Coupled resonator phase shift oscillator

A phase shift oscillator formed with a surface acoustic wave coupled resonator in series with an amplifier and wherein the coupled resonator has a substantially 180.degree. phase shift occurring between the 3dB points of the frequency response curve of the coupled resonator.

BACKGROUND OF THE INVENTION 
The present invention relates to a phase shift oscillator and in particular 
to a phase shift oscillator formed with a surface acoustic wave coupled 
resonator having at least a 180.degree. phase shift occurring between the 
3 dB points of the frequency response curve of the resonator. In 
particular the invention relates to a surface acoustic wave coupled 
resonator phase shift oscillator which oscillates at any desired 
frequency, either VHF or UHF, does not require inductors or capacitors, is 
relatively inexpensive, is shock insensitive and is temperature sensitive 
only as determined by the surface acoustic wave resonator. 
Oscillators formed from surface acoustic wave devices are well-known in the 
art such as those disclosed in U.S. Pat. No. 3,868,595 and 3,950,713. 
These surface acoustic wave oscillators are formed from surface wave delay 
line devices used as the feed back network in the oscillator circuit. The 
surface acoustic wave delay line oscillator disclosed in U.S. Pat. No. 
3,950,713 is for operation at relatively low frequencies since at high 
frequencies reflection problems become so pronounced that alternative 
techniques must be used. U.S. Pat. No. 3,868,595 also discloses a surface 
wave delay line oscillator operating at UHF frequencies. However, in order 
to provide stable oscillations, the input and output of the amplifier must 
be coupled to the output and input of the surface acoustic wave device 
through matching and/or phase shifting networks including inductors, 
capacitors and resistors. 
The trend in the technology, of course, is to make the devices much less 
expensive, very small and compact. Such devices can be used in 
applications such as garage door opener transmitters, security devices, 
local oscillators and the like. In addition, military applications for 
stable oscillators are numerous. Military oscillators must be shock 
insensitive and temperature insensitive. Oscillators such as that 
disclosed in U.S. Pat. No. 3,950,713 are relatively shock insensitive and 
temperature insensitive but cannot operate at high frequencies. The 
surface acoustic wave oscillator disclosed in U.S. Pat. No. 3,868,595 can 
be used at high frequencies but is shock sensitive as well as temperature 
sensitive since the coils and capacitors vary the phase shift and the 
resultant frequency and the vibration of these devices or their change in 
physical shape because of temperature change respectively causes phase 
noise and changes the frequency of the oscillator. 
The present invention overcomes the difficulties of the prior art by 
allowing the construction of a surface acoustic wave oscillator using 
coupled resonators having a 180.degree. phase shift occurring between the 
3 dB points of the frequency response curve thus allowing the oscillator 
to operate at any design frequency without the use of inductors or 
capacitors and which is shock insensitive and temperature insensitive. 
SAW coupled resonators are well-known in the art such as that disclosed in 
U.S. Pat. No. 3,970,970. A coupled resonator consists of two SAW 
resonators acoustically coupled to one another and yielding a to pole 
narrow band pass response. In addition, betweer the 3 dB points of the 
frequency response curve a phase shift of 180.degree. occurs. Thus with a 
phase shift of 180.degree. at the center frequency the phase shift can 
vary from 90.degree. at the lower 3 dB point to 270.degree. at the higher 
3 dB point. Simply by reversing the bonding or changing the signal 
connection to the input pads of the SAW device, the phase shift at the 
center frequency is shifted to 0.degree. and thus covers the range from 
270.degree. to 90.degree.. Thus the entire 360.degree. spectrum is covered 
with the coupled resonator oscillator. 
Therefore, it is an object of the present invention to provide a phase 
shift oscillator having an amplifier with an input and an output and a 
surface acoustic wave device having an input and an output, a frequency 
response passing through 3 dB points and at least a 180.degree. phase 
shift occurring between the 3 dB points of the frequency response curve 
and means for connecting the output of the amplifier to the input of the 
SAW device and the input of the amplifier to the output of the SAW device 
to form an inductorless phase shift oscillator. 
It is also an object of the present invention to provide a phase shift 
oscillator which utilizes a coupled resonator as the surface acoustic wave 
device coupled to an amplifier and having at least a 180.degree. phase 
shift occurring between the 3 dB points of the frequency response curve of 
the coupled resonator. 
It is still another object of the present invention to provide a phase 
shift oscillator having a transistor as an amplifier which is coupled to a 
surface acoustic wave device formed as a coupled resonator and having at 
least a 180.degree. phase shift occurring between the 3 dB points of its 
frequency response curve. 
It is yet another object of the present invention to provide a phase shift 
oscillator which utilizes a coupled resonator formed with a single phase 
unidirectional transducer and having at least a 180.degree. phase shift 
occurring between the 3 dB points of its frequency response curve. 
It is also an object of the present invention to provide a phase shift 
oscillator which utilizes a surface acoustic wave resonator structure 
having a piezoelectric substrate, first and second spaced gratings of 
.lambda./4 electrodes attached to the substrate, input and output spaced 
transducer structures having .lambda./4 electrodes and attached to the 
substrate between the first and second gratings for generating standing 
waves in the substrate and a resonant cavity formed on the substrate 
between one of the gratings and one of the transducers. 
SUMMARY OF THE INVENTION 
Thus the present invention relates to a phase shift oscillator comprising 
an amplifier having an input and an output, a surface acoustic wave device 
having an input and an output and having a frequency response curve 
passing through 3 dB points and at least a 180.degree. phase shift 
occurring between the 3 dB points of said frequency response curve, and 
means for connecting the output of the amplifier to the input of the 
surface acoustic wave device and the input of the amplifier to the output 
of the surface acoustic wave device to form an inductorless phase shift 
oscillator. 
The invention also relates to a method of forming a phase shift oscillator 
comprising the steps of forming an amplifier having an input and an 
output, forming a surface acoustic wave device having an input and an 
output and having a frequency response curve passing through 3 dB points, 
and at least a 180.degree. phase shift occurring between the 3 dB points, 
and connecting the output of the amplifier to the input of the surface 
acoustic wave device and the input of the amplifier to the output of the 
surface acoustic wave device thereby forming an inductorless phase shift 
oscillator.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a block diagram of an existing prior art phase shift oscillator 
utilizing a surface acoustic wave delay line as the phase shift element. 
Amplifier 10 has an output which is coupled to an impedance matching 
network 12. The oscillator output is on line 14. The output is also 
coupled through matching network 12 to surface acoustic wave delay line 
16. The delay line 16 is formed on a quartz substrate 24 and includes 
input and output interdigitated transducers 20 and 22 respectively. The 
signal generated by output transducer 22 is coupled to an impedance 
matching network 18 whose output is coupled as the input to amplifier 10. 
The matching networks 12 and 18 are formed with inductors, capacitors and 
resistors. A power source 20 provides the power to drive amplifier 10. 
Amplifier 10 is designed to have sufficient gain to overcome the losses in 
the feedback circuit and provide an output on line 14. 
Not only do the matching networks 12 and 18 provide for maximum power 
transfer between the amplifier 10 and the delay line 16 but they also fix 
the phase shift around the loop so that it is exactly 0.degree. or some 
multiple of 360.degree. so the device will oscillate. The amplitude and 
phase response of the matched delay line are shown in FIG. 2 and it can be 
seen therein that the phase response is linear through the pass band and 
that there is a 360.degree. phase shift through the 6 dB points. 
There are several problems with this type of circuit. First, it is 
relatively expensive but because it includes the inductors, capacitors and 
resistors. Secondly, its size is increased because of the use of the 
inductors, capacitors and resistors. In addition, it is shock sensitive 
because vibrations cause the coils or capacitors, which are frequency 
determining elements, to vibrate and thus cause a slight change in 
inductance or capacitance which causes a slight change in the phase and 
frequency. That shows up in the site bands causing phase noise. Further, 
coils and capacitors are temperature sensitive and change values with the 
change in temperature thus again causing a shift in frequency with 
temperature. It would be desirable therefore to have a phase shift 
oscillator utilizing a surface acoustic wave device without coils and 
capacitors because the device would then be entirely dependent upon the 
SAW device temperature characteristics and also would be shock 
insensitive, smaller in size and more economical to construct. 
Surface acoustic wave resonators are well-known in the art and such a 
device is illustrated in FIG. 3. The device 22 includes input transducer 
24 and output transducer 26 as well as reflective gratings 30 and 32. The 
input signals are supplied from the generator 34 to the input transducer 
24 and the load 36 is coupled to the output transducer 26. 
The amplitude or phase response of the device in FIG. 3 with respect to 
frequency is illustrated in FIG. 4. It will be noted that there is a 
90.degree. phase shift between the 3 dB points of the amplitude curve. 
While it is true that the phase shift is greater than 90.degree. beyond 
the 3 dB points, the amplitude is decreasing so rapidly that the amplifier 
has a difficult time finding enough excess gain to oscillate. The real 
frequencies of interest are between the 3 dB points and as shown in FIG. 
4, as in the case of a conventional resonator, the phase shift between 
those points is 90.degree.. Thus the graph in FIG. 4 is the typical single 
pole narrow band pass response of the conventional SAW resonator. The 
phase of the resonator center frequency can be 0.degree. or 180.degree. 
depending upon the bonding pad connections. In other words, one of the 
bonding pads of the input transducer and output transducer would be 
grounded whereas the other one would have the signal on it. With one such 
connection the center frequency is 0.degree. and when the connections are 
reversed so that the signal connection now becomes the ground connection 
and vice versa, the center frequency is 180.degree.. In either case, the 
phase can shift only 45.degree. from the center frequency on either side 
thus making a total of 90.degree. phase shift. From 0.degree. the signal 
can vary from 45.degree. to 315.degree.. At 180.degree., the signal can 
vary from 135.degree. to 225.degree.. As stated previously, in either case 
there is a phase discontinuity of 180.degree. which would require the use 
of tuning coils and tuning capacitors to shift the phase into those 
regions. This of course would bring us back to the prior art type of phase 
shift oscillators which are shock sensitive and temperature sensitive as 
well as more expensive and require more space. 
The problem can be solved with the use of a coupled resonator which is 
well-known in the prior art. Such resonator is shown in FIG. 5. The 
coupled resonator 38 is formed on a piezo-electric crystal 39 and includes 
an input transducer 40 and an output transducer 42 separated by a resonant 
cavity 44 which is a-periodic with the transducers 40 and 42 and with 
reflective gratings 46 and 48. The device has two electrodes per wave 
length and thus an electrode width of .lambda./4. Such prior art resonator 
is disclosed in U.S. Pat. No. 3,970,970. The frequency and phase response 
curve of the prior art coupled resonator shown in FIG. 5 is illustrated in 
FIG. 6. Note that the SAW coupled resonator has a 180.degree. phase shift 
between the 3 dB points instead of 90.degree.. The reason of course, as is 
well-known in the art, is that the two resonators in a coupled resonator 
device are in effect in series. When the frequency of one of the 
transducers is offset slightly from the design frequency of the other, the 
overall frequency response is a two-pole band pass with twice the phase 
shift of a conventional resonator. Thus, maintaining the same band width 
will enable twice the phase shift per hertz. Thus as can be seen in FIG. 6 
between the 3 dB points, there is a phase shift of 180.degree. which will 
allow a phase shift oscillator to be constructed without the use of coils 
or capacitors. The phase of the coupled resonator center frequency can be 
either 0.degree. or 180.degree. so once again if the center frequency is 
at 0.degree., the frequency can vary from +90.degree. to -90.degree. 
(270.degree.). In like manner, if the resonator center frequency is 
180.degree., the frequency can vary from +90.degree. to -90.degree. 
(270.degree.). Again, if the center frequency is at 0.degree., the band 
pass which is greater than 90.degree. but less than 270.degree. is 
omitted. In like manner, if the center frequency is at 180.degree., the 
band pass which is greater than 270.degree. but less than 90.degree. is 
missing. Again, once it is determined in which of the quadrants the 
circuit needs to operate, the bonding of the connections to the input and 
output pads can be reversed to cause the unit to operate with the center 
frequency of either 0.degree. or 180.degree.. 
Thus the coupled resonator circuit shown in FIG. 5 is ideal for use in the 
novel surface acoustic wave coupled resonator oscillator disclosed in FIG. 
7. Before constructing the SAW coupled resonator, the amount of phase 
shift that will be contributed by the amplifier must be determined. For 
example, assume a transistor amplifier coupled in a common emitter 
configuration. Ideally the phase shift through this circuit or amplifier 
is 180.degree.. But that is true only under ideal conditions. As the 
frequency applied to the amplifier begins to increase, there is an 
increasing delay between the time the signal is applied and the time that 
the output is produced. The delay produces an additional phase shift on 
top of the ideal 180.degree. phase shift. As an example, if a transistor 
has a design frequency of 3 GHz and it is operated at 10 MHz the total 
phase shift may be very close to 180.degree.. However, if it is operated 
at 300 MHz its phase shift is not now 180.degree.. It may be more in the 
order of 230.degree.. In other words there is an added 50.degree. phase 
shift that has been contributed to the normal operation of the transistor 
simply because of the frequency at which it is operated. 
Again, assume that the same transistor is operated in the common collector 
mode. Ideally the phase shift would be 0.degree.. However, if again, it is 
run at a much higher frequency in the order of 300 MHz, the phase shift is 
now 50.degree.. In the UHF range there is a considerable phase shift in 
the active devices such as transistors. There is not as much phase shift 
in the VHF range. 
Consider the standard resonator shown in FIG. 3 and its attendant amplitude 
or phase response curve shown in FIG. 4 when used in the VHF range. 
Suppose for example in the UHF range utilizing a standard resonator, a 
50.degree. phase shift error occurs. Since the standard resonator has a 
90.degree. phase shift, (from 0.degree. to +45.degree. and -45.degree.) 
the phase shift has exceeded the 3 dB band width of the standard resonator 
by 5.degree.. In addition, the higher the frequency at which the device is 
used, the larger the phase shift error becomes. For example, at 1000 MHz, 
the phase shift may be 280.degree.. Obviously, this is totally outside the 
capabilities of the standard resonator no matter which way it is bonded. 
The only way to use that type of a device is to add phase shifters with 
the use of coils and capacitors to make the device oscillate at the proper 
frequency. 
However, utilizing the SAW coupled resonator shown in FIG. 5, having a 
180.degree. phase shift, the devices will not only work at VHF but also at 
UHF frequencies. Thus, in that case, .+-.90.degree. (a total of 
180.degree.,) includes both the 50.degree. phase shift and the 280.degree. 
phase shift. Thus the present invention utilizes the entire phase 
continuum in which oscillations occur without the use of coils or 
capacitors simply by determining the initial phase shift of the amplifier 
and bonding the input and output connections to the coupled resonators to 
cause the device to oscillate in the proper region. 
Thus in the novel circuit shown in FIG. 7 which utilizes a coupled 
resonator in conjunction with an amplifier to form a phase shift 
oscillator, it is noted that amplifier 50 produces an output on line 52 
which is also coupled through line 54 to the input of the coupled 
resonator 56. The output of the coupled resonator 56 on line 58 is coupled 
as the input to amplifier 50. The device is powered by a power supply such 
as battery 60. By utilizing as the coupled resonator 56 the device shown 
in FIG. 5, and by knowing the phase shift through amplifier 50 for a given 
frequency, the coupled resonator can have its input and output leads 
bonded to the transducers so as to cause the resonator to have a phase 
shift in the particular region in which amplifier 50 operates. Thus, 
assume that the delay through amplifier 50 at high frequencies is 
230.degree.. SAW coupled resonator 56 can then be bonded to have a center 
frequency of 180.degree. .+-.90.degree. which will obviously cover the 
230.degree. phase shift of the amplifier 50 and thus allow the device to 
oscillate at the design frequency without the need for coils or capacitors 
to adjust the phase shift of the amplifier 50 to match that of the coupled 
resonator. 
It is also obvious that NPN or PNP transistor amplifiers could be utilized 
as amplifier 50. In addition, the amplifier may be coupled in the common 
emitter configuration, common base or common collector configurations to 
obtain the advantages of any particular transistor configuration. In 
addition, an integrated circuit could be used as the amplifier 50. 
The coupled resonator 56 may be of any well-known type but in particular 
may be well suited for constriction with use of a single phase 
unidirectional surface acoustic wave transducer and in particular of the 
type disclosed in commonly assigned co-pending U.S. Pat. application Ser. 
No. 677,513, entitled SINGLE PHASE UNIDIRECTIONAL SURFACE ACOUSTIC WAVE 
TRANSDUCER and filed Dec. 3, 1984 and which is incorporated herein by 
reference in its entirety. The maximum frequency operation may be obtained 
with such device inasmuch as it utilizes quarter wave length electrodes. 
In like manner, the resonator structure disclosed in commonly assigned 
co-pending application Ser. No. 822,233, entitled, RESONATOR STRUCTURE and 
filed Jan. 24, 1986, and incorporated herein by reference in its entirety, 
may also be used for the same reason. The resonator structure disclosed 
therein comprises a piezo-electric substrate, first and second spaced 
gratings of .lambda./4 electrodes attached to the substrate, input and 
output spaced transducer structures having .lambda./4 electrodes and 
attached to the substrate between the first and second gratings for 
generating standing waves in the substrate, and the resonant cavity formed 
on the substrate between one of the gratings and one of the transducers. 
Thus a phase shift oscillator has been disclosed which has no coils or 
capacitors, which operates from the VHF through the UHF ranges, is 
relatively inexpensive, is relatively small and compact, is shock 
insensitive and temperature insensitive. It is shock insensitive inasmuch 
as there are no coils or capacitors to vibrate thereby causing phase noise 
or jitter in the carrier. Further, it is well-known that depending upon 
the crystal cut used, surface acoustic wave devices are very temperature 
stable. 
While the invention has been described in connection with a preferred 
embodiment, it is not intended to limit the scope of the invention to the 
particular form set forth, but, on the contrary, it is intended to cover 
such alternatives, modifications, and equivalents as may be included 
within the spirit and scope of the invention as defined by the appended 
claims.