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2.1.7

Mixer

FromNCO

FromChannelDelay

Round

ToSumChain

2.1.8

NCO

Reg

Clr

Reg

ZeroPhaseSync

FrequencySync

FrequencyWord

S /C

Table

Reg

Dither

Gen

DitherSync

PhaseOffset

PhaseOffsetSync

GC5318

HIGH-DENSITY DIGITAL UPCONVERTER

SLWS187 – JULY 2006

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 = 2mixer_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

DESCRIPTION

mixer_gain

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

Figure 2-12. NCO

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

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