This is an excerpt from T. Carter, K. Taylor, K. Taylor, and M. Naylor, " Audio Impedance Meter," ECE 4522: Design II, Department of Electrical and Computer Engineering, Mississippi State University, Fall 2003:

Our device will measure the impedance of standard systems, including 70.7-volt systems, which are standard in the United States [3]. We will use a PIC to produce the desired wave, as well as perform all of our calculations. This PIC will be programmed using C++. The PIC will generate a 1 kHz pulse-width modulation signal to the active filter. The active filter has been tested and converts this signal from a 1 kHz-input wave to a sine wave. This desired result has been confirmed using P-SPICE and actual bread boarded tests. We are inputting a square wave from the Function Generator to our filter design, and getting the desired sine wave. We have used the oscilloscope to confirm the results. This sine wave will be passed through the amplifier, which will in turn amplify the voltage of the signal leaving the filter to 5 volts. The amplifier has also been tested using P-SPICE simulations and in lab implementations. The voltage is also being amplified as desired. The rectifier circuit will convert voltage signals from AC to DC. The rectifier is still in the process of being implemented using P-SPICE. The DC signals will be relayed back to the PIC so it can perform the calculations for the device.