High thermal gain oven with reduced probability of temperature gradient formation for the operation of a thermally stable oscillator

An oven assembly for a crystal resonator and oscillator utilizes a thermally symmetrical design to provide a high thermal gain. The oven assembly includes an encasement that forms a hermetically sealed oven chamber that is substantially cylindrical. Concentric with the oven chamber is an annular oven mass that functions as a heat reservoir for the crystal resonator that is contained within the oven mass. The cylindrical oven chamber and the concentric annular oven mass provide two levels of circular symmetry that help achieve a thermally isotropic oscillator environment. Wide-area uniform heat transfer promotes high thermal gain and minimizes thermal gradients. Another factor is the geometry and circuitry for temperature monitoring. Temperature sensors are equidistantly spaced from each other and are equidistant from the center of the oven chamber. Signals from the various thermistors are averaged to provide a more accurate temperature determination for regulating the heaters.

TECHNICAL FIELD 
The invention relates generally to ovenized crystal oscillators and more 
particularly to a temperature-controlled oven assembly for a crystal 
oscillator. 
BACKGROUND ART 
Ovenized crystal oscillators are used in applications that require a highly 
stable output frequency. For example, radio communication applications 
require strict stability of a precise frequency. The telecommunications 
industry also includes applications in which output frequency must be 
subjected to exacting tolerances. 
The crystals that are used to control the output of an oscillator are 
temperature dependent. In order to minimize the effects of static and 
dynamic shifts in the ambient temperature, the crystals are typically 
placed within a temperature-controlled oven. The fundamentals of such an 
oven are described in U.S. Pat. No. 4,317,985 to Wilson, which is assigned 
to Hewlett-Packard Company, the assignee of the present invention. The 
oven assembly isolates a crystal resonator from the external environment, 
with a thermistor being housed within the oven in order to monitor the 
temperature at a location close to the crystal. The thermistor is coupled 
to an oven controller that regulates the power applied to a heater of the 
oven. Consequently, the temperature of the quartz crystal is relatively 
insensitive to variations in the temperature of the atmosphere surrounding 
the oven, i.e. "ambient temperature." 
"Thermal gain" is defined herein as a primary figure of merit for the 
temperature stabilization achieved by a crystal oven. More specifically, 
thermal gain is the ratio of the change in ambient temperature to the 
change in crystal temperature resulting from the ambient temperature 
variation. Thermal gain values of 8,000 have been reported for a 
single-level oven. If the oven is placed within one or more larger ovens, 
the thermal gain of the multi-level oven assembly may greatly exceed this 
value. 
A number of techniques have been utilized to increase thermal gain. The 
Wilson patent describes a method that employs two heaters for heating a 
thermally conductive base on which the quartz crystal is positioned. The 
power applied to the first heater is responsive to the temperature sensed 
by a thermistor, while the power applied to the second heater is a 
multiple of the power applied to the first heater. The temperature is 
again sensed by the thermistor and the multiple is adjusted based upon the 
second temperature sensing. The patent states that by adjusting the ratio 
of the powers applied to the two heaters, the thermal gain between the 
thermistor and the external environment can be increased. Temperature 
control techniques are also described in U.S. Pat. No. 4,839,613 to Echols 
et al., U.S. Pat. No. 4,396,892 to Frerking et al., and U.S. Pat. No. 
4,157,466 to Herrin. The use of compensating systems to offset influences 
of the external atmosphere on the temperature sensing of a crystal 
oscillator oven is described by F. Walls in "Analysis of High Performance 
Compensated Thermal Enclosures," 41st Annual Frequency Control 
Symposium--1987, pages 439-443, and by R. Brendel et al. in "Analysis and 
Results of Compensated Quartz Crystal Oscillator Ovens," 1992 IEEE 
Frequency Control Symposium, pages 485-491, IEEE Transaction 
0-7803-0476-4/92 (1992). 
In order to improve thermal gain, special construction techniques, in 
addition to circuit techniques, are utilized. In an article entitled "A 
Temperature-Controlled Crystal Oscillator," 43rd Annual Symposium on 
Frequency Control--1989, IEEE Transaction CH2690-6/89/000-055 (1989), Les 
Hurley describes an ovenless crystal oscillator that utilizes a 
double-sided beryllium oxide substrate having two power FET's, each in 
series with a current limiting resistor. The FET's and resistors form a 
heater structure that is arranged in a semi-circle around the crystal. 
Midway around the semi-circle, on the opposite side of the substrate, is a 
temperature sensing chip thermistor. The flow of heat from the power FET's 
to the crystal is through short, wide paths in the beryllium oxide 
substrate. The crystal is mounted inside a close-fitting copper ring which 
is soldered to the substrate metallization. Void spaces between the 
crystal and the copper ring are filled with a thermally conductive 
material. The crystal leads are soldered directly to the substrate 
metallization. The article asserts that because beryllium oxide has 
thermal properties nearly equal to those of metallic aluminum, and because 
the substrate area is small, the temperature gradients across the entire 
circuit are small. However, the article also reports some frequency 
instability due to the instability of the crystal temperature. 
A thermal gain of greater than 2400 is reported by M. Mourey and R. Besson 
in an article entitled "A Space Oscillator with Cylindrical Oven and 
Symmetry" in 45th Annual Symposium on Frequency Control, pages 431-441, 
IEEE Transaction CH2965-2/91/0000-431 (1991). A cylindrical oven contains 
a resonator crystal. The oscillator is operated in space, with several 
reflectors being used to limit thermal exchanges by radiation. The 
oscillator is fixed by a titanium spacer, so as to limit exchanges by heat 
conduction. While the reported thermal gain is satisfactory for many 
applications, further improvements are necessary if oscillator performance 
is to be significantly enhanced. 
What is needed is an oven and oscillator assembly having a high ratio of 
change in ambient temperature to a time-corresponding change in crystal 
temperature, so that the output frequency of the crystal oscillator has a 
low sensitivity to variations in the ambient temperature. 
SUMMARY OF THE INVENTION 
A crystal oscillator oven assembly combines a thermally symmetrical design 
with temperature sensing and heat distribution techniques that promote a 
high thermal gain, while reducing the sensitivity of the oven assembly to 
thermal gradients. In the preferred embodiment, the oven assembly includes 
an encasement that forms a sealed oven chamber that is substantially 
disk-shaped. Concentric with the cylindrical oven chamber is an annular 
oven mass that provides a heat reservoir for a crystal located within the 
oven mass. The shape of the oven chamber and the oven mass provide two 
levels of circular symmetry that achieve an approximately thermally 
isotropic oscillator environment. 
Both the encasement and the oven mass are formed of a metal having a high 
thermal conductivity, such as copper. Preferably, the oven mass and at 
least a portion of the encasement are unitary. Such a one-piece structure 
is particularly advantageous if the oven chamber and oven mass are to be 
heated from the exterior of the encasement, since the one-piece structure 
does not present an oven mass-encasement interface through which thermal 
energy must pass. Moreover, the encasement preferably forms a hermetically 
sealed oven chamber. 
Rather than providing localized heating, the assembly preferably includes 
wide-area heat transfer. A flexible circuit may be attached to one or both 
of the planar exterior surfaces of the cylindrical encasement. In the 
preferred embodiment, there is a heating element on each of the planar 
exterior surfaces and an independently controllable heating element on the 
exterior rim. The heating elements can be separately adjusted to establish 
a condition in which temperature about the encasement is substantially 
uniform. However, at the corners of the cylindrical encasement, there is 
often a benefit to increasing the pitch of the resistive traces that 
generate heat when connected to a source of electrical power. The 
wide-area heat transfer reduces the likelihood that thermal gradients will 
be introduced within the oven chamber. 
Temperature monitoring within the oven chamber includes utilizing a number 
of heat sensors, such as thermistors, that are equidistantly spaced from 
each other and symmetrically located about the oven axis. In one 
embodiment, the heat sensors are all on a plane intersecting the effective 
thermal center of the crystal. The monitoring utilizes an averaging 
approach. Thus, the heat sensors are connected in series or in parallel or 
are connected to a separate averaging circuit for determining the 
temperature of the crystal oscillator based upon averaging signals from 
the heat sensors. By equidistantly spacing the heat sensors and by 
utilizing an averaging approach, there is a reduced risk that any 
temperature gradient that is present in the oven chamber will adversely 
affect performance of the crystal oscillator. The circuitry for 
determining temperature, or temperature error, is located within the oven 
chamber, so that external influences are minimized. However, the output of 
the circuitry is conducted to the exterior of the hermetically sealed oven 
chamber, allowing the output to be used to control operation of the 
heaters. Preferably, each signal output to the exterior of the oven 
chamber is passed through a plastic dielectric capacitor. The plastic 
dielectric capacitors do not affect electrical conductivity of the 
outputs, but inhibit the passage of thermal energy along the output paths. 
As an alternative to the single-plane embodiment, the heat sensors may be 
symmetrically arranged in a non-coplanar arrangement that permits 
three-dimensional averaging. 
The heat sensors are embedded within the oven mass, as close as possible to 
the crystal, and are equidistant from the center of the oven chamber. This 
places the heat sensors within the thermal reservoir that plays perhaps 
the largest role in determining and maintaining the temperature of the 
crystal oscillator. The assembly includes a thermal guard for the leads of 
the heat sensors, so that the leads do not reduce the reliability of heat 
sensing by conducting thermal energy to or from the sensors. The thermal 
guard may be a wrap of insulating material on which the leads are 
deposited or placed around the outside diameter of the annular oven mass, 
guarding against heat leaks from the sensor leads to the exterior of the 
thermal reservoir. 
In the single-plane embodiment, accurate determination of the temperature 
of the crystal resonator is further enhanced by locating the heat sensors 
and the crystal blank of the oscillator in the single plane. Because the 
thermal power is introduced from the exterior surfaces of the encasement, 
the position of the crystal blank relative to the exterior surfaces may be 
relevant to the determination of the crystal temperature. The coplanar 
arrangement accounts for this positioning in the determination of the 
crystal temperature. Moreover, the height of the assembly is less than 
conventional crystal oven encasements, so that the time delay from the 
heater or heaters to the heat sensors is minimized. A height of less than 
one centimeter is preferred, in order to reduce temperature differences 
and thermal delays and to facilitate the use of the oscillator in reduced 
height applications, such as card cages. 
The encasement is contained within a metallic housing that insulates the 
encasement from the surrounding environment. A standard insulative foam 
may be utilized without a concern that outgassing from the insulative foam 
will enter the hermetically sealed region, i.e. the oven chamber. 
Outgassing into the area of the oscillator circuit may adversely affect 
performance. 
The assembly may include the oscillator circuit within the oven chamber. 
The circuitry is preferably located between the outside diameter of the 
oven mass and the cylindrical interior surface of the encasement. 
An advantage of the invention is that the thermally symmetrical preferred 
embodiment provides an ovenized environment that has reduced 
susceptibility to temperature gradients. That is, the oven assembly is 
substantially thermally isotropic. The small size of the assembly reduces 
effects of thermal gradients, since the spatial variations are likely to 
be smaller. The reduced size of the encasement also reduces power 
consumption. 
Another advantage is that the equidistant spacing of the heat sensors from 
each other and the equidistant spacing of the heat sensors from the center 
of the oven chamber, combined with the averaging approach to determining 
crystal temperature, significantly increase the reliability of temperature 
detection for heater regulation. Additionally, the wide-area heat transfer 
from the heaters enhances the thermal characteristics of the assembly.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to FIGS. 1 and 2, an oven assembly 10 is shown as having an 
outer metallic housing that includes a cover 12 and a base 14. Contained 
within the housing is at least one piece of insulative foam 16 and 18. The 
housing is formed of metal, such as aluminum for thermal reasons or steel 
for magnetic shielding reasons, but this is not critical. Conventionally, 
the insulation is a synthetic foam, but the material is not critical to 
the invention. 
In the preferred embodiment, the height of the oven assembly 10 does not 
exceed 19.05 mm. An exemplary diameter of the cover 12 of the housing is 
76.2 mm. 
Supported within the insulative foam 16 and 18 is a metallic encasement 
that includes a cylindrical member 20 and a circular member 22. The 
cylindrical and circular members are attached to form a hermetically 
sealed oven chamber 24. As will be explained more fully below, the oven 
chamber is heated by conducting current through one or more resistive 
traces on an upper and lower heater 26 and 28. Preferably, there is a 
third heater 29 on the annular rim. In the preferred embodiment, the 
heaters are flex circuits having serpentine patterns that provide a 
wide-area heating approach. Referring briefly to FIG. 3, a resistive trace 
30 is formed on an insulative material 32. The insulative material may be 
adhered to the planar surface of the cylindrical member 20 of FIG. 1. An 
advantage of the wide-area heating approach is that thermal power is 
introduced into the oven chamber relatively uniformly until a thermal 
equilibrium condition is reached. After equilibrium is established, the 
wide-area thermal coupling reduces the likelihood that temperature 
gradients will be established in the oven chamber 24. While embodiments 
have been contemplated for providing localized heating without diverging 
from the invention, the preferred embodiment is one in which generalized 
heating establishes wide-area thermal coupling from the heaters 26, 28 and 
29 to the encasement that forms the oven chamber. This preferred 
embodiment also includes providing heaters at both planar surfaces and the 
rim of the encasement, since the multi-sided thermal coupling further 
promotes uniformity of thermal characteristics throughout the encasement 
and, therefore, throughout the oven chamber, The heater 29 on the rim is 
separately controlled and/or designed in order to provide a consistent 
thermal coupling along the exterior of the encasement. For example, the 
pattern of resistive traces of each heater may vary to compensate for 
variations in isolation wall thickness, or the ratio of power supplied to 
the rim heater to power supplied to the upper and lower heaters may be 
adjusted to reduce the likelihood that a thermal gradient will be formed 
within the oven chamber. 
Referring now to FIGS. 1, 2 and 4, formed with the cylindrical member 20 of 
the encasement is an oven mass 34 that is utilized to provide a thermal 
reservoir for maintaining a fixed temperature for a crystal 36. The oven 
mass extends from the interior surface of the cylindrical member 20, and 
contacts the interior surface of the circular member 22 of the encasement. 
Since the oven mass and the cylindrical member 20 have a one-piece 
structure, thermal power introduced from the upper heater 26 does not 
encounter an interface in a thermal coupling from the cylindrical member 
to the oven mass. While there is such an interface at the contact with the 
oven mass and oven chamber with the circular member 22, the heat-transfer 
characteristics may be improved by maintaining tight manufacturing 
tolerances and by applying a conductive material at the interface. 
Moreover, the thermal contact of the circular member 22 to the oven 
chamber could be improved by partially evacuating the hermetically sealed 
oven chamber and allowing the external ambient pressure to thus increase 
the contact pressure. 
The hermetic seal of the oven chamber 24 is maintained by an O-ring 38 at 
the connection of the cylindrical and circular members 20 and 22 of the 
encasement. Screws pass through an array of holes 35 in the circular 
member and are received within internally threaded bores 37 in the oven 
mass 34 to fix the circular member to the cylindrical member. While not 
shown, each screw passes through a small 0-ring prior to insertion into 
one of the holes 35, so that the integrity of the hermetic seal is 
preserved. An advantage of this fastening arrangement is that the 
oscillator remains accessible to a user. Mounted within the oven chamber 
24 is a printed circuit board 40. The printed circuit board is an annular 
substrate having circuitry for exciting and operating the crystal 
oscillator 36. Preferably, the heat-generating components of the circuit 
are symmetrically arranged around the printed circuit board 40, thereby 
preserving the thermal symmetry promoted by the geometries of the 
encasement and the oven mass 34. 
Embedded within the oven mass 34 are a number of heat sensors, such as the 
two thermistors 42 and 44 shown in FIG. 1. The sectional view of FIG. 1 
illustrates two diametrically opposed thermistors, but there are 
preferably at least three thermistors. Structure for a three-sensor 
arrangement is shown in FIG. 4. Each thermistor is equidistantly spaced 
from immediately adjacent thermistors. Moreover, there is a fixed distance 
between the center of the oven chamber 24 and a thermistor. FIG. 4 
illustrates three openings 45 for receiving thermistors. Such an 
arrangement is designed to provide a highly accurate determination of 
crystal temperature. As will be described more fully below, the signals 
from the thermistors are utilized in an averaging approach to determine 
crystal temperature for controlling the heaters 26, 28 and 29. As a 
result, any adverse effects on oscillator performance that may result from 
creation of temperature gradients within the thermal reservoir formed by 
the oven mass 34 are minimized. 
In the preferred embodiment, the thermistors 42 and 44 are positioned along 
the same cross sectional plane as the crystal blank 46 in FIG. 1. While 
the use of the oven mass 34 to create a thermal reservoir significantly 
reduces the risk of thermal gradients that are dependent upon the distance 
of the crystal 36 from the heaters 26 and 28, the positioning of the 
thermistors along the same plane as the crystal blank provides a further 
safeguard against thermal gradients adversely affecting crystal 
performance. Alternatively, three-dimensional averaging can be achieved if 
more than three thermistors are arranged in a non-coplanar but symmetrical 
manner. 
In the preferred embodiment, leads from the thermistors 42 and 44 are 
shielded from the exterior surface of the oven mass 34 by a thermal guard 
48. The thermal guard is a thin electrically insulative material that 
contains conductors and that is wrapped about the cylindrical outer 
surface of the oven mass. The thermal guard may include embedded 
conductors that channel the signals from the thermistors to a region from 
which the signals are to be conducted to the averaging circuit. The 
thermal guard functions to protect the thermistor leads from conducting 
thermal energy to and from the thermistors, thereby increasing the 
accuracy of monitoring the temperature of the crystal 36. 
The method of conducting signals and power to and from the interior of the 
encasement formed by the cylindrical and circular members 20 and 22 is 
critical if the hermetic seal is to be preserved. In one embodiment, 
hermetic feedthroughs are mounted through the upper surface of the 
cylindrical member 20, with leads extending through cylindrical insulative 
members to prevent electrical shorts to the metallic encasement and to 
preserve the seal. A flexible circuit may then be used to conduct the 
signals and utilities to the exterior of the housing formed by the cover 
12 and base 14. Preferably, each input and output signal passes through a 
plastic dielectric capacitor. The plastic dielectric capacitors permit 
electrical conduction into and out of the housing, but inhibit thermal 
conduction. 
The operation of the oven assembly 10 is described with reference to FIG. 
5. Three thermistors 42, 44, and 50 are shown as being contained within 
the hermetically sealed oven chamber 24. Signals from the thermistors are 
conducted to an averaging circuit 52. The assembly of FIG. 1 may be 
surface mounted to a printed circuit board that includes a control circuit 
54. The three thermistors are substantially identical and are of the type 
that could be used individually in a temperature monitoring and regulating 
circuit. However, in FIG. 5, the three thermistors provide inputs to the 
averaging circuit 52 which determines the crystal temperature based upon 
an averaging process. 
Referring now to FIG. 6, a preferred embodiment for executing the 
thermistor averaging is shown as including a bridge circuit 58. This 
bridge circuit is less complex than an embodiment that requires a separate 
averaging circuit, such as shown in FIG. 5. One leg of the bridge circuit 
58 includes the three thermistors 42, 44 and 50 in electrical series. Each 
of the other three legs includes a resistor 60, 62 and 64 having a value 
selected to permit the bridge to operate in the manner of a conventional 
Wheatstone bridge. The resistors are located on the oscillator circuit 
within the oven chamber, so that only the power lines 66 and 68 and the 
signal lines 70 and 72 exit the oven chamber. Since the error is the 
electrical difference between the two signal lines, the temperature 
sensing is implemented with little susceptibility to exterior thermal 
influences. 
As an alternative to the bridge circuit 58 of FIG. 6, the three thermistors 
42, 44 and 50 may be connected in electrical parallel to form one leg of 
the bridge circuit. With either the series or parallel arrangement, 
averaging occurs in determining an input to a heater control circuit. 
Returning to the less preferred embodiment of FIG. 5, the components of the 
averaging circuit 52 may merely be additive and divide-by-three 
components. That is, the three output signals from the thermistors 42, 44 
and 50 are summed and an average of the three detected temperatures is 
calculated. Averaging circuits are known to persons skilled in the art. 
The output of the averaging circuit 52 is connected to the control circuit 
54 via a plastic dielectric capacitor 55 that inhibits thermal 
conductivity. Control circuits are conventionally used in the operation of 
ovenized crystal oscillators. A power supply 56 is shown as being part of 
the control circuit. Operation of the circuits 52 and 54 may be used to 
regulate dynamically current flow to the upper, lower and rim heaters 26, 
28 and 29, or may be used to vary the duty cycles for heat generation 
based upon the calculation of the temperature at the crystal resonator 36. 
The desired operating temperature depends upon a number of factors, 
particularly the crystal resonator 36 that is used. As an example, the 
operating temperature may be maintained at 80.degree. C. Referring now to 
FIGS. 1-6, tests of the oven assembly 10 have shown a thermal gain of in 
excess of 100,000. This is more than an order of magnitude beyond 
conventional single oven assemblies for crystal oscillators. The testing 
was performed using the B-mode of SC cut crystal operation, which has a 
large temperature coefficient and is therefore very useful for temperature 
measurement. 
FIGS. 7 and 8 are graphs of data acquired for a crystal oscillator oven 
assembled according to the invention. The graph 74 illustrates a regulated 
change in the ambient temperature of the crystal oscillator oven. The 
ambient temperature was changed between -5.degree. C. and 75.degree. C. 
three times over a period of approximately twenty-four hours. The graph 76 
illustrates the measured change in crystal temperature over the same 
twenty-four hour period. While small spikes occurred at each ambient 
temperature switch, the crystal temperature quickly stabilized and the 
static change was less than 1.times.10.sup.-3 .degree. C. The substantial 
temperature variations used in acquiring the data of FIGS. 7 and 8 
provided increased understanding of the performance of the ovenized 
crystal oscillator assembly. 
The high thermal gain is at least partially a result of the geometry of the 
oven assembly 10. A thermally symmetrical design reduces the 
susceptibility of the assembly to thermal gradients. The oven assembly 24 
is circular in horizontal cross section, as viewed in FIG. 1. Compared to 
conventional rectangular oven chambers, the oven chamber of the 
illustrated invention enhances temperature uniformity. Locating the 
thermal reservoir that contains the crystal 36 concentrically with the 
oven chamber further enhances uniformity. 
The wide-area thermal coupling provided by the upper, lower and rim heaters 
26, 28 and 29 further reduces the risk of temperature gradients within the 
oven chamber 24. Again referring to the orientation of FIG. 1, the 
relatively small height and the provision of the thermal reservoir operate 
to provide temperature uniformity in the vertical direction. The 
insulative foam 16 and 18 increases heating efficiency without the risk of 
outgassing causing chemical migration that would adversely affect 
operation of the crystal oscillator 36. Since the insulative foam is 
outside of the hermetic seal, gases generated as a result of heating the 
insulation will not enter the oscillator. 
Symmetry is also considered in the monitoring of crystal temperature. The 
thermistors 42, 44 and 50 are equidistantly spaced from each other and 
from the center of the crystal 36. Moreover, the thermistors are 
preferably along the same horizontal plane as the crystal blank 46, so 
that the monitoring is at a position that corresponds to the distance of 
the crystal blank from the two heaters 26 and 28.