Temperature compensated surface acoustic wave oscillators

A plurality of parallel-connected surface acoustic wave (SAW) resonators, having different oscillation frequencies and different turnover temperatures, are connected to an active element containing circuit to form a SAW oscillator. The oscillator frequency deviation due to temperature is kept within a small allowable range .DELTA. fa by satisfying the following formulas: EQU f.sub.H -f.sub.L .ltoreq..DELTA.fa EQU T.sub.p1 <T.sub.p2. . .<T.sub.pn where PA1 f.sub.H : a maximum frequency among those frequencies of a plurality of SAW resonators connected to the active element-containing circuit which are produced at turnover temperatures on a temperature curve; PA1 f.sub.L : a minimum frequency among those of the plurality of SAW resonators; and PA1 T.sub.p1 to T.sub.p2 : the turnover temperatures corresponding to the individual SAW resonators.

BACKGROUND OF THE INVENTION 
This invention relates to a surface acoustic wave (SAW) oscillator 
comprising a plurality of parallel-connected surface acoustic wave 
resonators, each of which includes a pair of interdigitated electrodes 
mounted on a quartz substrate for propagating a surface acoustic wave. 
The prior art surface acoustic wave oscillator comprises as shown in FIG. 
1, an active element-containing circuit 2 which is connected to a surface 
acoustic wave oscillator 1 through the terminals 1a, 1b. With the 
conventional surface acoustic wave oscillator 1, a pair of interdigitated 
electrodes 4, 5 (formed of electrode elements 6, 7 respectively) are 
mounted on the main plane of a substrate 3 prepared from, for example, 
quartz for connection to said active element-containing circuit 2. 
FIG. 2 shows an equivalent circuit of the surface acoustic wave oscillator 
of FIG. 1. According to this equivalent circuit, a resonator section 1 
comprises a series circuit of an inductance L and capacitor C connected in 
parallel to another capacitor C.sub.T. A circuit section 2 including an 
active element consists of a series circuit of a capacitor C.sub.L and 
negative resistor -R. The resonance circuit section 1 and circuit section 
2 are connected by terminals 1a, 1b. 
The constants L, C, C.sub.T of the equivalent circuit can be freely 
selected by changing the number of the respective paired electrode 
elements or fingers of the interdigital electrodes, the thickness of said 
interdigital electrodes and the length of those portions of the respective 
paired electrode elements or fingers which are actually interdigitated 
with each other. 
Where the resonance circuit section 1 of a surface acoustic wave oscillator 
represented by the above-mentioned equivalent circuit has an impedance Z, 
then said oscillator is oscillated at a frequency f satisfying the 
condition expressed by the equation (1) below: 
EQU 1/(j2.pi.fC.sub.L)+z=0 (1) 
where j is imaginary symbol. 
Referring to a single resonator, the impedance z thereof generally varies 
with ambient temperature. This means that the surface acoustic wave 
oscillator has a frequency largely governed by ambient temperature. 
FIG. 3 is a curve diagram of the deviation .DELTA.f of the aforesaid 
oscillation frequency f of the oscillator relative to ambient temperature 
T, where the substrate thereof is formed of quartz and the main plane of 
the quartz is represented by the rotated Y cut plane. As apparent from 
FIG. 3, the frequency deviation .DELTA.f is substantially reduced to zero 
at the turnover temperature T.sub.p. Where, however, the turnover 
temperature T.sub.p is shifted either upward or downward, then the 
frequency deviation .DELTA.f increases along a second degree temperature 
coefficient. Compensation for the temperature dependency of oscillation 
frequency has hitherto been undertaken in a circuit including an active 
element. Yet said compensation has proved unsatisfactory. 
A crystal oscillator which has been proposed to date to compensate for the 
temperature dependency of oscillation frequency includes the U.S. Pat. No. 
3,821,666. According to this prior art, three bulk wave crystal vibrators 
are connected in parallel. This parallel circuit is connected to an active 
element-containing circuit. Said U.S. Patent oscillator is the 
temperature-compensation type which is intended to reduce frequency 
deviation over a prescribed temperature range by connecting in parallel 
the three crystal vibrators which collectively display a particular 
frequency deviation characteristic relative to ambient temperature. 
The bulk vibrators included in a temperature compensation type oscillator 
present greater difficulties in manufacture, according as said oscillator 
is demanded to have a higher frequency. The reason is that a substrate of 
bulk wave crystal vibrator has to be made thinner in the inverse ratio to 
the increased frequency. Further, provision of, for example, wiring 
supports unavoidably give rise to variations in the properties of said 
temperature compensation type oscillator, whose practical application is 
therefore undesirably limited. 
With a surface acoustic wave oscillator of this invention, the rotated Y 
cut plane of quartz is used as a substrate. A pair of interdigitated 
electrodes whose electrode elements are mounted on the main plane of the 
substrate in the alternately adjacent form are connected to an 
active-element containing circuit through connection terminals. According 
to the SAW resonator of this invention comprising a surface acoustic wave 
resonator presents a noticeable difference from the prior art bulk 
vibrator type oscillator in the properties resulting from the operation 
principle, construction and resonance condition, a plurality of surface 
acoustic wave resonators are connected in parallel and are so constructed 
that a prescribed relationship exists between the frequencies of the 
respective surface acoustic wave resonators as well as between the 
turnover temperatures. The technique of producing a surface acoustic wave 
oscillator embodying this invention has not been known to date, nor can be 
inferred from any prior art. 
SUMMARY OF THE INVENTION 
It is accordingly the object of this invention to provide a surface 
acoustic wave oscillator whose frequency varies little with ambient 
temperature. 
To this end, the invention provides a surface acoustic wave oscillator 
which comprises a resonance circuit formed of a plurality of surface 
acoustic wave resonators having different oscillation frequencies and 
displaying different turnover temperatures on different curves denoting 
frequency deviations relative to ambient temperature, said each surface 
acoustic wave resonator being formed of a pair of interdigital electrodes 
whose electrode elements are mounted in the alternately adjacent form on a 
substrate for propagating a surface acoustic wave; and an active 
element-containing circuit connected to the resonance circuit, and wherein 
the plurality of surface acoustic wave resonators have such oscillation 
frequencies and turnover temperatures as meet the conditions expressed by 
the following formulas: 
EQU f.sub.H -f.sub.L .ltoreq..DELTA.fa 
EQU T.sub.p1 &lt;T.sub.p2 &lt; . . . T.sub.pn 
where: 
f.sub.H =a maximum frequency among those frequencies of a plurality of 
resonators collectively connected to the active element-containing circuit 
which are produced at turnover temperatures on a temperature 
characteristic curve. 
f.sub.L =a minimum frequency among said frequencies of the plurality of 
resonators. 
.DELTA.fa=allowable frequency deviation of the surface acoustic wave 
oscillator. 
T.sub.p1 to T.sub.pn =those turnover temperatures of the plurality of 
oscillators each comprising a single resonator which are indicated on a 
temperature characteristic curve. 
This invention has newly developed the arrangement of the plurality of 
surface acoustic wave resonators mounted on the surface of a quartz 
substrate and the construction of electrodes constituting said resonators 
in order to meet the requirements denoted by the above formulas, thereby 
providing a surface acoustic wave oscillator whose frequency varies little 
with ambient temperature over a broad range. 
This invention has the various prominent advantages that the SAW resonator 
used with the respective embodiments can be constructed by the technique 
of photolithography to admit of tonnage production; it is unnecessary to 
reduce the thickness of a substrate in inverse proportion to the resonance 
frequency as in the case of a bulk wave crystal vibrator, thereby ensuring 
the easy manufacture of the SAW oscillator with high precision in the 
frequency range from scores of Mega Hertz (MHz) units to several Giga 
Hertz (GHz) units; application of printed wiring on the same substrate 
enables the vibrating section and substrate to be separately supported, 
thereby eliminating variations in the properties of the SAW oscillator; 
and the constant of an equivalent circuit of the SAW oscillator can be 
freely selected by changing the number of the respective paired electrode 
elements or fingers of the interdigital electrodes and the width defined 
between said respective paired electrode element fingers, thereby enabling 
temperature compensation to be effected over a substantially unlimited 
range.

DETAILED DESCRIPTION 
There will now be described by reference to the accompanying drawings a 
surface acoustic wave (SAW) oscillator embodying this invention. FIG. 4 
schematically illustrates the principle by which the surface acoustic wave 
oscillator 1 of this invention is operated. A plurality of surface 
acoustic wave resonators M.sub.1 to M.sub.n are connected in parallel on 
the main plane of a quartz substrate 3 prepared, for example, by the ST 
cut. This resonator section is connected to an active element-containing 
circuit 2 through terminals 1a, 1b. The arrangement of FIG. 4 is 
equivalently shown in FIG. 5. The respective surface acoustic wave 
resonators and active element-containing circuit 2 have the same 
arrangement as those of FIG. 2. 
The SAW resonator M.sub.1 is indicated by a circuit formed by connecting a 
series circuit of an inductor L.sub.1 and capacitor C.sub.1 in parallel 
with another capacitor C.sub.T1. The SAW resonator M.sub.i (i=1 to n) is 
represented by a circuit formed by connecting a series circuit of an 
inductor L.sub.i and capacitor C.sub.i in parallel with another capacitor 
C.sub.Ti. (The suffix i denotes the serial positions of the actually used 
SAW resonator, inductor and capacitor respectively.) These circuits 
constituting the SAW resonators M.sub.1, M.sub.i are connected to an 
active element-containing circuit through the connection terminals 1a, 1b. 
The constants L, C, C.sub.T of the equivalent circuit can be freely 
selected by changing the number of the respective paired electrode 
elements or fingers of the interdigital electrodes, the thickness of said 
interdigital electrodes and the length of those portions of the respective 
paired electrode elements or fingers which are actually interdigitated 
with each other. 
Assuming that the respective resonators M.sub.1 to M.sub.n of FIG. 4 have 
impedances Z.sub.1 to Z.sub.n, then the surface acoustic oscillator 1 has 
an oscillation frequency f expressed as follows: 
EQU 1/(j2.pi.fC.sub.L)+1/(1/Z.sub.1 +1/Z.sub.2 + . . . 1/Z.sub.N)=0 (2) 
As seen from the above equation, the frequency of the surface acoustic wave 
oscillator 1 varies with the impedance of the parallel-connected 
resonators. If the impedance is governed by ambient temperatures, then the 
frequency f of said oscillator should also be affected by the ambient 
temperature. Now let it be assumed that the frequency of the surface 
acoustic wave oscillator of this invention is represented by f.sub.i (Hz); 
an intermediate temperature on a curve denoting the deviation of said 
frequency relative to ambient temperature T is indicated by T.sub.pi ; an 
allowable frequency deviation is denoted by .DELTA.fa. Then, the plural 
surface acoustic wave resonators M.sub.1 to M.sub.n of the surface 
acoustic wave oscillator satisfy the conditions expressed by the following 
formulas: 
EQU f.sub.H -f.sub.L .ltoreq..DELTA.fa (3) 
EQU T.sub.p1 &lt;T.sub.p2 &lt; . . . &lt;T.sub.pN (4) 
where: 
f.sub.H =a maximum frequency among those frequencies of a plurality of 
resonators collectively connected to the active element-containing circuit 
which are produced at turnover temperatures on a temperature 
characteristic curve. 
f.sub.L =a minimum frequency among said frequencies of the plurality of 
resonators. 
T.sub.p1 to T.sub.pN =peak temperatures on a curve denoting a frequency 
deviation relative to ambient temperature. 
Now let it be assumed that a surface acoustic wave oscillator comprises a 
resonator section formed of two resonators M.sub.1, M.sub.2 included in 
those (M.sub.1 to M.sub.n) shown in FIG. 4. Where the resonators M.sub.1, 
M.sub.2 are used separately, then the frequency deviation .DELTA.f of said 
oscillator has such temperature dependency that as shown by the broken 
line curves Q.sub.1, Q.sub.2 of FIG. 6, the frequency deviation .DELTA.f 
is reduced to zero only at turnover temperatures T.sub.p1, T.sub.p2. In 
contrast where the resonators M.sub.1, M.sub.2 are used in the 
parallel-connected form, then the frequency deviation .DELTA.f 
approximates zero, as shown by the solid line curve Q.sub.0 of FIG. 6, 
over a temperature range T.sub.pD extending between turnover temperatures 
T.sub.p1 and T.sub.p2 with an intermediate temperature T.sub.pi taken as 
the center. Further over a broader temperature range T.sub.pA, the surface 
acoustic wave oscillator of this invention indicates a smaller frequency 
deviation than at least the allowable frequency deviation .DELTA.fa. The 
reason is that the parallel-connected surface acoustic waves resonators 
M.sub.1, M.sub.2 are considered to satisfy the condition of f.sub.2 
-f.sub.1 .ltoreq..DELTA.fa derived from the aforesaid formula (3) and also 
the condition of T.sub.p1 &lt;T.sub.p2 resulting from the previously 
mentioned formula (4). 
A solid line curve Q.sub.0 of FIG. 7 indicates the frequency deviation 
relative to ambient temperature of a surface acoustic wave oscillator 
which comprises three parallel-connected resonators M.sub.1, M.sub.2, 
M.sub.3. 
The curves Q.sub.1, Q.sub.2, Q.sub.3 of FIG. 7 denote the temperature 
characteristic curves of the frequencies of the respective resonators 
M.sub.1, M.sub.2, M.sub.3 when connected to an active element-containing 
circuit. The temperature characteristic curve Q.sub.0 of the resonator 
frequency is more prominently improved than the temperature characteristic 
curves Q.sub.1, Q.sub.2, Q.sub.3 of the frequencies of the resonators 
M.sub.1, M.sub.2, M.sub.3 each comprising a single resistor. Therefore, 
the three parallel-connected surface acoustic wave resonators M.sub.1, 
M.sub.2, M.sub.3 are regarded to meet the condition of f.sub.3 -f.sub.1 
.ltoreq..DELTA.fa derived from the formula (3) and the condition of 
T.sub.p1 &lt;T.sub.p2 &lt;T.sub.p3 resulting from the formula (4). 
There will now be described the concrete arrangement and construction of a 
plurality of surface acoustic wave resonators capable of satisfying the 
conditions expressed by the aforesaid formulas (3), (4). 
Referring to the embodiment of FIG. 8, the surface acoustic waves delivered 
from the surface acoustic wave resonators M.sub.1, M.sub.2 are propagated 
in different directions as indicated by the arrows K.sub.1, K.sub.2, and 
at different rates of temperature dependence. Due to the propagating in 
the respective directions K.sub.1, K.sub.2, the surface acoustic waves are 
designed to intersect each other at a prescribed angle, and the impedances 
Z.sub.1, Z.sub.2 of the surface acoustic wave resonators M.sub.1, M.sub.2 
are now rendered subject to temperature dependence. Since, as the result, 
the frequencies f.sub.1, f.sub.2 of the surface acoustic wave resonators 
M.sub.1, M.sub.2 and in consequence the turnover temperatures thereof vary 
with the impedances Z.sub.1, Z.sub.2, the surface acoustic wave oscillator 
of the embodiment of FIG. 8 can satisfy the conditions of the formulas 
(3), (4). Eventually, therefore, the frequency of the surface acoustic 
wave oscillator of FIG. 8 is little affected by ambient temperature over a 
broad range. 
FIG. 9 illustrates the arrangement of a surface acoustic wave oscillator 
comprising three surface acoustic wave resonators M.sub.1, M.sub.2, 
M.sub.3. Surface acoustic waves are propagated from said oscillator in 
different directions, thereby attaining the same result as the embodiment 
of FIG. 8. 
FIG. 10 sets forth a curve plotted from experiments conducted with the 
embodiments of FIGS. 8 and 9. The curve shows the extent to which turnover 
temperture varies with an angle defined by the propagation direction of 
surface acoustic waves indicated by, for example, the arrows K.sub.1, 
K.sub.2. 
FIG. 11 shows a curve plotted from experiments carried out with the 
embodiments of FIGS. 8 and 9. The curve indicates the extent to which the 
propagation velocity of a surface acoustic wave varies with an angle 
defined by the directions of said propagation. As is apparent from FIG. 10 
or FIG. 11, the more broadened the angle defined by the propagation 
directions of a surface acoustic wave, the higher the turnover temperature 
and the propagation velocity. Namely, where a plurality of surface 
acoustic wave resonators are so arranged as to cause a surface wave to be 
propagated over the surface of said resonators in different directions, 
then the conditions denoted by the formulas (3), (4) can be satisfied. 
There will now be described by reference to FIG. 12 a surface acoustic wave 
oscillator according to still another embodiment of this invention. The 
quartz substrate 3 has two main planes 3a, 3b formed of different rotated 
Y cut planes. A surface acoustic wave is propagated in the same direction 
as indicated by the arrows K.sub.1, K.sub.2 over said two main planes 3a, 
3b. Where, as in the embodiment of FIG. 12, the main planes 3a, 3b of the 
quartz substrate 3 did not lie on the same horizontal plane but were so 
inclined as to indicate a certain rotation angle difference relative to a 
referential plane, experiments were made to determine the extent to which 
the turnover temperature and surface wave propagation velocity of the 
resonators M.sub.1, M.sub.2 varied, the results being set forth in the 
curve diagram of FIG. 13. In FIG. 13, line T corresponds to turnover 
temperature and line V corresponds to velocity. The broader the rotation 
angle difference between the two main planes 3a, 3b, the lower the 
turnover temperature, and conversely the higher the propagation velocity 
of a surface acoustic wave. The above-mentioned rotation angle difference 
of the main planes 3a, 3b positively causes the frequency of both surface 
acoustic wave resonators M.sub.1, M.sub.2 to be more affected by ambient 
temperature. Eventually, therefore, the surface acoustic wave oscillator 
of FIG. 12 comprising a plurality of parallel-connected resonators can 
meet the conditions expressed by the aforesaid formulas (3), (4). 
There will now be described by reference to FIGS. 14 and 15 a surface 
acoustic wave oscillator according to a further embodiment of this 
invention. Two surface acoustic wave resonators M.sub.1, M.sub.2 are 
mounted on the same main plane formed of the ST cut plane of the quartz 
substrate 3. As in the embodiment of FIG. 12, a surface acoustic wave is 
propagated over the surface of said resonators M.sub.1, M.sub.2 in the 
same direction as indicated by the arrows K.sub.1, K.sub.2. With the 
embodiment of FIGS. 14 and 15, however, the electrodes 4, 5 of one (for 
example, M.sub.1) of said two resonators have a different thickness from 
the electrodes 4, 5 of the other resonator (M.sub.2). Experiments were 
conducted to find variations in the turnover temperature and surface 
acoustic wave propagation velocity of said resonators M.sub.1, M.sub.2, 
the results being set forth in the curve diagram of FIG. 16. In this 
figure broken line T corresponds to turnover temperature and line V 
corresponds to velocity. In the above-mentioned experiments, the 
electrodes were prepared from aluminium. As seen from FIG. 16, the larger 
the difference between the thicknesses of both aluminium electrodes, the 
lower the turnover temperature and surface acoustic wave propagation 
velocity of the two resonators M.sub.1, M.sub.2. Therefore, a surface 
acoustic wave resonator according to the embodiment of FIGS. 14 and 15 can 
also meet the conditions represented by the aforesaid formulas (3), (4). 
FIG. 16 relates to the case where the two electrodes 4, 5 are made of 
aluminum. FIG. 17 relates to the case where two electrodes 4, 5 are made 
of gold. In FIGS. 16 and 17, line T corresponds turnover temperature and 
line V corresponds to velocity. Where, however, both electrodes 4, 5 are 
prepared from gold, then, as shown in FIG. 17, the turnover temperature 
and surface acoustic wave propagation velocity more prominently vary than 
in the case of aluminium electrodes, even when the gold electrodes have 
the same thickness difference as the aluminium electrodes. As mentioned 
above, where the two surface acoustic wave resonators M.sub.1, M.sub.2 are 
connected in parallel, the temperature dependence of the frequency of said 
respective resonators M.sub.1, M.sub.2 can be made to vary with the 
electrode material thereof. 
The process of causing the frequencies of two adjacent resonators, for 
example, M.sub.1, M.sub.2 to have different degrees of temperature 
dependence can be effected either by depositing, as shown in FIG. 18, on 
the surface of the resonator M.sub.1 a thin insulating layer 10 having a 
different temperature coefficient from the substrate 3, or by providing 
said thin insulating layer 10, as shown in FIG. 19, between the substrate 
3 on one hand and the interdigital electrodes 4, 5 on the other. Said thin 
insulating layer 10 should advisably be formed of aluminium oxide 
(Al.sub.2 O.sub.3) or magnesium fluoride (MgF.sub.2) which admits of easy 
production. Where particularly in the case of a thin insulating layer of 
magnesium fluoride, kH (kH being the relative film thickness which is 
normalized according to wavelength) is set at 0.03 (with the resonator 
frequency taken to be 300 MHz and the thickness of said insulating layer 
to be 50 nm), then a turnover temperature can be reduced to about 
80.degree. C. which appears on a temperature dependence characteristic 
curve of the frequencies of SAW oscillators each comprising a plurality of 
or single resonator. It is experimentally proved that the turnover 
temperature can be varied aproximately in proportion to the thickness of 
said thin insulating layer of magnesium fluoride. This means that the 
conditions expressed by the previously shown formulas (3), (4) can be 
satisfied also by forming the above-mentioned thin insulating layer, 
namely, the resonance arrangements of FIGS. 18 and 19 enable the 
respective resonators to have different degrees of temperature dependence 
from each other. The conditions of the formulas (3), (4) can obviously be 
satisfied by combinations of the aforesaid processes of changing the 
propagating directions of a surface acoustic wave, the main plane of, for 
example, a quartz substrate and the raw materials of interdigital 
electrodes. The parts of the embodiments of FIGS. 18 and 19 the same as 
those of FIG. 15 are denoted by the same numerals, description thereof 
being omitted. 
The foregoing embodiments refer to the case where the parallel-connected 
surface acoustic wave resonators comprised a pair of interdigitated 
electrodes respectively. However, this invention is not restrictively 
applied to such case, but may be used with a cavity type resonator clamped 
between a pair of reflection latices. 
With a surface acoustic wave oscillator of the invention, a resonator 
circuit formed of a plurality of parallel-connected surface acoustic wave 
resonators is connected to an active element-containing circuit. The 
resonators are so constructed as to indicate different turnover 
temperatures and surface acoustic wave propagation velocities. If the 
different oscillation frequencies and turnover temperatures are so related 
to each other as to satisfy the prescribed conditions, then it is possible 
to provide a surface acoustic wave oscillator whose frequency is little 
affected by ambient temperature over a broad range.