DAC38RF86 datasheet(56/147 Pages) TI2

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MIXERCD GAIN

INPUTCD

NCOCD

INPUTCD

NCOCD

Output

cos

ŒI

VLQ

ŒI

MIXERAB GAIN

INPUTAB

NCOAB

INPUTAB

NCOAB

Output

cos

ŒI

VLQ

ŒI

AB or CD

PHASE

NCOAB or CD

u DAC

FREQ NCOAB or CD

Frequency

Accumulator

RESET

CLK

FDAC

NCO SYNC

via

syncsel_NCO(3:0)

Phase

Look Up

Table

sin

cos

DAC38RF86, DAC38RF96

DAC38RF87, DAC38RF97

SLASEF4B – FEBRUARY 2017 – REVISED JULY 2017

www.ti.com

Product Folder Links: DAC38RF86 DAC38RF96 DAC38RF87 DAC38RF97

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8.3.17.4 Digital Quadrature Modulator

Each DUC in the DAC38RFxx has digital quadrature modulator (DQM) blocks with independent Numerically

Controlled Oscillators (NCO) that converts the complex input signal to a real signal with flexible frequency

placement between 0 and fDAC/2. The NCOs are enabled by fields NCOAB_ENA and NCOCD_ENA in register

MULTIDUC_CFG2 (8.5.14). The NCOs have 48-bit frequency registers (FREQ_NCOAB (8.5.25) and

FREQ_NCOCD (8.5.26)) and 16-bit phase registers (PHASE_NCOAB (8.5.23) and PHASE_NCOCD (8.5.24))

that generate the sine and cosine terms for the complex mixing. The NCO block diagram is shown in Figure 50.

Figure 50. NCO Block Diagram

Synchronization of the NCOs occurs by resetting the NCO accumulators to zero. The synchronization source is

selected by fields SYNCSEL_NCOAB and SYNCSEL_NCOCD in register SYNCSEL1 (8.5.29). The frequency

word in the FREQ_NCOAB and FREQ_NCOCD registers are added to the accumulators every clock cycle, fDAC.

The frequency and phase offset of the NCOs are:

(1)

(2)

Treating the complex channels as complex vectors of the form I + j Q, the output of the DQM is:

(3)

(4)

Where t is the time since the last resetting of the NCO accumulator and the fields MIXERAB_GAIN and

MIXERCD_GAIN in register MULTIDUC_CFG2 (8.5.13) are either 0 or 1.

The maximum output amplitude of the DQM occurs if IIN(t) and QIN(t) are simultaneously full scale amplitude and

the sine and cosine arguments are equal to an integer multiple of π/4.

With MIXERAB_GAIN or MIXERCD_GAIN = 0, the gain through the DQM is sqrt(2)/2 or -3 dB. This loss in signal

power is in most cases undesirable, and it is recommended that the gain function be used to increase the signal

by 3 dB to compensate. With MIXERAB_GAIN or MIXERCD_GAIN = 1, the gain through the DQM is sqrt(2) or

+3 dB, which can cause clipping of the signal if IIN(t) and QIN(t) are simultaneously near full scale amplitude and

should therefore be used with caution.

8.3.17.5 Low Power Coarse Resolution Mixing Modes

In addition to the NCO the DAC38RFxx also has a coarse mixer block capable of shifting the input signal

spectrum by the fixed mixing frequencies ±N x fDAC/8. Using the coarse mixer instead of the full mixers will result

in lower power consumption.

Treating the two complex channels as complex vectors of the form I(t) + j Q(t), the outputs of the coarse mixer is

equivalent to:

DAC38RF86 Datasheet(PDF) 56 Page - Texas Instruments

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