HP 8753E User Manual page 449

In standard vector error-correction, a thru (delay = 0) is used as a calibration standard. The
solution to this problem is to use a calibration mixer with very small group delay as the
calibration standard.
An important characteristic to remember when selecting a calibration mixer is that the delay
of device should be kept as low as possible. 'Ib do this, select a mixer with very wide
the
bandwidth (wider bandwidth results in smaller delay).
6-166 Applicationmd OperationConcepts