GC5316_14 Datasheet, PDF (37/76 Pages) Texas Instruments – HIGH-DENSITY DIGITAL DOWNCONVERTER AND UPCONVERTER

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3.1.7 Transmit Mixer
18
I
Fro m C hannel Delay
18
Q
GC5316
SLWS154A − JANUARY 2004 − REVISED MARCH 2004
21
Round ₂₁ To Sum Chain
20
C os
S in
From N CO
Figure 30. Transmit Mixer
The transmit mixer, like the receive 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.
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 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 11. Programming
VARIABLE
mixer_gain
DESCRIPTION
When asserted adds 6 dB of gain in the mixer. Should always be set.
3.1.8 Transmit NCO
Frequency Syn c
32
32
Fre quency Word
R eg
32
R eg
Clr
20
23
18 _{Sin/ Cos}
Cos
Table
20
S in
Zero Phase Sync
5
Phase Offset Sync
R eg
D ith er
G en
16
Phase Offset
Dither Sync
Figure 31. Transmit NCO
The NCO is a digital complex oscillator that is used to translate (or upconvert) interpolated and filtered baseband
signals to a programmable carrier frequency.
The block produces programmable complex digital sinusoids by accumulating a frequency word which is
programmed by the user. The output of the accumulator is a phase argument that indexes into a Sin/Cos ROM table
which produces the complex sinusoid. A phase offset can be added prior to indexing if desired for channel calibration
purposes. This changes the Sin/Cos phase with respect to other channels’ NCOs.
A 5-bit dither generator is provided and generates a small level of digital pseudo-noise that is added to the phase
argument below the bottom bit and is useful for reducing NCO spurious outputs.
Table 12. Programming
VARIABLE
dither_ena
DESCRIPTION
When set turns dither on. Clearing turns dither off.
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