Multi-bandwidth saw filter

A multi-bandwidth SAW filter for an input signal is provided which includes an SAW transducer having a selectable length corresponding to the desired bandwidth of the filter. The SAW filter is responsive to a control signal which corresponds to a selectable bandwidth of the filter to present the input signal to an appropriately lengthed SAW transducer.

TECHNICAL FIELD 
This invention relates generally to Surface Acoustic Wave (SAW) structures 
and more particularly to a SAW filter structure having multiple 
bandwidths. 
BACKGROUND 
SAW components use acoustic waves which travel at the speed of sound. The 
SAW components are preferred over widely used transmission line components 
because acoustic waves have a substantially shorter wave length at 
operating frequency than electromagnetic waves which travel at the speed 
of light. Therefore, for a given operating frequency, a SAW filter 
provides a smaller structure than a transmission line structure, 
therefore, making them suitable for miniaturized radio frequency 
applications. Furthermore, SAW structures are easily integratable with 
other active circuits, such as amplifiers and mixers, which are produced 
using conventional integrated circuit technologies. For the above reasons, 
the popularity of SAW structures in radio frequency applications, 
especially in filter applications, has been increasing steadily. 
SAW filters are particularly used in communication devices to provide 
selectivity at various stages of a receiver, such as at the front-end 
stage or at the IF stage of the receiver. The selectivity of a SAW filter 
is determined by its bandwidth which is defined as the frequency spectrum 
limited between the 3 dB points of the filter's frequency response. 
When used in the IF stage of the receiver, the bandwidth of the IF SAW 
filter is dependent upon the type of modulation used in the receiver. With 
recent developments in telecommunication technology, particularly with the 
advent of the personal communication systems, such as cordless telephone 
second-generation also known as CT2, European and Japanese digital 
cordless telephone systems, respectively known as DECT and JDCT, more 
complex modulation techniques are used for communicating messages. These 
new digital services require wider bandwidths than usually used on 
conventional communication systems. For example, the CT2 system uses time 
division duplex (TDD) with frequency division multiple access (FDMA) 
technology which supports digital data rate of 72 kilobits per second per 
channel with a 32 kilobits per second adaptive differential pulse code 
modulated (ADPCM) voice signal. The CT2 system requires a 100 KHz IF 
bandwidth for communicating messages. On the other hand, the European 
digital cordless telephone system (DECT) utilize a time division multiple 
access (TDMA) system supporting digital data at 1152 kilobits per second 
per channel which uses a 32 kilobits per second ADPCM voice signal. The 
DECT requires approximately 1200 KHz of IF bandwidth, whereas, the 
Japanese digital cordless telephone system (JDCT) uses QPSK modulation at 
384 kilobits per second per channel with 32 kilobits per second ADCPM 
voice which requires an IF bandwidth of 240 KHz. In the United States of 
America, several systems with different modulation and IF bandwidth 
requirements are under consideration. The modulations under consideration 
are GFSK or QPSK with IF bandwidths of approximately 400-500 KHz. 
It is desirable to provide a universal IF circuit which is capable of 
servicing different modulation techniques and accommodate their bandwidth 
requirements accordingly. A universal IF circuit, in addition to providing 
a more flexible radio receiver, reduces design and manufacturing costs as 
well. However, conventional SAW filter designs provide a fixed bandwidth 
and are not suitable for applications requiring multiple bandwidths. 
Therefore, there exists a need for a multi-bandwidth SAW filter which may, 
for example, be used in a radio capable of operating in communication 
systems having different modulation requirements. 
SUMMARY OF THE INVENTION 
Briefly, according to the invention, a multi-bandwidth SAW filter includes 
a plurality of SAW transducers disposed on a piezoelectric substrate. The 
SAW transducers have different lengths corresponding to selectable 
bandwidths of the SAW filter. The SAW filter is responsive to a control 
signal for presenting an input signal to one of the selected SAW 
transducers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description of the invention is now presented and is best understood from 
a consideration of the following text in conjunction with the drawing 
figures, in which like reference numerals are carried forward. 
Referring to FIG. 1, a block diagram of a radio 100 which incorporates a 
multi-bandwidth filter, according to the principles of the present 
invention, is shown. The radio 100 is a two-way communication unit which 
is capable of operating in receive and transmit modes. The radio 100 is a 
microprocessor controlled communication unit comprising a controller 110 
which controls its overall operation. In the receive mode, a communicative 
signal is received by an antenna 101 which applies it to a filter 103 for 
initial selectivity at the receiver front end. The received signal is 
passed through an antenna switch 105 which under the control of the 
controller 110 applies it to a preamplifier stage 107. The preamplifier 
stage 107 amplifies the received signal and applies it to a mixer 109. The 
mixer 109 receives a local oscillator (LO) signal generated by a 
synthesizer 127 through a LO amplifier 139 to provide an IF signal 108. As 
is well known under the control of the controller 110, the synthesizer 127 
generates proper local oscillator frequency to tune the radio to receive 
the communication signals at a predetermined carrier frequency. 
The IF signal 108 is applied to an IF filter stage comprising 
multi-bandwidth SAW filter 120 of the present invention. The IF stage is 
responsive to a control signal generated by the controller 110 for 
presenting, based on a predetermined criteria, a proper bandwidth to the 
IF signal 108. Therefore, the IF signal may be selectively switched 
between ports 10 and 20 which present different bandwidths to the IF 
signal 108. The out of the IF stage is provided at a port 30. The proper 
bandwidth may be presented based on the type of modulation technique used 
by the communication system in which the radio 100 operates. Such 
bandwidth information may be pre-stored in the controller 110 to select a 
particular IF bandwidth commensurate with the modulation technique offered 
by the communication system. Alternatively, the modulation information may 
be modifiably stored in the controller 110 for accommodating situations in 
which the radio 100 operates in more than one communication system 
offering different modulation techniques. In this way, the radio 100 may 
determine the type of modulation used in the particular communication 
system and cause the controller 110 to generate corresponding control 
signal for presenting the proper IF bandwidth. For example, if the 
communication unit 100 operates in a DECT cordless telephone system, the 
control signal generated by the controller 110 causes the multi-bandwidth 
SAW filter 120 to present a 1200 KHz IF bandwidth to the IF signal 108. On 
the other hand, if the radio 100 operates in a JDCT system, the control 
signal generated by the controller causes the multi-bandwidth filter 120 
to present a 240 KHz bandwidth. In this way, the IF signal 108 is 
appropriately filtered by the multi-bandwidth SAW filter 120 as specified 
by the control signal. 
The output of the multi-bandwidth SAW filter 120 is applied to a well known 
demodulator/audio stage 129 which also operates under the control of the 
controller 110. The modulator/audio stage 129 may be arranged using well 
known software or hardware controlled schemes to provide more than one 
demodulation technique. In this way, the controller 110 may also specify, 
again, based on the modulation offered by the communication system, the 
type of demodulation technique needed for recovering the received message. 
The output of the demodulator/audio stage 129 is applied to a speaker 131 
to render the transmitted messages audible. 
In transmit mode, a communicated message as inputted through a microphone 
133 is applied to a modulator 125 which operates under the control of the 
controller 110. The controller 110 may also specify to the modulator what 
type of modulation technique is desired according to the communication 
system in use. The output of the modulator is applied to a transmitter IF 
mixer 135 which receives a transmitter local oscillator signal from the 
synthesizer 127. The output of the transmitter IF mixer 135 is amplified 
by an amplifier 137. The communication messages from the communication 
unit 100 are radiated through the antenna 101 after being applied to the 
antenna switch 105 and the filter 103. 
Having described the operation of the radio 100, different structural 
embodiments of the multi-bandwidth SAW filter, according to the present 
invention, will be described now. As is well known, SAW filters utilize 
piezoelectric materials to achieve conversion of electrical energy to 
acoustic energy and vice-versa. The piezoelectric material may comprise 
quartz, lithium niobate, and lithium tantalate which are formed as 
substrates upon which acoustic transducer structures are disposed for 
achieving the required acousto-electric and electro-acoustic energy 
conversion. Because the bandwidth of an acoustic wave transducer is 
inversely proportional to its length, the applicants of the present 
invention contemplate providing a mechanism whereby an input signal is 
presented to one or more transducers with length corresponding to the 
bandwidth requirements of the SAW filter. 
Referring to FIG. 2, a structural diagram of a multi-bandwidth SAW filter 
200 capable of presenting two distinct and selectable bandwidths is shown. 
Structurally, the filter 200 is comprised of a piezoelectric substrate 205 
upon which conductive patterns forming a first input transducer 202, a 
second input transducer 204, and an output transducer 206 are disposed. 
The conductive patterns may be disposed on the piezoelectric substrate 205 
utilizing any number of techniques such as thin-film or thick-film 
processes. The transducers are acoustically coupled by positioning the 
output transducer 206 between the input transducers 202 and 204. Thus, the 
output transducer 206 receives, at its opposing sides, acoustic waves 
propagated from the input transducers 202 and 204. The transducers 202, 
204 and 206 are each patterned to include pairs of electrodes 203 having 
the interdigitated fingers 207 as illustrated. Each electrode pair 
includes a first grounded electrode and a second non-grounded electrode 
which provides the input or output ports 10, 20 and 30 of the transducers 
202, 204 and 206. The properties of the piezoelectric material, as well as 
the spacing between the interdigitated fingers 203, determine the 
frequency response of the SAW filter. 
According to the invention, the input transducers 202 and 204 have 
different lengths which corresponds to the different bandwidths of the 
multi-bandwidth SAW filter 200. As shown, the input transducer 202 has a 
length L.sub.c and the input transducer 204 has a length L.sub.b which is 
shorter than the length Lc. Because the length of acoustic transducers are 
inversely proportional to their bandwidth, the shorter transducer 204 
provides a wider bandwidth than the longer transducer 202. The SAW filter 
200 is capable of receiving input signals at input ports 10 and 20 to 
provide an output signal at output port 30. Therefore, if a narrow 
bandwidth is desired an input signal may be applied to the input port 10, 
and if a wide bandwidth is desired the input signal is applied to the 
input port 20. For example, in the radio receiver application described in 
conjunction with FIG. 1, the IF signal 108 may be switched between the 
input ports 10 and 20 based on the bandwidth requirement of the modulation 
technique. Thus, the multiple bandwidths of the filter 200 are provided by 
selecting different lengths for the input transducer 202 and 204. 
Therefore, dependent upon which port the input signal is applied to, the 
SAW transducer 200 provides a bandwidth commensurate with the length of 
the input transducer. 
It may be appreciated by one of ordinary skill in the art that the terms 
"input transducers" and "output transducers" as referred to herein may be 
used interchangeably because multi-port SAW transducers are bidirectional 
devices capable of interchangeably receiving input and output signals at 
their port terminations to provide a frequency response commensurate with 
their geometry. That is, a port referred to as an input port may also be 
used as an output port and vice versa. Therefore, an input signal may be 
applied to port 30 of FIG. 2 to provide output signals at ports 10 and 20 
with bandwidth characteristics corresponding to the length of their 
respective SAW transducer. Thus, the SAW transducer 206 may be used as an 
input transducer, whereas the SAW transducers 202 and 204 may be used as 
output transducers. For simplicity of understanding the applicants have 
made references to "input transducer" and "output transducer" throughout 
the remainder of the specification. However, it should be noted that the 
input and output transducers are interchangeable and the true spirit of 
the invention is not limited by such references. 
Referring to FIG. 3, another embodiment of multi-bandwidth SAW filter 300 
of present invention is shown. The multi-bandwidth SAW filter 300 includes 
four input transducers 302, 304, 306 and 308 and an output transducer 310 
making it capable of presenting four distinct and selectable bandwidths. 
The four input transducers 302, 304, 306 and 308 and the output transducer 
310 comprise conductive patterns forming electrodes with interdigitated 
fingers 309. As before, the conductive patterns are disposed on a 
piezoelectric substrate 305. In this embodiment, the input transducers 302 
and 304 are paired together by means of opposing electrodes which share a 
common grounded track 303. As such, the fingers of the opposing electrodes 
extend from the opposing sides of the common track 303. Similarly, the 
input transducers 306 and 308 are paired together through opposing 
electrodes which share a common grounded track 307. The input transducers 
302, 304, 306 and 308 respectively receive signals at input ports 40, 50, 
60 and 70. The output transducer 310, which is positioned between the 
input transducer pairs 302-304 and 306-308 provides an output signal at an 
output port 80. Again, according to the invention, the length of the input 
transducers 302, 304, 306 and 308 are different from each other and 
correspond to the selectable bandwidth of the filter 300. Under this 
arrangement, an input signal, such as the IF signal 108 of FIG. 1, may be 
presented to differing bandwidths by selectively applying it to the input 
ports 40, 50, 60, or 70. The input signal may be applied to a selected one 
of the input ports by means of well-known control circuits which are 
responsive to a control signal corresponding to a desired bandwidth. 
Referring now to FIG. 4, another embodiment of a multi-bandwidth filter 400 
is shown. The filter 400 is comprised of a piezoelectric substrate 405 
upon which conductive patterns 407, 422, 426, 428, and 431 having 
interdigitated fingers 403 are disposed. These conductive patterns form a 
variable length input transducer 402 and an output transducer 420. In this 
embodiment the input transducer 402 is comprised of a plurality of 
sequentially positioned sub-transducers 412, 414, 416 and 418, having 
lengths La, Lb, Lc and Ld, respectively. The input transducer 402 includes 
a single conductive pattern 407 which serially integrates grounded 
electrodes of the sub-transducers 412, 414, 416 and 418. The conductive 
pattern 407 comprises a common track 409 from one side of which extend the 
fingers of the grounded electrodes of the sub-transducers 412, 414, 416, 
and 418. Also disposed on the substrate 405 are a number of conductive 
patterns 422, 424, 426 and 428 having finger arrangements which are 
interdigitated with the fingers of the single conductive pattern 407 
forming the SAW sub-transducers 422, 424, 426 and 428. The SAW filter 400 
also includes a control circuit 430 which, in response to binary control 
signals 432, is capable of closing or opening a plurality of serially 
coupled relay switches a, b and c. The relay switches a, b, and c are 
positioned between an input port 90 which receives an input signal, such 
as the IF signal 108 of FIG. 1, and the SAW sub-transducers 422, 423, 426 
and 428. In response to the control signal 432 which may be a binary 
signal representing a predetermined condition, such as bandwidth 
requirements of the filter, the switches a, b and c may be selectively 
closed or opened to present a variable length input transducer to the 
input signal. The terminals of the switches a, b and c are connected to 
the input sub-transducers 412, 414, 416, and 418 such that the length of 
the input transducer 402 is increased by sequential closing of the 
switches a, b, and c. Conversely, the length of the input transducer 402 
is decreased by sequential opening of the switches a, b, and c. As shown, 
one terminal of switch c is connected to the electrode 428, the common 
terminals of switches b and c are connected to electrode 426, the common 
terminals of switches a and b are connected to electrode 424, and the 
other terminal of switch a is connected both to the electrode 422 and the 
input port 90. It can be appreciated that by sequentially switching 
switches a, b and c, the length of the input transducer of the filter 400 
may be varied. If, for example, all the switches are open, the input 
signal is presented only to the sub-transducer 412 the sole length, La, of 
which sets the bandwidth of the filter 400. If the switch a is closed and 
the remaining switches, i.e., switches b and c, are kept open the input 
transducer comprises the combination of the sub-transducers 412 and 414 
having a length equal to L.sub.a +L.sub.b. However, if switches a and b 
are closed, the input signal is applied to an input transducer comprising 
sub-transducers 412, 414 and 416 having a length equal to L.sub.a +L.sub.b 
+L.sub.c. And, if all switches are closed, the input signal is applied to 
an input transducer having a length equal to L.sub.a +L.sub.b +L.sub.c 
+L.sub.d and comprising sub-transducers 412, 414, 416 and 418. Therefore, 
depending on which switches are closed and which are opened, the input 
signal at port 90 may be presented to a variable length input transducer 
402 comprising one or more of the sub-transducers 412, 414, 416 and 418. 
Thus, in this arrangement, a SAW filter is provided which has multiple 
bandwidths which includes means for selectively varying the length of the 
input transducer in response to a control signal. 
It may be appreciated by one of ordinary skill in the art that the control 
circuit 430 and the switches a, b, c, may be disposed on the piezoelectric 
substrate 405 utilizing conventional integrated circuit technologies to 
provide an integrated multi-bandwidth SAW filter package. 
Referring to FIG. 5, a diagram of a multiple bandwidth SAW filter 500 is 
shown which has a parallel switch arrangement as opposed to the serial 
switch arrangement for FIG. 4. The filter 500 includes conductive patterns 
disposed on a piezoelectric substrate 505 forming an input transducer 
comprising a plurality of SAW sub-transducers 512, 514, 516, 518, and 520 
and an output transducer 522. A plurality of parallel coupled relay 
switches a', b', c', d' and e' which are responsive to a control signal 
532, are coupled between an input port 95 and the SAW sub-transducers 512, 
514, 516, 518, and 520 for presenting the input signal to a selected one 
of the SAW transducers having a length corresponding to the desired 
bandwidth. A control circuit 530 in response to the control signal 532 
controls the switches a', b', c', d' and e'. When the control circuit 530 
closes any of the switches in response to the control signal 532, the 
input signal is presented to one or more of the appropriately lengthed 
input sub-transducer 512, 514, 516, 518 or 520. 
Referring to FIG. 6, a multi-bandwidth SAW filter 600 is shown which 
provides multiple bandwidth by selectively controlling the direction of 
acoustic waves and then applying through SAW transducers with lengths 
corresponding to a desired bandwidth. The SAW filter 600 includes a SAW 
center transducer 612 which is positioned between two SAW transducers 616 
and 614, the transducers being disposed on a piezoelectric substrate 605. 
As such, the SAW transducers 616 and 614 are positioned on the opposing 
sides of the center SAW transducer 612. The SAW transducers 614 and 616 
have lengths corresponding to selectable bandwidths of the filter 600. 
The center transducer 612 comprises a multi-phase unidirectional transducer 
which is capable of selectively directing acoustic waves to travel through 
one or the other of its opposing sides in response to a particular phase 
difference between a pair of phase differentiated signals 607. If for 
example, the phase difference between the phase differentiated signals is 
+90 degrees, the acoustic waves could, bidirectionally, travel within the 
right side of the center SAW transducer 612 with no acoustic wave 
propagating from the left side. Conversely, if the phase difference is -90 
degrees the acoustic waves propagate through the left side with none 
propagating through the right side. The phase differentiated signals 607 
are provided by applying an input signal at port 613 to a phase control 
circuit 630 which is responsive to a control signal 632 for setting the 
phase difference. In the preferred embodiment, the phase difference may be 
selectively set to +90 and -90 degrees based on a selectable bandwidth of 
the SAW filter 600. The phase differentiated signals 607 are applied to a 
pair of parallel tracks 609 one of which is positioned farther below the 
center SAW transducer 612. The multi-phase unidirectional transducers are 
well known and consist of groups of interdigitated fingers 603 which are 
offset from each other by integral number (n and m) of wave-lengths plus 
or minus one quarter wave length. Certain interdigitated fingers at 
predetermined positions along the center SAW transducer 612 are coupled to 
the farther one of the tracks 609 by jumpers 615, thereby creating the 
selectively unidirectional characteristic of the center SAW transducer 
612. As such, an input signal which is selectively split into plus (+) or 
(-) 90 degrees phase differentiated signals is applied to the 
unidirectional SAW transducer 612. Within the SAW transducer 612, 
depending on the phase difference, the phase differentiated signals will 
combine constructively in one direction and destructively in the other 
direction. Thus, the transducer will convert the electrical input signal 
to an equivalent acoustic signal in a unidirectional manner. The 
propagation direction (i.e., left-side or right-side) will depend on the 
phase difference of the phase differentiated signals 607. 
According to this embodiment of the invention, the bandwidth of the SAW 
filter is selected by selecting propagation direction of the acoustic 
waves within the center SAXV transducer 612. If left-side propagation 
direction is selected, the SAXV filter 600 provides a bandwidth which 
corresponds to the length of the SAW transducer 616. If however, a 
right-side propagation direction is selected, the SAW bandwidth of the SAW 
filter 600 is determined by the length of the SAW transducer 614. 
As described above, the various embodiments of the present invention 
provide a filter which takes advantage of small size and integratability 
of SAW structures while providing capability for providing selectable 
bandwidths which facilitate multi-system operation as well as 
manufacturing of communication devices with various bandwidth requirements 
.