A DC-RF Substitution Error in Duel-Element Bolometer Mounts

Most of the coaxial type bolometer mounts in current use employ a pair of bolometer elements which are connected in series for the dc or audio-frequency bias powers, and in parallel at radio or microwave frequencies. In the frequency range where these techniques are most often employed, the dc-RF substitution error has been generally believed to be negligible. It is quite possible, however, for this to be true of the elements individually, and yet fail to be true of a pair of these elements as used in a typical coaxial mount. If only the sum of the resistances of the two elements is maintained at a constant value, and if the resistance division between the two elements changes with the application of RF power, an error is introduced which is given by the equation: e = [(1/γb)-(1/γa)]Δr, where γa and γb are the "ohms per milliwatt" coefficients of the two bolometer elements, and γr is the shift in resistance division. An experimental study indicates that, in the existing state-of-the-art, this error may be ignored in many applications but is large enough to be important in others, particularly at the higher power levels.