ELEKTRONIKOS ĮTAISAI
1
2009
Bulk Acoustic Wave Devices
In electronic devices, electrical signals are used for the excitation of acoustic
waves. Mechanical vibrations and acoustic waves are excited due to
piezoelectric effect or magnetostriction.
The piezoelectric effect, discovered by Pierre Curie and Jacques Curie in
1880, is exhibited by certain crystals, e.g., quartz and ceramic materials.
The production of a voltage between opposite sides of a piezoelectric crystal
as a result of pressure or twisting. Also the reverse effect which the
application of a voltage to opposite sides causes a deformation to occur at
the frequency of the applied voltage. (Converts mechanical energy into
electrical energy and electrical energy into mechanical energy.)
Magnetostriction is a property of ferromagnetic materials that causes them
to change their shape when subjected to a magnetic field. The effect was
first identified in 1842 by James Joule when observing a sample of nickel.
The essence of the effect is in the mechanical deformation of a feromagnetic
material when it is subjected to a magnetic field. Mechanical deformations
depend on magnetic induction (magnetic flux density). Conversely, when the
material is subjected to mechanical stress, changes in its magnetisation and
magnetic permeability are observed.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
2
2009
Electromechanical resonators and transducers
The structure of an electromechanical resonator or transducer
The disc is a mechanical resonator. Due to the
piezoelectric and reverse piezoelectric effects the
device has properties of an electrical resonator.
L=
2 l
π 2 SE
f r1 =
fr2 =
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
1
C0 C
2π L
C0 + C
C=
lSρ
2
1
2π LC
= f r1 1 +
C
C0
ELEKTRONIKOS ĮTAISAI
3
2009
Overtone Response of a Quartz Crystal
jX
Reactance
Spurious
responses
Spurious
responses
Spurious
responses
0
Frequency
5th overtone
-jX
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3rd overtone
Fundamental mode
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
4
2009
Electromechanical resonators and transducers
The resonant frequencies and their electrical quality depend on the crystal
dimensions, how the surfaces are oriented with respect to its axes and how the
device is mounted. Frequencies ranging from a few kilohertz to a few hundred
megahertz and quality values in the range from several thousand to several
hundred thousand are commercially available. Quartz resonators are stable, but
at the same time expensive. Therefore piezoelectric ceramics are used for
piezoelectric resonators. Piezoelectric ceramics are often used for piezoelectric
transducers that convert electric oscillations to mechanical vibrations and
mechanical vibrations to electric oscillations in filters, delay lines and other
devices.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
5
2009
The Piezoelectric Effect in Quartz
STRAIN
X
X
EXTENSIONAL
along:
Y
Y
Z
√
√
X
Z
X
SHEAR
about:
Z
FIELD along:
√
Y
√
Z
√
Y
In quartz, the five strain components shown may be generated by an electric field.
The modes shown on the next page may be excited by suitably placed and shaped
electrodes. The shear strain about the Z-axis produced by the Y-component of the
field is used in the rotated Y-cut family, including the AT, BT, and ST-cuts.
VGTU EF ESK
3-3
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
6
2009
Modes of Motion
Flexure Mode
Extensional Mode Face Shear Mode
Thickness Shear Fundamental Mode Third Overtone
Thickness Shear Thickness Shear
Mode
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stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
7
2009
Quartz resonators: modes of vibrations
The Flexure Mode is used in the low frequency range.
The Thickness Shear Mode is used
in the high frequency range.
fr =
VGTU EF ESK
1,6...2,8
s
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Third Overtone
Thickness Shear
ELEKTRONIKOS ĮTAISAI
8
2009
Zero Temperature Coefficient Quartz Cuts
90o
60o
AT
30o
FC
SC
LC
θ
0
-30o
z
IT
SBTC
BT
-60o
-90o
θ
Do
gl y d
n
i
S tate
Ro t
Cu
ub
Ro l y
ta t
Cu e d
t
θ
y
φ
x
VGTU EF ESK
0o
10o
φ
20o
The AT, FC, IT, SC, BT, and SBTC-cuts are some
of the cuts on the locus of zero temperature
coefficient cuts. The LC is a “linear coefficient”
cut that has been used in a quartz thermometer.
Y-cut: ≈ +90 ppm/0C
(thickness-shear mode)
xl
30o
X-cut: ≈ -20 ppm/0C
(extensional mode)
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
9
2009
Singly Rotated and Doubly Rotated Cuts’
Vibrational Displacements
Z
Singly rotated resonator
θ
gl y
Sin ated
t
Ro
t
Cu
Do
u
Ro bly
ta
Cu ted
t
θ
Y
Doubly rotated resonator
ϕ
X
X’
VGTU EF ESK
3-18
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
10
2009
Resonator Packaging
Two-point Mount Package
Quartz
blank
Three- and Four-point Mount Package
Electrodes
Bonding
area
Cover
Mounting
clips
Seal
Base
Pins
Quartz
blank
Bonding
area
Cover
Mounting
clips
Seal
Pins
Base
Top view of cover
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stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
11
2009
Piezoelectric resonators and transducers
Ceramic resonators are made of high-stability piezoelectric ceramics,
generally lead zirconium titanate (PZT) which functions as a mechanical
resonator. A ceramic resonator is often used in place of quartz crystals in
electronic circuitry because of its lower cost and smaller size. It is used on
circuits where frequency specifications aren't highly critical (quartz has a
0.001% frequency tolerance, while PZT has a 0.5% tolerance).
Piezoelectric ceramics are often used for piezoelectric transducers that
convert electric oscillations to mechanical vibrations and mechanical
vibrations to electric oscillations in filters, delay lines and other devices.
Acoustic
load
Piezoelectric
plate
Acoustic
waveguide
Metal
electrodes
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
12
2009
Magnetostrictive resonators and transducers
Magnetostriction is also used for transducers.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
13
2009
Magnetostrictive resonators and transducers
Mechanical
resonator
The alternating current flowing in the coil creates magnetic flux in the
resonator and vibration of the resonator arises due to the magnetostrictive
force. Because the characteristic δl(B) is nonlinear at the origin, a
permanent magnet is used for bias. So the device may be used to convert
electric energy to mechanical.
If mechanical vibrations of the resonator are excited, its permeability
changes periodically. Due to the change of permeability, magnetic flux
created in the resonator by the permanent magnet varies periodically.
Then the electromotive force is induced in the coil due to the
electromagnetic induction effect. Thus, the device may be also used for the
conversion of mechanical vibrations to electric oscillations.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
14
2009
Piezoelectric and magnetostrictive resonators
Piezoelectric resonators are used in crystal oscillators and piezoelectric filters.
Piezoelectric and magnetostrictive transducers are used in electromechanical
filters and bulk acoustic wave delay lines.
Electromechanical and piezoelectric filters are used as band (usually – bandpass) filters having high selectivity.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
15
2009
Electromechanical filters
Excitement of torsion
vibrations
Electromechanical filters:
100 Hz – 1 MHz
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
16
2009
Piezoelectric filters
Piezoelectric filters are used in the frequency range to 100 MHz.
Quartz or ceramic resonators may be used in these filters. Quartz
resonators have high quality and are very stable. Due to the high quality a
few resonators can ensure high selectivity.
The prices of ceramic resonators are lower. But the quality of these
resonators is also lower and more resonators are needed to obtain high
selectivity. Application of ceramic resonators is also limited by requirements
of high stability.
Electrical coupling of resonators…
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
17
2009
Piezomechanical and monolithic filters
In piezomechanical filters
piezoelectric resonators are
coupled mechanically
Mechanical coupling of piezoelectric
resonators is also used in monolithic
piezoelectric filters. Metal electrodes
with piezoelectric material between
them function as piezoelectric
resonators. The space between the
adjacent metal electrodes functions as
a coupling element.
Monolithic filters are applied in the
frequency rage from 2 to 200 MHz.
They are small in size like monolithic
ICs.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt
ELEKTRONIKOS ĮTAISAI
18
2009
Ultrasonic Delay Lines
td = L / va
Ultrasonic delay lines are used in the frequency range to 100 MHz. Their
frequency response is like that of the band-pass filters. For this reason
ultrasonic delay lines may be used for delay of radio wave pulses having
band spectrum.
VGTU EF ESK
stanislovas.staras@el.vgtu.lt