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ELEKTRONIKOS ĮTAISAI 2009
VGTU EF ESK [email protected]
1
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.
ELEKTRONIKOS ĮTAISAI 2009
VGTU EF ESK [email protected]
2
Electromechanical resonators and transducers
The structure of an electro-
mechanical 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.
SE
lL
2π
2=
2
ρlSC =
LCf
π2
1r1 =
0r1
0
0
2r 1
π2
1
C
Cf
CC
CCL
f +=
+
=
ELEKTRONIKOS ĮTAISAI 2009
VGTU EF ESK [email protected]
3
0
jX
-jX
Reacta
nce
Fundamental mode
3rd overtone
5th overtone
Frequency
Spurious
responsesSpurious
responses
Spurious
responses
Overtone Response of a Quartz Crystal
ELEKTRONIKOS ĮTAISAI 2009
VGTU EF ESK [email protected]
4
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.
ELEKTRONIKOS ĮTAISAI 2009
VGTU EF ESK [email protected]
5
3-3
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.
STRAIN
EXTENSIONAL
along:
SHEAR
about:
FIELD along:
X
Y
Z
X
Y
Z
X Y Z
√√√√
√√√√
√√√√
√√√√
√√√√
X
Y
Z
The Piezoelectric Effect in Quartz
ELEKTRONIKOS ĮTAISAI 2009
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6
Flexure Mode Extensional Mode Face Shear Mode
Thickness Shear
Mode
Fundamental Mode
Thickness Shear
Third Overtone
Thickness Shear
Modes of Motion
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7
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.
sf
8,2...6,1r =
Third Overtone
Thickness Shear
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8
x xl
y
φ
z
θ
θ
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)
X-cut: ≈ -20 ppm/0C
(extensional mode)
90o
60o
30o
0
-30o
-60o
-90o0o 10o 20o 30o
AT FC IT
LC SC
SBTCBT
θ
φ
Singly
Rotated
Cut
Doubly Rotated Cut
Zero Temperature Coefficient Quartz Cuts
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9
Singly
Rotated
Cut
Doubly Rotated Cut
X
X’
Y
θ
θ
ϕ
Z
3-18
Singly Rotated and Doubly Rotated Cuts’
Vibrational Displacements
Singly rotated resonator
Doubly rotated resonator
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10
Base
Mounting
clips
Bonding
area
ElectrodesQuartz
blank
Cover
Seal
Pins
Quartz
blank
Bonding
area
Cover
Mounting
clips
SealBase
Pins
Two-point Mount Package Three- and Four-point Mount Package
Top view of cover
Resonator Packaging
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11
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.
Metal
electrodes
Acoustic
waveguide
Piezoelectric
plateAcoustic
load
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12
Magnetostriction is also used for transducers.
Magnetostrictive resonators and transducers
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13
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.
Magnetostrictive resonators and transducers
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14
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 – band-
pass) filters having high selectivity.
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15
Electromechanical filters
Excitement of torsion
vibrations
Electromechanical filters:
100 Hz – 1 MHz
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16
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…
ELEKTRONIKOS ĮTAISAI 2009
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17
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.
ELEKTRONIKOS ĮTAISAI 2009
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18
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.