AD9789BBCZ datasheet(39/76 Pages) AD

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AD9789

Rev. A | Page 39 of 76

THEORY OF OPERATION

QAM Encoder

The AD9789 is a flexible digital signal processing (DSP) engine

combined with a high performance, 2400 MSPS, 14-bit DAC

(Figure 70). The DSP blocks include a QAM encoder, a 2×

upsampling square root raised cosine (SRRC) filter, selectable

interpolation from 16× to 512×, a rate converter, and a complex

modulator. The digital interface can accept up to four channels

of complex data. The QAM encoder supports constellation sizes

of 16, 32, 64, 128, and 256. The on-chip rate converter allows

fine resolution of baud rates with a fixed DAC sampling clock.

The digital upconverters can place the input signals from dc to

0.5 × fDAC. An analog mix mode extends the output spectrum

into the second and third DAC Nyquist zones.

The QAM encoder supports seven different standards-compliant

mappings. (For illustrations of the supported mappings, see the

QAM Constellation Maps section.) The QAM encoder receives

input data-words of 8 bits in width and maps them into 16, 32,

64, 128, or 256 point constellations. It outputs 5-bit complex

QAM modulated samples. The mode in which the QAM

encoder runs is selected via the QAM/SRRC configuration

QAM

ENCODER

FROM INPUT

INTERFACE

Control of the AD9789 functions is via a serial peripheral

interface (SPI).

Figure 72. QAM Encoder I/O

Table 50 lists the available QAM mapper modes along with the

corresponding input bits and output range. The operation of the

QAM encoder when configured in DOCSIS 64-QAM mode is

described in this section. The operation of the QAM encoder in

the other modes is conceptually the same; only the input data

bit encoding and scale factors are different.

DATA

QAM/

FILTER/

NCO

QAM/

FILTER/

NCO

QAM/

FILTER/

NCO

QAM/

FILTER/

NCO

32 INPUT

PINS

AND

2 PARITY

PINS

DCO

16×

INTERPOLATOR

AND BPF

+ SCALARS

14-BIT

2.4GSPS

DAC

SPI

IRQ RS

DATA

CMOS

0TO 15

LVDS

RISE

CMOS

16 TO 31

LVDS

FALL

The DOCSIS 64-QAM constellation diagram is shown in

Figure 73. The constellation diagram shows how the QAM

encoder input is mapped into the QAM constellation. For

example, an input data-word of 111111 maps to the constellation

point in the upper right corner of the 64-QAM constellation.

Figure 70. Top Level Functional Block Diagram

DATAPATH SIGNAL PROCESSING

110,111 111,011 010,111 011,011 100,101 101,111 110,101 111,111

110,100 111,000 010,100 011,000 100,000 101,010 110,000 111,010

100,111 101,011 000,111 001,011 000,101 001,111 010,101 011,111

100,100 101,000 000,100 001,000 000,000 001,010 010,000 011,010

010,011 011,001 000,011 001,001 000,001 001,101 100,001 101,101

010,110 011,100 000,110 001,100 000,010 001,110 100,010 101,110

110,011 111,001 100,011 101,001 010,001 011,101 110,001 111,101

110,110 111,100 100,110 101,100 010,010 011,110 110,010 111,110

C5 C4 C3, C2 C1 C0

The DSP blocks included on the AD9789 can be grouped into

two sections. The first is the datapath signal processing. Four

identical datapaths, or channels, can be used. A block diagram

of a single channel is shown in Figure 71. Enabling and disabling

each DSP block within the datapath takes effect on all channels.

There is independent control of the scaling and the frequency

placement of each channel.

(N = 0 TO 5)

(P/Q = 0.5 TO 1)

RATE

CONVERTER

P/Q 24-BIT

CH GAIN

0× TO 2×

24-BIT NCO

0TO

fDAC/16

SRRC

BYPASS

SRRC

BYPASS

QAM

INSCALE

QAM

MAPPER

Figure 71. Datapath Block Diagram

The following sections describe each of the DSP blocks

included in the datapath.

Figure 73. DOCSIS 64-QAM Constellation

AD9789BBCZ Datasheet(PDF) 39 Page - Analog Devices

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