An Instrument for Investigating the Large Angular Scale Polarization of the Cosmic Microwave Background

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.