JAJSSV2G March 2011 – January 2024 DAC3482
PRODUCTION DATA
For typical IF based systems, the IF location is selected such that the image location and the LO feedthrough location is far from the signal location. The minimum distance is based on the bandpass filter roll-off and attenuation level at the LO feedthrough and image location. If sufficient attenuation level of these two artifacts meets the system requirement, then further digital cancellation of these artifacts may not be needed.
Although the I/Q modulation process will inherently reduce the level of the RF sideband signal, an IF based transmitter without sufficient RF image rejection capabilities or an zero-IF based system (detail in the next section) will likely need additional sideband suppression to maximize performance. Further, any mixing process will result in some feedthrough of the LO source. The DAC3482 has build-in digital features to cancel both the LO feedthrough and sideband signal. The LO feedthrough is corrected by adding a DC offset to the DAC outputs until the LO feedthrough power is suppressed. The sideband suppression can be improved by correcting the gain and phase differences between the I and Q analog outputs through the digital QMC block. Besides gain and phase differences between the I and Q analog outputs, group delay differences may also be present in the signal path and are typically contributed by group delay variations of post DAC image reject analog filters and PCB trace variations. Since delay in time translates to higher order linear phase variation, the sideband of a wideband system may not be completely suppressed by typical digital QMC block. The DAC3482 has integrated group delay correction feature to provide delay adjustments. (The maximum group delay correction ranges from 30 ps to 100 ps and is dependent on DAC sample clock. Contact TI for specific application information.) Moreover, system designer may implement additional linear group delay compensation in the host processor to the DAC to perform higher order sideband suppression.