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8.7
Sideband Rejection
Basic Test Procedure
Figure 10. Single Sideband Spectrum Output Before DAC Offset and QMC Adjustments
An iterative process is used to achieve the best performance.
•
Place a normal marker at the peak upper sideband, place a delta marker at the carrier signal, and note
the initial delta value.
•
Set initial DACA offset to 1000 and DACB offset to -1000
•
Change DACA offset by 1000 steps and monitor the output performance change.
•
If performance gets better, then repeat the process with an additional 1000 steps. If the performance
gets worse or doesn't change, then change the offset in the other direction by 1000 steps.
•
Once the performance remains basically unchanged, repeat the process on DACB offset with 1000
step changes.
•
Once optimized, go back to the A side and repeat the tuning process with a step size of 100.
•
Continue tuning. After each complete cycle, reduce the step size down (i.e., to 10, then to 1 if desired).
•
A performance greater that 65 dBc should be achievable.
Sideband rejection is determined by the two quadrature signals to the modulator being exactly 180
degrees out of phase and exactly the same amplitude. Amplitude and phase imbalance between the two
paths yield an unwanted lower sideband. The amplitude variation between the two paths can be
compensated for by adjusting the DAC fine gain controls or by adjusting the QMC gain controls if the
device is operating with the QMC on. The phase can be compensated by using the QMC phase
adjustment. Note this is only possible when the coarse mixer is not used in the f
DAC
/4 mode. Coarse
mixing in the f
DAC
/4 mode causes the relative phase information between I and Q paths to be mixed. In the
f
DAC
/2 mode there are no cross terms (terms are 0) and the relative phase information is maintained
between I and Q paths.
22
SLWU013A – March 2004 – Revised September 2005
