  The Heterodyne Instrument for the Far Infrared (HIFI) on board the Herschel
Space Observatory is composed of a set of fourteen double sideband mixers. We
discuss the general problem of the sideband ratio (SBR) determination and the
impact of an imbalanced sideband ratio on the line calibration in double
sideband heterodyne receivers. The HIFI SBR is determined from a combination of
data taken during pre-launch gas cell tests and in-flight. The results and some
of the calibration artefacts discovered in the gas cell test data are presented
here along with some examples of how these effects appear in science data taken
in orbit.
