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Claims  |
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What is claimed is:
1. In combination:
means for providing a signal having a predetermined amplitude;
means for feeding at least a portion of said signal back to an input of
said amplitude means, said feedback means further comprising:
(a) means operative over a predetermined temperature range for providing
said signal portion with a a predetermined delay relative to the signal at
an output of said amplitude means and a predetermined delay variation over
the operative temperature of said delay means, said delay means further
comprising:
(i) a substrate comprising ST-cut quartz, said substrate having a surface
which supports surface wave propagation and having a surface wave velocity
characteristic which varies nonlinearly over said operative temperature
range to provide said delay means with the delay variation over said
operative temperature range;
(ii) an input transducer and an output transducer, each one being coupled
to said surface wave propagating surface, with said signal portion from
said amplitude means being fed to the input transducer, and said output
transducer providing said signal portion fed to the input of said
amplitude means having the predetermined delay and predetermined delay
variation
(b) means including at least one passive reactive component electrically
coupled to said delay means and having a reactance characteristic
responsive to the temperature of the delay means, for providing a phase
shift variation as a function of temperature to compensate for the delay
variations provided to signal fed to the input of said amplitude means to
provide said signal fed to the input of said amplitude means with a
substantially reduced phase shift variation over the operative temperature
range.
2. The combination of claim 1 wherein said delay variation reducing means
further comprises at least one inductive element and at least one
capacitive element connected in series and coupled to one of said
transducers, with at least one of said inductive and capacitive elements
having the predetermined reactance variation.
3. The combination of claim 2 wherein said inductive element has the
predetermined reactance variation and comprises a magnetic material having
a magnetic permeability which varies in a predetermined nonlinear manner
over said operative range.
4. The combination of claim 3 wherein said inductive element has the
predetermined reactance variation and comprises:
a pair of solenoids;
a magnetic material disposed within portions of said solenoids; and
means for displacing said magnetic material within said solenoids as a
function of changes in temperature.
5. The combination of claim 4 wherein said displacing means further
comprises a member having a predetermined coefficient of thermal
expansion, said member being placed in a cooperative with said magnetic
material.
6. The combination of claim 2 wherein said capacitive element has the
predetermined reactance variation.
7. The combination of claim 6 wherein said capacitor comprises:
a center conductive member;
a pair of mutually spaced conductive members, each being dielectrically
spaced from said center conductive member, and each member arranged to be
capacitively coupled to a portion of said center conductive member; and
means for displacing in accordance with variations in temperature a first
one of the center conductive member and pair of spaced conductive members
with respect to a second one of the center conductive member and pair of
spaced conductive members.
8. The combination of claim 7 wherein said displacing means comprises a
dielectric having a predetermined coefficient of thermal expansion.
9. The combination of claim 8 wherein said dielectric comprises a fluid
having a predetermined coefficient of thermal expansion.
10. The combination of claim 8 wherein said dielectric comprises a member
having a predetermined coefficient of thermal expansion.
11. In combination:
a surface acoustic wave device to provide in response to an input signal,
an output signal having a first predetermined phase shift and a first
predetermined phase shift variation as a function of temperature with
respect to the input signal; and
means, disposed to have at least one of said input and output signals
coupled therethrough, said means including a passive component having an
electrical characteristic which varies as a function of the temperature of
the component, for providing a second temperature dependent predetermined
phase shift variation to reduce the temperature dependent phase shift
variation of said output signal.
12. The combination of claim 11 wherein the passive component is a reactive
component and wherein said means for providing a second phase shift
variation comprises:
at least one inductive element and at least one capacitive element
connected in series with a first one of said input and output transducers,
with at least one of said inductive and capacitive elements having the
predetermined reactance variation.
13. The combination of claim 12 wherein the inductive element has the
predetermined reactance variation.
14. The combination of claim 13 wherein said inductor comprises a magnetic
member having a magnetic permeability which varies in a predetermined
manner as a function of temperature to provide said predetermined
reactance variation.
15. The combination of claim 13 wherein said inductor comprises:
a pair of solenoids;
a magnetic material disposed within portions of said solenoids; and
means for displacing said magnetic material within said solenoids as a
function of changes in temperature.
16. The combination of claim 15 wherein said displacing means comprises a
member having a predetermined coefficient of thermal expansion, said
member being placed in a cooperative relationship with said magnetic
material.
17. The combination of claim 12 wherein passive element is a capacitor said
capacitor element has the predetermined reactance variation.
18. The combination of claim 17 wherein said capacitor comprises:
a center conductive member;
a pair of mutually spaced conductive members, each being dielectrically
spaced from said center conductive member, and each member arranged to be
capacitively coupled to a portion of said center conductive member; and
means for displacing in accordance with variations in temperature a first
one of the center conductive member and pair of spaced conductive members
with respect to a second one of the center conductive member and pair of
spaced conductive members.
19. The combination of claim 18 wherein said displacing means comprises a
dielectric having a predetermined coefficient of thermal expansion.
20. The combination of claim 19 wherein said dielectric comprises a fluid
having a predetermined coefficient of thermal expansion.
21. The combination of claim 19 wherein said dielectric comprises a member
having a predetermined coefficient of thermal expansion.
22. An oscillator comprising:
(a) means for providing a signal at an output thereof having a
predetermined amplitude;
(b) means for feeding at least a portion of said signal back to an input of
said amplitude means, said feedback means comprising:
(i) means for providing a predetermined phase shift characteristic to said
signal fed to the input of the amplitude means relative to the phase of
the signal provided from the output of the amplitude means comprising:
(a) means for supporting surface wave propagation having a surface wave
velocity characteristic which varies in a first predetermined manner as a
function of temperature;
(b) input and output transducers, each coupled to said surface wave
propagation means;
(c) wherein said signal is fed to the input transducer and received at the
output transducer and is provided with a predetermined phase shift and a
predetermined phase shift temperature dependent variation in accordance
with the surface wave velocity characteristic and temperature dependent
variation in surface wave velocity of the surface wave support means;
(ii) means, including at least one passive reactive element having a
predetermined reactance variation as a function of the temperature of the
element, and disposed to react to the temperature of the phase shift means
for providing, said signal with a substantially invariant delay variation
as a function of temperature.
23. The combination of claim 22 wherein said passive reactive element is an
inductor.
24. The combination of claim 23 wherein said inductor comprises a magnetic
member having a magnetic permeability which varies in a predetermined
manner as a function of temperature to provide the predetermined reactance
variation.
25. The combination of claim 23 wherein said inductor comprises:
a pair of solenoids;
a magnetic material disposed within portions of said solenoids; and
means for displacing said magnetic material within said solenoids as a
function of changes in temperature.
26. The combination of claim 25 wherein said displacing means comprises a
member having a predetermined coefficient of thermal expansion, said
member being placed in a cooperative relationship with said magnetic
material.
27. The combination of claim 22 wherein said passive element is a
capacitor.
28. The combination of claim 27 wherein said capacitor comprises:
a center conductive member;
a pair of mutually spaced conductive members, each being dielectrically
spaced from said center conductive member, and each member arranged to be
capacitively coupled to a portion of said center conductive member; and
means for displacing in accordance with variations in temperature a first
one of the center conductive member and pair of spaced conductive members
with respect to a second one of the center conductive member and pair of
spaced conductive members.
29. The combination of claim 28 wherein said displacing means comprises a
dielectric having a predetermined coefficient of thermal expansion.
30. The combination of claim 29 wherein said dielectric comprises a fluid
having a predetermined coefficient of thermal expansion.
31. The combination of claim 29 wherein said dielectric comprises a member
having a predetermined coefficient of thermal expansion.
32. A delay element comprising:
means, operative over a predetermined temperature range, for providing an
output signal having a predetermined, nominal delay with respect to an
input signal fed to an input of the delay means, such delay varying from
such nominal delay over the predetermined operating temperature range of
the delay means, said means further comprising:
(i) means for supporting surface wave propagation having a surface wave
velocity characteristic which varies in a predetermined manner over said
temperature range;
(ii) an input transducer fed by the input signal and an output transducer
providing said output signal, said transducers each being coupled to said
surface wave propagation means;
means, including an electrical component having a temperature dependent
electrical characteristic and disposed to have the temperature dependent
electrical characteristic thereof respond to the operative temperature of
the delay means, said electrical component being electrically coupled to
the delay means, for providing a temperature compensating delay to the
signal fed to the delay element, such signal passing through the delay
means and the electrical component of the temperature compensating delay
means, such compensating delay varying with the temperature dependent
electrical characteristic of the electrical component to provide the delay
element with a substantially temperature invariant delay characteristic
over the predetermined operating range.
33. The combination of claim 32 wherein said electrical component is an
inductor.
34. The combination of claim 33 wherein said inductor comprises a magnetic
member having a magnetic permeability which varies in a predetermined
manner as a function of temperature.
35. The combination of claim 33 wherein said inductor comprises:
a pair of solenoids fed by the signal passing through the temperature
compensating means;
a magnetic material disposed within portions of the magnetic fields
provided in response to said signal fed to said solenoids; and
means for displacing said magnetic material within said solenoids as a
function of changes in temperature.
36. The combination of claim 35 wherein said displacing means comprises a
member having a predetermined coefficient of thermal expansion, said
member being placed in a cooperative relationship with said magnetic
material.
37. The combination of claim 32 wherein said electrical component is a
capacitor.
38. The combination of claim 37 wherein said capacitor comprises:
a center conductive member;
a pair of mutually spaced conductive members, each being dielectrically
spaced from said center conductive member, and each member arranged to be
capacitively coupled to a portion of said center conductive member; and
means for displacing in accordance with variations in temperature a first
one of the center conductive member and pair of spaced conductive members
with respect to a second one of the center conductive member and pair of
spaced conductive members.
39. The combination of claim 38 wherein said displacing means comprises a
dielectric having a predetermined coefficient of thermal expansion.
40. The combination of claim 39 wherein said dielectric comprises a fluid
having a predetermined coefficient of thermal expansion.
41. The combination of claim 39 wherein said dielectric comprises a member
having a predetermined coefficient of thermal expansion.
42. An inductor comprising:
a pair of solenoids connected in parallel;
a body comprised of a magnetic material disposed within portions of each of
said solenoids; and
means for displacing said magnetic material within said solenoids as a
function of changes in temperature to provide said inductor with a
parabolic inductance variation in accordance with said changes in
temperature.
43. The combination of claim 42 wherein said displacing means comprises a
member having a predetermined coefficient of thermal expansion, said
member being placed in a cooperative relationship with said magnetic
material.
44. A capactior comprising:
a center conductive member;
a pair of mutually spaced conductive members, coaxially disposed around and
each being dielectrically spaced from said center conductive member, and
each member arranged to be capacitively coupled to a portion of said
center conductive member; and
means for displacing in an axial direction in accordance with variations in
temperature a first one of the center conductive member and pair of spaced
conductive members with respect to a second one of the center conductive
member and pair of spaced conductive members.
45. The combination of claim 44 wherein said displacing means comprises a
dielectric having a predetermined coefficient of thermal expansion and
said center conductor is disposed around said dielectric.
46. The combination of claim 44 wherein said means for displacing comprises
a fluid having a predetermined coefficient of thermal expansion.
47. The combination of claim 45 wherein said dielectric comprises a member
having a predetermined coefficient of thermal expansion. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates generally to temperature compensation circuitry and,
more particularly, to temperature compensation of surface acoustic wave
devices.
As is known in the art, components generally have a characteristic which
varies with temperature. As is also known, surface acoustic wave devices
(SAW devices) are used in a variety of applications such as a delay
element for oscillator circuits. SAW devices may be fabricated as a delay
line or resonator, for example, for use in such oscillators, as well as
filters and pressure transducers. Generally, a SAW device includes a pair
of transducers, with each transducer having a set of conductive members
which are disposed on a common piezoelectric substrate. At least the
surface of said substrate supports surface wave propagation. As a delay
line, for example, an electrical signal is coupled to a first one of the
transducers and in response to such signal, surface waves are launched.
These surface waves propagate along the surface of the piezoelectric
substrate and are received by the second one of the transducers. At this
second transducer an electrical signal, a replica of the original signal,
is produced in response to such surface waves. The time between launching
of the surface waves at the first transducer and arrival at the second
transducer provides a predetermined delay. When a SAW device is used in an
oscillator application, the SAW device is generally used as the delay
element in a feedback loop of an amplifier. The SAW device therefore
provides the requisite phase shift characteristics to an input signal fed
to the input of the amplifier. Thus, if the gain of the amplifier is
greater than the losses in the feedback loop, and the input signal is in
phase with respect to the output signal, then positive feedback is
provided around the amplifier and the amplifier will oscillate at the
frequency for which the input signal is in phase with the output signal.
Also commonly disposed in the feedback loop of the amplifier is a variable
phase shifter. The variable phase shifter, in response to an input control
signal, provides an output signal having a predetermined phase variation
with respect to the phase of an input signal. One type of variable phase
shifter commonly employed in SAW oscillators is a varactor diode coupled
in series with an inductor. A control bias signal is fed to the varactor
diode to vary its capacitance and hence the phase characteristic of the
variable phase shifter.
In many applications for SAW devices, particularly with respect to
applications involving oscillators, the SAW device is used because it is a
relatively stable delay element. Many different types of piezoelectric
materials may be employed with SAW devices. However, since the surface
wave velocity characteristic of the piezoelectric material as well as the
propagation length between the transducers generally vary as a function of
variations in the temperature of the material, the phase shift or delay
characteristic of the SAW device will also vary with temperature. One of
the most common types of substrate materials employed with SAW devices is
ST or rotated ST-cuts of quartz. ST or rotated ST-cuts of quartz have a
temperature dependent delay characteristic which is substantially
parabolic. That is, over temperatures less than the so-called "turn-over
temperature" of the ST or rotated ST-cuts of quartz, the delay
characteristic or phase shift decreases with increasing temperature, and
at temperatures greater than the turn-over temperature of the substrate
material the delay characteristics or phase shift increases with
increasing temperatures.
In highly stable precision oscillator applications, it is generally
required to compensate for these temperature dependent changes in the
surface wave velocity and propagation length of the SAW device and hence
for changes in the delay or frequency characteristics of the SAW device.
Otherwise, if left uncompensated, these temperature dependent variations
will cause a concomitant change in the resonant frequency of the
oscillator.
One type of temperature compensation scheme commonly used in oscillators
employing SAW devices as delay elements involves parametric compensation
of the phase of the input signal fed, via the feedback loop, to the input
of the amplifier. More particularly, an active device such as a
thermocouple is used to sense the temperature of the SAW device substrate
(or piezoelectric material). A signal is generated from the thermocouple
which is representative of the sensed temperature and this signal is then
fed to an analog multiplier or a set of parametric amplifiers which
provide, in response to the temperature sensor signal, an output signal
having an amplitude which varies in a predetermined manner as a function
of temperature. For example, in order to compensate a SAW device
fabricated on ST-cut or rotated ST-cut quartz, the output signal from the
multiplier will provide a signal having a quasiparabolic amplitude
characteristic. This output signal provides the control signal which is
fed to the varactor diode portion of the variable phase shifter described
above. In response to this control signal, the capacitance of the varactor
diode varies to provide, in combination with the series inductor, a phase
shift characteristic which varies oppositely with respect to the phase
variation generated by the temperature dependence of the SAW device. Thus,
with this arrangement, the frequency of the oscillator is relatively
stable with respect to temperature. This solution, however, presents
several problems. The temperature compensation circuitry, that is, the
active temperature sensor and the analog multiplier or parametric
amplifiers increase the weight, size, power consumption, cost and circuit
complexity of the oscillator. Also, due to the presence of these extra
components, the reliability of the oscillator may be reduced.
SUMMARY OF THE INVENTION
In accordance with the present invention, an input signal is fed to a first
device which provides a device output signal having a predetermined
nominal phase shift relative to the phase of the input signal and a
predetermined phase shift variation as a function of temperature. A
compensating network is provided to reduce the temperature dependent phase
shift variation of the device output signal. The compensating network
includes at least one passive element having an electrical characteristic
which varies in a predetermined manner as a function of the temperature of
the passive element. The variation of the electrical characteristic is
selected to provide the compensating network with a phase shift
characteristic which varies with temperature such that a temperature
compensated output signal is provided having a phase shift which is
substantially invariant with changes in temperature. With this
arrangement, the temperature compensated output signal has a phase shift
characteristic relative to the input signal which is substantially
invariant with changes in temperature. Since the compensation network
provides the compensating phase shift characteristic in response to
temperature-produced changes in the electrical characteristic of the
passive element, the complex circuitry generally used for temperature
dependent phase shift compensation is eliminated. Therefore, with this
arrangement, the cost, size, complexity, power consumption and weight of
the circuit are reduced.
In accordance with an additional aspect of the present invention, a surface
wave device has a predetermined surface wave velocity variation and a
predetermined propagation length variation as a function of temperature. A
signal is provided in response to the surface wave velocity and
propagation length variation having an electrical characteristic which
varies as a function of variations in said surface wave velocity. The
surface wave device is thermally and electrically coupled to a
compensating network. The compensating network includes a passive reactive
component having a predetermined temperature dependent reactance variation
as a function of the temperature of the passive reactive component to
compensate for the variations in the electrical characteristic resulting
from the temperature-produced surface wave velocity variation and
propagation length variation provided from the surface wave device. With
this arrangement, a signal is provided with a temperature compensated
electrical characteristic which is substantially invariant with
temperature.
In accordance with an additional aspect of the present invention, an
oscillator includes: means for producing a first signal having a
predetermined amplitude; feedback means, disposed around the amplitude
signal means, said feedback means including means, fed by said first
signal, for providing said first signal with a predetermined phase shift
and a predetermined phase shift variation as a function of temperature,
and means, thermally and electrically coupled to said phase shift means,
including at least one passive reactive element having a predetermined
reactance variation as a function of the temperature of the passive
reactive component, for providing a compensated signal to said amplitude
means having a phase shift with respect to said first signal which is
substantially invariant with temperature. With this arrangement, an
oscillator is provided having a frequency which is substantially invariant
with changes in temperature.
In accordance with an additional aspect of the present invention, a phase
shift device having a first predetermined phase shift variation with
respect to temperature is thermally and electrically coupled to a phase
compensating network. The phase compensation network includes an inductor,
having a coiled wire disposed around a magnetic member which is connected
in series with a capacitor. The capacitor has a capacitance variation
which is substantially invariant with temperature. The magnetic
permeability characteristic of the magnetic member is selected to provide
the inductor with a predetermined inductance variation with respect to
variations in the temperature of the inductor and thereby to provide, in
combination with the capacitor, a network having a second predetermined
phase shift variation with respect to temperature. With this arrangement,
the temperature dependent phase variation may be selected in accordance
with the phase shift variation provided by the phase shift means, to
thereby provide a phase shifter having a phase shift characteristic which
is substantially invariant with changes in temperature.
In accordance with a further aspect of the present invention, a composite
inductor element includes a pair of solenoids connected in parallel and a
magnetic member disposed within portions of the magnetic field provided by
said solenoids. The magnetic member is provided in a cooperative
relationship with a nonmagnetic member. The nonmagnetic member axially
displaces the magnetic member within regions of each of the solenoids in
accordance with changes in temperature. The magnetic member is displaced
within portions of the magnetic field regions provided by each of said
solenoids such that the inductance of a first one of said solenoids
increases linearly as a function of temperature, while concomitant
therewith, the inductance of a second one of said pair of solenoids
decreases linearly as a function of temperature. With this arrangement, by
connecting said solenoids in parallel, an inductor element is provided
having an inductance which varies parabolically as a function of
temperature. This composite inductor may then be connected with a
capacitor having a capacitance which is substantially invariant with
temperature to provide a network having a phase shift variation as a
function of temperature which is substantially parabolic. This network may
be used to compensate for temperature dependent parabolic phase shift
variations.
In accordance with a further aspect of the present invention, a composite
capacitor element includes a center conductive member, and a pair of
mutually spaced conductive members, each being dielectrically spaced from
said center conductive member, with each member being arranged to be
capacitively coupled to portions of the center conductive member. The
center conductive member is provided in a cooperative relationship with
means for axially displacing, as a linear function of the temperature of
the composite capacitor element, the center conductive member with respect
to the pair of spaced conductive members. The center conductive member
dielectrically spaced from the pair of mutually spaced conductive members
provides, in combination, a pair of series connected capacitors.
Therefore, in accordance with changes in the temperature of the composite
capacitor element, the first one of the pair of capacitors will have a
capacitance which increases linearly with temperature, whereas, the second
one of the pair of capacitors will have a capacitance which decreases
linearly with temperature. With this arrangement, the composite capacitor
element is provided having a change in capacitance as a function of
temperature which is parabolic. This capacitor may then be used with an
inductor having an inductance which is substantially invariant with
temperature to provide a network having a phase shift which varies
substantially parabolically as a function of temperature. This network may
then be used to compensate for temperature dependent parabolic phase shift
variations.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing features of this invention, as well as the invention itself,
may be more fully understood from the following detailed description of
the drawings, in which:
FIG. 1 is a block diagram showing a SAW delay line used as a delay element
in an oscillator circuit;
FIG. 2 is an equivalent circuit schematic diagram of the oscillator circuit
of FIG. 1 in accordance with a preferred embodiment of the invention;
FIG. 3 is a phasor representation useful in understanding the present
invention;
FIG. 4 is an isometric view of one embodiment of an inductor used in
accordance with the present invention;
FIG. 5 is a diagrammatical plan view of an oscillator circuit of FIG. 2
packaged in a hybrid microelectronic circuit package;
FIG. 6 is an equivalent circuit schematic diagram of the oscillator of FIG.
1 in accordance with an alternate embodiment of the present invention;
FIG. 7 is a block diagram of an alternate embodiment of an oscillator
circuit having a SAW device used as a delay element;
FIG. 8 is an equivalent circuit schematic diagram of an embodiment of the
oscillator of FIG. 7;
FIG. 9 is a schematic diagram of an alternate embodiment of the oscillator
in accordance with the block diagram of FIG. 7;
FIG. 10 is an isometric view of a capacitor having a capacitance which
varies parabolically as a function of temperature particularly suitable
for use in the embodiment of the oscillator shown in FIG. 7;
FIG. 10A is a cross-sectional view taken along line 10A--10A of FIG. 10;
FIG. 11 is an isometric view of an alternate embodiment of a capacitor
having a capacitance which varies parabolically as a function of
temperature particularly suitable for use in the embodiment of the
oscillator shown in FIG. 7;
FIG. llA is a cross-sectional view taken along lines 11A--11A of FIG. 11;
FIG. 12 is an isometric view of an inductor element having an inductance
which varies parabolically as a function of temperature particularly
suitable for use in the embodiment of the oscillator shown in FIG. 7; and
FIG. 12A is a cross-sectional view taken along lines 12A--12A of FIG. 12.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, an oscillator circuit 10 is shown to include a SAW
device, here a SAW delay line 12 having a piezoelectric substrate (not
shown) which supports surface wave propagation, a high temperature phase
compensation network 14, here for phase compensation at temperatures
greater than the turn-over temperature of the piezoelectric substrate of
the SAW device, a first amplifier 16, an electronic phase shifter 18, a
power divider 20, a second amplifier 22 and a low temperature phase
compensation network 24, here for phase compensation at temperatures less
than the turn-over temperature of the piezoelectric substrate. The SAW
delay line 12 is used in a feedback loop denoted by arrow 13 around
amplifiers 16 and 22 to provide the requisite phase shift characteristics
to the signal propagating in the feedback loop. The output of the SAW
delay line 12, therefore, is coupled to the high temperature phase
compensation network 14 as will be described in conjunction with FIG. 2.
The output of phase network 14 is coupled to the input of amplifier 16.
Amplifier 16 is biased to provide an output signal having a predetermined
amount of gain. The output signal from amplifier 16 is fed to an
electronic phase shifter 18. Electronic phase shifter 18 is used to
electronically fine tune the frequency of the oscillator 10 over a
predetermined bandwidth. The output of the electronic phase shifter 18 is
coupled to a power divider 20, here a center tapped transformer. Other
types of power dividers may alternatively be used, such as a microstrip
type power divider. The power divider divides an input signal at terminal
"IN" into a pair of here equal amplitude and opposite phase output signals
at terminals OUT 1, OUT 2. A first out-of-phase output signal is fed to an
oscillator output signal terminal 19 (at OUT 1) and the second in-phase
output signal (at OUT 2) is fed to the input of amplifier 22. Amplifier 22
is also biased to provide a predetermined amount of gain to the network
10. The output of amplifier 22 is coupled to a low temperature phase
compensation network 24. Low temperature phase compensation network 24 is
coupled to the input of the delay line 12.
SAW delay line 12 provides a substantially major portion of the phase delay
characteristics in the oscillator circuit 10. Amplifier 16 and amplifier
22 are selected to provide a sufficient amount of gain in the feedback
loop such that the composite gain of amplifiers 16, 24 exceeds the losses
in the feedback loop. With proper phase characteristics at each input
being substantially provided by the delay line 12 and the electronic phase
shifter 18, positive feedback to amplifiers 16 and 22 is provided and, as
a result, sustained oscillations are provided by the oscillator 10, with
the electronic phase shifter 18 providing a relatively small, variable
phase shift characteristic. The SAW delay line 12 provides relatively
stable frequency characteristics to the oscillator 10 thereby providing a
relatively stable, high precision output frequency from oscillator 10.
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