Tunable oscillator having a non-reflective saw resonator

A tunable oscillator circuit (200) includes surface acoustic wave transducer (303) disposed on a piezoelectric substrate (301) having a high SAW coupling coefficient. The SAW transducer is (303) non-reflective and self-resonant comprising a pair of electrodes with interdigitated fingers. A tuning mechanism, such as a varactor, is coupled across the transducer allowing for output frequency tuning of the oscillator circuit.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to U.S. patent application Ser. No. 08/085,526 
entitled "A Three-Terminal Surface Acoustic Wave (SAW) Device," by Robert 
J. Higgins, Jr., filed on Jun. 30, 1993 and assigned to Motorola, Inc. 
TECHNICAL FIELD 
This invention relates generally to voltage controlled oscillators (VCO) 
structures and more particularly to a VCO which uses a Surface Acoustic 
Wave (SAW) structure. 
BACKGROUND 
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, non-linear 
capacitor, such as a semiconductor varactor, which is responsive to a 
control signal for controlling the value of 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 an oscillator with the variable capacitor. As the 
inductor's component size is being reduced in response to the 
miniaturization drive in the electronics industry, 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 
desence performance. 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. 
There have been attempts in the past to use SAW devices in VCOs. SAW 
devices use acoustic waves which travel at the speed of sound. The SAW 
devices are preferred over widely used transmission line, and discrete 
components because acoustic waves have a substantially shorter wave length 
at operating frequency than electromagnetic waves which travel at the 
speed of light. These devices are less susceptible to stray magnetic and 
other problems associated with resonators which utilize magnetic 
inductors. Furthermore, for a given operating frequency, a SAW resonator 
provides a smaller structure than a transmission line structure, 
therefore, making them suitable for miniaturized radio frequency 
applications. Additionally, SAW structures are potentially integratable 
with other active circuits, such as amplifiers and varactors, which are 
produced using conventional integrated circuit technologies. 
For the above reasons, the popularity of SAW structures in radio frequency 
applications has been steadily increasing, especially in resonator filter 
applications. FIG. 1 depicts the diagram of a conventional SAW resonator 
structure 100 which shows a SAW transducer 110 and a pair of reflectors 
120 disposed on a piezoelectric substrate 105. As is well known, the 
reflectors increase 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 a stable, high frequency (within 100 MHz-1000 MHz 
range) source is desired. 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 kilohertz, whereas, 
in land-mobile applications, a typical VCO in that frequency range must 
tune in megahertz range. 
Therefore, there exists a need for an oscillator circuit which overcomes 
the problems associated with magnetically induced oscillators while 
providing a substantially wide tuning range. 
SUMMARY OF THE INVENTION 
Briefly, according to the invention, an oscillator circuit for providing an 
output frequency comprises a SAW resonator and a capacitive tuning means 
coupled across the SAW resonator for tuning the output frequency. The SAW 
resonator is non-reflective and self-resonant comprising a piezoelectric 
substrate upon which a SAW transducer is disposed. The SAW transducer 
includes a pair of electrodes with a large number of interdigitated 
open-ended fingers, wherein the surface acoustic waves emanating from 
transducer sides are not reflected back. In order to provide wide tuning 
range, the piezoelectric material is made of material having substantially 
high surface acoustic wave coupling coefficient.

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 schematic diagram of a voltage controlled oscillator 
(VCO) 200, according to the present invention, is shown. The VCO 200 
provides a frequency output which is tunable within a predetermined tuning 
range. The VCO includes a SAW resonator 300 coupled across a varactor 209 
which is responsive to a tuning signal generated by a tuning signal source 
201. The tuning signal is received at a tuning port 203 and applied to the 
varactor 209 through a resistor 205. The varactor 209 is coupled to ground 
through a capacitor 211. The present invention contemplates using 
inductive properties of the SAW resonator 300 to resonate the oscillator 
portion of the VCO 200, thus, replacing the electromagnetic inductor used 
in conventional VCO designs. The combined effect of the parallel 
arrangement of the inductive property of the SAW resonator 300 and the 
capacitive property of the varactor 209 which is coupled across the 
resonator 300, causes the VCO to be resonant at a specific frequency. As 
is well known, capacitive variations across the varactor 209 in response 
to the tuning signal tunes the frequency output within the tuning range. 
As such the varactor 209 constitutes a tuning means coupled across the SAW 
resonator 300 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 resonator 30 to tune the output frequency. Furthermore, as described 
later in detail, the SAW resonator 300 posses certain 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 resonator 
300 and the varactor 209, is coupled to an amplifier stage through a 
capacitor 213. The amplifier stage comprises a transistor 219 which 
provides the frequency output of the VCO. Resistors 217, 215, and 221 
provide the biasing for the transistor 219, as is well known in the art. 
The output of the VCO is fed back to the oscillating portion of the VCO by 
coupling transistor 219's emitter to capacitor 211. 
Referring to FIG. 3, a top plan view of the SAW resonator 300 used in the 
oscillator circuit of the present invention is shown. The SAW structure is 
comprised of a piezoelectric substrate 301 upon which conductive patterns 
constituting a SAW transducer 303 is disposed. The substrate 301 is made 
of suitable piezoelectric material which, as described later, possesses 
the appropriate properties for producing the desired frequency tuning 
characteristics. The transducer 303 may be disposed on the piezoelectric 
substrate 301, utilizing any number of suitable techniques, such as thin 
film evaporation, or spattering with photo-lithographic definition. The 
SAW transducer 303 is patterned to include a pair of electrodes 305 having 
interdigitated open ended fingers 307 which are equally spaced according 
to the resonant frequency of the SAW resonator 300. Each of the electrodes 
terminate at terminals 309 which provide for interfacing the SAW resonator 
300 with the other elements of the oscillator circuit 200 of the present 
invention. As shown in FIG. 2 the capacitive element of the oscillator 
circuit, that is, the varactor 209 is coupled across the terminals 309. 
The center-to-center spacing of the interdigitated fingers determines the 
resonant frequency of the resonator 300 and is equal to 1/2 the wave 
length of the surface acoustic wave at resonant frequency. Contrary to 
most conventional SAW resonators, the SAW resonator 300 is non-reflective 
and self-resonant. That is, the SAW resonator 300 does not include 
reflecting means for reflecting the surface acoustic waves emanating from 
the sides of the SAW transducer 303. 
In order to provide wide tuning range essential to the VCO circuits, the 
SAW resonator 300 posses certain characteristics which are described 
below. Referring to FIG. 4, an equivalent circuit 300' for the SAW 
resonator 300 of FIG. 3, 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 comprising capacitance 
C.sub.0, and serially coupled motional capacitance C.sub.m, motional 
inductance L.sub.m and resistance Rs. It has been determined that the 
tuning range is directly proportional to the ratio of C.sub.m /C.sub.0. 
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 =8.times.K.sup.2 /.pi..sup.2 (1). 
Where K.sup.2 is coupling coefficient of substrate 301. The coupling 
coefficient represents the electro-mechanical property of piezoelectric 
substrate to convert electrical power to the surface-acoustic-wave's 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 VCO 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 resonator 300 is a non-reflective, 
self-resonant resonator with no reflectors disposed on its opposing outer 
sides. As such the acoustic waves emanating from sides of the transducer 
303 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 resonator 300 can be 
expressed by the following equation: 
EQU Q.sub.u =N.pi./4 (2). 
Where N is the number of interdigitated fingers in the SAW transducer 303. 
However, elimination of reflective elements from the self-resonant 
resonator 300 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 
transducer 300. 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 offers 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. 
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 VCO 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. 5, a block diagram of such a PLL 
circuit is shown. As shown, the phase-lock loop circuit 500 includes a VCO 
507 having its output fed to a well known programmable divider 509. The 
VCO 507 comprises a VCO constructed according to the principles of the 
present invention including the resonant SAW resonator 300. The 
programmable divider 509 receives divider signals 511 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 503 which compares the phase of the divider's output with the 
phase of a reference signal as provided by a reference frequency source 
501. 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 505. The output of the low pass 
filter is applied to the VCO 507 as a tuning signal for providing a 
desired output frequency of the VCO. Output of the VCO is fed back to the 
divider 509 and, as such, any variations in the frequency output is 
compensated by minimizing the error signal at the output of the phase 
detector 503. 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 507, thereby allowing for 
a wide tuning range as contemplated by the present invention. 
Referring to FIG. 6, the voltage controlled oscillator and the PLL 
synthesizer of the present invention are utilized in a radio 600. The 
radio 600 comprises a two-way radio, which may operate in either receive 
or transmit modes. The radio 600 includes a receiver section 610, and a 
transmitter section 620 which comprise means for communicating 
communication messages, on a receiver and transmitter carrier frequencies. 
The radio 600 also includes a PLL synthesizer section 630 of the present 
invention which under control of a controller 640 tunes the transmitter 
and the receiver sections 610 and 620 to operate within a desired 
frequency band. As is well known in the art, the controller 640 provides 
the control signals for setting the phase locked loop synthesizer at a 
particular receive or transmit frequency. The PLL synthesizer 630 
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 600 is tuned to receive a 
communication signal via an antenna 601. A transmit/receive (T/R) switch 
602 couples the received communication signal to a filter 603 which 
provides the desired selectivity for the received communication signal. 
The output of the filter 603 is applied to a well-known receiver IF 
section 604 which recovers the base band signal. The output of the 
receiver IF section is applied to a well-known audio section 605 which, 
among other things, amplifies audio messages and presents them to a 
speaker 606. It may be appreciated by one of ordinary skill in the art 
that the control voltage for setting the output frequency of the 
synthesizer 630 and consequently the carrier frequency of the receiver is 
provided by the controller 640, which also controls the entire operation 
of the radio 600. 
In the transmit mode, audio messages are inputted via a microphone 607, the 
output of which is applied to a well-known modulator 608 to provide a 
frequency modulating signal for the PLL synthesizer section 630. A 
transmitter power amplifier 612 amplifies the output of the modulated PLL 
synthesizer and applies it to the antenna 601 through the T/R switch 602 
for transmission of the communication signal. Similar to receiver carrier 
frequency, the transmitter carrier frequency is provided by the PLL 
synthesizer 630 of the present invention under the control of the 
controller 640. 
As described above, the VCO of the present invention utilizes properties of 
the SAW resonator 300 to provide resonance in the presence of the 
capacitive element of the VCO, i.e. the varactor 209. One of the main 
advantage offered by the VCO 200 is that the SAW resonator 300 eliminates 
the electromagnetic inductive element used in conventional VCO circuits. 
As such, unlike conventional VCOs, the VCO 200 is immune to external 
factors, such as microphonics, which effect the electromagnetic field 
surrounding the oscillator. Furthermore, the VCO of present invention may 
be fabricated in much smaller sizes than those possible with 
electromagnetic inductive elements without sacrificing Q.sub.u and 
consequently sideband noise performance. 
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.