Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Electrical Engineering


The purpose of the research reported in this thesis was to develop a microcontroller-based system to control an ultrasonic surface acoustic wave (SAW) dewpoint sensor unit that had two RF signals as outputs. Commercially available microcontroller-based dew point sensors are of the optical chilled-mirror type. Drawbacks of these system include high cost, poor performance at the frost point transition and susceptibility to contamination. The expected advantages of a SAW dew point instrument are better frost point transition performance, less susceptibility to contamination, and lower cost. The Motorola 68HC711E9 microcontroller was chosen for this system. The system included the microcontroller, frequency measurement circuit, clock circuit, RS- 232 communication port, liquid crystal display, DIA driven current source for cooling device and operating software. Two frequency measurement methods were studied: pulse accumulator and external counter system. The software's functions included determining condensation density, output of appropriate TE voltage, and display of temperature and was written in Motorola assembly language. A crystal resonator was found to be more stable than a ceramic resonator and therefore used in the microcontroller clock circuit to provide a stable time base for the frequency measurements. The design parameters established for this project were a frequency measurement range of 10 kHz to 900 kHz and resolution and accuracy of± 100 Hz. These specifications were met with an external frequency counting circuit operated by the microcontroller. The advantages of the external counting circuit were the ability to count to 25 MHz, which was well above the 1 MHz for the pulse accumulator, and compatibility with other microcontrollers. The sensor measurement results when compared to a commercial optical dew point instrument were a stability of ± 1 °C versus 0.2 °C for the commercial instrument. The development of a more stable SAW oscillator unit is critical for more stable and accurate SAW dew point instruments. A sensor with a higher Q value, which is expected to improve the SAW oscillator stability, is currently under development.

Library of Congress Subject Headings

Acoustic surface wave devices Dew point -- Measurement



Number of Pages



South Dakota State University