Oscillators – Frequency stabilization – Temperature or current responsive means in circuit
Reexamination Certificate
2001-10-22
2004-05-04
Kinkead, Arnold M. (Department: 2817)
Oscillators
Frequency stabilization
Temperature or current responsive means in circuit
C331S068000, C331S069000, C331S158000, C310S341000, C310S343000, C310S344000
Reexamination Certificate
active
06731180
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to the field of temperature controlled structures, and more particularly, to a temperature controlled structure for resonator devices. The present structure includes direct bonding of a surface acoustic wave (SAW) device and incorporating multiple layers.
2. Background of the Invention
Oven controlled crystal oscillators (OCXO) are well known in the industry. These devices typically contain crystal resonators that resonate at a certain frequency. The resonant frequency varies with temperature, therefore stable frequencies require stable temperature environments.
The OCXO devices are packaged into some form of shell or casing that allows the device to be transported, handled, and incorporated into other electronic goods. There may be heaters associated inside the package along with temperature sensors and temperature controllers to maintain a stable thermal management of the device.
Integrated oscillator assemblies in particular are highly sensitive to temperature changes and operate non-linearly with temperature fluctuations. The oscillators arc generally used for providing highly precise reference frequencies or frequency sources. A typical precision oscillator, which includes a quartz crystal, normally has temperature sensors and some control means to stabilize temperature instability.
Despite the efforts of thermal management, in many cases the precision components are so sensitive to temperature that accurately measuring the temperature and controlling the circuit is not possible. The inability to maintain a precise temperature results in thermal drift from the desired temperature. Typically the temperature varies about the desired goal temperature and the amount of deviation produces frequency instability.
In addition, there may be thermal variations at different locations of the same circuit. The thermal distribution across the circuit board may result in differing temperatures and therefore differing performance even on the same circuit. Attempting to compensate for these thermal inconsistencies or gradients is difficult to implement.
Some of the prior art methods for dealing with temperature variations involve bonding a circuit board to a thermally conductive plate or temperature controlled plate. Other packages involve temperature controlled structures using heaters, while even further concepts use an oven structure placed inside another oven, appropriately called a double oven.
Temperature stability is improved by placing the oscillator device in an oven and maintaining the temperature of the device at a higher level than outside ambient. Such OCXO devices are well-known in the art and are commonly used in applications such as radio base stations that encounter large temperature variations. The oven keeps the temperature range of the oscillator within a very small window. In some stringent applications an additional oven houses the temperature controlled oven, producing what is termed a double oven system.
Despite all the efforts and variations of the prior art, temperature control is not generally uniform. The thermal control may be sufficient in one section of the board and inadequate in another section. There is a general lack of uniform thermal flow across the entire circuit.
Thermal gain is a figure of merit for quantifying the temperature stabilization of a structure. It is defined as the ratio of change in external or ambient temperature &Dgr;Ta, to the change in temperature of a small volume &Dgr;Tv reference.
Thermal Gain=&Dgr;
Ta/&Dgr;Tv
When the thermal gain is small, there is more temperature change with a change in ambient temperature. A higher thermal gain equates to a more stable environment because there is little temperature change with a corresponding change in ambient temperature. On a given circuit is possible to have both high and low thermal gain, and possibly even a negative thermal gain.
Besides OCXO devices exhibiting thermal management problems, the problem is even more prevalent for surface acoustic wave (SAW) devices. SAW devices suffer from the same problems as the OCXO counterparts, but SAW devices operate at much higher frequencies. More specifically, SAW devices operate as much as ten times higher frequencies than the standard crystal resonators. SAW devices also have a higher percentage of error than BAW devices. Thus, very small percentages of error result in a greater frequency difference, and this has been one of the primary drawbacks of SAW devices in these applications.
A SAW device depends upon acoustic wave generation to function properly. In most cases the acoustic waves are in close proximity to the surface of the substrate, which makes the devices very sensitive to external conditions such as temperature, stresses and vibrations.
Thermal conductivity is a major factor in the design of ovenized oscillators, whether OCXO or SAW. It has been demonstrated that the performance of ovenized oscillators can be significantly enhanced by evacuating the entire package prior to sealing, which results in a lower thermal loss. Evacuated miniature SAW (EMSO) devices have been utilized with some success.
While temperature considerations are important, it is also necessary to consider vibration conditions. The mounting of the SAW device is important in order to avoid interfering vibrations. Generally the SAW devices are mounted onto carriers, or otherwise disposed in a manner that supports the SAW device and reduces vibration troubles. EMSO devices have a smaller overall mass, which further reduce vibration problems.
There have been many attempts to alleviate the aforementioned problems. In U.S. Pat. No. 4,213,104, a SAW device is vacuum encapsulated for improvements of the thermal characteristics. Packaging methods are disclosed in U.S. Pat. Nos. 5,594,979 and 5,059,848. These patents lay out the foundation of the problems related to the thermal management of SAW devices and various attempts to resolve these problems.
The issued U.S. Pat. No. 5,917,272 ('272) to the same applicant, incorporated herein in its entirety by reference, describes an ovenized oscillator mounted over a heat conducting substrate, wherein the substrate is thermally isolated from the base by insulated posts. The resonator is suspended above the heated substrate and held in place by clips that permit radiative thermal flow. The '272 device did not disclose en enabling description of using SAW devices.
U.S. Pat. No. 4,317,985 ('985) describes an OCXO device with dual heater apparatus to achieve very high thermal gains at the location of the sensor, or by using two heaters, at a selected location. This location normally coincides with the most sensitive component—generally the crystal. It also describes adjusting the ratio of power between the heaters. A major drawback to this approach is the temperature gradients in other locations of the circuit is excessively poor.
In U.S. Pat. No. 5,919,383 a package for a temperature sensitive optical device is described. The package contains an inner and outer container of low thermal polymer and with an insulating material disposed therebetween. The temperature sensitive circuit is contained within the inner container along with a temperature sensor. The control circuit is outside the inner container and controls the heating element(s) that are located inside the inner compartment and in contact with the temperature sensitive circuit. Within the inner container is a thermally conductive plate that attaches to the circuit, the heater and the temperature sensor.
Despite the previous attempts to address the thermal management issues of SAW devices, significant problems remain in the industry. What is needed is a device that provides a stable environment for temperature and vibration sensitive devices. The control structure should work with electronics, namely SAW devices and OCXO devices, and have very high thermal gain and zero gradient. Such as invention should be a cost-effective solution and rugged in order to work
Adler Joseph V.
Clark Roger L.
Li Jacob M.
Deleware Capital Formation Inc.
Kinkead Arnold M.
Maine & Asmus
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