The tuning band of the 3mm1 receiver is 81 -- 116 GHz; its receiver temperature is between 100 and 140 K, the image side band rejection is between 25 dB and 30 dB. The tuning band of the 3mm2 receiver is 82 -- 116 GHz; its receiver temperature is between 70 and 90 K with the same range of image side band rejection.
The high rejection of the USB improves the system temperature and the calibration accuracy, particularly for 115 GHz observations, for which the receiver image side band sees the bright oxygen 118.75 GHz atmospheric line.
It is important to check your calibration on strong reference sources (see IRAM line catalog and updates). Beware also of possible interference between the `second' 1.3 mm receiver, 230G2, and this receiver when operating at harmonic frequencies (the two receivers receive the same polarization; the interference will be a strong and narrow line).
This receiver is also used as the standard pointing receiver.