AD9789BBCZ datasheet(40/76 Pages) AD

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AD9789

Rev. A | Page 40 of 76

Table 50. QAM Mapper Input and Output Range vs. Mode

ITU-T J.83

Annex

Description

SPI Register 0x07,

MAPPING[2:0] Bits

Bit Range

at Output

Input Bits

B7 B6 B5 B4 B3 B2 B1 B01

DOCSIS 64-QAM

000

−14 to +14

X X C5 C4 C3 C2 C1 C0

DOCSIS 256-QAM

001

−15 to +15

C7 C6 C5 C4 C3 C2 C1 C0

DVB-C 16-QAM

010

−15 to +15

X X X X C3 C2 C1 C0

DVB-C 32-QAM

011

−15 to +15

X X X C4 C3 C2 C1 C0

A and C

DVB-C 64-QAM

100

−14 to +14

X X C5 C4 C3 C2 C1 C0

A and C

DVB-C 128-QAM

101

−11 to +11

X C6 C5 C4 C3 C2 C1 C0

A and C

DVB-C 256-QAM

110

−15 to +15

C7 C6 C5 C4 C3 C2 C1 C0

Unused

111

1 X = don’t care.

Each constellation point corresponds to an I and Q coordinate

pair, as shown in Figure 74. In the figure, two symbols are high-

lighted in a 64-QAM constellation: I = 14, Q = 14 (Pair 1) and

I = 6, Q = −10 (Pair 2).

To represent the I and Q coordinate points, 5-bit, twos com-

plement numbers are used. For example, an input of 011101

into the QAM encoder maps to the I = 6, Q = −10 position of

the QAM-64 constellation and results in output samples of

I = 00110, Q = 10110.

–2

–6

–10

–14

–10

–6

–2

SYMBOL I = 14, Q = 14

I = 01110, Q = 01110

SYMBOL I = 6, Q = –10

I = 00110, Q = 10110

Figure 74. I and Q Symbol Mapping

SRRC

BYPASS

SRRC

BYPASS

QAM

INSCALE

QAM

MAPPER

Figure 75. QAM Mapper and SRRC Filter Detail

(I and Q Paths Are Identical So Only One Is Shown)

Input Scalar

The input scalar block is active only when the QAM mapper

is bypassed. The value of INSCALE[7:0] is programmed in

is calculated as follows:

INSCALE[7:

ScaleFacto 

This factor provides a scaling range of the input data from 0 to

7.96875 in steps of 0.03125. The default value of 0x20 provides a

scale factor of 1. As shown in Figure 76, the output of the input

scalar block is rounded to the nearest 16-bit value. If the output

exceeds the maximum or minimum value, it is clipped to either

positive or negative full scale (0x7FFF or 0x8000).

ROUND

SATURATE

INSCALE

Figure 76. Input Scalar Block Diagram

SRRC Filter

The square root raised cosine (SRRC) filter performs a 2×

interpolation and filtering operation on the input data. The

SRRC filter has a pass band, transition band, and stop band

requirement as per the DOCSIS, Euro-DOCSIS, and DVB-C

standards.

To cover all the standards, the value of alpha can be set to 0.12,

0.13, 0.15, or 0.18. This value is programmed in Register 0x07[5:4].

The frequency, fN, is determined by the input data baud rate.

The response of the SRRC filter is illustrated in Figure 77.

The SRRC filter accepts only five bits at its input and can be

bypassed (Register 0x06[6]). If the SRRC filter is the first block

enabled in the datapath, these five bits are the five MSBs of the

8-bit data-word.

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