Note: Descriptions are shown in the official language in which they were submitted.
CA 02294896 2000-O1-OS
THERMOSTATICALLY CONTROLLED CRYSTAL OSCILLATOR
Technical Field
This invention relates to radio electronics and more particular to frequency
oscillators with piezoelectric crystal resonators.
Background Art
Various versions of stable thermostatically controlled quartz crystal
oscillators are
known to the prior art (see, for example US Patent No. 4,985,687 and French
Patent No.
2,660,499, 2). To achieve temperature stability up to t 1x10'8 prior art
frequency
oscillators usually employed a one-stepped thermostatic circuit. This allows a
most simple,
economical and compact design of oscillator to be achieved. To achieve
stability above t
1x10'9 a two-stepped thermostatic circuit is used. A higher precision is
achieved in this
case at the cost of more complex and cumbersome design (see. Ingberman M.L,
Fromberg
E.M, Graboi L.P., Thermostatic Methods in Communication Technology., M.,
Sviaz,
1979, page 17, and also Godkov V.D., Gromov S.S., Nikitin N.V., Thermostatic
Methods
and Wireless Appliances, Moscow, Goskomizdat, 1979, page 46).
RU 2081506 describes a crystal oscillator with a one-stepped thermostatic
circuit,
which circuit contains a circuit board with elements of the oscillator system
mounted
thereon. The circuit board is installed in a hermetically sealed outer
housing. All
temperature controlled elements of the system, including a thermostat with
built-in crystal
resonator, heating elements, a temperature detector and a thermostatic
regulator with a
thermosensitive bridge circuit are located in the central part of the circuit
board. The
central part of the circuit board is separated from its peripheral part by
means of through
cuts, both parts being connected at the ends of the cuts by narrow bridging
strips.
This oscillator has obvious advantages, such as a simple design, small size
and low
cost, but it does not allow temperature stability of ~ 1x10-9 to be achieved.
This limitation
is explained by insufficient uniformity of temperature distribution and
insufficient
precision in maintaining a constant temperature.
Summary of the Invention
The main object of the present invention is to provide a thermostatically
controlled
crystal oscillator attaining a high temperature stability of the frequency
without use of a
double thermostatic circuit.
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A more specific object of the present invention is to resolve a technical
contradiction between a necessity for uniform temperature distribution at the
crystal
resonator, which requires placing the resonator out of a thermal flow field,
for example by
increasing the distance between a crystal resonator and a heater and a
necessity for very
accurate control of temperature, which requires a temperature detector and the
heater being
placed as close to the resonator as possible.
It is also a further object of the invention to improve accuracy of
temperature
control.
The foregoing objects are achieved by using the combination of essential
features
as specified in independent claims.
Thus, according to one embodiment of the present invention, the required
uniformity of the temperature field at the crystal resonator is achieved in a
thermostatically
controlled crystal oscillator with a circuit board divided into central and a
peripheral part
by means of through cuts (similar to those employed in the oscillator
disclosed in RU
2081506). These parts are joined together at the end of each cut with narrow
bridging
strips. All thermostatically controlled elements of the oscillator are located
in the central
part of the circuit board. They include a crystal resonator in a separate
casing, and a
thermostatic regulator with heating elements. The circuit board is installed
in a
hermetically sealed outer housing. In addition, according to the present
invention, the
crystal resonator in the separate casing is tightly adjoined to the bottom of
an inner case:
This inner case is shaped as an inverted open box made of material that is an
excellent heat
conductor (or, in other words, possesses a high thermal conductivity). The
inner case is
fixed to the circuit board using heat-conductive rods, that go through the
central part of the
circuit board in close proximity to the narrow bridging strips. The bottom of
the inner case
is tightly adjoined to the circuit board, preferably by a layer of highly heat-
conductive
glue. The heating elements and a temperature detector are mounted on the outer
side walls
of the inner case.
Preferably, the open side of the box is covered by a thin cover that is made
of
material that is an excellent heat conductor, and there is a heat-insulating
space between
the cover and the body of the crystal resonator. Further, according to the
preferred
embodiment, a cover is attached to the those ends of the heat-conducting rods
which
extend from the opposite side of the circuit board. A heat-insulating space is
provided
CA 02294896 2000-O1-OS
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between said cover and the thermostatically controlled elements located in the
central part
of the circuit board, while the cover is made from material of a high thermal
conductivity.
Owing to the presence of the above-described features, for any point chosen at
random on the surface of thermostatically-controlled elements, a certain
matching point
can be found along the path of the heat flow being dispersed into the
surrounding medium
in any possible direction. Said matching point, while having virtually the
same temperature
as the selected one, is separated from it by a heat-insulating space
possessing a large heat
resistance. In this way, the crystal oscillator as well as other
thermostatically controlled
elements of the circuit (also located in the central part of the circuit
board) are in a zone of
uniform temperature distribution, outside of the heat flow path. This
constitutes one of the
major preconditions of achieving a high temperature stability.
The described embodiment of the present invention ensures minimum heat loss
directly from the surface of the thermostatically controlled elements, and
consequently
minimises temperature variations through the volume of these elements,
especially of the
resonator.
According to another embodiment of the present invention, aimed at providing
high
accuracy of thermostatic regulation, the circuit board of a thermostatically
controlled
crystal oscillator is divided into a central part and a peripheral part by
means of through
cuts (similar to those described in RU 2081506). Both parts are joined
together at the end
of each cut with narrow bridging strips. All thermostatically controlled
elements of the
oscillator are located in the central part of the circuit board. These
elements include a
crystal resonator in its own casing, a thermostatic regulator with heating
elements and a
thermosensitive bridge circuit having a reference arm and a main
thermosensitive arm with
a main temperature detector. The circuit board is installed in a hermetically
sealed outer
housing. According to the present invention, the thermosensitive bridge
circuit is provided
with an additional thermosensitive arm with an additional temperature
detector. This
additional arm is coupled to the main thermosensitive arm by means of a
coupling resistor.
The aforementioned additional temperature detector can be installed at any
location in the
oscillator, as long as the temperature at this location (taken under normal
outside
conditions) is lower than the temperature of the crystal resonator, at least
by 0.5° C.
Introduction of the additional temperature detector makes it possible to
minimise
the inherent design static error of the thermostatic regulator which is caused
by that the
CA 02294896 2000-O1-OS
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heaters are physically remote from the thermostatically controlled elements of
the circuit.
Moreover, the above-specified temperature difference between the crystal
resonator and
the additional temperature detector provides the opportunity for the effective
tuning of the
thermostatic regulator.
The most complete attainment of the above-specified objects of the present
invention is ensured by employment of the preferred embodiment of the
temperature
controlled crystal oscillator according to the present invention as disclosed
in claim 9.
Brief Description of the Drawings
The invention will be further described with reference to attached drawings
which
show:
Figurel, a simplified general presentation of the crystal oscillator in a
cross-sectional view;
Figure2, a front elevation of the circuit board;
Figure3, an electric circuitry of the thermosensitive bridge.
Best Mode for Carrying out the Invention.
Fig. l shows the crystal oscillator design of the preferred embodiment of the
invention. It
comprises a hermetically sealed outer housing 1 inside which a circuit board 2
bearing all
elements of the oscillator is located. A central part 3 of the circuit board 2
is separated
from a peripheral part thereof by means of through cuts 4, both parts being
connected at
the ends of the cuts with narrow bridging strips 5 (shown in Fig.2). All
thermostatically
controlled elements 16 of the oscillator are concentrated in the central part
3 of the circuit
board 2. The outer housing 1 holds inside an inner case 6. The inner case 6 is
shaped as an
inverted open box and is made of material that is an excellent heat conductor,
such as
copper or aluminium alloy. The bottom of the inner case 6 is tightly adjoined
to the central
part 3 of the circuit board 2. To improve the thermal contact between the
inner case 6 and
the circuit board 2 a layer 7 of a heat conductive glue is placed
therebetween. The inner
case 6 is fixed to the circuit board 2 by rods 8. Material of the rods 8 must
be an excellent
heat conductor, such as copper. These heat-conducting rods 8 go through the
central part 3
of the circuit board 2 in close proximity to each bridging strip S (as can be
seen from Fig.
2). The ends of the heat-conducting rods 8 extend from the opposite side of
the central part
3 of the circuit board 2. A crystal resonator in a separate casing 9 is fixed
to the inner side
CA 02294896 2000-O1-OS
on the bottom of inner case 6. Heating elements 10, such as transistors, and a
first, main,
temperature detector 11 are attached to the side walls of the inner case 6.
The open side of the inner case 6 is preferably covered with a thin cover 12.
The
cover is made of a heat conducting material, preferably copper, and the
thickness of this
cover does not exceed 0.5 mm. This cover is fixed with screws that hold the
heating
elements 10. There is a heat-insulating space, or gap, between the resonator
body and the
cover 12.
A thin copper cover 13 is attached to those ends of the heat-conducting rods 8
that
extend from the opposite side of the central part 3 of the circuit board 2.
All the
thermostatically controlled elements 16 of the oscillator system are located
in the central
part 3 of the circuit board 2. There is a heat-insulating space between the
cover 13 and the
elements 16 of the oscillator system.
The empty space inside the outer housing 1 and the inner case 6 is filled with
heat
insulation 14.
In the preferred embodiment of the invention illustrated by Figs.l and 3 the
thermostatic regulator is designed using a bridge principle. A thermosensitive
bridge
circuit of the thermostatic regulator consists of
- a fixed arm with resistors R6, R, having fixed resistance,
- a main thermosensitive arm with a first, main temperature detector 11
constituted by a
temperature-sensitive resistor (thenmistor) RS (see Fig. 3) and with a
specially selected
resistor R'4, and
- an additional thermosensitive arm with an additional temperature detector 1
S constituted
by a thermistor marked as Rz in the Fig. 3 and a fixed value resistor R,. The
additional
thermosensitive arm is coupled to the main thermosensitive arm by a coupling
resistor R',
having a fixed resistance.
The additional temperature detector 15 is mounted on the peripheral part of
the
circuit board 2. The through cuts 4 in the circuit board 2 thermally insulate
the central part
3 of the circuit board 2 from its peripheral part, which guarantees
temperature difference of
more than 0.5°C between these parts.
The thermostatically controlled oscillator according to the present invention
functions as follows. The main part of total heat flow generated by the
transistor heaters 10
is spreading by the following route: the heater 10 - a side wall of the case 6
- the rods 8 -
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the bridging strips 5 - the peripheral part of the circuit board 2 - the outer
housing 1 -
surrounding medium. Owing to the symmetry of the design, all rods 8 have the
same
temperature. Consequently, the bottom of the inner case 6 is only subject to
that part of the
heat flow which travels as follows: the bottom of the inner case 6 - the
central part 3 of the
circuit board 2 - the heat insulation 14 - the outer housing 1 - surrounding
medium. The
heat can also navel along another route: the bottom of the inner case 6 - the
resonator
casing 9 - the heat insulation 14 - the outer housing 1 - surrounding medium.
In both latter
instances, there is the heat insulation 14 in the way of the heat flow, so the
heat flow along
corresponding routes will be quite insignificant. Installation of the covers
12 and 13 will
further decrease those flows.
Mathematical analysis of a performance of a thermostatic regulator with a
thenmosensitive bridge circuit as described above shows that it is possibleto
achieve, with
the correct tuning of the bridge circuit, the temperature accuracy in the
range of 0.01 °C -
0.05°C, even if the static error of the thermostatic regulator and its
design static error (the
difference in temperatures between the crystal resonator and the heater) are
of the order of
0.5°C.
During tests of the crystal oscillator model made according to the preferred
embodiment of the invention on the base of a quartz crystal of SC-cut in a HC-
37 case
(produced by the Israeli Company NEL), the following results were obtained:
Temperature stability ~ 4x10-'
Accuracy of thermostatic stabilisation
(evaluated by measuring a frequency +0,02C
in the B mode)
within temperature interval from -20C-0,01 C
to +70C
Power consumption (at 25C) 1,5 V
Overall size 51 x41 x 19 mm
Thus, the temperature stability of the order of 1x10-9 and higher can be
provided
with the oscillator of the invention without using a double thermostatic
circuit.
Though only one of the preferred embodiments of the present invention has been
described in detail, the scope of the present invention is determined only by
the attached
claims and covers all multiple embodiments and modifications of the invention,
which will
CA 02294896 2000-O1-OS
be obvious to those skilled in the art. For example, according to one
alternative
embodiment, the thermostatically controlled oscillator of the invention is
built with use of
only one main temperature detector provided in the main thermosensitive arm of
the
thermosensitive bridge circuit. With moderate demands on the precision of
stabilisation,
the oscillator can be designed without the use of the heat conductive covers
12 and 13, or
the layer 7 of glue could be substituted with another suitable material with a
high thermal
(or heat) conductivity, and so on.
