The following non-ideal situations in the proposed SAW devices are studied experimentally.
The effects of the change in separation between NCIDT and piezo-substrate, tilt between NCIDT and piezo-substrate, and change in orientation of NCIDT with respect to the piezo- substrate are analyzed by measuring the resonance frequency from S11 parameter using network analyzer. The experimental study of these effects is described below.
6.3.1 Effect of change in separation
The effect of separation between NCIDT and the piezo-substrate on the resonance frequency of a one port SAW resonator is studied experimentally by separating NCIDT from the piezo- substrate with a uniform known gap. The separation between NCIDT and the piezo-substrate is maintained by using stacks of pieces of transparency (OHP sheet) used in overhead projectors (OHP). The nominal thickness of transparency is 100 m. The pictorial representation of the device is shown in Figure 6.5. The separation between NCIDT and the surface of the piezo-substrate is varied from 0.1 mm to 1.0 mm in steps of 0.1 mm. The measurement setup is shown in Figure 6.3a. The scattering parameter S11 is measured using the network analyzer. The plots of S11 obtained from the network analyzer are shown overlapped in Figure 6.6. The resonance frequency and the corresponding value of S11 are tabulated in Table 6.1.
Figure 6.4 Plot of scattering parameter S11 for the SAW device with NCIDT fabricated on copper clad FR4 sheet.
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0
4.388 4.390 4.392 4.394 4.396 4.398 4.400
S
11(d B)
Frequency (MHz)
As the separation between NCIDT and the surface of piezo-substrate increases, the coupling strength decreases [3], [67] and the reflection coefficient S11 changes from – 9.142 dB to – 3.167 dB for separation of 0.1 mm to 1.0 mm. The resonance frequency changes from 4.393
Figure 6.6 Plots of scattering parameter S11 for various values of separation between NCIDT and the piezo-substrate.
Piezo-substrate Non-piezoelectric
substrate NCIDT
Changing the separation
Figure 6.5 Pictorial representation showing the separation between NCIDT and the piezo-substrate.
MHz to 4.396 MHz (change of about 0.07%) for separation of 0.1 mm to 1.0 mm. The main reason for the slight increase in frequency is due to the decrease in effective permittivity as the separation increases.
More detailed analysis is carried out in simulation using FEM and the simulation results are given in section 6.5.
6.3.2 Effect of tilt between NCIDT and piezo-substrate
The effect of tilt between NCIDT and piezo-substrate is analyzed by observing the change in the resonance frequency of the proposed SAW resonator. The resonance frequency of the proposed device is measured from the scattering parameter S11 using the network analyzer.
We have investigated the effects for tilt in two ways. In the first case, the NCIDT is tilted with the axis of rotation in the direction of wave propagation (Figure 6.7(a)). The tilt between NCIDT and piezo-substrate is made by placing stack of OFC sheets between of NCIDT and piezo-substrate on one side. The tilt angle is calculated from the stack height and the distance of the stack from the axis of rotation and taking tan-inverse of their ratio. For the stack height ranging from 0.1 mm to 1 mm, the tilt angle is found to vary from 0.145⁰ to 1.447⁰. The table
TABLE 6.1
RESONANCE FREQUENCY AND S11 AT VARIOUS VALUES OF SEPARATION BETWEEN NCIDT AND PIEZO-SUBSTRATE
Separation
(mm) Scattering parameter S11
(dB) Resonance frequency (MHz)
0.1 -9.14 4.393085
0.2 -8.44 4.393633
0.3 -6.92 4.394585
0.4 -6.48 4.394833
0.5 -5.42 4.395103
0.6 -5.11 4.395358
0.7 -4.39 4.395500
0.8 -4.32 4.395620
0.9 -3.36 4.395950
1.0 -3.17 4.396055
of resonance frequency and S11 at various tilt angles is given in Table 6.2. In the second case, the NCIDT is tilted with the axis of rotation normal to the direction of wave propagation (Figure 6.7(b)) and the tilt angle varies from 0.076⁰ to 0.758⁰.
Piezo-substrate NCIDT
λ
Stack of OHP Sheets
Tilt angle
(0.145° – 1.447°) Axis of rotation
Piezo-substrate NCIDT
λ Tilt angle (0.076° – 0.758°) Stack of
OHP Sheets
Axis of rotation
(a) (b)
Figure 6.7 Pictorial representation showing tilting of NCIDT for axis of rotation (a) in the direction of wave propagation, (b) normal to the direction of wave propagation.
Figure 6.8 Photograph of the measurement setup used to analyze the effect of tilt on the device characteristics.
TABLE 6.2
RESONANCE FREQUENCY AND S11 AT VARIOUS TILT ANGLES BETWEEN NCIDT AND THE PIEZO-SUBSTRATE: THE FIRST CASE
Tilt angle
(degree) Scattering parameter
S11 (dB) Resonance frequency
(MHz) Deviation in resonance frequency (kHz)
0 –11.02 4.392043 0
0.145 –9.71 4.392935 0.892
0.289 –9.33 4.393445 1.402
0.434 –8.65 4.393903 1.860
0.579 –7.84 4.394188 2.145
0.723 –7.34 4.394435 2.392
0.868 –6.55 4.394705 2.662
1.013 –6.33 4.394810 2.767
1.158 –5.13 4.394975 2.932
1.302 –4.79 4.395148 3.105
1.447 –4.06 4.395215 3.172
TABLE 6.3
RESONANCE FREQUENCY AND S11 AT VARIOUS TILT ANGLES BETWEEN NCIDT AND PIEZO- SUBSTRATE: THE SECOND CASE
Tilt angle (degree)
Scattering parameter S11 (dB)
Resonance frequency (MHz)
Deviation in resonance frequency (kHz)
0 –11.02 4.392043 0
0.076 –11.78 4.392168 0.125
0.152 –11.29 4.392736 0.693
0.227 –9.76 4.393297 1.254
0.303 –8.39 4.393654 1.611
0.379 –7.44 4.394053 2.010
0.455 –6.89 4.394402 2.359
0.531 –6.22 4.394727 2.684
0.606 –5.55 4.395003 2.960
0.682 –5.04 4.395255 3.212
0.758 –4.80 4.395336 3.293
Figure 6.9 Scattering parameter S11 at various tilt angles between NCIDT and the piezo-substrate:
The first case.
Figure 6.10 Scattering parameter S11 at various tilt angles between NCIDT and the piezo-substrate:
The second case.
-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1
4.388 4.390 4.392 4.394 4.396 4.398 4.400
S
11(d B)
Frequency (MHz)
0.145°
0.289°
0.434°
0.579°
0.723°
0.868°
1.013°
1.158°
1.302°
1.447°
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1
4.388 4.390 4.392 4.394 4.396 4.398 4.400
S
11(d B)
Frequency (MHz)
0.076°
0.152°
0.227°
0.303°
0.379°
0.455°
0.531°
0.606°
0.682°
0.758°
The table of resonance frequency and S11 at various tilt angles is given in Table 6.3. The measurement setup for studying the tilt effects on the device characteristics is shown in Figure 6.8. The plots of S11 parameter for various tilt angles for the two cases are given in Figures 6.9 and 6.10. From Tables 6.2 and 6.3, it is observed that the deviations in the device resonance frequency caused by tilt are small about 0.07% over the range of tilt angles under study. The main reason for the deviation in the resonance frequency is due to the change in effective permittivity with the tilt angle. The plot of resonance frequency versus tile angle for the first case is shown in Figure 6.11 (a) and for the second case is shown in Figure 6.11 (b).
6.3.3 Effect of orientation between NCIDT and piezo-substrate
The effect of relative orientation between NCIDT and piezo-substrate is analyzed by observing the change in the resonance frequency of the proposed resonator device. The resonance frequency of the proposed device is obtained from the scattering parameter S11
using the network analyzer. The pictorial representation of the device to study the effect of orientation is shown in Figure 6.12. The measurement setup for studying the effect of orientation of NCIDT with respect to the piezo-substrate is shown in Figure 6.8.
(a) (b)
Figure 6.11 Change in resonance frequency versus various tilt angles between NCIDT and piezo- substrate for (a) the first case, and (b) the second case.
TABLE 6.4
RESULTS OF EXPERIMENT ON ORIENTATION BETWEEN NCIDT AND PIEZO-SUBSTRATE Relative Orientation
( ⁰) Resonance frequency
(MHz)
Deviation (kHz)
0 4.392043 0
1 4.392101 -0.058
2 4.391978 0.065
3 4.391895 0.148
4 4.391779 0.264
5 4.391774 0.269
10 4.391548 0.495
15 4.391348 0.695
20 4.391450 0.593
25 4.391004 1.039
30 4.391231 0.812
Piezo-substrate λ
Changing the orientation
Non-piezoelectric substrate NCIDT
Figure 6.12 Pictorial representation showing relative orientation between NCIDT and the piezo- substrate.
To facilitate the measurement of relative orientation between NCIDT and the piezo-substrate, a plane paper with angles marked is kept under the semi-transparent piezo-substrate and the direction of wave propagation is marked on the top surface of the NCIDT as shown in Figure 6.8. The resonance frequency is measured from the S11 parameter at various relative orientations between NCIDT and the piezo-substrate. The values of resonance frequency and the angle of relative orientation are given in Table 6.4. It is observed that the variation in the resonance frequency is within 1 kHz (about 0.02%). The plot of resonance frequency versus relative orientation is shown in Figure 6.13. The decrease in resonance frequency is due to decrease in velocity as orientation angle increases [67].