Page 358 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

338 IDT MICROSENSOR PARAMETER MEASUREMENT

1
not so surprising, as acoustic sensors are digital by nature with a good dynamic range
and linearity. Therefore, state-of-the-art system instrumentation can readily exploit the
digital acoustic microsensor. One of the main advantages digital instrumentation has
over the earlier system technologies is that there is no requirement to perform any
frequency to voltage (for output) conversion - a process that often causes a consider-
able loss of resolution. In the state-of-the-art systems, frequency to digital conversion
takes place without loss 2 through the use of simple electronic counters. However, other
measuring system instrumentation must be considered and compared in terms of its
viability, functionality, and suitability. Having accepted that either frequency or multiple-
period measurements (timing signal) is the most appropriate parameter to measure, consid-
eration will now be given to past and current forms of such instrumentation (Campbell
1998).
There are three electronic configurations that may be used to measure the response of
a SAW (two-port) microsensor:
1. Where the microsensor is connected to a network analyser (or vector voltmeter) and
scanned by a narrow radio frequency (RF) band on either side of its fundamental
resonant frequency (see Section 11.3).

2. Where the microsensor is used as the passive element, being driven from a fixed RF
source (amplitude or phase measurements) (see Sections 11.4 and 11.5).
3. Where the microsensor is used as the active feedback-determining element, controlling
the frequency of an oscillator circuit (frequency measurement) (see Section 11.6).

11.3 NETWORK ANALYSER AND VECTOR
VOLTMETER

Primarily, the network analyser and vector voltmeters are used by radio and electronic
engineers for network analysis and design. Although SAW resonators have equivalent
electric circuit analogues, network analysis is usually performed on the basis of the idea
of electrically matching resonators with then- corresponding oscillator circuits. This is
particularly true for the SAW-sensor oscillator design. From either the network analysers
or the vector voltmeters, the characteristic admittance Y or impedance Z can be measured.
These measurements will produce a locus of the admittance or impedance parameters over
a set of discrete frequency data points. The admittance (or impedance) can be expressed in
either rectangular (real and imaginary) or polar (magnitude and phase) coordinates. Anal-
ysis from such instruments will deliver the most information possible about an acoustic
transducer or the sensor that is under load, detailing shifts in resonant frequency and any
changes in the quality factor Q (Subramanian 1998; Piscotty 1998).
Despite the advantages that either the network analyser (or vector voltmeter) has in
delivering very detailed information on a number of important parameters, both these
instruments are impractical as general-purpose laboratory tools. The reasons for this are
that the resonator setup is:

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Digital in the sense that they are active frequency elements in oscillator circuits where frequency counters
are used to measure the output frequency.
2
High resolutions are achieved through long counting periods.