Electro-acoustic device with a variable acoustic wave...

Details Electro-acoustic device with a variable acoustic wave... Electro-acoustic device with a variable acoustic wave...

1996-10-30

2001-07-31

Lee, Benny (Department: 2817)

Wave transmission lines and networks

Coupling networks

Electromechanical filter

C333S195000, C310S31300R

Reexamination Certificate

active

06268782

ABSTRACT:

FIELD OF INVENTION
The present invention relates to an electro-acoustic device, in particular but not exclusively, to a device for exciting Surface Acoustic Waves (SAW), or Surface Skimming Bulk Waves (SSBW).
BACKGROUND OF INVENTION
Typically, communication systems such as mobile telephones or cellular radio telephone systems require frequency band-pass filters having frequency ranges from tens of Megahertz to Gigahertz, and having fractional bandwidths covering a range from less than 0.01% to about 5.0%. The foregoing frequency ranges and bandwidths are suitable for both centre frequency and bandwidth for the intermediate frequency (IF) and radio frequency (RF) stages of such communication systems.
Well-known frequency filtering techniques for these frequency ranges are lumped LC filters, spiral or helical filters, dielectric filters and bulk acoustic wave filters. In addition to the foregoing there are also known electro-acoustic devices such as SAW coupled resonator filters, which have found particular applications in mobile communication systems since they are generally of lower volume and size, have better electrical performances, manufacturing uniformity and stability. Additionally, SAW devices can be manufactured to a substantially consistent standard, and hence there is less need to tune devices after fabrication. Comparison of coupled resonator filter techniques to other types of SAW band-pass filter techniques shows that the coupled resonator filter has a relatively low loss over its operating bandwidth with a high out of band rejection and small transition bandwidth.
Due to the low loss, high out of band rejection and small transition bandwidths typically associated with SAW coupled resonator filters including acoustic transversely coupled resonator filters and acoustic in-line coupled resonator filters, SAW coupled resonator filters are found to be particularly suitable for frequency band-pass filtering from narrow to moderate bandwidths. They are particularly suitable for miniaturised analogue and digital mobile communication systems as RF and IF filters.
FIG. 1
shows a schematic diagram of a conventional 2-pole transversely coupled resonator filter (TCF)
100
. The TCF is composed of two acoustic tracks
102
,
104
which are disposed adjacent to each other. Each track consists of one inter-digital transducer (IDT)
106
,
108
and two reflection gratings
110
,
112
,
114
,
116
symmetrically disposed at each side of respective IDTs
106
,
108
. Each IDT
106
,
108
comprises an array of transducer electrodes
120
,
122
arranged in a comb-like fashion and interleaved with each other. A common bus bar
118
is coupled to the transducer electrodes on adjacent sides of the respective IDTs
106
,
108
. A common bus bar is advantageous in that adjacent acoustic tracks
102
,
104
can be disposed closer together than if separate bus bars were used, which results in a stronger acoustically transverse coupling between adjacent tracks than for separate bus bars. Typically, the common bus bar
118
extends to the reflection gratings which in the example shown in
FIG. 1
comprise earthed electrodes
124
. When one or other of the IDTs
106
,
108
is electrically excited an acoustic wave is excited within the respective acoustic track
102
,
104
. Due to the proximity of the acoustic tracks, an acoustic wave which is guided in a first acoustic track,
102
for example, is coupled into the second acoustic track
104
by virtue of an overlap of the guided wave profile tail in the first acoustic track, thereby exciting an acoustic wave in the second acoustic track. The acoustic wave in the second acoustic track then generates an appropriate electric signal in the IDT,
108
in this example, for the second acoustic track, and an electrical signal is output from the filter. The spaces between the IDT
106
,
108
and each reflection grating can be covered with a conductive film or may be a free surface. Further details of transversely coupled resonator filters may be found in European Patent Application EP 0 100 503.
FIG. 2
shows a typical in-line coupled resonator filter
200
. The in-line coupled resonator filter shown in
FIG. 2
consists of three IDTs
202
,
204
and
206
and two reflection gratings
210
and
212
collinearly positioned with respect to each other. The two outer IDTs
202
and
204
are parallel-connected and IDT
206
is disposed between them. The whole system shown in
FIG. 2
, including the reflection gratings
210
and
212
comprises a cavity resonant system having two electric ports. A first electric port is formed by IDTs
202
and
204
and a second electric port is formed by IDT
206
. As discussed with reference to
FIG. 1
each of the IDTs
202
,
204
,
206
consist of arrays of interleaved transducer electrodes
208
. The reflection gratings comprise earthed electrodes
214
. In operation, when one electric port of the in-line coupled resonator filter is driven by an applied voltage acoustic cavity modes are excited within the resonant system and the other electric port couples to these acoustic modes to produce an electric output signal.
Both the conventional coupled resonator filters described above are suitable for only unbalanced driving and loading at their input and output ports. Thus, they are only capable of being directly coupled to devices having unbalanced inputs or outputs. Thus, they are not particularly suitable for a number of applications, for example, an IF band-pass filter coupled to a balanced mixer. For applications where the conventional coupled resonator filter is to be coupled to balanced inputs or outputs an appropriate balanced-unbalanced (BALUN) transition is required. Such transitions are typically lossy and furthermore take up space either on a circuit board upon which they are typically etched or by virtue of the lumped element components comprising the BALUN. Additionally, when more than one coupled resonator filters are cascaded together to form multi-track, multi-pole filters, the necessary ground connections between respective tracks of the cascaded filters result in cross-talk which seriously degrades the performance of such multi-pole filters. Such degradation in performance is particularly noticeable in the out of band regions of such filters. Furthermore, having to provide ground connections between tracks of multi-track devices makes the layout of the device more complex. In many cases the connections can only be performed by using bonding wires. Such bonding wires or flying leads introduce parasitic electrical components into the electrical characteristics of the device and thereby further degrade the device's performance.
BRIEF SUMMARY OF THE INVENTION
In a first aspect of the invention there is provided an electro-acoustic device, comprising a piezo-electric substrate, a first transducer supported by the substrate and including a pair of interdigital transducer electrode arrays, the electrodes of the arrays being interleaved with each other, a second transducer supported by the substrate and including a pair of interdigital transducer electrode arrays, the electrodes of the arrays being interleaved with each other, the first transducer being associated with a first electrical signal and the second transducer being associated with a second electrical signal having a different phase from the first electrical signal, wherein the first and second transducer are disposed opposing each other in a propagation direction of acoustic waves excitable by the first and second transducers, and the first and second transducers are spaced apart such that substantially in phase acoustic waves propagating along the substrate are incident on the first transducer in phase with the first electrical signal and on the second transducer in phase with the second electrical signal.
This has the advantage that out of phase signals may be input or output from a device such that the acoustic waves generated or received by respective transducers are in-phase.
Preferably, the first and second electric signals are (

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