Abstract :
A small-signal dynamic equivalent circuit is established for the output voltage of a dc-biased bolometer (barretter) detector. The circuit consists of a voltage generator υg, whose output is an undistorted replica of the incident RF-power modulation envelope, followed by a series resistor R1 of dynamic origin, a shunt capacitor C that represents heat storage in the bolometer wire, and a series resistor R0 equal to the dc resistance, usually 200 ohms. The resistance R1 is independent of signal level, and is typically about 220 ohms for an 8.75-mA bolometer and about 120 ohms for a 4.5-mA bolometer. At a modulation frequency fm near 0 Hz, the equivalent audio source impedance of the bolometer is R1 +R0. The common belief that the source impedance is R0 in the weak-signal case is, therefore, refuted. Formulas are derived giving vg / /PRF and R1 as functions of basic, easily determined bolometer parameters. The time constant for open-circuit load is τoc= R1C, where τoc is determined best by measurement, since catalog values of τoc often are seriously in error. The capacitance is C=τoc / /R1. With one type of bolometer τoc measures about 110 μs, while various catalogs state values of 250 to 350 μs. The equivalent circuit is confirmed quantitatively by measurements of output voltage and source impedance versus modulation frequency.