FIELD OF THE INVENTION
The present invention relates to electronic devices wherein temperature stabilization is utilized in
order to achieve the required performance characteristics. Such devices include temperature
stabilized frequency control products used in a variety of applications where accurate and stable
frequency reference and/or timing signals are required.
BACKGROUND OF THE INVENTION
Temperature stabilization is often utilized in electronic devices as a way of achieving the required
performance characteristics.
An Oven-Controlled Oscillator (OCO) is an example of a temperature stabilized frequency control
device. OCOs are frequency control devices that generate output signals characterized by high
frequency stability that is achieved, to a large extent, through internal temperature stabilization:
i.e., while the ambient operating temperature for an OCO may vary over a wide range, such as, for
example, from -40°C to +85°C, the OCO’s frequency determining components are kept at a stable
temperature, which results in a stable output signal frequency, despite the ambient temperature
variations.
In an oven-controlled crystal oscillator (OCXO), for example, frequency determining components
that are sensitive to temperature variations comprise the crystal resonator and the active and
passive components of the oscillator circuit. Maintaining a stable temperature of the frequency
determining components is therefore a key factor in achieving a stable output frequency in OCXOs.
Historically, several different approaches to internal temperature stabilization in OCOs have been
deployed, including, -
• placing frequency determining components in a heated enclosure (commonly called “an
oven”), with one or more heating elements and a temperature control loop arranged to
maintain the oven’s temperature as stable as possible over the operating ambient temperature
range;
• arranging a second heated enclosure (a second “oven”) wherein the first heated enclosure,
containing frequency determining components, is placed - i.e., a double-oven arrangement.
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In both of the aforementioned approaches, the oven’s temperature is maintained at a temperature
setpoint that is usually set higher than the higher limit of the device’s operating ambient
temperature range. The aforementioned approaches are associated with such downsides as
relatively large device size and weight, high power consumption required to maintain the oven(s)
at a stable temperature, and a relatively long warm-up time during a cold start.
Other approaches, aiming at reducing power consumption and enabling device miniaturization,
have been known. One example of such an approach is a structure wherein a heating element is
attached directly to the resonator’s case in order to maintain a stable temperature of the main
frequency determining component - the resonator. This approach, while capable of reducing the
device’s power consumption, compromises the temperature stabilization effectiveness and results
in thermally induced stresses in the resonator. Another known approach, illustrated in Fig. 1, is to
place the resonator 7 in close thermal contact with a heat distribution block 4 in order to spread
the heat along the resonator’s case and thus achieve a more even temperature distribution across
the resonator 7 and reduce the thermally induced stresses in the resonator 7. This approach has
the downsides of (a) insufficient temperature stability of other frequency determining electronic
components 8 and (b) exposure of the other frequency determining electronic components 8 to
convection airflow. Fig. 2 shows simulated thermal distribution inside the device of Fig. 1 at ambient
temperature of -40°C: the temperatures of frequency determining components 8 can differ by
anywhere between 0.3°C and 28°C. The resultant output frequency variation over the ambient
temperature range of -40°C to +85°C is illustrated in Figures 3 and 3a. As shown in Fig. 3, when
ambient temperature (graph t) varies in a cycle from +25°C to +85°C to -40°C to +25°C, the device’s
output frequency (graph f) varies in the range from -6ppb (parts per billion) to +8ppb (total change
magnitude of 14ppb), with frequency hysteresis of 0.3ppb as shown in Fig. 3a.
The present invention offers a solution enabling to produce temperature stabilized frequency
control devices with substantially higher internal thermal stability and more uniform thermal
distribution for the frequency determining components, which results in higher output frequency
stability when ambient temperature varies.
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SUMMARY OF THE INVENTION
The term “comprising” as used in this specification means “consisting at least in part of”. When
interpreting each statement in this specification that includes the term “comprising”, features
other than that or those prefaced by the term may also be present. Related terms such as
“comprise” and “comprises” are to be interpreted in the same manner.
In temperature stabilized frequency control devices of the present invention, comprising frequency
determining components including a resonator and frequency determining components other than
the resonator, a heat distributor, and a substrate carrying at least some of the frequency
determining components, the heat distributor is arranged to (a) define, in conjunction with the
substrate, a space in which at least some of the frequency determining components are located,
and (b) spread the heat throughout the resonator and throughout the said space.
Preferably, the heat distributor is made of a thermally conductive material such as, for example,
copper.
In some embodiments, the space for placing at least some of the frequency determining
components defined by the heat distributor and the substrate, can be a semi-enclosed space that
is at least partially isolated from convection airflow outside of the said space.
In some embodiments, the space for placing at least some of the frequency determining
components defined by the heat distributor and the substrate, can be a fully enclosed space that is
completely isolated from convection airflow outside of the said space.
In some embodiments, the heat distributor can be thermally connected to a thermally conductive
layer, such as a metallic plane, in the substrate.
In some embodiments, one or more heating elements, utilized to maintain a stable temperature
inside the temperature stabilized frequency control device, are located outside of the space defined
by the heat distributor and the substrate.
In some embodiments, one or more heating elements, utilized to maintain a stable temperature
inside the temperature stabilized frequency control device, are located within the space defined by
the heat distributor and the substrate.
In some embodiments, one or more heating elements, utilized to maintain a stable temperature
inside the temperature stabilized frequency control device, are located on the surface of, and
thermally linked to, the heat distributor.
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In some embodiments, one or more heating elements, utilized to maintain a stable temperature
inside the temperature stabilized frequency control device, are located on the surface of, and
thermally linked to, the substrate carrying frequency determining components. Preferably, the
position of the one or more heating elements is chosen to ensure a close thermal connection
between the one or more heating elements, the heat distributor, and the substrate.
Without limiting the scope of the present invention, a person skilled in the art will be able to devise
a suitable spatial arrangement for placing the constituent elements of a temperature stabilized
frequency control device in order to take advantage of the possibilities offered by the present
invention.
Various temperature stabilized frequency control devices could be constructed utilizing the present
invention, including, but not limited to, Oven-Controlled Crystal Oscillator (OCXO) comprising an
oscillator circuit with a crystal resonator, Oven-Controlled MEMS Oscillator (OCMO) comprising an
oscillator circuit with a microelectromechanical systems (MEMS) resonator, Oven-Controlled SAW
Oscillator (OCSO) comprising an oscillator circuit with a surface acoustic wave (SAW) resonator, and
others.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described with reference to the accompanying figures in which, -
Fig. 1 shows an example of a prior art temperature stabilized oscillator device wherein a resonator
is placed in close thermal contact with a heat distribution block.
Fig. 2 shows simulated thermal distribution in a prior art temperature stabilized oscillator device
shown in Fig. 1.
Figures 3 and 3a show the extent of output frequency variations exhibited by the prior art
temperature stabilized device shown in Fig. 1.
Fig. 4 shows an example of an OCXO implemented according to the present invention.
Fig. 5 shows simulated thermal distribution in a temperature stabilized frequency control device of
the present invention.
Fig. 6 and 6a show the output frequency variations exhibited by the OCXO device shown in Fig. 4.
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DETAILED DESCRIPTION OF THE INVENTION
Without limiting the scope of the present invention, the invention is illustrated herein by the
following specific embodiment of an Oven-Controlled Crystal Oscillator (OCXO).
The constituent elements of the OCXO, including the heat distributor 1, can be arranged as shown
in Fig. 4. The printed circuit board (PCB) substrate 2 carries all of the OCXO’s electronic components.
The PCB has an internal metal layer 3, which is, preferably, a ground plane; the ground plane serves
both the electrical functions related to distributing ground level reference, and, importantly in the
context of the present invention, improves thermal coupling between the OCXO components. The
heat distributor 1 can be made of copper and is shaped to comprise a flat portion 4 and two portions
5 and 6 that are orthogonal to the flat portion 4. In this embodiment, the cross-section shape of
the heat distributor 1 comprising the flat portion 4 and the two portions 5 and 6 shown in Fig. 4
resembles the English capital letter “F”, and the heat distributor 1 will be referred to herein as the
“F-shaped heat spreader” or “F-shaped heat radiator”. A crystal resonator 7 is installed onto the
flat portion 4 of the F-shaped heat spreader 1 and is closely thermally coupled to it. The other
frequency determining components 8 are located in the semi-enclosed space 9 defined by the two
portions 5 and 6 of the F-shaped heat radiator 1 and the PCB substrate 2. At least one heating
element 10 is located on the substrate 2 in close vicinity, both spatially and thermally, to the Fshaped heat spreader 1. The F-shaped heat spreader 1 is installed onto the PCB 2 by through-hole
soldering of at least one of the orthogonal portions 5 and 6. Preferably, the through-hole soldered
potions 5 and 6 are electrically and thermally connected to the PCB internal metal layer 3.
In normal operation, when the OCXO’s ambient temperature varies, power dissipated in the heating
element 10 is controlled in such a way as to maintain a stable temperature of the F-shaped heat
spreader 1 and of all components and elements thermally linked to the F-shaped heat spreader 1,
including the resonator 7 and the other frequency determining components 8, which results in
highly stable output signal frequency of the OCXO device.
There are several advantages of utilizing the F-shaped heat spreader 1 as shown in Fig. 4, including:
• Closer thermal coupling between all of the main frequency determining components of the
OCXO and, as a result, higher thermal stability of all of the main frequency determining
components. As illustrated in Fig. 5 showing simulated thermal distribution inside a device
utilizing an F-shaped heat distributor, at ambient temperature of -40°C the temperatures of
frequency determining components 8 differ by only 0.1°C to 2.0°C.
• Reduced thermally induced stress experienced by the resonator 7.
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• The frequency determining components 8 located in semi-enclosed space 9 are significantly
less affected by convection airflow.
• Unlike a traditional oven, the F-shaped heat spreader 1 is characterized by low mass and small
size, which allows to achieve shorter warm up time for the device, and facilitates device
miniaturization.
The higher thermal stability of all of the main frequency determining components results, in turn,
in higher output frequency stability of the OCXO device, as illustrated in Figures 6 and 6a. As shown
in Fig. 6, when the ambient temperature (graph t) varies in a cycle from +25°C, to +85°C, to -40°C,
and back to +25°C, the device’s output frequency (graph f) varies in the range from -1.25ppb (parts
per billion) to +0.75ppb (total change magnitude of 2ppb), with frequency hysteresis of 0.06ppb as
shown in Fig. 6a – this is a 7-fold improvement in output frequency stability and a 5-fold
improvement in frequency hysteresis when compared to the performance of the prior art device
shown in Fig. 1.
Temperature stabilized frequency control devices constructed according to the present invention
can be used in a variety of electronic equipment and apparatus where accurate and stable
frequency reference and/or timing signals are required, including, but not limited to,
telecommunication infrastructure equipment, precise frequency and timing reference equipment,
and test and measurement instrumentation.
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1. A temperature stabilized frequency control device comprising frequency determining
components including a resonator and frequency determining components other than the
resonator, a heat distributor, and a substrate carrying at least some of the frequency
determining components other than the resonator, wherein
the heat distributor is arranged to define, in conjunction with the substrate, a space in which
at least some of the frequency determining components other than the resonator are located,
the heat distributor is arranged to spread heat throughout the resonator and throughout the
said space, and
wherein the resonator is installed onto the surface of, and in close thermal connection with,
the heat distributor.
2. A temperature stabilized frequency control device according to claim 1, wherein the space in
which at least some of the frequency determining components are located is a fully enclosed
space.
3. A temperature stabilized frequency control device according to claim 1, wherein the space in
which at least some of the frequency determining components are located is a semi-enclosed
space.
4. A temperature stabilized frequency control device according to any one of claims 1 to 3,
wherein the heat distributor is thermally connected to a thermally conductive layer in the
substrate.
5. A temperature stabilized frequency control device according to any one of claims 1 to 4,
wherein one or more heating elements are located outside of the space in which at least some
of the frequency determining components are located.
6. A temperature stabilized frequency control device according to any one of claims 1 to 4,
wherein one or more heating elements are located within the space in which at least some of
the frequency determining components are located.
7. A temperature stabilized frequency control device according to any one of claims 1 to 6,
wherein one or more heating elements are located on a surface of the heat distributor.
8. A temperature stabilized frequency control device according to any one of claims 1 to 6,
wherein one or more heating elements are located on a surface of the substrate.
We Claim:
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9. An electronic apparatus comprising the temperature stabilized frequency control device
according to any one of claims 1 to 8.
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