Page 357 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

11

IDT Microsensor Parameter

Measurement

11.1 INTRODUCTION TO IDT SAW SENSOR
INSTRUMENTATION

There is no specific design procedure for interdigital transducer (IDT) microsensors based
on surface acoustic wave (SAW) delay lines. Although substantial work has been done
on delay line designs for filtering and signal-processing applications, the requirements
for SAW-based devices are essentially different from those for commercial non-SAW
oscillator-based sensors (Avramov 1989). The SAW device should not only have the
appropriate frequency-transfer characteristics, but its physical dimensions should also
allow for miniaturisation and remote-sensing of a variety of physical and chemical media.
This chapter deals with the instrumentation and measurement aspects of a typical
IDT-SAW sensor during the course of its operation and so covers the different measure-
ment techniques available and makes a comparison between them.
Specifically, Section 11.3 describes the basic principles of a 'network analyser,' whereas
subsequent sections describe its use to measure the amplitude (Section 11.4), phase
(Section 11.5), and frequency (Section 11.6) of signals. Finally, a brief overview of a
network analyser system is given with particular emphasis on the topics of forward-
matching, reverse-matching, and transmission (see Gangadharan 1999).

11.2 ACOUSTIC WAVE SENSOR INSTRUMENTATION

11.2.1 Introduction

Acoustic wave sensors convert the physical or chemical property of interest into a signal
suitable for measurement. Ultimately, the measured sensor data must be processed so that
they can be presented to the user in both a sensible and meaningful way. The role of
system instrumentation is to implement this task.
Ever since the early work by Sauerbrey and King (Avramov 1989), and many of
those that have followed, the vast majority of acoustic-sensing applications has involved
acoustic sensors being used as the active frequency-control elements in oscillator circuits.
The oscillator output frequency is then used as the (desired) measured parameter. This is