Active and Passive Direct-Reading Ratio Sets for the Comparison of Audio-Frequency Admittances

Design considerations and constructional details for two audio-frequency direct-reading ratio sets are presented. These devices allow the comparison of admittances with accuracies up to one part in 109. The first direct-reading ratio set (DRRS) is based upon operational amplifier circuitry, and utilizes a two-stage amplifier system. With this technique, only a moderate voltage amplification per stage is required, and the circuitry is therefore not difficult to stabilize to prevent oscillation. The second DRRS is purely passive and makes use of two-stage voltage transformers to reduce the detrimental effects of transformer excitation current upon the linearity and ratio stability of the device. Equivalent circuits representing the behavior of critical parts of the two DRRSs are presented and analyzed. A convenient procedure for calibrating a DRRS is treated mathematically.