If the PSG/MXG/EXG/ESG I/Q calibration has not been performed recently, or the instrument's environment has changed, it is recommended that you run the I/Q calibration procedure. The I/Q calibration adjusts the DC offsets into the I/Q modulator to minimize carrier leakage. It also insures that the I/Q modulator is in Quadrature to help minimize images. It may be possible to reduce the carrier leakage even further by using the I and Q offset adjustments on the PSG/MXG/EXG/ESG. The N7620B software does not control the I and Q offset levels.
Make sure there is no signal present at the wideband I/Q inputs of the PSG with Options 015, 016, H16, or HBQ while executing the IQ calibration. There are no internal switches to disconnect the I/Q inputs for Option 015 during a calibration. Having any signal present on the wideband I/Q inputs will cause the calibration to be incorrect and in some cases to fail outright.
The PSG has two I/Q calibrations, one for the normal I/Q path, and one for the wideband I/Q path. Select the desired I/Q path before pressing Execute Cal. Any DC offsets present on the external Arb outputs are not accounted for by the signal generator I/Q Calibration.
E8267D Option H16 and N8212A Option 016 have an improved I/Q calibration, available with firmware Rev C.04.61.