Response time and noise power gain of electrical substitution radiometers with feedback control

We present criteria for choosing filter and control parameters for absolute radiometers and true RMS meters that employ feedback controlled electrical substitution. Expressions are obtained for the response of the electrical heating power to radiation and to detector noise. The gains and time constants that minimize the response time for a given variance in a single power measurement are obtained from analyses of second- and third-order systems. Near optimal behavior is obtained in the third-order system comprising a first-order filter and a proportional-plus-integral controller with an integration time equal to the detector time constant. A procedure for tuning the control system is presented.