Three terminal surface acoustic wave (SAW) device

A three-terminal resonator device (200) resonant at a resonant frequency has a first terminal (201), a second terminal (202), and a common terminal (203). The resonator includes a surface acoustic wave transducer (290) disposed on a piezoelectric substrate (201). The SAW transducer (290) is comprised of a first sub-transducer (210) and a second sub-transducer (220) which respectively include a first set of electrodes (211,213) and a second set of electrodes (221,223). One of the electrodes of each of the first set and the second set of electrodes respectively provide the first terminal (201) and the second terminal (203). The other terminals of the first and second set of electrodes are connected at the common terminal (203) to provide a serial connection between the first sub-transducer (210), and the second sub-transducer (220).

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is a related to U.S. patent application entitled "A 
Tunable Oscillator Having a Surface Acoustic Wave Transducer," by Robert 
J. Higgins, Jr., Attorney Docket No. CM01515J, filed concurrently herewith 
and assigned to Motorola, Inc. 
TECHNICAL FIELD 
This invention relates generally to Surface Acoustic Wave (SAW) devices and 
more particularly to a three-terminal SAW device. 
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, SAW devices provide 
a smaller structure than a transmission line structure, making them 
suitable for miniaturized radio frequency applications. Furthermore, SAW 
devices are 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 resonator filter applications, 
has been steadily increasing. 
FIG. I depicts a conventional SAW resonator structure 100 which includes a 
SAW transducer 110 and a pair of reflectors 120 disposed on a 
piezoelectric substrate 105. As is well known, the reflectors increase the 
quality factor of the SAW resonator by preventing dissipation of surface 
acoustic waves emanating from the SAW transducer 110 near the resonant 
frequency. The SAW transducer 110 comprises a first electrode 112 having a 
first set of open-ended fingers 114 and a second electrode 116 having a 
second set of open-ended fingers 118. The first electrode 112 and the 
second electrode 116 comprise conductive layers patterned on the 
piezoelectric substrate such that a first set of fingers 114 and a second 
set of fingers 118 are interdigitated in relation to each other. 
Conventionally the substrate 105 is made of a material with low 
temperature coefficient, such as quartz. As such, the SAW resonator is 
used in applications where stable, high frequency (within 100 MHz-1000 MHz 
range) source is desired. 
Electric oscillators are extensively used in many applications where there 
is a need to generate one or more predetermined frequency output. For 
example, in frequency modulation (FM) radio frequency (RF) communication, 
RF receivers and RF transmitters use oscillators for generating carrier 
frequencies. Generally, oscillators operate at a particular resonant 
frequency. An oscillator's resonant frequency is determined by reactive 
elements of a tank circuit which comprises capacitive and inductive 
components. In conventional voltage controlled oscillators (VCOs), a 
variable reactive element in the tank circuit controls the resonant 
frequency of the oscillator. Often, the variable element is a 
two-terminal, nonlinear capacitor, such as a semiconductor varactor, which 
is responsive to a control signal for controlling its capacitance. Such 
tank circuit resonators provide a very wide tuning range (i.e. in the 
range of 3 MHz to 30 MHz) which makes them particularly suitable in 
frequency synthesized land-mobile communication applications. 
Conventionally, transmission lines and coaxial distributed structures have 
been used as a substitute for the inductive component of the tank circuit. 
However, the fundamental companion element to the variable capacitor has 
relied upon magnetic field storage to provide the inductive reactance 
necessary to resonate the oscillator with the variable capacitor. As the 
inductor's size is reduced, the resonator's quality factor, Q, decreases. 
Decreased quality factor, has a number of undesired consequences, namely, 
degradation of VCO's sideband noise performance and desensitization of the 
receiver in the presence of a strong adjacent channel carrier signal. 
Additionally, because an inductor's magnetic field is very difficult to 
shield, VCO generated signals, spurious or otherwise, are undesirably 
coupled to the surrounding circuit. 
Another problem frequently encountered in conventional VCOs, is a 
phenomenon known as "microphonics" which adversely affects an FM 
receiver's performance. Microphonics is a phenomenon whereby mechanical 
vibrations around the VCO structure are picked up by the electromagnetic 
inductor, thereby changing its effective inductance. As such, the resonant 
frequency of the VCO is changed. Since, in FM systems, the frequency 
changes are demodulated, the resonant frequency changes are manifested as 
undesired hum and noise which adversely affect the receiver's audio 
output. 
Historically, SAW resonators have not been used as oscillators in 
land-mobile communication because it is extremely difficult to get the 
resonant frequency to change, over a wide tuning range, with a variable 
capacitor. Typically, the tuning range of for a 900 MHz SAW resonator has 
been in the range of only a few kilo-hertz, whereas, in land-mobile 
applications, a typical VCO in that frequency range must tune in megahertz 
range. In a related co-pending U.S. patent application entitled "A Tunable 
Oscillator Having A Surface Acoustic Wave Transducer", filed concurrently 
herewith and assigned to assignee of the present application, which is 
hereby incorporated by reference, the applicant of the present invention 
has disclosed a two- terminal SAW device structure which uses inductive 
properties of a SAW resonator to provide a substantially wide tuning range 
in an oscillator circuit. This two-terminal SAW device is coupled to 
external components which receive oscillation feed back from an active 
device. In some conventional oscillator designs, the oscillation feed back 
from the active device is fed into an electromagnetic three terminal 
device, such as a tapped inductor. Having described the problems 
associated with electromagnetic device, there exist a need for a 
three-terminal SAW device which behaves as a tapped inductor. 
SUMMARY OF THE INVENTION 
Briefly, according to the invention, a three-terminal SAW device resonant 
at a resonant frequency has a first terminal, a second terminal, and a 
common terminal. The SAW device includes a piezoelectric substrate upon 
which a SAW transducer for generating acoustic waves is disposed. The SAW 
transducer has a first sub-transducer and a second sub-transducer. The 
first sub-transducer consists of a first set of two electrodes, and the 
second sub-transducer which is acoustically coupled to the first 
sub-transducer, consists of a second set of two electrodes. One of the 
electrodes of the first set of electrodes provides the first terminal, and 
one of the electrodes of the second set of electrodes provides the second 
terminal. The other electrodes of the first and the second set of 
electrodes are connected to each other at the common terminal to provide 
serial connection between the first and the second sub-transducers such 
that the acoustic waves generated by the SAW transducers are in phase at 
the resonant frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the specification concludes with claims defining the features of the 
invention that are regarded as novel, it is believed that the invention 
will be better understood from a consideration of the following 
description in conjunction with the drawing figures, in which like 
reference numerals are carried forward. 
Referring to FIG. 2, a top plan view of the SAW device of the present 
invention is shown. The SAW device 200 is a three-terminal SAW device 
having a first terminal 201, a second terminal 202, and a common terminal 
203. The SAW device 200 is comprised of a piezoelectric substrate 201 upon 
which conductive patterns constituting a SAW transducer 290 is disposed. 
As such, the SAW device 200 is resonant at a frequency as determined by 
the physical characteristics of the transducer 290 and the substrate 20 1. 
The substrate 20 1 is made of a suitable piezoelectric material which, as 
described later, possesses the appropriate properties for producing a 
desired frequency response characteristics. The transducer 290 may be 
disposed on the piezoelectric substrate 301, utilizing any number of 
suitable techniques, such as thin film evaporation, or sputtering with 
photo-lithographic definition. When an electrical signal is applied across 
any two terminals of the SAW device 200, surface acoustic waves propagate 
on the surface of the piezoelectric substrate creating desirable frequency 
characteristics. 
The SAW transducer 290 is patterned to include a plurality of 
sub-transducers consisting of a first sub-transducer 210, and a second 
sub-transducer 220. The first sub-transducer 210 and the second 
sub-transducer 220 are disposed side by side to be acoustically coupled to 
each other. The first sub-transducer 210 comprises a first upper electrode 
213 and a first lower electrode 211 with a first number of interdigitated 
fingers 212. The second sub-transducer 220 has a second upper electrode 
223 and a lower electrode 221 with a second number of interdigitated 
fingers 222. As is well known, the interdigitated open ended fingers 221 
and 222 are equally spaced and the spacing corresponds to the resonant 
frequency of the SAW device 200. According to the invention, the first and 
the second sub-transducers 210 and 220, are electrically coupled to each 
other in series such that the acoustic waves generated by the SAW 
transducer 290 are in phase at the resonant frequency. In the SAW device 
200, the first terminal 201 is provided by the first upper electrode 213, 
and the second terminal 202 is provided by the second lower electrode 221. 
The first lower terminal 211 and the second upper terminal 223 are 
electrically coupled to each other in series via a common electrode 240 
and terminate at the common terminal 203. In this arrangement, the 
electrodes 211 and 223 are serially coupled to each other where the phase 
of the propagating surface acoustic waves are not changed as they travel 
across the transducer 290 of the resonant frequency. As such, the 
electrodes of the acoustically coupled first sub-transducer 210 and the 
second sub-transducer 220 are electrically coupled to each other in series 
such that the acoustic waves generated by the transducer 290 are in phase 
with each other. 
It is also contemplated that a second transducer portion may be added on 
the piezoelectric substrate 201 to complement the performance of the SAW 
device 200 to produce a desired frequency response. One such transducer 
portion is shown by a dotted line and is designated by the reference 
numeral 290' which is electrically and acoustically coupled to the first 
transducer 290 as illustrated. The second transducer portion 290 portion 
is arranged identical to that of the transducer portion 290 and includes 
sub-transducers 210' and 220'. The sub-transducers 210' and 220' are 
electrically coupled to each other identical to the arrangement described 
in conjunction with the transducer 290. Furthermore, the transducer 290 is 
electrically coupled to the transducer 290' by connecting the 
sub-transducer 210' and 210 in parallel to each other. The parallel 
arrangement is provided by connecting the upper electrodes of the 
sub-transducers 210' and 210 to each other and likewise by connecting the 
lower electrodes of the sub-transducers 220 and 220' to each other. As 
illustrated, the lower electrode of the sub-transducers 210 and the upper 
electrode of the sub-transducer 220 are directly coupled to each other via 
the extension of the common track 240 which terminates at the common 
terminal 203. It may be appreciated that the parallel coupling of the 
sub-transducers 210 and 210' and parallel connection of the 
sub-transducers 220 and 220' correspondingly increases the number of 
interdigitated fingers of the SAW device 200. As a result the quality 
factor of the resonant SAW device 200 is increased as described later in 
detail. Accordingly, the further including a second transducer comprising 
a plurality of serially coupled sub-transducers, wherein at least one of 
either the sub-transducers 210' or 220' of the second transducer 290' is 
connected in parallel to one of the first sub-transducer 210 or the 
second-sub-transducer 220. 
It may be appreciated that the SAW device 200 may include acoustic wave 
reflectors disposed on each side of the transducer 290 (or the combination 
of transducer 290 and 290') to reflect the propagating acoustic waves. 
However, as described later in detail, in tunable oscillator application 
according to one aspect of the present invention, it is contemplated that 
the SAW device 200 be non-reflective (i.e., without reflectors) in order 
to provide wider tuning range. 
Referring to FIG. 3, an equivalent circuit 300 for the SAW device 200 of 
FIG. 2, which models effects of the acoustic circuit from an electrical 
perspective, is shown. As illustrated, the equivalent circuit 300 is 
represented by a parallel network of an electrically capacitive branch 
comprising capacitors C.sub.01 and C.sub.02 and a branch of serially 
coupled motional capacitance C.sub.m, motional inductance L.sub.m and 
resistance Rs. The capacitive branch of capacitors C.sub.01 and C.sub.02 
correspond to the electrical serial connection between the first 
sub-transducer 210 and the second sub-transducer 220. The resultant 
capacitance of C.sub.01 and C.sub.02 is designated by a capacitance 
C.sub.0 which represents the overall electrical capacitance of the 
transducer 290. Depending on the frequency of operation, SAW devices are 
capable of exhibiting both capacitive and inductive characteristics. For 
frequencies above the series resonance of C.sub.m and L.sub.m and below 
the parallel resonance across terminals 201 and 203 the circuit 300 
exhibits inductive characteristics. For frequencies below the series 
resonance of C.sub.m and L.sub.m and above the parallel resonance across 
terminals 201 and 203, the circuit 300 exhibits capacitive 
characteristics. The frequency range within which the SAW device 200 
exhibits inductive properties is designated as inductive frequency range. 
Similarly, the frequency range within which the SAW device 200 exhibits 
capacitive properties is designated as capacitive frequency range. 
Referring to FIG. 4, an equivalent circuit 400 is shown representing the 
inductive properties of the SAW device 200 within its inductive frequency 
range by a tapped inductive network. As shown, the circuit 400 comprises 
two serially coupled inductors L1 and L2 being tapped at the common 
terminal 203. It may be appreciated that the SAW device 200 can appear as 
a tapped capacitor in its capacitive frequency range. However, in tunable 
oscillator application, the inductive properties of the SAW device 200 are 
more desirable as they can be tuned to resonate with a variable capacitive 
tuning mechanism, such as a varactor. The tap ratio corresponds to the 
inductive characteristics of the inductors L.sub.1 and L.sub.2. As is well 
known, in electromagnetic inductors, the tap ratio is related to the 
winding ratio of the inductors L.sub.1 and L.sub.2. In the SAW device 200, 
the tap ratio is related to the ratio of the first number of fingers of 
the first sub-transducers 210 to the second number of fingers of the 
second sub-transducers 220. In other words, the amount of inductive (or 
capacitive) coupling between the common terminal 203 and the first 
terminal 201 and the second terminal 202 is dependent upon the ratio of 
first number of fingers to the second number of fingers. 
Referring to FIG. 5, the schematic diagram of a simple oscillator circuit 
500 which utilizes the three-terminal SAW device 200 of the present 
invention is shown. As shown, the SAW device 200 is coupled to an active 
stage comprising a transistor 530 which is biased, in a well known manner, 
by a supply voltage Vcc and resistors 510, 520 and 540 to provide a gain 
of greater than unity. The first terminal 201 of the SAW device 200 is 
coupled to the input base of the transistor 530, and the second terminal 
202 is grounded. The common terminal 203 is coupled to the emitter of the 
transistor 203. It is well known that an oscillator circuit is a closed 
loop circuit having an active stage with a gain of greater than unity, 
where its output is positively fed back to the input to cause oscillation. 
In the oscillator circuit 500, the output of the active transistor stage 
530 is positively fed back to the input by means of the three-terminal SAW 
device 200. The output the oscillator circuit 500 taken at the emitter of 
the transistor 530 is coupled to the common terminal 203 which in this 
arrangement acts as the feed back coupling port. 
Referring to FIG. 6, a schematic diagram of a voltage controlled oscillator 
(VCO) 600, according to the present invention, is shown. The VCO 600 
provides a frequency output which is tunable within a predetermined tuning 
range. The VCO 600 includes the SAW device 200 of the present invention 
constructed to be resonant at an output frequency. The three-terminal SAW 
device 200 is coupled across a varactor 601 via terminals 201 and 203 
producing the frequency oscillating portion of the VCO 600. The varactor 
601 is responsive to a tuning signal generated by a tuning signal source 
602 for varying the output frequency of the VCO 600 across the tuning 
range. The tuning signal is received at a tuning port 604 and applied to 
the varactor 601. Through a coupling capacitor 603, the VCO 600 utilizes 
inductive properties of the SAW device 200 to resonate the oscillator 
portion of the VCO 600, thus, replacing the electromagnetic inductor used 
in conventional VCO designs. The combined effect of the parallel 
arrangement of the inductive property of the SAW device 200 and the 
capacitive property of the varactor 209 causes the VCO to be resonant at a 
specific frequency. As is well known capacitive variations across the 
varactor 601 in response to the tuning signal tunes the frequency output 
within the tuning range. As such, the varactor 601 constitutes a tuning 
means coupled to terminals 201 and 202 of the SAW device 200 for tuning 
the output frequency of the tunable oscillator of the present invention. 
It may be appreciated that besides a varactor, other tuning means, such as 
a mechanically variable capacitor or a combination of a variable capacitor 
and an inductor, may be used across the SAW device resonator structure 20 
to tune the output frequency. As described above, the SAW device 200 
possesses characteristics which provide a wider tuning range for the VCO 
than the substantially narrow tuning range available from the conventional 
SAW resonators. The frequency of the oscillating portion of the VCO, i.e. 
the SAW device 200 and the varactor 601, is coupled to an amplifier stage. 
The amplifier stage comprises a transistor which provides the frequency 
output of the VCO. Resistors 605, 607, and 611 provide the biasing for the 
transistor 609 to have greater than unity gain, as is well known in the 
art. The output of the VCO 600 is fed back to the oscillating portion of 
the VCO by coupling transistor 609's emitter to common terminal 203 of the 
SAW device 200. As such the three-terminal SAW device 200 provides the 
means by which output of the VCO 600 is positively fed back. 
In order to provide the wide tuning range essential to VCO circuits, the 
SAW device 200 possesses certain characteristics which are described 
below. It has been determined that the tuning range is directly 
proportional to the ratio of C.sub.m /C.sub.0 (see FIG. 3). Therefore, 
widest tuning range is provided when the ratio of C.sub.m /C.sub.0 is as 
high as possible. For a self resonant transducer, the ratio of C.sub.m 
/C.sub.0 can be expressed by the following equation: 
EQU C.sub.m /C.sub.0 .congruent.8.times.K.sup.2 /.pi..sup.2 Eq. (1). 
Where K.sup.2 is coupling coefficient of the substrate on which the SAW 
device 200 is implemented. The coupling coefficient represents the 
electro-mechanical property of piezoelectric substrate to convert 
electrical power to the surface-acoustic-wave power and vice versa. As 
seen from Equation 1, there exists a direct relationship between the 
coupling factor K.sup.2 and the C.sub.m /C.sub.0 ratio. Thus, according to 
the invention, in order to achieve a substantially wide tuning range, a 
substrate having substantially high coupling coefficient, K.sup.2, is 
used. It has been determined that piezoelectric substrates with high 
coupling coefficients, particularly those exceeding 2 percent, are 
suitable for use in the SAW device 200 of the present invention. 
Piezoelectric substrates such as lithium niobate, lithium tantalate, or 
lead zirconate titanate, when cut at proper angles, offer high coupling 
coefficient. For example, the lithium niobate when cut at a 41 degree 
angle exhibits coupling coefficient of approximately 17%. Other exemplary 
high coupling coefficient piezoelectric substrates suitable for use in the 
oscillator circuit of the present invention include 36 degree-cut lithium 
tantalate and 64 degree-cut lithium niobate. 
Another factor affecting the C.sub.m /C.sub.0 ratio relates to the 
reflectors. As described above, the SAW device 200 as used for tunable 
oscillator applications is a non-reflective and self-resonant with no 
reflectors disposed on its opposing outer sides. As such the acoustic 
waves emanating from sides of the transducer are not reflected back by 
reflecting means conventionally used in SAW resonator structure. It has 
been determined that the self-resonant transducers offer a C.sub.m 
/C.sub.0 ratio of approximately 4 times that of transducers with 
reflective elements. 
Furthermore, it is desirable for the oscillator to exhibit a high unloaded 
quality factor, Q.sub.u, within the tuning range of the oscillator. The 
unloaded quality factor, Q.sub.u, of the SAW device 200 can be expressed 
by the following equation: 
EQU Q.sub.u =N.pi./4 Eq.(2) 
Where N is the number of interdigitated fingers in the SAW transducer 303. 
However, elimination of reflective elements from the self-resonant SAW 
device 200 substantially reduces the unloaded quality factor Q.sub.u 
thereof. According to the present invention the reduction in the quality 
factor is compensated by increasing the number of fingers in the SAW 
device 200. In current state of art (that is, VCOs which utilize inductive 
and capacitive elements), Q.sub.u is typically within 50-250 range with 
greater value being more desirable. Based on Equation 2, it has been 
determined that a transducer with approximately 70 fingers offers Q.sub.u 
of 50 and one with 320 fingers after Q.sub.u of 250. Therefore, in the 
present invention the requirement for high Q.sub.u and wide tuning range 
are balanced by utilizing a high coupling coefficient piezoelectric 
substrate and a non-reflective, self-resonant resonator having a 
transducer with a number of fingers for compensating some of Q.sub.u 
degradation caused by elimination of the reflectors. As described above, 
the number of fingers may be increased by disposing one or more transducer 
portions having sub-transducers which are connected in parallel to the 
sub-transducers 210 and 220 to increase the number of interdigitated 
fingers of the SAW device 200. 
Substrates such as lithium niobate and lithium tantalate despite offering 
high coupling coefficient, exhibit rather poor temperature coefficient. 
The poor temperature coefficients exhibited by the high coupling 
coefficient substrates are compensated for by utilizing the SAW device 200 
of the present invention in a phase-locked loop (PLL) synthesizer. This is 
because output feedback of the PLL would automatically compensate for 
instabilities caused by poor ambient temperature coefficient of the high 
coupling substrate 301. 
Referring to FIG. 7, a block diagram of such a PLL circuit is shown. As 
shown, the phase-lock loop circuit 700 includes a VCO 707 having its 
output fed to a well known programmable divider 709. The VCO 707 comprises 
a VCO constructed according to the principals of the present invention 
including the SAW device 200. The programmable divider 709 receives 
divider signals 711 from a controller or other suitable source for setting 
the desired frequency output of the PLL. The output of the programmable 
divider is applied to a well known phase detector 703 which compares the 
phase of the divider's output with the phase of a reference signal as 
provided by a high stability reference frequency source 701. An error 
signal produced by the phase detector and corresponding to the phase 
difference between the programmable divider and the reference frequency is 
applied to a low pass filter 705. The output of the low pass filter is 
applied to the VCO 707 as a tuning signal for providing a desired output 
frequency of the VCO. Output of the VCO is fed back to the divider 709 
and, as such, any variations in the frequency output is compensated by 
minimizing the error signal at the output of the phase detector 701. 
Because of the output feedback loop in the phase-lock loop circuit, any 
temperature variation would be compensated for by the loop itself and, 
therefore, the need for a highly-stable temperature coefficient substrate 
in the VCO is eliminated. As a result, a high coupling coefficient 
substrate (with poor temperature coefficient) could be used in the SAW 
transducer section of the VCO 707, thereby allowing for a wide tuning 
range as contemplated by the present invention. 
Referring to FIG. 8, the voltage controlled oscillator and the PLL 
synthesizer of the present invention are utilized in a radio 800. The 
radio 800 comprises a two-way radio, which may operate in either receive 
or transmit modes. The radio 800 includes a receiver section 810, and a 
transmitter section 820 which comprise means for communicating 
communication messages, on a receiver and transmitter carder frequencies. 
The radio 800 also includes a PLL synthesizer section 830 of the present 
invention which under control of a controller 840 tunes the transmitter 
and the receiver sections 810 and 820 to operate within a desired 
frequency band. As is well known in the art, the controller 840 provides 
the control signals for setting the phase locked loop synthesizer at a 
particular receive or transmit frequency. The PLL synthesizer 830 
incorporates the VCO of the present invention for providing frequency 
outputs corresponding to receiver and transmitter carrier frequencies. 
In the receive mode, the portable radio 800 is tuned to receive a 
communication signal via an antenna 801. A transmit/receive (T/R) switch 
802 couples the received communication signal to a filter 803 which 
provides the desired selectivity for the received communication signal. 
The output of the filter 803 is applied to a well-known receiver IF 
section 804 which recovers the base band signal. The output of the 
receiver IF section is applied to a well-known audio section 805 which, 
among other things, amplifies audio messages and presents them to a 
speaker 806. It may be appreciated by one of ordinary skill in the art 
that the control signal for setting the output frequency of the 
synthesizer 830 and consequently the carrier frequency of the receiver is 
provided by the controller 840, which also controls the entire operation 
of the radio 800. 
In the transmit mode, audio messages are inputted via a microphone 807, the 
output of which is applied to a well-known modulator 808 to provide a 
frequency modulating signal for the PLL synthesizer section 830. A 
transmitter power amplifier 812 amplifies the output of the modulated PLL 
synthesizer and applies it to the antenna 801 through the T/R switch 802 
for transmission of the communication signal. Similar to receiver carrier 
frequency, the transmitter carrier frequency is provided by the PLL 
synthesizer 830 of the present invention under the control of the 
controller 840. 
As described above, the three-terminal SAW device of the present invention 
provides a simple and reliable means for producing an oscillator circuit 
using SAW technology. The SAW device of the present invention eliminates 
the need for using electromagnetic inductive element of conventional VCO 
circuits while reducing the number of parts necessary for providing the 
positive feed back necessary in oscillator circuits. Furthermore, unlike 
conventional VCOs, the SAW device of the present invention makes SAW 
oscillator circuit immune to external factors, such as microphonics, 
caused by electromagnetic field changes surrounding the oscillator 
circuit. 
While the preferred embodiments of the invention have been illustrated and 
described, it will be clear that the invention is not so limited. Numerous 
modifications, changes, variations, substitutions and equivalents will 
occur to those skilled in the art without departing from the spirit and 
scope of the present invention as defined by the appended claims.