DAC5687IPZP datasheet(70/79 Pages) TI

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Application Example: CMTS/VOD Transmitter

B0044-01

DAC

Ch2

Ch1

GC5016

DUC

DAC5687

CDC7005

DUC

DAC

QAM1

QAM2

QAM3

QAM4

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

The exceptional SNR of the DAC5687 enables a dual-cable modem termination system (CMTS) or video on

demand (VOD) QAM transmitter in excess of the stringent DOCSIS specification, with > 74 dBc and 75 dBc in

the adjacent and alternate channels.

A typical system using the DAC5687 for a cost-optimized dual-channel two-QAM transmitter is shown in

Figure 81. A GC5016 would take four separate symbol rate inputs and provide pulse shaping and interpolation to

~128 MSPS. The four QAM carriers would be combined into two groups of two QAM carriers with intermediate

frequencies of approximately 30 MHz to 40 MHz. The GC5016 would output two real data streams to one

DAC5687. The DAC5687 would function as a dual-channel device and provide 2

× interpolation to increase the

frequency of the second Nyquist zone image. The two signals are then output through the two DAC outputs,

through a transformer and to an RF upconverter.

Figure 81. Dual-Channel Two-QAM CMTS Transmitter System Using DAC5687

The DAC5687 output for a two-QAM256 carrier signal at 33-MHz and 39-MHz IF with the signal and system

properties listed in Table 19 is shown in Figure 82. The low DAC5687 noise floor provides better than 75 dBc

(equal bandwidth normalized to one QAM256 power) at > 6-MHz offset.

Table 19. Signal and System Properties for Complex IF System Example in Figure 82

Signal

QAM256, 5.36 MSPS,

α = 0.12

IF frequencies

33 MHz and 39 MHz

DAC5687 input rate

5.36 MSPS

× 24 = 128.64 MSPS

DAC5687 output rate

257.28 MSPS (2

× interpolation)

DAC5687 mode

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