User Manual

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Figure 11. Dielectric permittivity and dissipation factor
vs. frequency
Resonance values thus depend on film thickness.
They range from low MHz for thick films
(1,000 ) to >100MHz for very thin films.
µ
m
Figure 11 shows the effect that frequency has on
permittivity and dissipation factor at room
temperature. As a result of its very low permittivity
… (1 percent that of piezo ceramics), the film
exhibits g-constants (voltage output coefficients)
that are significantly greater than piezo ceramics
(g = d/…).
PIEZO FILM AT LOW FREQUENCIES
Introduction
The behavior of a piezo film component at low frequencies is fairly straightforward to describe in
electrical terms, yet is quite frequently misunderstood. Since any practical application of the
technology will most likely involve some consideration of this topic, it is the intent of this article to
examine the subject at some length. The treatment is made as non-mathematical as possible, with
verbal descriptions and real-world examples being used to illustrate the concepts. Some familiarity
with the use of FFT techniques to transform between time-domain and frequency-domain
descriptions is assumed, but not essential.
Connecting Up
In most instances, the first evaluation of piezo film begins with connecting a piezo component to an
oscilloscope via a probe ("scope probe"). Under normal electronics circumstances, a scope probe
can be considered to be an "infinite impedance" - so high, that its effect on the circuit under test can
be neglected. Not so with piezo film - in many cases, a scope probe can act almost like a short-
circuit. Typical probes, when plugged in to an oscilloscope, have an effective resistance of 1MΩ (one
million ohms). Others may be fixed at 10MΩ, while many are conveniently switchable between "x1"
( 1MΩ) and "x10" (10MΩ). Note that the physical element comprising the 1MΩ resistance is usually
built into the oscilloscope input stage, rather than being a discrete component within the probe itself.
A "x1" probe is thus basically a length of shielded cable with suitable contacts attached to each end.
Source Capacitance
To analyze what will happen when the probe is connected, we now need to consider the properties
of the piezo film element. Perhaps the most important characteristic (after the piezoelectric property,
of course) is the material's capacitance. Capacitance is a measure of any component's ability to store
electrical charge, and is always present when two conductive plates are brought close together. In
our case, the conductive plates are the conductive electrodes printed or metallized onto each surface
of the film. The capacitance of the device is strongly affected by the properties of the insulator
serving to space the plates apart, and the measure of the insulator's capacity to store charge is given
by its dielectric constant or permittivity.