Datasheet

AD9709
Rev. B | Page 22 of 32
APPLICATIONS INFORMATION
QUADRATURE AMPLITUDE MODULATION (QAM)
USING THE AD9709
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.
A common and traditional implementation of a QAM
modulator is shown in Figure 43. 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 between the two baseband channels. A quadrature
mixer modulates the I and Q components with the in-phase and
quadrature carrier frequencies and then sums the two outputs
to provide the QAM signal.
QUADRATURE
MODULATOR
DAC
8
8
DAC
CARRIER
FREQUENCY
NYQUIST
FILTERS
TO
MIXER
DSP
OR
ASIC
Σ
0°
90°
00606-044
Figure 43. 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 44 helps improve
the matching between the I and Q channels, and it shows a path
for upconversion using the AD8346 quadrature modulator. The
AD9709 provides both I and Q DACs with a common reference
that will improve the gain matching and stability. R
CAL
can be
used to compensate for any mismatch in gain between the two
channels. The mismatch may be attributed to the mismatch
between R
SET1
and R
SET2
, 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
AD9709 are fed into the respective differential inputs of the
AD8346 via matching networks.
I
OUT
A
I
OUT
B
I
OUT
A
I
OUT
B
DCOM1/
DCOM2
SLEEP
DVDD1/
DVDD2
AVDD
VPBF
BBIP
BBIN
BBQP
BBQN
LOIP
LOIN
VOUT
WRT1/IQWRT
FSADJ1
ACOM
+
SPECTRUM ANALYZER
AD8346
CLK1/IQCLK
PORT Q PORT I
DIGITAL INTERFACE
I
DAC
WRT2/IQSEL
FSADJ2MODE REFIO
C
FILTER
VDIFF = 1.82V p-p
RLRL
RB
RB
RB
RL
RL
RL
RL
LA
LA
LA
LA
RL
CA
CA
RB
RA RA
RA
AD9709
RL
RB
RA
0 TO I
OUTFS
AD8346
AVDD
AD976x
A
VDD
TEKTRONIX
AWG2021
WITH
OPTION 4
I DAC
LATCH
Q DAC
LATCH
Q
DAC
2kΩ
20kΩ
0.1µF
NOTES
1. DAC FULL-SCALE OUTPUT CURRENT = I
OUTFS
.
2. RA, RB, AND RL ARE THIN FILM RESISTOR NETWORKS
WITH 0.1% MATCHING, 1% ACCURACY AVAILABLE
FROM OHMTEK ORNXXXXD SERIES OR EQUIVALENT.
V
MOD
V
DAC
DIFFERENTIAL
RLC FILTER
RL = 200Ω
RA = 2500Ω
RB = 500Ω
RP = 200Ω
CA = 280pF
CB = 45pF
LA = 10µH
I
OUTFS
= 11mA
AVDD = 5.0V
VCM = 1.2V
RL
CB
0.1µF
RA
CB
PHASE
SPLITTER
ROHDE & SCHWARZ
FSEA30B
OR EQUIVALENT
ROHDE & SCHWARZ
SIGNAL GENERATOR
00606-045
256Ω
22nF
2kΩ
20kΩ
256Ω
22nF
Figure 44. Baseband QAM Implementation Using an AD9709 and AD8346