Low loss wide bandwidth parallel channel acoustic filter

Abstract: An acoustic wave filtering apparatus includes an acoustic wave propagating substrate (120) for supporting propagation of acoustic waves and a plurality of acoustic wave filters (217, 219) for filtering an electrical signal. Each of said plurality further comprises an input (201) for supplying electrical input signals, an input unidirectional acoustic wave transducer (203, 205, 250; 223, 225, 260) for converting electrical signal energy into acoustic energy, an output unidirectional acoustic wave transducer (210, 213, 255; 230, 233, 265) for converting acoustic signals to electrical signals, and an output (215) for combining and delivering electrical output signals to external electrical apparatus.

Description: Field of the Invention The present invention pertains to frequency selection components and more particularly to wide bandwidth, low insertion loss surface acoustic wave filters. Background of the Invention Acoustic wave filters comprise a class of frequency selection components having the advantages of small size, light weight, and large out-of-band signal rejection. Periodic or quasi-periodic acoustic wave transducer structures are employed for achieving acousto-electric and electro-acoustic energy conversion required in surface acoustic wave filters. The bandwidth of an acoustic wave transducer (and filter) is inversely proportional to the length of the acoustic wave transducer. A first problem which all acoustic wave filters incur is that such structures typically provide efficient energy conversion, but also impose bandwidth limitations on the frequency response of the completed acoustic wave filter. A second problem which many acoustic wave transducers suffer is "ripple" or nonuniformity of the pass-band frequency response of the completed filter due to what is termed "triple-transit" distortion of the filter response. Triple transit distortion results from acoustic reflections within an acoustic wave filter, which occur when a propagating acoustic wave impinges upon, for example, a simple, bi-directional acoustic wave transducer. Such transducers are often preferred because they are easily manufactured using a single photolithography step, by techniques which are very similar to those employed to fabricate semiconductor-based integrated circuit devices, as is well known in the art. The triple transit distortion level can be minimized by increasing the pass-band insertion loss of the acoustic wave filter, without increasing the filter's fabrication complexity. However, this approach requires large filter insertion losses in order to satisfy many system specifications for filter pass-band insertion loss uniformity. These losses are often deliberately introduced into acoustic wave filters in order to ameliorate triple-transit induced effects. In turn, these large filter insertion losses necessitate either pre- or post-filter gain, effected via amplifiers. Such amplifiers require power, occupy space, and impose weight requirements which are inconsistent with many applications for low-power, hand-portable communications equipment, wherein acoustic wave filters find substantial application. "Low Loss, Highly Stable SAW devices on Quartz" by T.N. Oliver et al. (40th Ann. Freq. Cont. Symp. - 1986) describes insertion loss and bandwidth improvements available in SAW filters through use of Single Phase Uni Directional Transducers disposed on quartz substrates. What is needed are means and methods for achieving wide bandwidth acoustic wave filters. What is further needed are means and methods for providing wide bandwidth acoustic wave filters also having low pass-band insertion loss, low triple transit spurious response, strong rejection of out-of-band signals, and single level photolithographic process fabrication requirements. Summary of the Invention In accordance with a first aspect of the present invention there is provided a wide bandwidth acoustic wave filter as claimed in claim 1. In accordance with a second aspect of the present invention there is provided, in a radio, at least one surface acoustic wave (SAW) filter (200) for providing a frequency selection function as claimed in claim 7. Brief Description of the Drawing FIG. 1 is a plan view of an acoustic wave filter; FIG. 2 represents an acoustic wave device in accordance with the present invention; FIG. 3 is an enlarged side view, in section, taken on section lines Z-Z' of FIG. 2, of a portion of an acoustic reflector structure in accordance with a first embodiment of the present invention; FIG. 4 is an enlarged side view, in section, taken on section lines Z-Z' of FIG. 2, of a portion of an acoustic reflector structure in accordance with a second embodiment of the present invention; FIG. 5 illustrates a response measured from a first acoustic transducer to a second acoustic transducer of the present invention; FIG. 6 illustrates a response measured from a third acoustic transducer to a fourth acoustic transducer of the present invention; FIG. 7 illustrates a combined response from first through fourth acoustic transducers of the present invention; FIG. 8 illustrates a combined response from first through fourth acoustic transducers of the present invention when the transducers are connected in accordance. with the present invention; and FIG. 9 is a block diagram of a radio receiver including surface acoustic wave filters in accordance with the present invention. Detailed Description of the Drawing A broad variety of different acoustic wave types have application in microwave acoustic devices for frequency selection. These include surface acoustic waves (SAWs), also known as Rayleigh waves; surface skimming bulk acoustic waves (SSBAWs); shallow bulk acoustic waves (SBAWs); surface transverse waves (STWs); Stonely, Sezawa, Love, and other plate and higher order acoustic guided waves; longitudinal and shear bulk acoustic waves (BAWs); line acoustic waves (LAWs); and so on. For convenience of explanation, the present invention is described in terms of SAWs, with the understanding that other varieties of acoustic propagation are also applicable, including but not limited to those listed above. The terms "surface acoustic wave", "acoustic wave", and "surface wave" or "SAW", are employed interchangeably herein to stand for any suitable type of acoustic wave propagation. The terms "substrate material", "substrate", and "acoustic wave propagating substrate" are employed interchangeably herein to stand for any substrate that supports propagation of acoustic waves. The terms "reflection element" and "reflection electrode" are employed interchangeably herein to stand for reflection elements comprising electrodes. Further, the terms "comb electrode" and "transducer electrode" are employed interchangeably herein to stand for acoustic wave transducer elements comprising electrodes. FIG. 1 is a plan view of an acoustic wave filter 100. Acoustic wave filter 100 comprises substrate 120 and transducers 105 and 110 which are connected to signal source 130 and load 140, respectively. Transducers 105 and 110 further comprise busses 115, 117, 119 and 121 and comb electrodes 107, 109, 112 and 114. Comb electrode 107 comprises electrodes connected to buss 115. Similarly, comb electrodes 109, 112 and 114 comprise electrodes connected to busses 117, 119 and 121, respectively. Transducers 105 and 110 convert electrical to acoustic energy, and vice versa. Interdigitated comb electrodes 107, 109, 112 and 114 are interconnected by busses 115, 117, 119 and 121. Electrodes 107, 109, 112 and 114, and busses 115, 117, 119 and 121 are made of thin-film metal, deposited, for example, by vacuum evaporation, on the polished surface of substrate material 120 which is in whole or in part piezoelectric. Comb electrodes 107, 109, 112 and 114 making up acoustic wave transducers 105 and 110 are typically defined photolithographically, using processes well known in the art. The piezoelectric nature of substrate material 120 causes mechanical waves to be emitted from transducer 105 when excited by electrical signals from signal source 130 having an appropriate frequency, and conversely transducer 110 delivers electrical output signals to load 140 when transducer 110 is appropriately insonified. The Applicants have discovered that the efficiency and low pass-band insertion loss of narrow-band acoustic filters can provide low pass-band insertion loss and broader bandwidth when a plurality of such filters are combined in parallel with the proper phasing of signals to and from each individual acoustic filter. FIG. 2 represents an acoustic wave device 200 in accordance with the present invention. Acoustic wave device 200 comprises acoustic wave filters 217 and 219. Acoustic wave filter 217 comprises acoustic reflector 203, acoustic transducer 205, optional conductive structure 208, and gaps 250 and 255. Acoustic wave filter 217 has a first center frequency. Acoustic wave filter 217 further comprises acoustic transducer 210 and acoustic reflector 213. Acoustic wave filter 219 comprises acoustic reflector 223, acoustic transducer 225, acoustic transducer 230, acoustic reflector 233, input connection 201, output connection 215, and gaps 260 and 265. Acoustic wave filter 219 has a second center frequency. Acoustic wave reflector 203 and acoustic transducer 205 are adjacent and substantially aligned as shown in Fig. 2, with gap 250 disposed therebetween. Acoustic transducer 210 is adjacent to and substantially aligned with acoustic transducer 205, with optional conductive structure 208 disposed therebetween. Acoustic wave reflector 213 is adjacent to and substantially aligned with acoustic transducer 210, with gap 255 disposed therebetween, as shown in FIG. 2. Acoustic wave reflector 223 and acoustic transducer 225 are adjacent and substantially aligned as shown in Fig. 2, with gap 260 disposed therebetween. Acoustic transducer 230 is adjacent to and substantially aligned with acoustic transducer 225, with optional conductive structure 208 disposed therebetween. Acoustic wave reflector 233 is adjacent to and substantially aligned with acoustic transducer 230, with gap 265 disposed therebetween, as shown in FIG. 2. One side of each of transducers 205, 210, 225 and 230 is connected to ground, as illustrated in FIG. 2. In operation, electrical energy is supplied to input 201, causing acoustic transducers 205 and 225 to emit acoustic energy towards acoustic reflectors 203 and 223, respectively, and also towards optional conductive structure 208. Acoustic energy of a frequency in the filter pass-band incident on acoustic reflector 203 is reflected back towards acoustic transducer 205, with the result that acoustic energy having a frequency in the filter pass-band is largely or wholly emitted from transducer 205 and acoustic reflector 203 towards optional conductive structure 208. Similarly, acoustic energy having a frequency in the filter pass-band incident on acoustic reflector 223 is reflected back towards acoustic transducer 225, with the result that acoustic energy having a frequency in the filter pass-band is largely or wholly emitted from transducer 225 and acoustic reflector 223 towards optional conductive structure 208. As well, acoustic energy in the filter pass-band incident on acoustic reflector 213 is reflected back towards acoustic transducer 210, with the result that acoustic energy having a frequency in the filter pass-band is largely or wholly emitted from transducer 210 and acoustic reflector 213 towards optional conductive structure 208. Further, acoustic transducer 230 and acoustic reflector 233 cooperate to form a unidirectional acoustic transducer emitting acoustic waves towards optional conductive structure 208. The output 215 is the sum of signals from transducers 210 and 230. The output on lead 215 provides wide bandwidth together with low insertion loss. The acoustic filters thus formed provide the advantages of low insertion loss and broad bandwidth signal output together with the simplicity and desirability of single-level photolithography requirements. The Applicants have further discovered that these advantages can be effected together with requirements for only one or two distinct comb electrode and reflection electrode line-widths for the acoustic filters when the distinct pass-band frequencies of the separate filter functions are realized through inclusion of gaps of different sizes in filters containing uni-directional acoustic transducers and acoustic reflectors separated by such gaps. Requirements for multiple linewidths, especially those having non-integer linewidth relationships, cause difficulties in photomask preparation and also in the photolithography required for acoustic wave filter fabrication. The Applicant's invention employs variations in the size of gaps 251 and 261 to effect different center frequencies for transducers having the same linewidths. FIG. 3 is an enlarged side view, in section, taken on section lines Z-Z'of FIG. 2, of a portion 305 of an acoustic reflector structure in accordance with a first embodiment of the present invention. FIG. 3 illustrates a portion of substrate 120, acoustic reflector elements 310, acoustic transducer electrode 315, and gap 250 of width 251. Acoustic reflector 203 (FIG. 2) comprises reflection elements 310, each having width 312 of approximately one-fourth of an acoustic wavelength, separated by gaps of width 308, width 308 also being approximately one-fourth of an acoustic wavelength. FIG. 4 is an enlarged side view, in section, taken on section lines Z-Z'of FIG. 2, of portion 405 of an acoustic reflector structure in accordance with a second embodiment of the present invention. FIG. 4 illustrates a portion of substrate 120, acoustic reflector elements 410, acoustic transducer electrode 415, and gap 250 of width 451. Acoustic reflector 203 (FIG. 2) comprises reflection elements 410, each having width 412 of approximately one-half of an acoustic wavelength, separated by gaps of width 408, width 408 being approximately one-half of an acoustic wavelength. FIGS. 5-8 depict measured responses for a set of acoustic wave filters constructed on a substrate of 128° Y-rotated X-propagating LiNbO 3 , having a wavelength of 4.158 micrometers, and utilizing the acoustic reflector arrangement depicted in FIG. 4. The electrodes in both the reflectors and the transducers are fabricated from Al having a thickness of 1900 Angstroms. Widths 251 and 261 (FIG. 2) were about 3/8 and 5/8 wavelengths, corresponding to 1.53 and 2.55 micrometers, respectively. The center frequencies of all of the pass-bands illustrated in FIGS. 5-8 are in the range of 930 MegaHertz. Filters in this frequency range are undergoing intense development at the present time. FIGS. 5-8 depict measured acoustic wave filter responses wherein the vertical scales show measured signal amplitudes on a scale of 5 dB per division, with 0 dB insertion loss corresponding to the top of the graph, and where the horizontal scale represents signal frequency on a scale of 25 MegaHertz per division, with the center of the graph corresponding to a frequency of 930.5 MegaHertz. FIG. 5 illustrates the response measured from a first acoustic transducer to a second acoustic transducer versus frequency. Response 510 shows a pass-band response to the right of the middle of the figure, having substantial ripple within the pass-band, a pass-band insertion loss of about 1.4 dB and a 3 dB bandwidth of about 10 MegaHertz. The out-of-band signal rejection characteristics surrounding the pass-band show about 20 to 25 dB of rejection. FIG. 6 illustrates the response measured from a third acoustic transducer to a fourth acoustic transducer versus frequency. Response 610 shows a pass-band response to the right of the middle of the figure, having substantial ripple within the pass-band, a pass-band insertion loss of about 1.8 dB and a 3 dB bandwidth of about 10 MegaHertz. The out-of-band signal rejection characteristics surrounding the pass-band show about 25 to 30 dB of rejection. The data of FIGS. 5 and 6 differ substantially while the devices from which the data were taken differ only in the width of the gaps 250, 255, 260 and 265. Gaps 250 and 255 are 3/8 of an acoustic wavelength in width while gaps 260 and 265 are 5/8 of an acoustic wavelength in width. FIG. 7 illustrates the combined response from first through fourth acoustic transducers of the present invention versus frequency, when they are combined such that the phase of the signal from the first and second filters is 180° removed from the ideal. Curve 710 illustrates a pass-band response having a minimum of 4 dB of insertion loss, a 3 dB bandwidth of about 10 MegaHertz, coupled with out-of-band signal rejection ranging from 15 to 30 dB, and shows substantially greater pass-band insertion loss than either of traces 510 and 610 of FIGS. 5 and 6, without any substantial change in bandwidth. FIG. 8 illustrates the combined response from first through fourth acoustic transducers of the present invention when the transducers are connected in accordance with the present invention versus frequency. Trace 810 illustrates a pass-band insertion loss of 2.5 dB, a 3 dB bandwidth of 20 to 25 MegaHertz, improved pass-band ripple compared to the data of FIGS. 5-7, and out-of-band rejection ranging from 30 to more than 45 dB. Trace 810 thus comprises a measured response showing substantial improvement over the combination of characteristics previously shown in FIGS. 5-7. The bandwidth of the data of FIG. 8 is roughly twice those illustrated in FIGS. 5-7. This is due to the differences in the gap widths and also to the relative phase of the signals applied to the inputs and combined from the outputs of the filters comprising the overall acoustic filter. The advantages of broader bandwidth and greater out-of-band signal rejection allow such filters to fulfill needs in a greater number of system applications since such filters provide a strong, nearly noise free output signal. FIG. 9 is a block diagram of a portion of radio receiver 900 including surface acoustic wave filters in accordance with the present invention. FIG. 9 depicts a radio receiver utilizing a number of surface acoustic wave (SAW) filters in accordance with the present invention. Radio receiver 900 includes antenna 901 which is used to receive and transmit signals. Diplexer 903 is connected to antenna 901 and to the transmitter portion (not shown). Diplexer 903 transmits received signals to filter 905. Filter 905 may be a SAW filter according to the present invention. Filter 905 is connected to amplifier 907. The output of amplifier 907 is transmitted to SAW filter 909. Filter 909 transmits its output to mixer 911 where the output is combined with a signal from local oscillator 913 via SAW filter 915. The output of mixer 911 is then filtered by SAW 917 to provide the IF output. A method for providing low insertion loss acoustic filters comprises the steps of (1) providing an acoustic wave propagating substrate, (2) coupling a plurality of acoustic wave filters to the acoustic wave propagating substrate, (3) supplying an input signal to the first and second acoustic wave filters; and (4) combining the output signals from the first and second acoustic wave filters in parallel. By now it should be appreciated that the invention discloses frequency selection components having the advantages of doubling the bandwidth together with small size and large out-of-band signal rejection. As described herein, the present invention meets the goals and advantages set forth earlier, namely, providing means and methods for broad bandwidth acoustic wave filters having low pass-band insertion loss, low triple transit spurious response, strong rejection of out-of-band signals, and single level photolithographic process fabrication requirements. A further advantageous feature results from incorporation of only one or two distinct line-widths in the photolithographic fabrication of the acoustic wave filter. Although the preferred embodiment of the invention has been illustrated, and that form described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the scope of the appended claims.

Claims: A wide bandwidth acoustic wave filter (200) comprising: a piezoelectric substrate (120) made of a piezoelectric material including a surface; a first acoustic wave filter (217) having a first center frequency disposed on said surface of said piezoelectric substrate (120); a second acoustic wave filter (219) having a second center frequency disposed on said surface of said piezoelectric substrate (120); an input (201) coupled to said first and second acoustic wave filters (217, 219); said first acoustic wave filter (217) providing a first output signal in response to said input (201); said second acoustic wave filter (219) providing a second output signal in response to said input (201); and an output (215) coupled to said first and second acoustic wave filters (217, 219) for combining said first and second output signals from said first and second acoustic wave filters (217, 219) to provide a wide bandwidth, low loss acoustic wave filter output, wherein said first acoustic wave filter (217) comprises: a first acoustic reflector (203) for reflecting acoustic waves; a first acoustic transducer (205) for converting between electrical and acoustic energy, said first acoustic transducer (205) being adjacent to said first acoustic reflector (203); and a first gap (250) having a first width (251) disposed between said first acoustic reflector (203) and said first acoustic transducer (205), said first acoustic reflector (203), said first acoustic transducer (205) and said first gap (250) comprising a first unidirectional acoustic transducer (203, 205, 250); and a second acoustic reflector (213) for reflecting acoustic waves; a second acoustic transducer (210) for converting between electrical and acoustic energy, said second acoustic transducer (210) being adjacent to said second acoustic reflector (213); and a second gap (255) having a first width (251) disposed between said second acoustic reflector (213) and said second acoustic transducer (210), said second acoustic reflector (213), said second acoustic transducer (210) and said second gap (255) comprising a second unidirectional acoustic transducer (210, 213, 255), said first (203, 205, 250) and second (210, 213, 255) unidirectional acoustic transducers comprising said first acoustic wave filter (217), CHARACTERIZED IN THAT said second acoustic wave filter (219) comprises: a third acoustic reflector (223) for reflecting acoustic waves; a third acoustic transducer (225) for converting between electrical and acoustic energy, said third acoustic transducer (225) being adjacent to said third acoustic reflector (223); and a third gap (260) having a second width (261) disposed between said third acoustic reflector (223) and said third acoustic transducer (225), said third acoustic reflector (223), said third acoustic transducer (225) and said third gap (260) comprising a third unidirectional acoustic transducer (223, 225, 260). An acoustic wave filter (200) as claimed in claim 1, wherein said second acoustic wave filter (219) further comprises: a fourth acoustic reflector (233) for reflecting acoustic waves; a fourth acoustic transducer (230) for converting between electrical and acoustic energy, said fourth acoustic transducer (230) being adjacent to said fourth acoustic reflector (233); and a fourth gap (265) having a second width (261) disposed between said fourth acoustic reflector (233) and said fourth acoustic transducer (230), said fourth acoustic reflector (233), said fourth acoustic transducer (230) and said fourth gap (265) comprising a fourth unidirectional acoustic transducer (230, 233, 260), said third (223, 225, 260) and fourth (230, 233, 260) unidirectional acoustic transducers comprising said second acoustic wave filter (219). An acoustic wave filter (200) as claimed in claim 2, wherein said first (203), second (213), third (223) and fourth (233) acoustic reflectors comprise pluralities of acoustic reflection elements (310) each approximately one-fourth of an acoustic wavelength in width. An acoustic wave filter (200) as claimed in claim 2, wherein said first (203), second (213), third (223) and fourth (233) acoustic reflectors comprise pluralities of acoustic reflection elements (410) each approximately one-half of an acoustic wavelength in width. An acoustic wave filter (200) as claimed in claim 2, wherein said first (251) and second (261) gap widths differ by one-fourth acoustic wavelength. An acoustic wave filter (200) as claimed in claim 1, wherein said piezoelectric substrate (120) comprises one-hundred twenty eight degree lithium niobate. A radio (900) including a surface acoustic wave filter as claimed in claim 1.

Inventors: CHO FREDERICK Y, HICKERNELL THOMAS, PENUNURI DAVID, CHO, FREDERICK Y., HICKERNELL, THOMAS, PENUNURI, DAVID
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1992
Main classification: H03H9
IPC codes: H03H9