AD9853 Analog Devices, AD9853 Datasheet - Page 25

• Manufacturer Part Number: AD9853
• Description: Programmable Digital OPSK/16-QAM Modulator
• Manufacturer: Analog Devices

REV. C
(01)
(00)
(0111)
(0101)
(0010)
(0011)
10
00
0001
0011
1011
1010
b. D16-QAM Symbol Mapping
a. QPSK Symbol Mapping
(0110)
(0100)
(0000)
(0001)
1001
1000
0000
0010
Q
Q
(1101)
(1100)
(1000)
(1010)
1110
1100
0100
0101
(1111)
(1110)
(1001)
(1011)
1111
1101
0110
0111
(11)
(10)
11
01
I
I
–25–
Figure 37. Symbol Mapping for QPSK, 16-QAM, and DQAM,
Spectrum = I
SIN)
MIXERS, ADDER, INVERSE SINC FUNCTIONS
At the output of the Interpolation filters, the pulse-shaped, up-
sampled I and Q baseband data is multiplied with digitized
quadrature versions of the carrier, cos(
tively, which are provided by a direct digital synthesizer (DDS)
block. The DDS block has a 32-bit tuning word that results in
an extremely fine frequency tuning resolution of f
well as extremely fast output frequency switching. The multiplier
outputs are then summed to form the QPSK/QAM-modulated
signal. This signal is then filtered by an inverse sinc filter to
compensate for the SINx/x roll-off function inherent in the
digital-to-analog conversion process. The inverse sinc filter
flattens the gain response across the Nyquist bandwidth. This is
most critical for higher data rate signals that are placed on carri-
ers at the high end of the spectrum where the uncompensated
SINx/x roll-off would be getting progressively steeper. Gain
attenuation across a channel will result in modulation quality
impairments, such as degraded error vector magnitude (EVM).
The spectral inversion bit, when enabled, inverts the Q data at the
input to the adder circuit in the quadrature amplitude modulator
section. This has the effect of reversing the direction of the phase
rotation around the constellation map. Positive phase rotation
on the I/Q constellation plane corresponds to counterclockwise
movement. For example, the symbols in parentheses on the
QPSK constellation in Figure 37 corresponds to a spectral
mapping of I
value 11 to 01 is a positive 90 degree rotation. Traversing
around the constellation in a positive direction, there are also
positive 90 degree rotations from 01 to 00, 00 to 10, and 10
back to 11. If the spectral invert bit is disabled, providing the
spectral map I
rotation from symbol value 11 to 01 now corresponds to a nega-
tive 90 degrees of phase rotation. Similarly, there are now nega-
tive 90 degree phase rotations from 01 to 00, 00 to 10 and 10
back to 11. In other words, the direction of phase rotation
c. 16-QAM Gray-Coded Symbol Mapping
(0111)
(0110)
(0010)
(0011)
0001
0011
1011
1001
COS – Q
COS + Q
COS + Q
(0101)
(0100)
(0000)
(0001)
1010
1000
0000
0010
SIN. The phase rotation from symbol
SIN (Spectrum = I
SIN as shown in Figure 37, a phase
Q
(1101)
(1100)
(1000)
(1001)
1110
1100
0100
0110
C
t) and sin(
(1111)
(1110)
(1010)
(1011)
1111
1101
0101
0111
AD9853
CLOCK
COS – Q
C
t) respec-
/2
n
, as
I