AD9765AST(RevB) View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD9765AST Datasheet PDF : 28 Pages
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AD9765
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
4.85
4.90
4.95
5.00
5.05
5.10
5.15
FREQUENCY – MHz
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.
12
DAC
DSP
OR
ASIC
12
CARRIER
FREQUENCY
DAC
0
Σ
90
TO
MIXER
NYQUIST
FILTERS
QUADRATURE
MODULATOR
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.
TEKTRONICS
AWG2021
W/OPTION 4
IQWRT
IQCLK
DCOM DVDD
AVDD
ACOM AVDD
“I†DAC
LATCH
RL
IOUTA
LA
“Iâ€
DAC
CA
IOUTB RL LA
AD9765
“Q†DAC
LATCH
RL
QOUTA
LA
“Qâ€
DAC
CA
QOUTB RL LA
RL
RA
RA
RB
CB RB
RL
0.1â®F
BBIP
BBIN
RL
RA
RB
CB RB CFILTER
RL
RA
BBQP
BBQN
VPBF
RHODE &
SCHWARZ
RFSEA30B
SPECTRUM
ANALYZER
VOUT
+
PHASE
SPLITTER
LOIP
LOIN
AD8346
IQSEL
SLEEP
FS ADJ I FS ADJ Q REFIO
MODE RSET
3.9kâ€
RSET
3.9kâ€
0.1â®F
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
DIFFERENTIAL
RLC FILTER
RL = 200â€
RA = 2500â€
RB = 500â€
RP = 200â€
CA = 280pf
CB = 45pf
LA = 10â®H
OUIFS = 11mA
AVDD = 5.0V
VCM = 1.2V
VDIFF = 1.82V p–p
AVDD
RHODE & SCHWARZ
SIGNAL GENERATOR
AD976X
RL
RB
0 TO IOUTFS
VDAC
RA AD8346
VMOD
Figure 42. Baseband QAM Implementation Using an AD9765 and AD8346
REV. B
–17–
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