HP 8517B - Manual (Page 22)

1-6 HP 8517B S-Parameter Test Set Manual
Getting Started
Maintaining Measurement Accuracy
Maintaining Measurement Accuracy
Precision measurements rely on a precision calibration of the network
analyzer. As a general rule, the shorter the time lapse between a calibration
and a device measurement, the more precise the measurement results will be
(within the limitations of your system).
Hewlett-Packard recommends calibrating your system every few hours, or at
least re-verifying your system’s calibration this frequently. Doing so helps
retain measurement precision.
The frequency of calibrations is determined by both the location of the
system and the ambient temperature stability in its operating area.