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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 (read below), 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 (discussed below).
Anne Dutrey