AD9853_15 datasheet(25/31 Pages) AD

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OBSOLETE

AD9853

–25–

REV. C

(01)

(00)

(10)

(11)

a. QPSK Symbol Mapping

1011

(0111)

1010

(0101)

1000

(0100)

1001

(0110)

0001

(0010)

0011

(0011)

0010

(0001)

0000

(0000)

0100

(1000)

0101

(1010)

0111

(1011)

0110

(1001)

1110

(1101)

1100

(1100)

1101

(1110)

1111

(1111)

b. D16-QAM Symbol Mapping

1011

(0111)

1001

(0110)

1000

(0100)

1010

(0101)

0001

(0010)

0011

(0011)

0010

(0001)

0000

(0000)

0100

(1000)

0110

(1001)

0111

(1011)

0101

(1010)

1110

(1101)

1100

(1100)

1101

(1110)

1111

(1111)

c. 16-QAM Gray-Coded Symbol Mapping

Figure 37. Symbol Mapping for QPSK, 16-QAM, and DQAM,

Spectrum = I

× COS + Q × SIN (Spectrum = I × COS – Q ×

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(

Ct) and sin(ωCt) respec-

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 fCLOCK/2

n, as

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

× COS – Q × SIN. The phase rotation from symbol

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

× COS + Q × SIN as shown in Figure 37, a phase

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

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