AD9148BBCZ datasheet(62/73 Pages) AD

62 / 73 page

AD9148

Preliminary Technical Data

Rev. PrA | Page 62 of 73

Reducing LO Leakage and Unwanted Sidebands

Analog Devices modulators can introduce unwanted signals at

the LO frequency due to dc offset voltages in the I and Q baseband

inputs as well as feedthrough paths from the LO input to the

output. The LO feedthrough can be nulled by applying the correct

dc offset voltages at the DAC output. This can be done either by

using the auxiliary DACs (Register 0x32, Register 0x33,

offset adjustments (Register 0x2C to Register 0x2F). Using the

auxiliary DACs has the advantage that none of the main DAC

dynamic range is used for performing the dc offset adjustment.

The disadvantage is that the common-mode level of the output

signal changes as a function of the auxiliary DAC current. The

opposite is true when the digital offset adjustment is used.

Good sideband suppression requires both gain and phase matching

of the I and Q signals. The phase adjust (Register 0x28 to

registers can be used to calibrate I and Q transmit paths to optimize

the sideband suppression. As an alternative to the digital gain

scaling, the DAC full-scale output current (Register 0x30,

adjusted to calibrate the I and Q transmit paths; however, changing

the DAC full-scale output current affects the common-mode

voltage level.

For more information on correcting imperfections in IQ

modulators to improve RF signal fidelity, refer to Application

Note AN-1039.

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