Datasheet

Quadrature Modulation Mode
DAC5686
www.ti.com ............................................................................................................................................................ SLWS147F – APRIL 2003 – REVISED JUNE 2009
The output of the mixer for the Nth sample is
A
out
(N) = A
in
(N)cos(2 π (Nf + f
0
)/f
DAC
+ φ ) – B
in
(N)sin(2 π (Nf + f
0
)/f
DAC
+ φ )
B
out
(N) = A
in
(N)sin(2 π (Nf + f
0
)/f
DAC
+ φ ) + B
in
(N)cos(2 π (Nf + f
0
)/f
DAC
+ φ )
where f = freq × f
DAC
/2
32
and φ = phase × π /2
15
and f
0
is the initial value of the NCO accumulator. When
f = f
DAC
/2, f
0
= 0, and φ = 0, the sine term is 0 and the equations simplify to
A
out
(N) = A
in
(N) × ( – 1)
N
B
out
(N) = B
in
(N) × ( – 1)
N
resulting in two independent, real up-conversion paths.
It is essential that the NCO accumulator initial value f
0
= 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 2
31
.
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
f
DAC
/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 f
DAC
/4 mix mode is implemented by
deasserting nco in register config_msb while in single-sideband or quadrature modulation mode.
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