GC5316 datasheet(37/75 Pages) TI

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GC5316

SLWS154A − JANUARY 2004 − REVISED MARCH 2004

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3.1.7 Transmit Mixer

Sin

Cos

Round

Fro m C hannel Delay

To Sum Chain

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 = 2mixer_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

DESCRIPTION

mixer_gain

When asserted adds 6 dB of gain in the mixer. Should always be set.

3.1.8 Transmit NCO

Fre quency Word

Phase Offset

Dith er

Gen

Dither Sync

Frequency Syn c

Zero Phase Sync

Phase Offset Sync

Reg

Sin/ Cos

Table

Cos

Sin

Clr

Reg

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

DESCRIPTION

dither_ena

When set turns dither on. Clearing turns dither off.

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