GC5318 Datasheet, PDF (17/45 Pages) Texas Instruments – HIGH-DENSITY DIGITAL UPCONVERTER

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2.1.7 Mixer
GC5318
HIGH-DENSITY DIGITAL UPCONVERTER
SLWS187 – JULY 2006
From Channel Delay ^{I 18}
Q ₁₈
20
21
Round
21
20
C_OS S_IN
From NCO
To Sum Chain
Figure 2-11. Mixer
The mixer is a complex multiplier which takes the baseband I and Q data that has been previously
pulse-shaped and interpolated, and translates to a carrier frequency programmed into the NCO. The mixer
data size is 18 bits for the signal path and 20 bits for the NCO path, as shown in Figure 2-11.
The gain through the mixer is –12 dB. It can be increased by 6 dB through a control bit. It is
recommended that this extra gain always be used. The output is then rounded to 21 bits.
Mixer gain = 2^{mixer_gain - 2}
The mixer output of each channel is combined in daisy-chain fashion in four sum chain adder blocks that
are described in a subsequent section.
For CDMA, the maximum output rate is txclk/2. The maximum output rate for UMTS is txclk.
Table 2-11. Programming
VARIABLE
mixer_gain
DESCRIPTION
When asserted, adds 6 dB of gain in the mixer. Should always be set.
2.1.8 NCO
Frequency Sync
32
Frequency Word ³²
Reg ³²
Reg
Clr
Zero Phase Sync
23
18
20
S_IN/C_OS
C_OS
Table
20
S_IN
5
Phase Offset Sync
Reg
Dither
Gen
16
Phase Offset
Figure 2-12. NCO
Dither Sync
The NCO, shown in Figure 2-12, is a digital complex oscillator that translates (or upconverts) interpolated
and filtered baseband signals to a programmable carrier frequency.
The block produces programmable complex digital sinusoids by accumulating a delta phase frequency
word that is programmed by the user. A phase offset can be added to the accumulated value if desired for
channel calibration purposes. The output of the accumulator is a phase value that indexes into a Sin/Cos
ROM table which produces the complex sinusoid.
A 5-bit dither generator is provided, and generates a small level of digital pseudo-noise that is added to
the phase accumulated value. It is useful for reducing NCO spurious outputs.
GC5318 Operation
17