The G2 receiver with the new mixer was tested off-line at Pico Veleta in February 1994. In this test a standard receiver configuration was used: MK3 cryostat with ambient temperature optics, local oscillator power injection through the dual beam interferometer diplexer, and the standard local oscillator. The IF chain has been changed. In the cryostat is now installed a 3.5-4.5 GHz Berkshire IF amplifier with a noise temperature of 6 K. The receiver output bandwidth is limited to 3.7--4.3 GHz by the filter in the uncooled IF amplifier. The infrared filters in the cryostat were replaced too.
First the receiver has been tuned in SSB operation. The backshort to junction distance was optimized in order to have a 16--20 dB rejection of the upper sideband. The SSB receiver noise temperature, measured in front of the receiver, is presented in Fig. 1. Upper sideband rejection for this SSB tuning is presented in Fig. 2. Noise temperature measured in the reference plane of the automatic cold and hot calibration loads may be 40--60 K higher than in Fig. 1 due to : a) cross-polarization of the receiver optics b) sidelobes of the receiver optics, and c) the loss in the multiplexing system of the receiver cabin.
Next the receiver noise temperature corresponding to the approximate balance between the sidebands ( dB) has been measured for the backshort positions close to SSB tuning. This ``DSB'' noise temperature is also presented in Fig. 1. Minimum receiver noise is about 65 K.
Figure 1: G2 receiver SSB noise temperature (SSB tuning) and DSB noise temperature (DSB tuning). Noise temperature is measured directly in front of the receiver. In regular operation, the noise temperature is measured at a standard reference plane, and includes some optics losses (see text).
Figure 2: Upper sideband rejection in the G2 receiver in SSB mode.