One-Port Resonator Operating with Surface Acoustic Waves

Abstract

The present invention relates to a one-port resonator (1) operating with surface acoustic waves, comprising an interdigital transducer (2) having a first busbar (6), a second busbar (7) and electrode fingers (8), wherein in an excitation region (10) of the interdigital transducer (2) the electrode fingers (8) are alternately connected to the first busbar (6) and the second busbar (7) in the longitudinal direction (L), wherein the interdigital transducer (2) comprises a first reversed region (11), in which the electrode fingers (8) are alternately connected to the first busbar (6) and the second busbar (7) in the longitudinal direction (L) and which is directly adjacent to the excitation region (10), and wherein that electrode finger (118) of the first reversed region (11) which is directly adjacent to the excitation region (10) in the longitudinal direction (L) and that electrode finger (118) of the excitation region (10) which is directly adjacent thereto are connected to the same busbar (6, 7).

Claims

1. A one-port resonator (1) operating with surface acoustic waves, comprising an interdigital transducer (2) having a first busbar (6), a second busbar (7) and electrode fingers (8), wherein in an excitation region (10) of the interdigital transducer (2) the electrode fingers (8) are alternately connected to the first busbar (6) and the second busbar (7) in the longitudinal direction (L), wherein the interdigital transducer (2) comprises a first reversed region (11), in which the electrode fingers (8) are alternately connected to the first busbar (6) and the second busbar (7) in the longitudinal direction (L) and which is directly adjacent to the excitation region (10), and wherein that electrode finger (118) of the first reversed region (11) which is directly adjacent to the excitation region (10) in the longitudinal direction (L) and that electrode finger (118) of the excitation region (10) which is directly adjacent thereto are connected to the same busbar (6, 7).

2-13. (canceled)

Description

[0040] The invention is explained in greater detail below with reference to figures.

[0041] FIG. 1 shows a first exemplary embodiment of a one-port resonator.

[0042] FIG. 2 shows a diagram in which the real part of the admittance for various exemplary embodiments of the one-port resonator is plotted on a logarithmic scale.

[0043] FIG. 3 shows an insertion loss of a basic element of a ladder-type structure comprising two one-port resonators.

[0044] FIG. 4 shows a second exemplary embodiment of the one-port resonator.

[0045] FIG. 5 shows a third exemplary embodiment of a one-port resonator.

[0046] The figures here illustrate schematic illustrations which are not true to scale. By way of example, the number of electrode fingers of the interdigital transducers is significantly reduced in the figures, in order to allow a more comprehensible illustration.

[0047] FIG. 1 shows a first exemplary embodiment of a one-port resonator 1. The one-port resonator 1 comprises an interdigital transducer 2. Furthermore, the one-port resonator 1 comprises a first reflector 3 and a second reflector 4. The interdigital transducer 2 is arranged between the first reflector 3 and the second reflector in the longitudinal direction L. Furthermore, the one-port resonator 1 comprises a piezoelectric substrate 5, on which the interdigital transducer 2 and the two reflectors 3, 4 are arranged. The piezoelectric substrate 5 can comprise lithium niobate or lithium tantalate, for example.

[0048] The interdigital transducer 2 comprises a first busbar 6 and a second busbar 7. Furthermore, the interdigital transducer 2 comprises electrode fingers 8 that serve for exciting a surface acoustic wave. Furthermore, the interdigital transducer 2 comprises stub fingers 9 that do not contribute to the excitation of the acoustic wave. Each of the electrode fingers 8 and of the stub fingers 9 is connected either to the first busbar 6 or to the second busbar 7. In this case, the first busbar 6 and the electrode fingers 8 connected to it form a comb-like structure representing a first electrode of the interdigital transducer 2. Correspondingly, the second busbar 7 and the electrode fingers 8 connected to it form a second comb-like structure, which forms a second electrode of the interdigital transducer 2. The two comb-like structures intermesh.

[0049] The interdigital transducer 2 comprises an excitation region 10. In the excitation region 10, the electrode fingers 8 are alternately connected to the first busbar 6 and the second busbar 7. The excitation region 10 is the region of the interdigital transducer 2 having the most electrode fingers 8.

[0050] Furthermore, the interdigital transducer 2 comprises a first reversed region 11 and a second reversed region 12. In the longitudinal direction L, firstly the first reversed region 11 is adjacent to the first reflector 3. The excitation region 10 is adjacent to the first reversed region 11. Furthermore the second reversed region 12 is adjacent to the excitation region 10. The second reflector 4 is adjacent to the second reversed region 12.

[0051] In each of the first reversed region 11 and the second reversed region 12, the electrode fingers 8 are alternately connected to the first busbar 6 and the second busbar 7 in the longitudinal direction L. In this case, the first reversed region 11 comprises an electrode finger 118 which is directly adjacent to an electrode finger 108 of the excitation region 10 in the longitudinal direction L. These two electrode fingers 108, 118 are connected to the first busbar 6. This has the effect that, upon an AC voltage being applied to the busbars 6, 7, surface acoustic waves that are in each case phase-shifted with respect to one another are excited in the excitation region 10 and in the first reversed region 11. In the first reversed region 11, as it were, a surface acoustic wave is excited which counteracts the surface acoustic wave excited in the excitation region 10 and performs a correction of said wave.

[0052] Since, furthermore, the two electrode fingers 108, 118 are connected to the same busbar, no electric field is built up between them upon an AC voltage being applied and, consequently, a piezoelectric excitation does not occur between them either.

[0053] In the case of the exemplary embodiment shown in FIG. 1, all the electrode fingers 8 of the inter-digital transducer 2 are at the same distance from one another. In this case, the electrode fingers 8 are arranged on a periodic grid. The distance between the electrode fingers 8 corresponds to half the wavelength of the resonant frequency of the one-port resonator 1.

[0054] Furthermore, the electrode finger 108b of the excitation region which is directly adjacent to an electrode finger 128 of the second reversed region 12 in the longitudinal direction L, and said electrode finger 128 of the second reversed region are both connected to the first busbar 6. Accordingly, a surface acoustic wave that is phase-shifted relative to the surface acoustic wave excited in the excitation region 10 is excited in the second reversed region 12 as well. Since, furthermore, the two electrode fingers 108b, 128 are connected to the same busbar, no electric field is, furthermore, built up between them upon an AC voltage being applied and, consequently, a piezoelectric excitation does not occur between them either.

[0055] In the exemplary embodiment shown here, the first and second reversed regions 11, 12 comprise the same number of electrode fingers 8.

[0056] FIG. 2 shows a diagram that clarifies the effect of the reversed regions 11, 12 on the admittance of the one-port resonator 1. The one-port resonator 1 shown in FIG. 1 is taken as a starting point here, wherein the two reflectors 3, 4 each comprise 50 reflector strips and the interdigital transducer 2 comprises a total of 181 electrode fingers.

[0057] FIG. 2 shows a diagram in which a frequency f is plotted on the abscissa axis and the real part of the admittance Re(Y) on a logarithmic scale is furthermore plotted on the ordinate axis. A reference curve K.sub.ref is plotted, which shows the admittance for a one-port resonator comprising no reversed regions. The further curves show the admittance for one-port resonators 1 comprising a first and a second reversed region 11, 12, wherein the two reversed regions 11, 12 respectively comprise three, four, five, seven, nine, eleven, 15, 19, 25 and 29 electrode fingers 8. By way of example, the curves which correspond to a one-port resonator 1 comprising two reversed regions 11, 12 comprising respectively four and 29 electrode fingers 8 are marked by K.sub.4 and K.sub.29, the index indicating the number of electrode fingers 8 of the reversed regions 11, 12.

[0058] It is clearly evident in FIG. 2 that the profile of the admittance near the resonant frequency becomes distinctly steeper in the case of the one-port resonators 1 comprising reversed regions 11, 12. The reversed regions 11, 12 lead to an increase in the real part of the admittance near the resonant frequency.

[0059] FIG. 3 shows the insertion loss S.sub.12 of a basic element of a ladder-type filter structure. The basic element is constructed from a series resonator and a parallel resonator. What is taken as a starting point here is a series resonator and a parallel resonator which are respectively formed by a one-port resonator 1 comprising an interdigital transducer 2 having 151 electrode fingers 8 and two reflectors 3, 4 each having ten reflector strips.

[0060] FIG. 3 illustrates three curves K.sub.10, K.sub.20 and K.sub.40 that respectively illustrate the insertion loss of the basic element for the case where the parallel resonator comprises an excitation region and, adjacent thereto, two reversed regions having respectively ten, 20 or 40 electrode fingers. The curve K.sub.0 is a reference curve illustrating the insertion loss of the basic element for the case where the parallel resonator comprises only the excitation region and no reversed regions.

[0061] On the abscissa axis the frequency f is plotted and on the ordinate axis the insertion loss S.sub.12 is plotted for the respective basic element of the ladder-type filter structure. It is clearly evident that the lower pass-band slope for the basic elements in which the parallel resonator is formed by a one-port resonator comprising reversed regions turns out to be significantly steeper, given the reference curve K.sub.0 describing a basic element in which the parallel resonator is formed by a one-port resonator without a reversed region.

[0062] Accordingly, in particular the use of the one-port resonators according to the invention as a parallel resonator in a ladder-type structure is of interest since the left slope of the insertion loss characteristic is crucially determined by the configuration of the parallel resonator.

[0063] FIG. 4 shows a second exemplary embodiment of the one-port resonator 1. The one-port resonator shown in FIG. 4 comprises only a first reversed region 11, which is arranged between the excitation region 10 of the interdigital transducer 2 and the first reflector 3. Furthermore, the excitation region 10 is directly adjacent to the second reflector 4 in the longitudinal direction L.

[0064] FIG. 5 shows a third exemplary embodiment of a one-port resonator 1. The one-port resonator 1 shown in FIG. 5 furthermore comprises a third reversed region 13 in addition to the first reversed region 11 and the second reversed region 12. In the longitudinal direction L, there are adjacent to the first reflector 3, in the following order, the third reversed region 13, the first reversed region 11, the excitation region 10, the second reversed region 12 and the second reflector 4. The first, second and third reversed regions 11, 12, 13 each comprises a number of electrode fingers 8 deviating from one another.

[0065] An electrode finger 138 of the third reversed region 13 which is directly adjacent to the first reversed region 11 in the longitudinal direction L and that electrode finger 118b of the first reversed region 11 which is directly adjacent thereto are connected in each case to the second busbar 7.

LIST OF REFERENCE SIGNS

[0066] 1 One-port resonator

[0067] 2 Interdigital transducer

[0068] 3 First reflector

[0069] 4 Second reflector

[0070] 5 Piezoelectric substrate

[0071] 6 First busbar

[0072] 7 Second busbar

[0073] 8 Electrode finger

[0074] 9 Stub finger

[0075] 10 Excitation region

[0076] 11 First reversed region

[0077] 12 Second reversed region

[0078] 13 Third reversed region

[0079] 108 Electrode finger of the excitation region

[0080] 108b Electrode finger of the excitation region

[0081] 118 Electrode finger of the first reversed region

[0082] 118b Electrode finger of the first reversed region

[0083] 128 Electrode finger of the second reversed region

[0084] 138 Electrode finger of the third reversed region

[0085] L Longitudinal direction