The quasioptical frontend uses a Martin-Puplett interferometer to suppress noise and atmospheric
signatures from the unused sideband and a second Martin-Puplett interferometer as diplexer to
feed the local oscillator (LO) to the mixer. The LO operating at 276.3 GHz uses a Gunn oscillator at
92.1 GHz and a tripler. For high frequency stability a phaselock system is applied. The downconversion
to the intermediate frequency of 2.1 ± 0.5 GHz is performed by a cryogenic single-ended Schottky mixer
operating at about 100K. The LN2 dewar also encloses the low noise IF amplifier and two cold
loads. The systems noise temperature including quasioptics and the low-noise cryogenic amplifier is
1200 K at center frequency. Baseline ripples caused by standing waves in the quasioptical transmission
path are suppressed by a rooftop like mirror reciprocating by about 0.4 mm. The high spectral
resolution of 1 MHz over an instantaneous IF bandwith of 1 GHz is achieved by an acusto-optical
spectrometer.
Schematic diagram of the 278 GHz radiometer (35 kB)
