  We describe the first instrument of a Cosmic Microwave Background experiment
for mapping cosmological structures on medium angular scales (the COSMOSOMAS
experiment) and diffuse Galactic emission. The instrument is located at Teide
Observatory (Tenerife) and is based on a circular scanning sky strategy. It
consists of a 1 Hz spinning flat mirror directing the sky radiation into a 1.8
m off-axis paraboloidal antenna which focuses it on to a cryogenically cooled
HEMT-based receiver operating in the frequency range 12--18 GHz. The signal is
split by a set of three filters, allowing simultaneous observations at 13, 15
and 17 GHz, each with a 1 GHz bandpass. A 1-5 degree resolution sky map
complete in RA and covering 20 degrees in declination is obtained each day at
these frequencies. The observations presented here correspond to the first
months of operation, which have provided a map of ~9000 square degrees on the
sky centred at DEC = +31 degrees with sensitivities of 140, 150 and 250 microK
per beam area in the channels at 13, 15 and 17 GHz respectively. We discuss the
design and performance of the instrument, the atmospheric effects, the
reliability of the data obtained and prospects of achieving a sensitivity of 30
microK per beam in two years of operation.
