AD9763/AD9765/AD9767
Data Sheet
between the two baseband channels. A quadrature mixer
QUADRATURE AMPLITUDE MODULATION (QAM)
EXAMPLE USING THE AD9763
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.
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
(that is, CDMA) based systems. A QAM signal is a carrier
frequency that is modulated in both amplitude (that is, AM
modulation) and phase (that is, 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 component 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 carrier frequency.
10
DAC
DSP
0°
90°
CARRIER
FREQUENCY
TO
MIXER
OR
Σ
ASIC
10
DAC
NYQUIST
FILTERS
QUADRATURE
MODULATOR
Figure 83. 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 84 helps improve the
matching between the I and Q channels, and it shows a path for
upconversion using the AD8346 quadrature modulator. The
AD9763 provides both I and Q DACs a common reference that
improves the gain matching and stability. RCAL can be used to
compensate for any mismatch in gain between the two channels.
The mismatch can be attributed to the mismatch between RSET1
and RSET2, the effective load resistance of each channel, and/or
the voltage offset of the control amplifier in each DAC. The
differential voltage outputs of both DACs in the AD9763 are
fed into the respective differential inputs of the AD8346 via
matching networks.
A common and traditional implementation of a QAM modulator
is shown in Figure 83. 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
component’s spectral envelope while minimizing intersymbol
interference. The DAC is typically updated at the QAM symbol
rate, or at a multiple of the QAM symbol rate 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
AVDD
ROHDE & SCHWARZ
FSEA30B
OR EQUIVALENT
0.1µF
DCOM1/ DVDD1/
DCOM2 DVDD2
ACOM AVDD
RA
RB
RA
SPECTRUM ANALYZER
RL
LA
RL
VPBF
I
I
A
B
BBIP
BBIN
OUT
TEKTRONIX
AWG2021
WITH
I DAC
LATCH
I
VOUT
CA
CB
DAC
RB
OPTION 4
+
OUT
LA
LA
RL
RL
RL
RL
CB
AD9763/
AD9765/
AD9767
WRT1/IQWRT
LOIP
LOIN
CLK1/IQCLK
RA
RA
PHASE
SPLITTER
BBQP
BBQN
I
I
A
B
RB
OUT
Q DAC
LATCH
Q
DAC
CA
C
FILTER
RB
RL
OUT
LA
RL
WRT2/IQSEL
AD8346
VDIFF = 1.82V p-p
SLEEP
MODE FSADJ1
FSADJ2 REFIO
0.1µF
DIFFERENTIAL
RLC FILTER
ROHDE & SCHWARZ
SIGNAL GENERATOR
RL = 200Ω
RA = 2500Ω
RB = 500Ω
RP = 200Ω
CA = 280pF
CB = 45pF
LA = 10µH
256Ω
22nF
256Ω
22nF
2kΩ
2kΩ
20kΩ
20kΩ
AVDD
RA
AD976x
AD8346
I
= 11mA
RL
RB
OUTFS
AVDD = 5.0V
VCM = 1.2V
NOTES
1. DAC FULL-SCALE OUTPUT CURRENT = I
V
MOD
.
OUTFS
2. RA, RB, AND RL ARE THIN FILM RESISTOR NETWORKS
WITH 0.1% MATCHING, 1% ACCURACY AVAILABLE
FROM OHMTEK ORNXXXXD SERIES OR EQUIVALENT.
0 TO I
OUTFS
V
DAC
Figure 84. Baseband QAM Implementation Using an AD9763 and an AD8346
Rev. G | Page 32 of 44
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