Physical Principles of RFID Systems
4.3 Surface Waves
4.3.4 The Sensor Effect
The velocity vof a surface wave on the substrate, and thus also the propagation time τ and the mid-frequencyf0of a surface wave component, can be influenced by a range of physical variables (Reindl and M´agori, 1995). In addition to temperature, mechanical forces such as static elongation, compression, shear, bending and acceleration have a particular influence upon the surface wave velocityv. This facilitates the remote interrogation of mechanical forces by surface wave sensors (Reindl and M´agori, 1995).
In general, the sensitivityS of the quantityx to a variation of the influence quantityy can be defined as:
Syx= 1 x·∂x
∂y (4.119)
The sensitivityS to a certain influence quantityyis dependent here upon substrate material and crystal section. For example, the influence of temperatureT upon propagation speedvfor a surface wave on quartz is zero. Surface wave transponders are therefore particularly temperature stable on this material. On other substrate materials the propagation speedvvaries with the temperatureT.
Measurement time t(s)
Interrogation distance (m)
2.45 GHz, IL = 40 dB 433 MHz, IL = 35 dB
10−5 10−4 10−3 10−2 10−1 1 101 10
1
0.1
101 1
Figure 4.100 Calculation of the system range of a surface wave transponder system in relation to the inte-gration timetiat different frequencies (reproduced by permission of Siemens AG, ZT KM, Munich)
Table 4.10 The properties of some common surface wave substrate materials
Material Crystal direction V k2 SvT(T k) Damping (dB/µs) Section* Prop** (m/s) (%) (ppm/◦C) 433 MHz 2.45 GHz
Quartz ST X 3158 0.1 0 0.75 18.6
Quartz 37◦ rot-Y 90◦rot-X 5092 =0.1 00 3 3
LiNbO3 Y Z 3488 4.1 94 0.25 5.8
LiNbO3 128◦rot-Y X 3980 5.5 75 0.27 5.2
LiTaO3 36◦ rot-Y X 4112 =6.6 30 1.35 3 20.9 3
LiTaO3 X 112◦rot-Y 3301 0.88 18 – –
∗Section – surface normal to crystal axis
∗∗Crystal axis of the wave propagation
3Strong dependency of the value on the layer thickness
The temperature dependency is described by the sensitivity STv (also called the temperature coefficientTk). The influence of temperature on the propagation speedv, the mid-frequencyf0and the propagation timeτ can be calculated as follows (Reindl and M´agori, 1995):
v(T )=v(T0)·[1−STv·(T−T0)] (4.120) f0(T )=f0(T0)·[1−STv·(T −T0)] (4.121) τ (T )=τ (T0)·[1+STv·(T−T0)] (4.122)
4.3.4.1 Reflective Delay Lines
If only the differential propagation times or the differential phases between the individual reflected pulses are evaluated, the sensor signal is independent of the distance between the reader and the
transponder. The differential propagation timeτ2−1, and the differential phaseθ2−1 between two received response pulses is obtained from the distanceL2−1between the two reflectors, the velocity vof the surface wave and the frequencyf of the interrogation pulse.
τ2−1= 2·L2−1
v (4.123)
ϕ2−1=2πf ·τ2−1=4πf ·L2−1
v (4.124)
The measurable changeτ2−1 orθ2−1when a physical quantityy is changed by the amount y is thus:
τ2−1=τ2−1·Syτ ·y (4.125)
ϕ2−1=2πf ·τ2−1·Syτ·y (4.126) The influence of the physical quantityyon the surface wave transponder can thus be determined only by the evaluation of the phase difference between the different pulses of the response signal. The measurement result is therefore also independent of the distance between reader and transponder.
For lithium niobate (LiNbO3, YZ section), the linear temperature coefficientTk=STvis approxi-mately 90 ppm/◦C. A reflective delay line on this crystal is thus a sensitivetemperature sensorthat can be interrogated by radio. Figure 4.101 shows the example of the pulse response of a temperature sensor and the temperature dependency of the associated phase values (Reindlet al., 1998c). The precision of a temperature measurement based upon the evaluation of the associated phase value θ2−1 is approximately±0.1◦C and this precision can even be increased by special measures such as the use of longer propagation pathsL2−1 (see Equation 4.124) in the crystal. The unambiguity of the phase measurement can be assured over the entire measuring range by three or four correctly positioned reflectors.
4.3.4.2 Resonant Sensors
In a reflective delay line the available path is used twice. However, if the interdigital transducer is positioned between two fully reflective structures, then the acoustic path can be used a much greater number of times due to multiple reflection. Such an arrangement (see Figure 4.101) is called a surface wave one-port resonator. The distance between the two reflectors must be an integer multiple of the half-wavelengthλ0at the resonant frequencyf1.
The number of wave trains stored in such aresonatorwill be determined by its loadedQfactor.
Normally aQ factorof 10 000 is achieved at 434 MHz and at 2.45 GHz aQfactor of between 1500 and 3000 is reached (Reindlet al., 1998b). The displacement of the mid-frequency f1 and the displacement of the associated phaseθ1of a resonator due to a change of the physical quantity y with the loaded Q factor are (Reindlet al., 1998a):
f1= −f1(y0)·Sy,1·y (4.127)
and
ϕ=2Q·f
f (4.128)
wheref1is the unaffected resonant frequency of the resonator.
In practice, the same sensitivity is obtained as for a reflective delay line, but with a significant reduction in chip size (Reindlet al., 1998c).
Amplitude (dB) Phase change (rad/2π)
Running time (µs) (a) 0
0 5 10 15 20 25 30 35 40 45 50
−100
0 1 2 3 4 5
−80
−60
−40
−20
1 2 3 4 5 6 7 8 9 10
∆t= 0.8µs
∆t= 0.8µs
∆t= 2.27µs
∆t= 2.27µs
Temperature change (°C) (b)
Figure 4.101 Impulse response of a temperature sensor and variation of the associated phase values between two pulses(τ=0.8µs)or four pulses(τ=2.27µs). The high degree of linearity of the measurement is striking (reproduced by permission of Siemens AG, ZT KM, Munich)
Antenna Electroacoustic transducer
Acoustic
reflector Sensor signal
Interrogation signal
Frequencyf0
Figure 4.102 Principal layout of a resonant surface wave transponder and the associated pulse response (reproduced by permission of Siemens AG, ZT KM, Munich)
Antenna Interdigital transducer
Piezoelectric crystal Reflector RF interrogation
signal
RF response
Figure 4.103 Principal layout of a surface wave transponder with two resonators of different frequency (f1, f2)(reproduced by permission of Siemens AG, ZT KM, Munich)
0
−20
−40
−60
−80
−100
0 5 10 15 20 25 30
Time (µs) S11 (dB)
Ungated
Gated S11 (dB)
0
−20
−40
−60
432 433 434
Frequency (MHz) 435 436
Figure 4.104 Left, measured impulse response of a surface wave transponder with two resonators of different frequency; right, after the Fourier transformation of the impulse response the different resonant frequencies of the two resonators are visible in the line spectrum, here: approximately 433.5 and 434 MHz (reproduced by permission of Siemens AG, ZT KM, Munich)
If, instead of one resonator, several resonators with different frequencies are placed on a crystal (Figure 4.103), then the situation is different: instead of a pulse sequence in the time domain, such an arrangement emits a characteristic line spectrum back to the interrogation device (Reindlet al., 1998c, d) which can be obtained from the received sensor signal by a Fourier transformation.
The difference f2−1 between the resonant frequencies of the two resonators is determined to measure a physical quantity y in a surface wave transponder with two resonators. Similarly to Equation (4.127), this yields the following relationship (Reindlet al., 1998c).
f2−1= −[f2(y0)·Sy,2−f1(y0)·Sy,1]·y (4.129)
4.3.4.3 Impedance Sensors
Using surface wave transponders, even conventional sensors can be passively interrogated by radio if the impedance of the sensor changes as a result of the change of a physical quantity y (e.g.
HF interrogation signal
HF response
Antenna
Piezoelectric crystal
Load impedance (switch or external sensor) r Z
Figure 4.105 Principal layout of a passive surface wave transponder connected to an external sensor (repro-duced by permission of Siemens AG, ZT KM, Munich)
photoresistor, Hall sensor, NTC or PTC resistor). To achieve this a second interdigital transducer is used as a reflector and connected to the external sensor (Figure 4.105). A measured quantityy thus changes the terminating impedance of the additional interdigital transducer. This changes the acoustic transmission and reflectionρof the converter that is connected to this load, and thus also changes the magnitude and phase of the reflected RF pulse, which can be detected by the reader.