The digital I information drives one

input port, while the digital Q information drives the other input

port. If no interpolation ﬁlter precedes the DAC, the symbol

rate will be the rate at which the system clock drives the CLK

and WRT pins on the AD9709. In interleaved mode, the digital

input stream at Port I contains the I and the Q information in

alternating digital words. Using IQSEL and IQRESET, the

AD9709 can be synchronized to the I and Q data stream. The

internal timing of the AD9709 routes the selected I and Q data

to the correct DAC output. In interleaved mode, if no inter-

polation ﬁlter precedes the AD9709, the symbol rate will be

half that of the system clock driving the digital datastream and

the IQWRT and IQCLK pins on the AD9709.

IOUTA

IOUTB

QOUTA

QOUTB

VMOD

AVDD

AD8346

AD976x

0 TO IOUTFS

VDAC

DCOM

FSADJI

REFIO

SLEEP

RSET

3.9k

0.1 F

DVDD

AVDD

0.1 F

VPBF

BBIP

BBIN

BBQP

BBQN

AD8346

LOIP

LOIN

VOUT

IQWRT

IQCLK

ACOM

AD9709

“I”

DAC

AVDD

CFILTER

DIFFERENTIAL

RLC FILTER

VDIFF = 1.82V p-p

“Q”

DAC

LATCH

PHASE

SPLITTER

ROHDE &

SCHWARZ

FSEA30B

SPECTRUM

ANALYZER

ROHDE &

SCHWARZ

SIGNAL

GENERATOR

PORT I

PORT Q

TEKTRONICS

AWG2021

W/OPTION 4

IQSEL

FSADJQ

RSET

3.9k

MODE

CB = 45pF

LA = 10 H

IOUTFS = 11mA

AVDD = 5.0V

VCM = 1.2V

NOTE:

RL = 200

RA = 2500

RB = 500

RP = 200

CA = 280pF

“Q”

DAC

“I”

DAC

LATCH

NOTE: DACs Full-Scale OUTPUT CURRENT = IOUTFS

RA, RB AND RL ARE THIN FILM RESISTOR NETWORKSWITH

0.1% MATCHING, 1% ACCURACY.

AVAILABLE FROM OHMTEK ORNXXXXD SERIES.

Figure 41. Baseband QAM Implementation Using an AD9709 and AD8346

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