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Effects of clock frequency stability on digital microwave radiometer performance

A new generation of digital microwave radiometer back ends replaces conventional analog square‐law detectors with a high‐speed digital signal processing (DSP) stage to enhance their performance. The enhancements can include frequency subbanding for improved spectral resolution and mitigation of radio frequency interference, complex cross‐correlation of orthogonally polarized signals for full Stokes polarimetry, and high‐order moment (kurtosis) detection for improved radio frequency interference detectability. In each of these cases, the quality of the performance enhancement can be degraded by the nonideal realization of the desired DSP algorithm. In particular, instability in the master clock, which times the cadence of the DSP stage, can affect the performance. Analytical expressions are developed to model critical radiometer performance criteria as functions of the clock frequency stability. Experimental results are presented to validate the expressions. These results can be used to derive clock frequency stability requirements for future digital microwave radiometers.