WO2001005030A2

WO2001005030A2 - Saw ladder filter with improved 'q' coupling capacitors

Info

Publication number: WO2001005030A2
Application number: PCT/US2000/015913
Authority: WO
Inventors: David Penunuri; David S. Hickernell
Original assignee: Cts Corp.
Priority date: 1999-06-11
Filing date: 2000-06-09
Publication date: 2001-01-18
Also published as: WO2001005030A3

Abstract

Surface acoustic wave ladder filter comprising coupling capacitors that include interdigitated electrodes at an angle theta to a surface acoustic wave propagation direction of the filter,where 0°<theta<90°

Description

SAW LADDER FILTER WITH IMPROVED "Q" COUPLING CAPACITORS

This is a provisional patent application flled under 37 C.F.R. § 1.53(c).

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates in general to SAW filters and in particular to a SAW ladder filter with improved "Q" coupling capacitors.

2. DESCRIPTION OF RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR 1.97 AND 1.98

A SAW ladder filter is ideally suited for RF front end application for portable communications equipment because of its very small size. A relatively low-loss prior art design can be achieved using several series connected SAW resonators that are coupled with shunt capacitors as seen in FIG. 1(a). An improvement of the prior art was made as shown in FIG. 1(b) where the configuration for the off-chip coupling shown in FIG. 1(a) has been replaced with interdigitated structures used only as capacitors which are fabricated on the SAW die or substrate itself as part of the electrode metalization.

The problem of the prior art coupling capacitors that are used in SAW resonator based ladder filters is that they are implemented as interdigitated electrode structures fabricated directly on the SAW substrate near the SAW resonators. This proximity minimizes the deleterious effects of parasitic elements such as bonding wire inductance and stray capacitance. However, since the IDT-type coupling capacitors are fabricated on the piezoelectric substrate with dimensions which are generally similar to those for ordinary SAW resonators, these coupling capacitors will radiate acoustic energy. The large substrate coupling coefficient corresponding to the value for rotation angle of 0 degrees, which leads to the high performance of the SAW resonators, also causes the poor performance of the coupling capacitors. For the coupling capacitors, this acoustic energy is undesirable since it robs the capacitors of stored energy thus reducing their "Q". Also in the prior art, the rotation angle of 90 degrees as shown in

FIG. 1(b) has been used. However, at this angle, significant acoustic radiation is still generated and it is attributed mainly to the ordinary surface wave mode in this direction and due to the bulk acoustic wave mode which exists with propagation velocity similar to the SAW resonator (leaky wave) velocity. Thus the performance of the prior art coupling capacitors rotated at 90 degrees as shown in FIG. 1(b) is poor.

Known prior art includes U.S. Patent No. 5,694,095 that discloses the use of a capacitor implemented as an interdigital array of electrodes fabricated on an insulator between the electrodes and the piezoelectric substrate to reduce the generation of surface acoustic waves.

In U.S. Patent No. 4,931,755 there is disclosed the use of a capacitor connected between the input terminal and the output terminal of the SAW filter. The capacitor is implemented as an interdigital array of electrodes as shown in FIG. 1(b). The patent discloses that the IDT should be rotated 90 degrees as shown in FIG. 1 (b) and that the electrode pitch should be changed in order to eliminate the generation of surface waves.

It would be advantageous to have a SAW ladder filter that has its spurious mode generation minimized and moved to a frequency range which eliminates their interference with the desired filter response. Further, it would be desirable to have such a SAW ladder filter wherein the losses of the coupling capacitor are significantly reduced.

SUMMARY OF THE INVENTION

The present invention relates to an IDT coupling capacitor used in a resonator ladder filter and whose electrodes are rotated to a specific non-0 angle (other than 90 degrees) with respect to the angular orientation of the resonator electrodes.

When the coupling capacitor is rotated to a particular angular range, θ, where 0°<θ<90° its spurious mode generation is minimized and moved to a frequency range which eliminates their interference with the desired filter response. Further, in such case, the losses of the rotated coupling capacitor are significantly reduced.

The present invention utilizing the rotation of the IDT coupling capacitor electrodes could also have stopband applications. The present invention, according to the preferred embodiment, is rotated to an angle θ other than 90 degrees or 0 degrees (0°<θ<90°) to cause most of the acoustic modes to have coupling coefficients to the EDT that decrease so that less acoustic energy is generated and the IDT coupling capacitor "Q" is higher since less energy is lost. Further, the velocities of the acoustic modes at which the parasitic acoustic energy propagates decrease so that their optimum frequency is shifted away from the critical frequencies of the filter.

Thus it is an object of the present invention to provide a coupling capacitor for a SAW ladder filter or stopband applications in which its spurious mode generation is minimized and moved to a frequency range which eliminates their interference with the desired filter response.

It is also an object of the present invention to provide an EDT coupling capacitor in which the losses of the coupling capacitor are significantly reduced.

It is a special object of the present invention to provide an EDT coupling capacitor having electrodes that are rotated to a specific non-0, non-90 degree angle with respect to the angular rotation of the resonator electrodes. BRIEF DESCRIPTION OF THE DRAWINGS

These and other object of the present invention will be more fully disclosed when taken in conjunction with the following Detailed Description of the Drawings in which: FIG. 1(a) is a schematic drawing of a prior art ladder filter showing the coupling capacitors to be externally located from the substrate itself; FIG. 1(b) is also a schematic diagram of a prior art improved SAW ladder filter in which the coupling capacitors are formed on the substrate as interdigitated transducers;

FIG. 2 has 3 separate figures (a), (b) and (c) illustrating various embodiments of the rotation of the EDT electrodes from 0 degrees and 90 degrees in a preferred angle range of 40 degrees to 50 degrees;

FIG. 3 is a simulation illustrating decreased coupling coefficients for the EDTs of the present invention;

FIG.4 illustrates the velocities of most of the acoustic modes at which the parasitic acoustic energy propagates decrease; and FIG. 5 is a sketch illustrating another version of using an EDT rotated at some angle θ between 0°<θ<90°.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a prior art ladder filter having a plurality of SAW resonators thereon and external coupling capacitors that are off of the substrate itself.

FIG. 1(b) is a schematic diagram illustrating an improved prior art SAW ladder filter wherein the coupling capacitors are formed as interdigitated transducers on the SAW substrate itself.

FIG. 2(a), (b) and (c) illustrate improved coupling capacitors according to the present invention wherein the EDT electrodes are at an angle θ, where 0°<θ<90° . In the preferred embodiment, the range of rotation is an angle from about 40° to

50° from the ordinary SAW propagation direction. Three different embodiments are illustrated in FIGS, (a), (b), and (c).

FIG. 3 is a graph illustrating the results of detailed simulations as the coupling capacitor is rotated at an angle according to a preferred embodiment of the present invention as depicted in FIG. 2 and shows that the coupling coefficients to most of the acoustic modes decrease so that less acoustic energy is generated and the EDT coupling capacitor "Q" is higher since less energy is lost.

FIG. 4 is a graph summarizing the velocities of most of the acoustic modes at which the parasitic acoustic energy propagates decrease so that their optimum frequency is shifted away from the critical frequencies of the filter. The exception is the longitudinal mode for which the coupling coefficient is maximum near 45 degrees. However, the velocity for this mode at this angle is much higher than that of the important surface wave modes. For example, in a SAW ladder filter designed for operation at 860 MHz, the corresponding longitudinal mode would not appear below

1355 MHz, almost 500 MHz above the passband of the .liter and therefore would have no performance impact on the filter. The simulations indicate that for the above reasons, at a rotation angle substantially equal to 45 degrees, the coupling capacitor achieves its best performance on substrates commonly used for SAW ladder filters such as a 36-degree cut and 45-degree cut lithium tantalite.

FIG. 5 is a logbook sketch of a ladder filter adding an EDT coupling capacitor to the substrate with the EDT itself being at an angle θ as shown to minimize the acoustic or SAW response. This results in a high "Q" capacitor.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements are intended to include any structure, material, or act for performing the function in combination with other elements as specifically disclosed.

Claims

1 . A surface acoustic wave device comprising coupling capacitors that include interdigitated electrodes at an angle θ to a surface acoustic wave propagation direction of the device, where 0^° < θ < 90°.

2. The device of claim 1 wherein the angle θ is from about 40° to 50°.

3. The device of claim 1 wherein the interdigitated electrodes are substantially parallel.

4. The device of claim 1 wherein the interdigitated electrodes form a substantially chevron configuration.

5. The device of claim 1 wherein some of the interdigitated electrodes are mirror images of other interdigitated electrodes.

6. The device of claim 1 wherein the angle θ is 45°.

7. The device of claims 1 or 6 further comprising a 36-degree cut lithium tantalite substrate.

8. The device of claims 1 or 6 further comprising a 45-degree cut lithium tantalite substrate.

9. A surface acoustic wave device comprising: a. a substrate; b. at least two resonators disposed on said substrate; and c. at least one coupling capacitor that includes interdigitated electrodes at an angle θ to a surface acoustic wave propagation direction of the device, where 0° < θ < 90°.

10. The device of claim 9 wherein the angle θ is 45°.

1 1. The device of claim 9 wherein the angle θ is from about 40° to 50°.

12. The device of claim 9 wherein the interdigitated electrodes are substantially parallel.

13. The device of claim 9 wherein the interdigitated electrodes form a substantially chevron configuration.

14. The device of claim 9 wherein some of the interdigitated electrodes are mirror images of other interdigitated electrodes.

15. The device of claims 9 or 10 wherein the substrate is comprised of 36-degree or 45-degree cut lithium tantalite.