Receiver Temperatures

The best indicator of the receiver performance is receiver temperature (T_rx), obtained by doing a calibration; e.g. see see Figure 1 for results obtained in the lab in March 2000. This quantity is independent of sky conditions and (hopefully) the telescope elevation angle, but obviously depends upon how well the receiver is tuned. Since both mixers have been replaced by tunerless versions there is rather little one can do about this except to ensure that the mixer operating point is optimum and that the Josephson effect is as well suppressed as possible. In fact the current tunerless mixers are slightly less good than the previous tuneable mixers (see Figure 2). Trx increased by about 10% typically over that shown in Figure 2, and there is a fairly sharp increase in Trx for both mixers above about 360 GHz.

System Temperatures

System temperature is directly related to the overall system sensitivity and is strongly dependent upon weather conditions. The tabulated values are typical system temperatures at 345 GHz near the zenith. The atmospheric opacity is measured at 225 GHz. At other frequencies the Tsys values can be significantly higher, since the atmospheric transmission is greater, e.g. especially around 330 GHz.

Note that receiver temperatures in SSB mode will be roughly twice the DSB values at the same frequency, but Tsys will be lower if one opts for SSB operation rather than DSB.