3.5 Local oscillator injection

As mentioned above, the local oscillator power must be added to the astronomical signal before it enters the mixer. When the mixers were based on Schottky diodes (10 years ago and more), they required an LO power of almost a mW. As a consequence, the LO power was coupled via a diplexer, which is like a frequency-selective coupler, allowing the mixer to be coupled with close to unity efficiency to both the input signal and the LO. With the advent of SIS mixers, and due to their modest LO power requirements (see Eq. 3.5), a new method can be used, based on frequency-independent couplers. A fraction f (typically 1%) of the LO power is coupled into the signal path; the rest is wasted! The fraction f must be kept small because the same amount f of room-temperature blackbody noise is also coupled into the signal path. The coupler requires no adjustment and is located close to the receiver, inside the cryostat (see Sec.3.8).