Page 426 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

406 MEMS-IDT MICROSENSORS

response resolution for the time band pass, using the minimum window, can be expressed
as a parameter r:
1.2
r = -— (14.3)
f span

where the frequency span f span is expressed in Hz. Thus, if a frequency span of 10 MHz
is used, the measurement system will not be able to distinguish between equal magnitude
responses separated by less than 0.12 us for transmission measurements.
Time domain range response is the ability to locate a single response in time. Range
resolution is related to the digital resolution of the time domain display, which uses the
same number of points as that of the frequency domain. The range resolution can be
computed directly from the time span and the number of points selected. If a time span
of 5 us and 201 points are used, the marker can read the location of the response with
a range resolution of 24.8 ns (5 us/201). The resolution can be improved by using more
points in the same time span.

14.4.4 Experimental

We now evaluate the suitability of using S 11 measurement for the measurement of reflec-
tions in SAW delay line.
The operating principle of the device is based on the perturbation in the velocity of the
acoustic wave due to the changes in the electrical boundary conditions. The two extreme
electrical boundary conditions were applied in turn. An attempt was then made to detect
these in various measurement options that are available in the vector network analyser.
These electrical conditions represent the maximum change possible for the device designed
and hence are useful in evaluating suitability of any measurement technique. The details
of this device are given in the following paragraph. Split finger electrodes were used in
order to reduce reflections from the electrodes:

• Number of finger pairs is 10

• Propagation path length is 6944 urn
• Operating frequency is 82.91 MHz

Two devices representing the two extreme electrical boundary conditions were used:

1. A SAW delay line with split finger IDTs and with an aluminum layer in the propagation
path. This device represents the electrical boundary condition in which the electric field
is shorted on the substrate surface.
2. A SAW delay line with split finger IDTs and without an aluminum layer in the prop-
agation path. This device represents the electrical boundary condition in which the
electric field decays at an infinite distance from the substrate surface.

Both these devices have a propagation path length of 6944 urn. The SAW propagation
2
velocity on the substrate is 3980 m/s and the crystal size is (10 x 10) mm . The equipment