  XPOL, the first correlation polarimeter at a large millimeter telescope, uses
a flexible digital correlator to measure all four Stokes parameters
simultaneously, i.e. the total power I, the linear polarization components Q
and U, and the circular polarization V. The versatility of the backend provides
adequate bandwidth for efficient continuum observations as well as sufficient
spectral resolution (40 kHz) for observations of narrow lines. We demonstrate
that the polarimetry specific calibrations are handled with sufficient
precision, in particular the relative phase between the Observatory's two
orthogonally linearly polarized receivers. The many facets of instrumental
polarization are studied at 3mm wavelength in all Stokes parameters: on-axis
with point sources and off-axis with beam maps. Stokes Q which is measured as
the power difference between the receivers is affected by instrumental
polarization at the 1.5% level. Stokes U and V which are measured as cross
correlations are very little affected (maximum sidelobes 0.6% (U) and 0.3%
(V)). These levels critically depend on the precision of the receiver
alignment. They reach these minimum levels set by small ellipticities of the
feed horns when alignment is optimum (<~ 0.3"). A second critical prerequisite
for low polarization sidelobes turned out to be the correct orientation of the
polarization splitter grid. Its cross polarization properties are modeled in
detail. XPOL observations are therefore limited only by receiver noise in
Stokes U and V even for extended sources. Systematic effects set in at the 1.5%
level in observations of Stokes Q. With proper precautions, this limitation can
be overcome for point sources. Stokes Q observations of extended sources are
the most difficult with XPOL.
