DAC5687MPZPEP datasheet(68/75 Pages) TI1

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Application Example: Wide Bandwidth Direct Baseband to RF Conversion

B0043-01

DAC

Processing

TRF3750

DAC5687

DAC

Phase/

Gain/

Offset

Adjust

TRF3701

TRF3702

TRF3703

GC1115

and DPD

Processor

DAC5687-EP

SGLS333 – JUNE 2006

Table 17. Signal and System Properties for Complex IF System Example in Figure 77

Signal

Three WCDMA Carriers, Test Model 1

Baseband Carrier Offsets

–7.5 MHz, 2.5 MHz, 7.5 MHz

DAC5687 Input Rate

122.88 MSPS

DAC5687 Output Rate

491.52 MSPS (4x Interpolation)

DAC5687 Mode

X4 CMIX

DAC5687 Complex IF

122.88 MHz (fDAC/4)

LO Frequency

2140 MHz

The complex IF has several advantages over the real IF architecture such as:

1. Uncalibrated side-band suppression ~ 35 dBc compared to 0 dBc for real IF architecture

2. Direct DAC – Complex mixer connection – no amplifiers

3. Non-harmonic clock-related spurious signals fall out of band

4. DAC 2nd Nyquist zone image is offset fDAC compared with fDAC– 2 × IF for a real IF architecture, reducing

the need for filtering at the DAC output.

5. Uncalibrated LO feed through for AQM is ~ 35 dBc and calibration can reduce or completely remove the LO

feed through.

A system example of the DAC5687 used in a wide bandwidth direct baseband to RF conversion is shown in

Figure 78. The DAC input would typically be generated by a crest factor reduction processor such as Texas

Instruments GC1115 and digital predistortion processor. With a complex baseband input, the DAC5687 would

be used to increase the data rate through interpolation. In addition, phase, gain and offset correction of the IQ

imbalance is possible using the QMC block, DAC gain and DAC offset features. The correction could be done

one time during manufacturing (see the TRF3701 data sheet (SLWS145) and the TRF3702 data sheet

(SLWS149)) for the variation with temperature, supply, LO frequency, etc. after calibration at nominal conditions)

or during operation with a separate feedback loop measuring imbalance in the RF signal.

Figure 78. Direct Conversion System Using DAC5687 in X4L Mode

Operating at baseband has the advantage that the DAC5687 output is insensitive to DAC sample clock phase

noise, so using the DAC PLL clock mode will have similar spectral performance to the External clock mode. In

addition, the non-harmonic clock-related spurious signals will be small due to the low DAC output frequency.

With a complex input rate specified up to 250 MSPS, the DAC5687 is capable of producing signals with up to

200-MHz bandwidth for systems such as digital predistortion (DPD).

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