SAW RESONATOR OR RESONATOR FILTER

Abstract: A resonator or resonator filter (134) has an output transducer (140) of typical design and conductance response and an input transducer (138) whose conductance response can be adjusted to offset adverse conductance response of the output transducer to provide an unique improved overall conductance response of the resonator or resonator filter, by forming the input transducer with one adjacent pair of electrodes (48,50) located substantially centrally and having a spacing forming a cavity (150) with a width of (N)λ/2 where N is an integer.

Description: This invention relates to a resonator or resonator filter having a transducer formed as a surface acoustic wave device which has better transduction characteristics for the same length transducer thus allowing a more compact device to be constructed, has a higher input conductance for the same length transducer and has a more symmetrical conductance thus providing less distortion to input signals applied thereto. When conventional types of SAW transducers with two electrodes per wavelength are used as impedance elements, the conductance is maximum at a frequency offset from the impedance element design frequency because of the acoustic wave reflections occurring within the device. Thus, the conductance of the device at the design frequency is decreased because of the electrical reflections within the element. More importantly the peak of maximum conductance is shifted from the design frequency. Prior art two-port resonators such as those disclosed in US-A-4144507 and US-A-4,454,488 also use SAW transducer elements having several spurious response peaks of different amplitude with the largest spurious peak at or near the frequency at which the resonator is desired to operate. However, under certain conditions it is possible that the device may lock onto the frequency exhibited at the highest amplitude adjacent peak. Of course, this is disadvantageous in as much as the resonator exhibits unstable operating conditions. Further, there may be a slight decrease in Q exhibited by the device due to the high amplitude adjacent peak. SAW devices find large use in resonator filters because there are size and cost advantages which lead to replacing conventional LC bandpass filters. These resonator filters typically involve a piezoelectric substrate with interdigital input and output transducers formed on the substrate. Although the frequency response has a major peak at the frequency at which it is desired that the resonator filter operate, the frequency response also shows several other spaced but lesser amplitude peaks at other frequencies close to the operating frequency. This, of course, allows unwanted frequencies to pass through the filter. To suppress the unwanted peaks, special steps must be taken, such as using grooves or changing the frequency of the transducers with respect to the gratings, which complicates both the construction and the operation of the resonator filter; see US-A-4325037. An object of the present invention is to overcome these problems. A surface acoustic wave resonator or resonator filter according to the invention comprising a substrate having at least a surface layer of piezoelectric material on which acoustic waves may be propagated; first and second grating structures provided on the piezoelectric surface layer in juxtaposed spaced relationship; input and output transducers interposed between the first and second grating structures and each comprising opposed conductive transducer pads on the substrate, and a plurality of interdigitated electrodes, having a width of λ/4 and a spacing of λ/4, extending from the opposed transducer pads; is characterised in that: the input transducer alone has one adjacent pair of electrodes located substantially centrally, extending from the same transducer pad with an electrode extending from the other transducer pad into the spacing between them, and having a spacing forming a cavity with a width of Nλ/2 where N is an integer. A resonator filter will also have a third grating structure interposed between the input and output transducers and being resonant at the same frequency as the input and output transducers. The resonator and resonator filters of the present invention utilise a modified input transducer and an output transducer of normal design. The input transducer provides an admittance response that more closely peaks at the design frequency of the transducer and has a null in its input conductance which can be adjusted by withdrawal weighting, or other types of weighting, to occur at or near any desired frequency. However, the output transducer has a peak in the conductance, depending upon the substrate material and overlay, which will occur offset from the design frequency. By adjusting the null of the input transducer to coincide with the conductance peak of the output transducer, spurious responses of the resonator or resonator filter may be suppressed. Thus, a good net frequency response that is far improved over prior art responses is obtained. These results are obtained with λ/4 electrodes with no recessed or split finger electrodes, and without grooves or frequency shifts between the transducers and the gratings thereby allowing such filters to be designed at much higher frequencies and with a greater economy in construction. A unique two port resonator can be formed using a modified transducer element as the input transducer, a standard prior art transducer element as the output transducer and standard gratings used in conjunction with the input and output transducers. It has all of the advantages of existing two port resonator structures as shown and described in US-A-4144507 and US-A-4454488 and, in addition, has essentially one major transmission peak at the design frequency instead of several. While there are other adjacent peaks, they are, to a great extent, suppressed below the one major peak thus preventing an oscillator formed from the resonator from locking on to the frequency of the lower adjacent peaks as can happen with the prior art oscillators. In addition, the Q of the input transducer is slightly better than the prior art transducers. The invention also includes a method of forming a surface acoustic wave resonator or resonator filter, comprising the steps of: forming a substrate having at least a surface layer of piezoelectric material on which acoustic waves may be propagated; providing first and second grating structures on the piezoelectric surface layer in juxtaposed spaced relationship; forming input and output spaced, aligned transducers, each comprising a pair of opposed conductive transducer pads on the substrate interposed between the first and second grating structures, and a plurality of interdigitated electrodes extending from the opposed pads, the electrodes having a width of λ/4 and a spacing of λ/4; characterised by forming, substantially centrally of only the input transducer, one adjacent pair of electrodes having a spacing of λ/2 where N is an integer, wherein said adjacent pair of electrodes extends from the same transducer pad with no electrode extending from the other transducer pad into the spacing between said adjacent pair of electrodes. One example of a resonator/resonator filter of the present invention will now be described in relation to the accompanying drawings, in which:- Figure 1 illustrates a SAW transducer impedance element of the prior art; Figure 2 is a graph of the typical input conductance versus frequency of the prior art impedance element of Figure 1; Figure 3 is an illustration of an input SAW transducer element for use in a single port resonator or resonator filter, of the present invention; Figure 4 is an alternative version of the SAW impedance element of Figure 3; Figure 5 illustrates the conductance curve of the impedance element of Figure 3 as compared with the typical conductance wave form shown in Figure 2 of the prior art transducer; Figure 6 illustrates a typical SAW two-port resonator or resonator filter of the prior art construction; Figure 7 is a graph of the wave form illustrating the frequency response of the prior art two-port resonator shown in Figure 6; Figure 8 is a graph of the wave form illustrating the frequency response of the prior art resonator filter shown in Figure 6; Figure 9 is a representation of a two port resonator or resonator filter of the present invention; Figure 10 illustrates the coupling between the input and output transducers of the resonator/resonator filter of Figure 9; and, Figure 11 illustrates the improved coupling that exists between the input and output transducers of the resonator/resonator filter of Figure 9. Figure 1 illustrates a typical prior art transducer element formed as a SAW impedance element which can be used in a single port resonator device with the addition of necessary reflectors, the impedance element 10 comprising a substrate 12 having at least a surface layer of piezoelectric material thereon through which acoustic waves may be propagated. Opposed conductive tranducer pads 14 and 16 are defined on said substrate 12 in a well known manner and include a plurality of interdigitated surface acoustic wave electrodes 18 and 20 extending from the opposed pads 14 and 16 respectively and forming an elongate transducer. Leads 24 and 26 are connected to the conductive transducer pads 14 and 16 respectively as input and output terminals. One of the transducer pads, such as the conductive pad 16, may be grounded as shown at 28. The number of interdigitated electrodes 18 and 20 may of course vary, but a large number of them are used under normal conditions to form the transducer 10. Only a few of the electrodes 18 and 20 are shown in Figure 1 for purposes of clarity in the drawings. The electrodes 18 and 20 have a width of λ/4 and are spaced apart by λ/4 and the device has two electrodes per wavelength. The typical input conductance characteristic for a prior art two electrode per wavelength transducer 10 as in Figure 1, shown looking into terminals 24 and 26, is illustrated in Figure 2. It will be noted that the conductance wave form 30 in Figure 2 is not entirely symmetrical. The conductance of the device is a maximum at peak 31 at a frequency offset below the design frequency of the impedance element 10 because of acoustic wave reflections occurring within the device. Thus, the conductance of the device at the design frequency 33, for example 100 MHz as shown in Figure 2, as can be seen, is substantially lower than peak 31 thus decreasing the efficiency of the device at the design frequency. The peak 31 may occur at a frequency offset above the design frequency depending on the choice of substrate material and electrode metal used in the construction of the transducer. The transducer element to be used with the present invention is disclosed in Figures 3 and 4 as an impedance element 32 which comprises a substrate 34 having at least a surface layer of piezoeletric material on which acoustic waves may be propagated. Opposed conductive transducer pads 36 and 38 are defined on substrate 34 in a well known manner and include a plurality of interdigitated electrodes 40 and 42 respectively. Each of the electrodes has a width of λ/4 as illustrated at 44 and the electrodes are separated by a space also having a width of λ/4 as illustrated at 46. A plurality of electrodes 40 extend from the conductive transducer pad 36 and are interdigitated with a plurality of electrodes 42 extending from the transducer pad 38. One adjacent pair of electrodes 48 and 50 extending from the same transducer pad 36 is located at approximately the centre of the elongate transducer 32 ±20% and the adjacent electrodes 48 and 50 are separated by a space of λ/2 as indicated at 52 to form a cavity 53. Because the electrodes 48 and 50, as shown, both extend from the transducer pad 36, both have the same polarity. The important feature of the present invention, however, is to change the periodicity between the electrodes on either side of the centre of the transducer. Thus, for example, in Figure 3, either electrode 48 or 50 could also be eliminated as shown in Figure 4 with little change in the operation of the device. The cavity 53 at the centre (±20%) of the transducer should be of a width of Nλ/2 where N is an integer. In Figures 3 and 4, N=1. Obviously, the greater N becomes, the less efficient the transducer element becomes. Thus, the ideal cavity width is λ/2. The conductance response characteristic of the transducer shown in Figure 3 is illustrated in Figure 5 as a curve 54 which is compared to the dashed curve 30, the same curve illustrated in Figure 2 for the typical prior art impedance element device. Note that the conductance characteristic 54 of the transducer in Figure 3 is shifted further to the right than curve 30 thus providing a greater conductance at the design frequency (100 MHz in this case) than the prior art device. The reason that the conductance response curve 54 is shifted to the right is because the cavity 53 causes poorer conductance of the transducer 32 at frequencies lower than the design frequency as shown by the null 55 in Figure 5 and the net effect is that the greatest peak of the conductance is nearer the design frequency. The position of null 55 can be varied or controlled by withdrawal weighting of the transducer electrodes as by adding an even number of dummy electrodes 51 on either side of cavity 53 in Figure 3 or an odd number of dummy electrodes 51 on only one side of cavity 53 as shown in Figure 4. In Figure 3, the device will cause the null to occur on the low side of the design frequency as shown in Figure 5 while the device in Figure 4 will cause the null to occur on the high side of design frequency. The total number of dummy electrodes 51 determines the exact location of the null 55. An alternative embodiment of the impedance element of Figure 3 is illustrated in Figure 4. The main difference in the two embodiments is that in Figure 4, the cavity width is caused by the adjacent electrodes 48 and 50 which depend from opposite pad 38. Effectively, the same impedance characteristic is obtained as illustrated in Figure 5 except that the peak will be on the low side and the null 55 on the high side of the design frequency. Figure 6 is an illustration of a simple prior art two-port resonator or resonator filter 102 utilizing SAW devices. If there is a change in periodicity between the input and output transducers, whether with or without discontinuities 118, the device is a two-port resonator with a single main peak in the frequency response. If the device is a resonator filter, intermediate grating 118 is necessary with cavity 111 creating a first change in periodicity and cavity 109 creating a second change in periodicity. The cavities 109 and 111 are separated by a coupling grating 118. As can be seen generally in Figure 6, piezoelectric substrate 104 has thereon an input transducer 106 and an output transducer 108 which are disposed in a spaced apart aligned relationship. The input transducer 106 is driven by a signal source 110 and a load 112 is coupled to the output transducer 108. As is well known in the art the surface acoustic waves induced by the transducer 106 travel through the piezoelectric substrate 104 and are recovered by the output electrodes of the transducer 108. Only a few interdigitated electrodes are shown forming each of the input and output transducers 106 and 108 for purposes of clarity. It will be understood that a large number of electrodes is normally used to form each of these transducers. First and second reflective grating structures 114 and 116 are provided on the substrate 104, and each of the reflective grating structures 114 and 116 comprises a respective plurality of discontinuities 120 and 122. These discontinuities 120 and 122 are formed at the surface of the substrate 104 so as to be capable of reflecting at least a portion of any surface waves incident thereon. As is well known in the art, the discontinuities 120 and 122 of the grating structures 114 and 116 are formed as a plurality of narrow thin film fingers or bars deposited on the surface of the substrate 104. They could, of course, also be formed by selectively etching portions of the surface of substrate 104. Grating structures 114 and 116 are arranged on the substrate 104 in spaced apart, aligned relationship. The input and output surface acoustic wave transducers 106 and 108 are disposed on substrate 104 in the space between the grating structures 114 and 116. The input transducer 106 is coupled to an input excitation source 110. The output transducer 108 is coupled to a load 112. As is well known in the art, the presence of the two reflecting grating structures 114 and 116 on the surface of substrate 104 results in a standing wave resonance being set up between the two reflecting structures with a band width which is controlled by the residual losses in the system. Proper realisation of the standing wave resonance requires that the two reflecting grating stures 114 and 116 be separated by approximately an integral number of half wavelengths along the surface of substrate 104. The third intermediate grating structure 118 may be added for uniformity of velocity throughout the structure when the device is a resonator. If the prior art device in Figure 6 is a resonator as disclosed earlier, the frequency response illustrating the frequency at which the device resonates is shown in Figure 7. Notice that curve 61 has a single main peak 63 and an adjacent suppressed peak 65 separated in amplitude by approximately 14 dB. There are still further adjacent peaks 67 and 69 which are suppressed even lower. These peaks 65, 67 and 69 are substantially large in magnitude because of the conductance characteristic of the standard SAW transducer as shown and explained in regard to Figure 2 wherein it was shown that the device conducts well at frequencies below the design frequency (see peak 31 in Figure 2). Note in Figure 7 that the peaks 65, 67 and 69 all occur between 99 MHz and 100 MHz in this example in which 100 MHz is the frequency at which the maximum peak 63 of conductance of curve 30 in Figure 2 occurs. Because of the substantially large amplitude of adjacent peak 65 shown in Figure 7, it is possible, when the device is used in an oscillator, that the frequency of the oscillator may suddenly lock on to the frequency at peak 65 instead of peak 63 because they are in such close proximity in magnitude and frequency. This, of course, creates instability in the oscillator and is a problem which is encountered in the prior art. When the prior art device in Figure 6 is a resonator filter as disclosed earlier, the frequency response or the coupling between transducers 106 and 108 is shown in Figure 8. As can be seen in Figure 8, two major peaks 125 and 126 occur at substantially the resonant frequency, for example 565 MHz in Figure 8. However, a substantial number of smaller peaks 128, 130 and 132 are also present at lower frequencies indicating that the filter is transducing or passing frequencies of substantial magnitude below the resonant frequency designated by curve 126. This, of course, creates a problem with a resonator filter because instead of just passing the frequencies in the narrow band designated by peaks 125 and 126, it also passes the frequencies designated by peaks 128, 130 and 132. In order to suppress peaks 128, 130 and 132 to a sufficient degree to allow the filter to be operable, the filter has to be modified, as is well known in the art, in a number of ways. One method is to utilise grooves for the discontinuities forming the grating structures 114 and 116. This, of course, requires highly accurate construction and is very expensive. Another method is to change the frequency of the transducers 106 and 108 with respect to the grating structures 114 and 116. This is disadvantageous in that the device is now responsive to two different frequencies which have a tendency to conflict with each other and create distortions. Such construction increases design complexity and is very expensive. These problems with the prior art resonator filter are overcome by utilising the transducer element of the present invention. The device 134 in Figure 9 may be either a resonator or a resonator filter. The device 134 again comprises a substrate 136 on which an input transducer 138 and an output transducer 140 are located. Again, first and second reflective gratings 142 and 144 are placed in spaced, aligned relationship with each other and are separated by input and output transducers 138 and 140. In the present case, however, input transducer 138 has the cavity 150 which is of a width of Nλ/2 as explained previously while the remainder of the electrodes in the input transducer 138 and all of the electrodes forming the transducer 140 are of a width of λ/4 and are separated by a space of λ/4. In addition, space 148 exists between a third grating structure 146 and the output transducer 140 when the device 134 functions as a resonator filter. The space or cavity 148 can be placed on the opposite side of the output transducer 140 if desired without significantly changing the operation of the device. The third grating structure 146 may be added for uniformity of velocity when the device is used as a resonator. The grating structure 146 forms a continuation of the discontinuities representing the electrodes of the input transducer 138. An input signal source 156 excites the input transducer 138 while a load 158 is coupled to the output transducer 140. The use of dummy electrodes 151 causes the null 55 shown in Figure 5 to be positioned as desired. Figure 10 is a graph illustrating the frequency response curve of the device 134 shown in Figure 9 when it is used as a resonator. Again, curve 96 has two major peaks, 98 and 100. However, almost 25 dB separates the magnitude of peaks 98 and 100 thus providing such a wide disparity of coupling that, in an oscillator, the device will not lock on to the frequency represented by the peak 100. The depressed peak 100 is caused by the null 55 in Figure 5 initiated by the cavity 150 and positioned by dummy electrodes 151 in the resonator of Figure 9. Thus, the resonator is extremely stable and does not have the frequency shift problems of the prior art. As stated, the frequency response of the element in Figure 3 and Figure 4 and the resonator filter shown in Figure 9 can be arbitrarily adjusted by positioning the null 55 as previously described as is well known in the art by modfying the reflection gratings as necessary. For instance, the electrodes of the resonator can be weighed as shown in Figure 3 and 4 or apodized to adjust the input admittance response curve null 55 as desired. Weighting of the electrodes can be done in a number of ways well known in the prior art such as withdrawal weighting, varying the thickness of the electrode overlay material, adding grooves to the substrate and the like. When the device in Figure 9 is used as a resonator filter as decribed previously, the resulting frequency response curve indicating the coupling between the input and output transducers in Figure 9 is shown in Figure 11. Note the large peak 160 with small lobes 162, 164, 166 and 168. All of these smaller lobes 162 through 168 are so suppressed in relation to the magnitude of peak 160 by the positioning of null 55 that they are of minimal effect and the resonator filter functions in the manner intended. The reason that the side lobes 162 and 164 are so suppressed is because of the combination of the input transducer 138 with the cavity 150 being utilized with a conventional output transducer 140 in Figure 9. The reasoning can be explained in relationship to Figure 5 with the conductive characteristics shown therein where the input transducer 138 of the invention is utilized in conjunction with the standard output transducer 140. These two transducers have the conductance responses shown in Figure 5. Thus, the dashed curve 30 is the conductance of the standard output transducer 140 while the curve 54 is the conductance of the novel transducer element 138. As can be seen in Figure 5, the input transducer 138 does not transduce well at frequencies below the design frequency as evidenced by null 55. Just the opposite occurs with a standard transducer 140. It conducts well at frequencies below the design frequency as evidenced by the peak 170. Using withdrawal weighting by adding the desired even number of dummy electrodes 151 in Figure 9, null 55 can be positioned with respect to peak 170 so as to reduce the transduction of the total device 134 at the frequency of peak 170. Thus, the two transducers tend to produce opposite effects at the frequency of peak 170 offset from the design frequency which tend to cancel each other and reduce the side frequency conduction as shown by peak 100 in Figure 10. Thus, as will be noted in Figure 5, the modified resonator used as the input transducer 138 in Figure 9 conducts better at frequencies higher than the design frequency than does the standard resonator 140. The magnitude of the frequency passed as indicated by peak 100 in Figure 10 is still so far below the maximum conduction frequency peak 98 that no ill effects are noted in the resonator-filter. Thus, there has been disclosed a two-port resonator and resonator filter which has a better impedance characteristic for the same length of transducer, is more compact, has a higher input conductance per length than the prior art, has a more symmetrical conductance and provides better coupling between the input and output transducers. As a resonator, the novel resonator has all of the advantages of existing resonator structures and it does not change frequency inadvertently as in the prior art where the response curve has two peaks which are relatively close together in magnitude. As a resonator filter, it attenuates the design frequencies very little while suppressing to a great degree the frequencies outside the design.

Claims: A surface acoustic wave resonator or resonator filter (134) comprising a substrate (136) having at least a surface layer of piezoelectric material on which acoustic waves may be propagated; first (142) and second (144) grating structures provided on the piezoelectric surface layer in juxtaposed spaced relationship; input (138) and output (140) transducers interposed between the first and second grating structures and each comprising opposed conductive transducer pads (36,38) on the substrate, and a plurality of interdigitated electrodes (40,42), having a width of λ/4 and a spacing of λ/4, extending from the opposed transducer pads; characterised in that: the input transducer (138) alone has one adjacent pair of electrodes (48,50) located substantially centrally, extending from the same transducer pad (36) with no electrode extending from the other transducer pad (38) into the spacing between them, and having a spacing forming a cavity (150) with a width of Nλ/2 where N is an integer. A resonator or resonator filter as in claim 1, wherein the adjacent pair of electrodes (48,50) is located at the centre of the elongate input transducer plus or minus 20% of the elongate transducer length. A resonator filter according to either of claims 1 and 2, further comprising a third grating structure interposed between said input and output transducers and being resonant at the same frequency as said input and output transducers. A method of forming a surface acoustic wave resonator or resonator filter (134), comprising the steps of: forming a substrate (136) having at least a surface layer of piezoelectric material on which acoustic waves may be propagated; providing first (142) and second (144) grating structures on the piezoelectric surface layer in juxtaposed spaced relationship; forming input (138) and output (140) spaced, aligned transducers, each comprising a pair of opposed conductive transducer pads (36,38) on the substrate interposed between the first and second grating structures, and a plurality of interdigitated electrodes (40,42) extending from the opposed pads, the electrodes having a width of λ/4 and a spacing of λ/4; characterised by forming, substantially centrally of only the input transducer (138), one adjacent pair (48,50) of electrodes having a spacing of Nλ/2 where N is an integer, wherein said adjacent pair of electrodes (48,50) extends from the same transducer pad (36) with no electrode extending from the other transducer pad (38) into the spacing between said adjacent pair of electrodes (48,50). A method as in claim 4, further comprising the step of locating the adjacent pair of electrodes (48,50) at the centre of the elongate input transducer plus or minus 20% of the elongate input transducer length. A method as in claim 4 or claim 5, further including providing an even number of dummy electrodes (151) on both sides of the cavity (150) to adjust the conductance of the input transducer. A method of forming a resonator filter according to any of claims 4 to 6, further comprising interposing a third grating structure between said input and output transducers, said third structure being resonant at the same frequency as said input and output transducers.

Inventors: WRIGHT, PETER V.
Applicants: R F MONOLITHICS, INC.
Application year: 1986
Main classification: H03H9
IPC codes: H03H9, H03H3