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The present invention relates to a temperature probe using a plate of
quartz in a frequency generator.
BACKGROUND OF THE INVENTION
In order to measure and control temperature accurately, thermometer sensors
are used that have a high degree of sensitivity and linearity in the
required temperature range. The ultimate resolution of the measuring
instrument is determined by the measuring chain and the physical
characteristics of the sensor.
A widde variety of physical phenomena have been used to provide temperature
probes capable of operating over various temperature ranges. For example,
probes based on the principle of expansion in bodies that are solid,
liquid (alcohol, mercury) or gaseous (hydrogen) have been used, as have
probes based on visible radiation (pyrometers). Other instruments rely on
temperature variations in electrical characteristics in resitors,
thermistors, and thermocouples.
At present, the highest performance temperature measuring instruments rely
on methods that cause the frequency of an oscillator to vary with
temperature. In particular, it has been known for some years that plates
of quartz used as frequency standards in high stability oscillators are
subject to temperature effects that perturb their accuracy.
Since quartz is anisotropic, crystal cut orientations can be found to
minimize, or alternatively to selectively increase sensitivity to
temperature.
A quartz orientation that can be used to produce plates with a linear
temperature coefficient over a wide temperature range is described by D.
L. Hammond, C. A. Adams & P. Schmidt in a paper entitled "A linear quartz
crystal temperature sensing element" given at the 19th annual conference
of the ISA in October 1964. This bulk wave quartz cut, known as the LC
(Linear Coefficient) cut, has been used to make a quartz thermometer by
inserting the plate in an amplified loop to obtain an oscillator whose
frequency varies linearly with temperature (D. L. Hammond &. A.
Benjaminson in "Unthermometre lineaire a quartz" (A linear quartz
thermometer), in the February 1966 issue of the journal "Mesure").
Measurements could be performed automatically with a resolution of
10.sup.-4 .degree. C. over a temperature range of -40.degree. C. to
+230.degree. C. A major drawback of such a probe is that its response time
is about 10 seconds, which is due to the fact that the bulk wave quartz
plate is held by three fixing points which are the seat of the main heat
exchangers. The sensitivity of this probe is about 35.times.10.sup.-6
/.degree. C., which corresponds to a frequency variation of 10.sup.3
Hz/.degree. C. at an operating frequency of 28 MHz (3rd partial).
Preferred embodiments of the present invention reduce the response time of
such a temperature probe and increase the accuracy of temperature
measurement.
SUMMARY OF THE INVENTION
The present invention provides a temperature probe comprising a quartz
crystal plate for use in determining the frequency of a frequency
generator, the frequency determined by said plate being variable as a
function of temperature, wherein the quartz plate constitutes the
substrate for a surface acoustic wave device, and wherein the orientation
in the quartz crystal of the plane over which the acoustic wave
propagates, and the direction of propagation in said plane, said defined
in terms of a doubly rotated frame of reference as follows:
a base frame of reference OXYZ in the quartz crystal is defined by the
optical axis OZ, one of the electrical axes OX, and a mechanical axis OY
at right angles to the axis OX;
said base frame of reference OXYZ is rotated through a first angle .phi.
about the optical axis OZ thereby obtaining a singly rotated frame of
reference OX'Y'Z';
said singly rotated frame of reference OX'Y'Z' is rotated through a second
angle .theta. about the axis OX' (that is itself rotated .phi. away from
said electrical axis OX) thereby obtaining a doubly rotated frame of
reference OX"Y"Z";
the plate is cut such that surface waves propagate in the OX"Z" plane
defined in said doubly rotated frame of reference; and
said surface wave are caused to propagate in a direction that is at a third
angle .psi. to the axis OX" in said OX"Z" plane; as such, the plate is a
double rotation cutting plate defined by yxwl: .phi./.theta. in which one
large surface is used for the propagation of a surface wave whose
direction of propagation makes the angle .psi. with the second axis of
rotation;
where the said first angle .phi. lies in the range 9.degree.24" to
13.degree.24', said second angle .theta. lies in the range 57.degree.24'
to 61.degree.24', and said third angle .psi. lies in the range 33.degree.
to 37.degree..
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention are described by way of example with reference
to the accompanying drawings, in which:
FIG. 1 shows the rotations of the frames of reference relative to the
quartz in a double rotation cut;
FIG. 2 shows the direction of propagation of a surface wave over a double
rotation plate;
FIG. 3 schematically shows a frequency generator circuit using a first type
of temperature probe in accordance with the invention;
FIG. 4 schematically shows another frequency generator circuit using the
first type of temperature probe in accordance with the invention;
FIG. 5 shows an alternative embodiment of the circuit of FIG. 3 using a
second type of temperature probe in accordance with the invention;
FIG. 6 shows an alternative embodiment of the circuit of FIG. 4 using the
second type of temperature probe in accordance with the invention;
FIGS. 7 and 8 show a temperature probe in accordance with the invention in
which the quartz plate is mounted in a housing in accordance with a first
embodiment;
FIG. 9 shows a temperature probe in accordance with the invention in which
the quartz plate is mounted in a housing in accordance with a second
embodiment;
FIG. 10 shows a temperature probe in accordance with the invention in which
the quartz plate is mounted in a housing in accordance with a third
embodiment;
FIG. 11 is a frequency-temperature graph for a temperature probe.
DESCRIPTION OF PREFERRED EMBODIMENTS
Temperature probes in accordance with the present invention are based on
the properties of quartz crystal. These properties are particularly
applicable to implementing resonators or delay lines in oscillators. The
main properties in question are a high degree of purity, high chemical
stability and great elasticity.
Because of the anisotropic structure of quartz, the frequency-temperature
characteristic of a quartz device can be varied by a suitable choice of
angular orientations of cut. Thus an ST cut gives rise to a parabolic
temperature characteristic, with inversion taking place at a temperature
of about +20.degree. C. Plates obtained from such a single rotation cut
are used for surface wave devices.
The frequency-temperature characteristic of a quartz crystal is adequately
represented over a fairly wide temperature range by a third order
polynomial of the form:
f(T)=f(T.sub.0)[1+a.sub.1 f(T-T.sub.0)+a.sub.2 f(T-T.sub.0).sup.2 +a.sub.3
f(T-T.sub.0).sup.3 ]
where T.sub.0 is a reference temperature, and a.sub.1, a.sub.2 & a.sub.3
are first, second and third order frequency coefficients.
A study of the coefficients and the variation of frequency as a function of
temperature can be found in an article entitled "High order temperature
coefficients of quartz SAW oscillators" by D. Hauden, M. Michel & J. J.
Gagneplan in the Proceedings of the 32nd Annual Symposium on Frequency
Control, May 1978.
A plate of quartz with an ideal frequency response as a function of
temperature would have zero second and third order coefficients a.sub.2 &
a.sub.3, in which case the frequency-temperature characteristic would be
perfectly linear.
The Applicants have discovered that, by taking advantage of the two degrees
of freedom offered by the angles .phi. & .theta. defining a double
rotation cut, they can obtain plates of quartz with lower value second and
third order coefficients than is possible with single rotation cuts.
Furthermore, the coefficients can be further reduced by choosing a
direction of propagation for surface waves different from the OX" axis
which is rotated through an angle o about the optical axis OZ of the
crystal away from one of the electrical axes OX. Such plates are obtained
from double rotation cuts in which the angles .phi. & .theta. lie in the
following ranges: 9.degree.24' to 13.degree.24' for the angle .phi.; and
57.degree.24' to 61.degree.24' for the angle .theta.. The direction of
surface wave propagation on such plates should be at an angle .psi. to the
OX" axis, with .psi. lying in the range 33.degree. to 37.degree..
FIG. 1 shows the rotation of the frame of reference OXYZ in a double
rotation cut for a plate initially having its thickness according to the Y
axis and its length according to the X axis.
The frame of reference is itself defined by the optical axis OZ of the
crystal, by an electrical axis OX and by a mechanical axis OY that is at
90.degree. to the axis OX.
The angle .phi. describes a first rotation of the frame of reference about
the axis OZ; this gives a frame of reference OX'Y'Z', with the axes OZ &
OZ' constituting the same axis.
The angle .theta. describes a second rotation of the frame of reference
OX'Y'Z' about the axis OX'; this gives a frame of reference OX"Y"Z", with
the axes OX' & OX" constituting the same axis. The cut plane, and hence
the plane of the plates is the plane OX"Z", the cut of such a plate being
defined by yxwl: .phi./.theta..
FIG. 2 shows the direction of propagation D of surface waves over a double
rotation cut plate L, having two transducers 1 & 2, said plate being
defined by yxwl: .phi./.theta..
The direction of propagation D makes an angle .psi. with the axis OX" in
the plane OX"Z", the OX" axis being confused with the second rotation axis
OX'.
The angles .phi., .theta. & .psi. are in the notation laid down by the 1949
IEEE convention.
To obtain zero second and third order coefficients, to within experimental
error, the angles 100 , .theta. & .psi. should have the following values:
.phi.=11.degree.24'
.theta.=59.degree.24'
.psi.=35.degree.
A plate defined by the cut yxwl: 11.degree.24'/59.degree.24' and on a large
surface of which a surface wave is propagated, according to a direction
making an angle .psi.=35.degree. with the axis of second rotation, to a
temperature coefficient of 30.times.10.sup.-6 /.degree.C. Such a metalized
plate is referred to herein as an LTS plate (Linearly temperature
sensitive).
LTS plates may be of the delay line type or of the resonator type.
A surface wave delay line is constituted by two transducers, each in the
form of two interdigitated combs of metal fingers deposited on one face of
the piezoelectric substrate. Since the transducers are bidirectional,
surface waves are emitted in opposite directions. Absorbant material
eliminates reflections due to the waves emitted towards the edge of the
plate.
A resonator comprises one transducer with a reflector on each side of the
transducer, which is generally in the form of two interdigitated combs.
For both the resonator and the delay line configurations, the combs are
oriented on the quartz plate such as to obtain a direction of surface wave
propagation that makes an angle .psi. with the axis of second rotation OX'
as shown in FIG. 2.
The plates of quartz are cut and measured using conventional techniques.
Reference should be made in this respect to the article "Goniometric
Measurements of the Angles of Cut of Doubly Rotated Quartz Plates" by J.
Clastre, C. Pegeot & P. Y. Leroy in the Proceedings of the 32nd Annual
Symposium on Frequency Control, U.S. Army Electronic Command, Ft Monmouth,
New Jersey--May 1978.
The combs and reflectors are then obtained by conventional techniques, and
in particular by photoengraving.
A temperature probe in accordance with the invention is essentially used in
a frequency generator that incorporates the LTS plate of the temperature
probe, with the plate used in an amplifier feedback loop, or in a phase
locking loop of voltage controlled local oscillator or of a frequency
synthesiser. The temperature response of the probe is obtained by plotting
its frequency-temperature characteristic.
FIG. 3 schematically represents a frequency generator circuit using a
temperature probe constituted by a delay line 3 accordance with the
invention comprising a delay line 3 connected across the terminals of an
amplifier 4, together with an output amplifier 5. The transducers 1 & 2 of
the delay line 3 are connected respectively to the input and to the output
of the amplifier 4. The delay line 3 and the amplifier 4 constitute an
oscillator.
To sustain oscillation, it is necessary for the amplifier 4 to have
sufficient gain to to compensate loss round the loop, and for the total
phase shift round the loop to be equal to an integer multiple of 2".
It is thus necessary for .phi..sub.A +.phi..sub.L =2k.pi., where
.phi..sub.A is the phase shift of the amplifier 4, the matching circuits
and the transducers, .phi..sub.L is the phase shift of the delay line 3,
and k is an integer.
The relative stability of the oscillator is a few parts in 10.sup.-9 over
one second.
FIG. 4 schematically represents another frequency generator circuit in
which the delay line 3 is used in a phase locking loop of a frequency
generator 7, which may equally well be a synthesiser or a voltage
controlled oscillator (VCO). The output of the frequency synthesiser is
connected to an output amplifier 5, to the transducer 1 of the delay line
and to one input to a phase comparator 8 whose other input is connected to
the other transducer 2 of the delay line. The output of the phase
comparator 8 is connected via an amplifier 9 to a control input of the
frequency generator 7. The phase comparator delivers a signal which is a
function of the phase shift between the signal at the output of the
frequency generator 7, and the same signal after the delay imposed by the
delay line 3. The delay imposed by the delay line is itself a function of
temperature. Thus temperature differences have the effect both of changing
the distance between the transducers (surface wave propagation path), and
of changing the speed of propagation because of the changes that occur in
the moduli of elasticity of the quartz. This results in variation in the
travel time between the input and output transducers of the delay line,
and hence in the synchronous frequency of the delay line. For an LTS
plate, this variation is substantially linear with varying temperature.
FIG. 5 represents an alternative embodiment of the circuit of FIG. 3 in
which the temperature probe is a resonator.
This figure is identical to FIG. 3, except that the delay line is replaced
by a resonator 10 comprising two interdigitated combs 11 & 12 and two
reflectors 13 & 14. The combs 11 & 12 are shown diagrammatically. The
input of the amplifier 4 is connected to the comb 12 and the output of the
amplifier 4 is connected both to the comb 11 and to the output amplifier
5.
FIG. 6 represents an alternative embodiment of the circuit of FIG. 4, in
which the temperature probe is a resonator 10 used in a phase locking loop
of a frequency generator 7, which may equally well be a synthesiser or a
voltage controlled oscillator (VCO). The output of the frequency
synthesiser is connected to an output amplifier 5, to the comb 11 of the
resonator 10 and to one input to a phase comparator 8 whose other input is
connected to the other comb 12 of the resonator 10. The output of the
phase comparator 8 is connected via an amplifier 9 to a control input of
the frequency generator 7.
FIGS. 7 & 8 show a first mode of encapsulating the quartz plate of a
temperature probe in accordance with the invention. FIG. 7 is an elevation
through a section VII--VII in FIG. 8, while FIG. 8 is a plan through a
section VIII--VIII in FIG. 7.
The quartz plate 3 of a delay line is glued on one face to the bottom of a
metal box 15, while the opposite face of the plate carries the transducers
1 & 2. The box is closed by a lid 16 which is also made of metal and which
is welded or soldered to the box. The lid has a hole 17 for use in
evacuating the interior of the box after it has been closed, and for
subsequently filling it with an inert gas such as helium. The hole 17 is
then closed, e.g. by a blob of solder 18. Two metal terminals 19 & 20 are
electrically insulated from the box by respective glass lead-throughs 21 &
22. The terminal 19 is connected to one comb of the transducer 2 whose
other comb is connected to the box. Likewise, the terminal 20 is connected
to one comb of the transducer 1 whose other comb is connected to the box.
FIG. 9 is an elevation in section of a second mode of encapsulating the
quartz plate of a temperature probe.
In this figure the metal box 25 is provided with a metal "window" 24 made
of very thin copper or nickel foil, for example, having a thickness of
about 0.05 mm, and to which the quartz plate is bonded. The box includes
two terminals insulated therefrom as in FIGS. 7 & 8, even though only one
of them is visible in FIG. 9.
The box further includes two auxiliary terminals, only one of which,
terminal 26, is visible in FIG. 9. Each of the auxiliary terminals is
connected both to one of the combs of a corresponding one of the
transducers and to a corresponding one of the main terminals by a length
of stainless steel, thereby providing thermal isolation between the main
terminals and the quartz plate. The auxiliary terminals are insulated from
the box by respective beads of glass 27, and they are not in direct
communication with the outside of the box. As in the example of FIGS. 7 &
8, one comb of each of the transducers is connected to the box, and the
box is evacuated and filled with a gas such as helium via a hole 29 in the
lid 28. The hole is sealed with a drop of solder 30.
FIG. 10 is an elevation in section through a third mode of encapsulating
the quartz plate of a temperature probe.
In this figure the quartz plate constitutes the bottom of the box 31, to
which it is bonded round its edges. As in FIG. 7, the box has two
terminals (only one of them, 19, being visible) connected to one comb each
of respective transducers whose other combs are connected to the box. The
box is closed by a lid 32 having a hole 33 that is sealed with a drop of
solder 34.
In FIGS. 7 to 10, the description has been in terms of a delay line
embodiment. Naturally, it is possible to use the same system for resonator
embodiments, in which case the two terminals are connected to respective
combs of the resonator's sole transducer, and there is no need to connect
combs to the box.
Temperature probes in accordance with the invention have low thermal
inertia since one face of the plate of quartz is either glued to a good
conductor of heat, e.g. copper, (as illustrated in FIGS. 7 to 9), or else
one face of the plate of quartz actually makes direct contact with the
medium whose temperature is to be measured (FIG. 10). Since the time taken
for heat to travel through the quartz is short, less than 1/10th of a
second, the response time of the probe as a whole is also short, depending
essentially on the thermal capacity of the box, and the smaller the box,
the less its thermal capacity. The intimate contact between the plate of
quartz and the box, makes it possible to obtain boxes which are
considerably smaller that those required for bulk wave quartz plates.
Another factor that leads to a small size of box, is that the higher the
frequency used the smaller the dimensions of the quartz plate.
Now a temperature probe in accordance with the invention uses surface waves
propagating over a quartz substrate, and this technology can be used at
very high frequencies, up to about 3 GHz, which is not possible with
temperature probes using waves propagating through the bulk of the
substrate.
Given the resolution limits of frequency measuring apparatus, the higher
the frequency, the more accurately temperatures can be measured in the
vicinity of a given temperature.
FIG. 11 shows, by way of example, a graph of the frequency-temperature
response of a temperature probe operating at a frequency of about 93 MHz.
The temperature range shown is from 0.degree. to 100.degree. C. with the
frequency varying from 92.914 274 MHz at 0.degree. to 93.174 274 MHz at
100.degree.. This gives a sensitivity of 2600 Hz/.degree.C., or about
26.times.10.sup.-6 /.degree.C. The greatest measured non linearity in the
range 0.degree. C. to 80.degree. C. is about 0.1.degree. C.
The temperature probes described and shown are useful both for high
performance thermometers for rapid temperature measurements, and for
temperature sensing devices in high quality thermostats.
Both analog and digital thermometers can be provided. For thermostats, the
probe can be used as a digital sensor for incorporation in a
microprocessor controlled thermostat system, having a local accuracy to
within a few micro degrees centigrade. Alternatively, a phase locking
analog thermostat can be arranged to give local accuracy to within about
one micro degree centigrade.
* * * * *
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