Low loss wide bandwidth parallel channel acoustic filter

An acoustic wave filtering apparatus includes an acoustic wave propagating substrate for supporting propagation of acoustic waves and a plurality of acoustic wave filters for filtering an electrical signal. Each of said plurality further comprises an input for supplying electrical input signals, an input unidirectional acoustic wave transducer for converting electrical signal energy into acoustic energy, an output unidirectional acoustic wave transducer for converting acoustic signals to electrical signals, and an output for combining and delivering electrical output signals to external electrical apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS 
The present application is related to co-pending U.S. patent applications 
Ser. Nos. 754,477, now U.S. Pat. No. 5,212,420, and 733,933, which are 
assigned to the same assignee as the present invention. 
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. 
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 the present invention, a novel method and apparatus for 
providing low pass-band insertion loss acoustic wave filters having 
simplified fabrication requirements is described. 
An acoustic wave filtering apparatus includes an acoustic wave propagating 
substrate for supporting propagation of acoustic waves and a plurality of 
acoustic wave filters for filtering an electrical signal. Each of the 
plurality further comprises an input for supplying electrical input 
signals, an input unidirectional acoustic wave transducer for converting 
electrical signal energy into acoustic energy, an output unidirectional 
acoustic wave transducer for converting acoustic signals to electrical 
signals, and an output for combining and delivering electrical output 
signals to external electrical apparatus. 
The 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
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 in accordance with the prior art comprises substrate 120 and 
bi-directional 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 21, 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 bi-directional 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 bi-directional transducer 110 is appropriately illuminated by 
acoustic waves. 
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 disposed atop substrate 120. Acoustic wave filter 217 
comprises acoustic reflector 203, acoustic transducer 205, optional 
conductive structure 208, and gaps 250 and 25. 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 (i.e., transducers 205 and 225 by themselves are 
bi-directional). Acoustic energy of a frequency in the filter 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 having width 317, and a gap of width 
251. Acoustic reflector 203, analogous to acoustic reflectors 213, 223 
and/or 233 (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 having width 417 a gap of width 415, 
analogous to gap 250 (FIG. 2) and separating acoustic transducer electrode 
415 from acoustic reflector elements 410. Acoustic reflector 203, 
analogous to acoustic reflectors 213, 223 and/or 233 (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.degree. Y-rotated X-propagating 
LiNbO.sub.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 A1 having a 
thickness of 1900 Angstroms. Widths 251 and 261 (FIG. 2; see also width 
451, FIG. 4) 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.degree. 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 spirit of the invention or from the scope of the 
appended claims.