Cosmic Background Imager

Receivers

Each antenna in the CBI is equipped with a cooled receiver. This has a low-noise 26-36 GHz HEMT amplifier followed by a cooled down-converter which shifts the band to 2-12 GHz. All the receiver components, including the feed-horn, are cooled to 6 K using a closed-cycle Gifford-McMahon refrigerator. A quarter-wave plate between the HEMT amplifier and feed-horn provides circular polarization. Each receiver also has a rotating half-wave plate phase shifter just behind the feed-horn. This can be used to provide some additional rejection of the false signals caused by inter-antenna coupling. All the phase shifters in the array are synchronized so that the wavefront incident on the array is not distorted, even when the phase shifters are rotated. However, noise coupled between antennas passes through two phase shifters so the phase of the false signals at the correlator output varies with the phase shifter setting.

Feed horn and receiver with dewar removed.

Four stage HEMT amplifier: RF enters at the top; horizontal structures are bias networks.

Noise temperature of the prototype CBI receiver across the 26-36 GHz band. There is a calibration uncertainty of ~ 2 K.

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