Rapid measurement of impedance and admittance

A method is described for direct, rapid, and simultaneous measurement of the real and quadrature components of a driving-point or transfer impedance and admittance over a wide and continuous range of frequency. Basically, the method employs a mixer which derives its signal and local oscillator voltages from the voltage across and current through the unknown impedance respectively. With the unknown driven by a sinusoidal constant current, the mixer d-c output is linearly related to the unknown resistance. If the phase difference between the two derived voltages is altered by 90 degrees, the mixer d-c output is linearly related to the unknown reactance. To measure the components of an admittance, the unknown is driven by a sinusoidal constant voltage and the signal and local oscillator voltages are interchanged. Apparatus using this method for measurement of impedance and admittance in the frequency range of 50 kc to 5 megacycles (mc) has been built. With suitable modifications of detail, the method appears to be usable from acoustic to microwave frequencies.