William H Couch

US Patent:

50639374, Nov 12, 1991

Filed:

Sep 12, 1990

Appl. No.:

7/581579

Inventors:

Bertram N. Ezenwa - Centerville OH
William P. Couch - Fairborn OH

Assignee:

Wright State University - Dayton OH

International Classification:

A61B 505

US Classification:

128723

Abstract:

The multiple frequency impedance measurement system of the present invention permits noninvasive examination of living tissue at any one of a plurality of frequencies. Precise frequency control is provided by a phase locked loop (PLL) circuit which is adjustable among at least a plurality of frequencies by locking the loop to the output signal generated by a voltage controlled oscillator (VCO) divided by a selectable divisor. By selecting the divisor of the output signal of the VCO, the frequency is changed and, due to the use of a PLL circuit, each selected frequency is stably maintained. Electrodes connecting the circuit to tissue to be monitored are driven by a high-Q filter and buffer amplifier which convert the square wave output from a voltage controlled clipper circuit to a sine wave signal. The clipper circuit receives the output signal from the VCO and is part of a feedback loop which maintains a constant examination current for the system. By monitoring the examination current via a precision resistor, a current reference signal is generated and used in the feedback loop to maintain the current at a substantially constant but selectable level.