Quadrature Modulation Mode

DAC5686

www.ti.com ............................................................................................................................................................ SLWS147F – APRIL 2003 – REVISED JUNE 2009

The output of the mixer for the Nth sample is

Aout(N) = Ain(N)cos(2π(Nf + f0)/fDAC + φ) – Bin(N)sin(2π(Nf + f0)/fDAC + φ)

Bout(N) = Ain(N)sin(2π(Nf + f0)/fDAC + φ) + Bin(N)cos(2π(Nf + f0)/fDAC + φ)

where f = freq × fDAC/2

32 and

φ = phase × π/215 and f

0 is the initial value of the NCO accumulator. When

f = fDAC/2, f0 = 0, and φ = 0, the sine term is 0 and the equations simplify to

Aout(N) = Ain(N) × (–1)

N

Bout(N) = Bin(N) × (–1)

N

resulting in two independent, real up-conversion paths.

It is essential that the NCO accumulator initial value f0 = 0 to eliminate the cross-terms between the A and B

channels. The accumulator is reset to 0 when the NCO is running by raising the PHSTR pin to IOVDD (when

sync_phstr is set to 0). Note that the accumulator remains at 0 until the PHSTR pin is lowered to GND. The

following steps ensure that the accumulator does not have a non-zero starting value:

1. Program the frequency register to 231.

2. Enable the NCO by asserting register bit nco in config_msb.

3. Raise PHSTR to IOVDD.

4. Lower PHSTR to GND.

In quadrature modulation mode, on-chip mixing of complex I and Q inputs provides the final baseband-to-IF

up-conversion. Quadrature modulation mode is selected by mode[1:0] = 10 in the config_lsb register. Figure 23

shows the data path architecture in quadrature modulation mode. Complex baseband I and Q from the

ASIC/FPGA are input to the DAC5686, which in turn quadrature modulates I and Q to produce the final IF

single-sideband spectrum. DAC A is held constant, while DAC B presents the DAC5686 quadrature modulator

mode output.

NCO mixing frequencies are programmed through 32-bit freq (4 registers); 16-bit phase adjustments are

programmed through phase (2 registers). The NCO operates at the DAC update rate; thus, increased amounts

of interpolation allow for higher IFs. More details for the NCO are provided in the NCO section. For mixing to

fDAC/4, the DAC5686 provides a specific architecture that exploits the {… –1 0 1 0 …} resultant streams from sin

and cos; the NCO is shut off in this mode to conserve power. The fDAC/4 mix mode is implemented by

deasserting nco in register config_msb while in single-sideband or quadrature modulation mode.

Copyright © 2003–2009, Texas Instruments Incorporated

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