A self-calibrating instrument for measuring conductance at radio frequencies

Accuracies obtainable in measuring resistance (co nductance) at radio fr equencies have lagged behind those obtainable in measuring reactance (s usceptancc), bccause frequencyd ependent changes in the values of r esistors ar c not as well known as are such changes in capacitors, and because no satisfactory method has existed for comparing resistors to capacitors at radio frequencies.An instrum ent based 011 the twin T null circuit can be made self-calibrating at a given frequency, allowing conductances in the proper range to be measured directly in terms of a change in capacitance, withou t any need for acc urate knowledge of t he values of the circuit elements.Analysis of a practical circuit s hows t hat s uch an instrum ent is potentially capable of m easuring conductance to within a few parts in 10 6 at frequencies below abo ut 15 MHz, and t hat accuracies actually obtained will be limi ted o nly by the acc uracy to which changes in effective capacitance are known.Conductances between 0.01 a nd 0.0001 mho were measured at 10 7 rad ians/sec with an estimated errol' of 0.05 percent, which is the errol' associated with the meas urement of changes in effective capacitance at that fr eq uency.' Radio Standards Engineering Divi sion, NBS Boulder Laboratories.Boulder, Colo.t Figures in brackets indicate tho literature references at the end of this paper.