EVLA Memo #168 Assessing the Impact of Using Three Cryogenic Compressors on the Performance of the EVLA

We present dewar temperature and system temperature measurements for the eight cryogenically cooled receivers of a subset of four EVLA antennas that have undergone a reconfiguration to use three cryogenic compressors instead of four. We compare the changes in the system temperatures in these antennas with those obtained from a subset of eight other EVLA antennas that have not undergone such a reconfiguration. The results show that the S, Ku, and K-band receivers have a small, but significant loss of sensitivity because of the change in the compressor configuration. The net loss in the sensitivity is ∼ 3.4%, 5.1%, and 2.7% for the S, Ku, and K-bands, respectively. Two other receivers, namely the C and X-band receivers, also show some degradation (. 2%) in their sensitivity. After the reconfiguration, the S, Ku, and K-band receivers have been connected to compressor Unit A, while the C and X-band receivers to compressor Unit B along with the Q-band receiver. Considering the overall load on compressor Unit A and the high thermal load of the S-band receiver, we suggest connecting the S, Ku, and Q-band receivers to this compressor unit. The K-band receiver will then be connected to compressor Unit B along with the C and X-band receivers.