TABLE OF CONTENTS The block diagram of a 17-GHz radiometer will be covered in some detail in order to discuss the different aspects of designing a radiometer. The chapter will first concentrate on the design of an instrument operating in the Dicke mode, followed by comments to the necessary additions and special considerations regarding the NIR mode. Finally, a total power instrument will be discussed. In order to enable the calculation and discussion of gains, signal levels, and so forth throughout the instrument, some major specifications will be given in Table 5.1.
| Frequency | 17 GHz |
| IF bandwidth | 500 MHz |
| Noise figure | 5 dB |
| Integration time | 5 ms |
| Input range | 0 313K |
Before proceeding to the specific examples, a few subjects of general nature are covered.
The radiometer is merely a very sensitive microwave receiver and, like any receiver, it employs front-end circuitry, which has two prime tasks: input frequency band selection, and amplifying the incoming signal to a proper level for the detector and subsequent low-frequency circuitry. This amplification may have to be very large, typically 50 80 dB for microwave radiometers. It can be obtained by two entirely different schemes, either by direct use of amplifiers at the input frequency (the direct receiver) or by use of a mixer, local oscillator, and IF frequency amplifiers (the superheterodyne receiver). In the direct receiver, all amplification takes place at the input frequency, and all selectivity is determined by filters in the same RF range.
