TABLE OF CONTENTS The radiometers of the system are used in pairs: H and V polarization, frequency for frequency (two pairs at 89 GHz). Hence, it is practical to construct them as dual receiver units: a pair of receivers for one given frequency.
Following the discussion in Section 5.1.1, the receivers operating at 10.65 to 36.5 GHz will be designed as direct receivers: Microwave amplifiers covering the frequency range are readily available, as well as good quality tunnel diode detectors; requirements to filter selectivity are not overwhelming for these broadband applications (which would otherwise favor the superheterodyne principle with its IF filter possibilities); and finally, for spaceborne applications, it is a strong argument to get rid of the power consuming local oscillator. The situation is not quite so favorable for the 89-GHz channel, and it will be designed as a DSB superheterodyne receiver without preamplifier.
The design and layout of the direct total power receiver is very close to that of the SSB receiver as discussed in Section 5.4.2, and it is illustrated in Figure 13.2. The input calibration switch is not present in this design where the calibration switching process is taken care of as part of the antenna scan. The mix-preamp and the local oscillator are not present and the IF filter is omitted. This has the consequence that all filtering takes place at the RF level, and thus the requirements to the RF filter may well be tighter than normally found...
