Datasheet

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Application Example: Wide-Bandwidth Direct Baseband-to-RF Conversion
B0043-01
y2
DAC
I
Q
RF
Processing
TRF3750
DAC5687
y2
y2y2
DAC
Phase/
Gain/
Offset
Adjust
TRF3701
TRF3702
TRF3703
GC1115
and DPD
Processor
DAC5687
SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006
Table 18. Signal and System Properties for Complex IF System Example in Figure 79
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 (4 × interpolation)
DAC5687 mode X4 CMIX
DAC5687 complex IF 122.88 MHz (f
DAC
/4)
LO frequency 2140 MHz
The complex IF has several advantages over the real IF architecture such as:
• Uncalibrated sideband suppression, ~35 dBc compared to 0 dBc for real IF architecture.
• Direct DAC – complex-mixer connection — no amplifiers
• Nonharmonic clock-related spurious signals fall out-of-band
• DAC second Nyquist zone image is offset f
DAC
compared with f
DAC
– 2 × IF for a real IF architecture, reducing
the need for filtering at the DAC output.
• Uncalibrated LO feedthrough for AQM is ~35 dBc and calibration can reduce or completely remove the LO
feedthrough.
A system example of the DAC5687 used in a wide-bandwidth direct baseband-to-RF conversion is shown in
Figure 80 . 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 80. 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 has similar spectral performance to the external clock mode. In
addition, the nonharmonic clock-related spurious signals are 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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