Also, an adjustment to the dual beam interferometer was required recently in order to correct poor sideband rejection. The receiver is usually tuned under remote computer control. In SSB mode the detectors look into a cold load at the image sideband frequency, thereby enhancing sensitivity under all sky conditions except the very best. There is only one local oscillator so the two channels will be tuned to the same frequency. B3 has good baseline stability and bandwidths of up to the full spectrometer coverage of 1.8 GHz can be used.
Calibration is achieved via on-board cold and ambient loads.
In Figure 6 the double-sideband receiver temperatures for receiver B3 are plotted as a function of LO frequency. These data are quite similar to those for the earlier tuneable mixers. Other details as for Figure 5 . For frequencies within a few GHz of the tuning limits, mixer performance is less good, and one or both mixers may not show true heterodyne performance. On the sky, SSB system temperatures well below 500 K are routine 11 for B3 under normal conditions.