Todd A. Ell

Date of Award


Document Type


Degree Name

Master of Science (MS)


Electrical Engineering


There exists a problem of accurately performing spirographic measurements under physical stress situations. Existing systems, which use mechanical structures in the measurement process, have response times that are too slow, or are too bulky to be considered portable.

The proposed system solves these problems and has a number of attractive characteristics. The system uses relatively inexpensive solid state electronic components which implies a minimal of mechanical parts; portability; and a linear, fast response time.

The system presented in this thesis determines the velocity and temperature fluctuations of the human breath by measuring the difference and sum of the transit times for two continuous sound waves travelling in opposite directions along the air path. The information about the transit times is contained in the phase differencces of the two sound waves across the path. A phase-locked loop is used to keep the differences across the parth constant, irrespective of air - and sound - velocity variations. Therefore, the phase information is converted to frequency variations in the phase-locked loop.