WHAT IS A TRANSPONDER ?
A communications satellite's transponder, is the series of
interconnected units which form a communications channel between
the receiving and the transmitting antennas. It is mainly used in
satellite communication to transfer the received signals.
In air navigation or radio frequency identification, a transponder is a
device that emits an identifying signal in response to an interrogating
In optical fibre communications, a transponder is the element that
sends and receives the optical signal from a fibre. A transponder is
typically characterized by its data rate and the maximum distance
the signal can travel.
WHAT IS SURFACE ACOUSTIC WAVE ?
A surface acoustic wave (SAW)
is an acoustic wave traveling
along the surface of a material
exhibiting elasticity, with
an amplitude that typically
decays exponentially with depth
into the substrate.
SAWs were first explained in
1885 by Lord Rayleigh.
Rayleigh waves are a type
of surface acoustic wave that
travel on solids. They can be
produced in materials in
many ways, such as by a
localized impact or by piezoelectric transduction, and are
frequently used in nondestructive testing for
detecting defects. They are
part of the seismic waves that
are produced on
the Earth by earthquakes.
When guided in layers they
are referred to as Lamb
waves, or generalized
Experimental image of surface
acoustic waves on a crystal of
WHAT IS A SAW TRANSPONDER ?
Surface acoustic wave (SAW) radio transponders
make it possible to read identification codes or
measurement values from a remote location. A
sensor system based on surface acoustic waves
(SAW) consists of a reader with transmitting and
receiving antenna(s), a SAW transponder that
may for example be designed as a reflective delay
line and a transponder antenna. The measurement
signals are contained in the SAW transponder’s
high-frequency response signal, which the reader
records and evaluates. Depending on the design
of the piezoelectric crystal in the SAW
transponder, its natural sensitivity for example to
temperature can be utilised.
Surface acoustic wave (SAW) devices are
based upon the piezoelectric effect and on the
surface related dispersion of elastic (= acoustic)
waves at low speed. If an (ionic) crystal is
elastically deformed in a certain direction,
surface charges occur, giving rise to electrical
voltages in the crystal (application: piezo
lighter). Conversely, the application of a
surface charge to a crystal leads to an elastic
deformation in the crystal grid (application:
piezo buzzer). Surface acoustic wave devices
are operated at microwave frequencies,
normally in the ISM range 2.45 GHz.
Schematic drawing of a wireless transponder system
based on a passive SAW transponder.
SAW sensors can be built with a SAW delay line element connected
to an antenna. The SAW delay line consists of a substrate, an interdigital transducer (IDT), and a reflector.
1. The transceiver sends RF interrogation signal which is received by
the antenna of the SAW sensor.
2. The IDT which is connected to the antenna, transforms the
received signal which is an electrical RF voltage applied between
the two opposing electrode combs into a SAW.
3. The SAW propagates on the piezoelectric crystal and is partially
reflected by reflectors placed in the acoustic path.
4. The reflected waves are reconverted into an electromagnetic
pulse train by the IDT and are retransmitted to the radar unit.
5. The high frequency electromagnetic signal is amplified and down
converted to the baseband frequency in the RF module of the radar
6. Then the sensor signals are analysed with a digital signal processor.
7. Finally the measurement results can be transferred to a personal
computer for post processing and data storage.
SAW PRESSURE SENSOR OPERATION :
The SAW propagates on a quartz diaphragm, bending under hydrostatic pressure. To
bend the diaphragm in a defined manner, there has to be a constant referencepressure at the other side of the diaphragm. This is realized by a hermetically closed
cavity with the reference pressure inside. Therefore with a sand-blast unit a blind-hole
was structured into a quartz cover plate, which is of the same substrate material as the
diaphragm. A monolithically packaged SAW radio transponder and pressure sensor
are developed for the application to a TPMS. The device contains the wireless
transponder, which converts analog signal into digital one without any auxiliary
electronic circuits and transmits the converted data wirelessly. The realization of the
mechanical A/D conversion is possible since the SAW radio transponder is connected
to the touch-mode capacitive pressure sensor.
A tire pressure monitoring
system (TPMS) is
an electronic system designed to
monitor the air pressure inside
the pneumatic tires on various types
of vehicles. TPMS report real-time
tire-pressure information to the
driver of the vehicle, either via a
gauge, a pictogram display, or a
simple low-pressure warning light.
TPMS can be divided into two
different types — direct
(dTPMS) and indirect (iTPMS)
Avoiding traffic accidents due to
under-inflated tires by early
recognition of the malfunction of
Reducing rolling resistance thus
increasing overall fuel efficiency
SUBJECTED TO TIRE PRESSURE AND A SEALED REFERENCE CHAMBER. THE ENERGY NEEDED IS PROVIDED
SIMPLE BENDING MODE. FIGURE BELOW SHOWS HOW THE SAW SENSOR IS USED IN TPMS.
Temperature measurement on high-voltage In metal working SAW sensors are used for
power lines is important additional information for
example to measure temperature in
optimum capacity utilisation of the grid. In this
refractory components. The measurements
case SAW sensors are mounted on the electricity
lines. The special benefit of SAW sensors in this
enable conclusions to be drawn regarding
application is that they operate completely
the state of the components, thus
passively and do not therefore require any
contributing to safety and reliability.
maintenance (e.g. battery replacement).
SAW sensors are also used to monitor the rotor They also can be used for contactless
temperature in high-speed electric motors. The
measurements in high vacuum process
sensor itself is located at the point of interest inside
the rotor and is connected to the antenna on the
front of the rotor. The reader is housed in the
heat, or strong radioactive radiation, where
the use of conventional sensors is
complicated, dangerous, or expensive.
Fixed Coded SAW ID Tags.
The decisive advantage of these SAW transponders lies in their passive operation
(i.e. no power-supply).
The possibility of wireless installation at particularly inaccessible locations.
The passive SAW transponders are maintenance free.
SAW radio transponders are advantageously placed on moving or rotating parts and
in hazardous environments such as contaminated or high voltage areas.
They can also be used for contactless measurements in high vacuum process
chambers, under concrete, extreme heat, or strong radioactive radiation, where the
use of conventional sensors is complicated, dangerous, or expensive.
IEEE paper on ―Theory and application of passive SAW radio transponders