  We describe the design and performance of a microwave polarimeter used to
make precision measurements of polarized astrophysical radiation in three
microwave frequency bands spanning 26-36 GHz. The instrument uses cooled HEMT
amplifiers in a correlation polarimeter configuration to achieve high
sensitivity and long-term stability. The instrument demonstrates long term
stability and has produced the most restrictive upper limits to date on the
large angular scale polarization of the 2.7 K cosmic microwave background
radiation.
