As analog-to-digital converter technology improves, direct sampling is emerging as a suitable architecture for radiometer systems. A digital front-end replaces the mixer and local oscillator stage found in a conventional receiver. A digital system is immune to gain fluctuations and spurious noise caused by unstable components found in analog systems. We are developing a digital L-band direct-sampling radiometer and have found that less than 0.1 K sensitivity is possible at 1.4 GHz using an analog-to-digital converter that has only 2 bits of quantization.
Direct-sampling makes the idea of a "receiver on a chip" possible. Future integrated digital receivers have several characteritics which make them ideal for use in satellite radiometer systems:
Two-dimensional array radiometer systems such as the Microwave Imaging Radiometer with Aperture Synthesis (MIRAS), an instrument proposed by the European Space Agency which would map soil moisure and ocean salinity, can have over 100 receivers. The use of integrated digital receivers will make these systems feasible for use on satellites.
For more information about direct-sampling, contact Mark Fischman.