Abstract:
Methods and apparatus for controlling frequency in a crystal oscillator are provided that allows for continued reception of GPS signal solution in a continuous high G environment. One method comprises measuring G-forces asserted on the crystal oscillator, determining a shift in frequency of the crystal oscillator due to the measured G-forces, determining a temperature that would shift the crystal oscillator&#39;s frequency back to a rate that would occur without the measured G-forces, and changing the temperature of the crystal oscillator based on the determined temperature to shift the crystal oscillator&#39;s frequency back to a rate that would occur if the G-forces were not present.

Description:
BACKGROUND 
   It is common for aeronautical devices to employ Global Positioning Systems (GPS) for navigational purposes. One common component of a GPS system is a crystal oscillator that provides electrical oscillations (clock signals) for use by components of the GPS at a frequency that is defined by the physical characteristics of a piezoelectric quartz crystal. In high spin aero-space devices that maintain a relative high acceleration, crystal oscillators exhibit a susceptibility to the associated G-forces. As the result of the susceptibility to G-forces, there is a shift in the fundamental frequency of the crystal oscillator. In continuous high G-force environments, this becomes a significant source of error. For example, since accurate timing of sent and received signals between the GPS and satellites is needed to determine location, any shift in frequency of the crystal oscillator that provides the timing will effect the determination of the location. Frequency error of the crystal oscillator cannot be easily detected or compensated for using current techniques. For example, monitoring the frequency of the crystal oscillator is difficult since a device used to compare frequencies (which would include another crystal oscillator) would also be affected by the G-forces. 
   For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a system for effectively and efficiently compensating for the effects of G forces on crystal oscillators so that a desired frequency of the crystal oscillator is maintained. 
   SUMMARY OF INVENTION 
   The above-mentioned problems of current systems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention. 
   In one embodiment, a method of controlling frequency in a crystal oscillator is provided. The method comprises measuring G-forces asserted on the crystal oscillator, determining a shift in frequency of the crystal oscillator due to the measured G-forces, determining a temperature that would shift the crystal oscillator&#39;s frequency back to a rate that would occur without the measured G-forces, and changing the temperature of the crystal oscillator based on the determined temperature to shift the crystal oscillator&#39;s frequency back to a rate that would occur if the G-forces were not present. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the detailed description and the following figures in which: 
       FIG. 1  is a block diagram of a device subject to G-forces of one embodiment of the present invention; and 
       FIG. 2  is a frequency control flow diagram illustrating one method of one embodiment of the present invention. 
   

   In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the present invention. Reference characters denote like elements throughout Figures and text. 
   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the claims and equivalents thereof. 
   Embodiments of the present invention provide a system and method to maintain a desired frequency of a crystal oscillator in a relatively high spin vehicle. In embodiments, acceleration of the vehicle is monitored. Based on the monitored acceleration and known relationships between G-forces Vs. frequency and temperature Vs. frequency of crystal oscillators, the temperature of the crystal oscillator is adjusted to main a desired frequency. 
   Referring to  FIG. 1 , a device  100  that is subject to G-forces with a frequency control system of one embodiment of the present invention is illustrated. In this embodiment, a high spin vehicle  102  includes a crystal oscillator  104 , an oven  106 , an accelerometer circuit  108 , a temperature circuit  110  and a compensation circuit  112 . As illustrated in  FIG. 1 , external pressures (G-forces) from vehicle spin due to centripetal and centrifugal accelerations are exerted on the vehicle  102 . The external pressure shifts the clock frequency of the crystal oscillator  104  away from its desired frequency. The affects of G-forces on the frequency of crystal oscillators (G-force Vs. frequency) are known in the art. 
   The accelerometer circuit  108  measures the amount of G-forces being applied to the vehicle  102 . In one embodiment, the accelerometer circuit  108  includes three orthogonal accelerometers and gyros that determine the G-force on the crystal. The accelerometer circuit  108  outputs an accelerometer measurement signal to the compensation circuit  112 . The temperature circuit  110  monitors the temperature of the crystal oscillator  104 . The relationship between a temperature of a crystal oscillator  104  and the frequency of the crystal oscillator (temperature Vs frequency) is also well known in the art. In fact, it is common to use oscillator ovens (such as oven  106 ) in vehicles with GPS/INS navigation systems to control the frequency of the crystal oscillator in order to maintain a precise lock on satellites. The temperature circuit outputs a temperature measurement signal to the compensation circuit  112 . 
   The compensation circuit  112  takes the accelerometer measurement signal which is present in a GPS/INS (Inertial Navigation System), and determines if the current G-force has affected the frequency of the crystal oscillator  104  and by how much. In one embodiment this in done with a processor  109 . Moreover, in one embodiment, the processor  109  uses data from a stored table that sets out G-force Vs. frequency affects. Hence in one embodiment, the compensation circuit  112  includes a memory  111  to store relationship tables. If the processor determines a frequency shift has occurred due to G-forces based on the accelerometer measurement signal and the G-force Vs. frequency table, the processor then determines what temperature is needed to shift the frequency back to the desired frequency. This is done by looking at the temperature Vs frequency relationship of crystal oscillators. In one embodiment, a temperature Vs frequency table is stored in the memory  111 . In this embodiment, the processor  109  simply looks at the temperature Vs frequency table to determine the temperature needed to shift the frequency back to the desired frequency. Once a temperature needed to shift the frequency back to the desired frequency has been determined, a control signal is sent to the oven  106 . In one embodiment, a compensation voltage is the signal used to control the oven  106 . The oven  104 , in response to the control signal from the compensation circuit  112  heats or cools the crystal oscillator  104  accordingly. Hence, in embodiments of the present invention, the temperature of the oven  106  is controlled via accelerometer feedback to keep the oscillator frequency constant. 
   In one embodiment, a navigation circuit  114  is included. The navigation circuit  114  is also illustrated in  FIG. 1 . The navigation circuit  114  is used to navigate the device  100  that is subject to G-forces. The navigation circuit  114  in one embodiment includes global positioning systems (GPS). In yet another embodiment, the navigation circuit  114  includes inertial navigation system (INS). In still another embodiment, the navigation circuit includes both GPS and INS. The GPS and INS use data from the accelerometer circuit  108  for navigation. In one embodiment, the accelerometer circuit  108  includes three orthogonal accelerometers and gyros that determine G-force. The navigation circuit  104  uses a clock (or frequency signal) from the crystal oscillator  104  for timing reasons. For example, with GPS precision timing between sent signals and received signals from satellites is needed to determine location. If the crystal oscillator&#39;s frequency is shifted because of G-forces, the timing will be off and a wrong location will be determined. Hence, the embodiments of the present invention maintain the frequency of the crystal oscillator at a desired frequency so that accurate timing by components using the frequency signal can occur. 
   Referring to  FIG. 2 , a frequency control flow diagram  200  illustrating one method of implementing one embodiment of the present invention is illustrated. As illustrated, the process starts by measuring G-forces being applied to a vehicle ( 202 ). As discussed above, in one embodiment this is done with an accelerometer circuit. Data regarding the measured G-force is then output to a compensation circuit ( 204 ). It is then determined if the G-force is strong enough to shift the frequency of the oscillator ( 206 ). If it is not strong enough to shift the frequency ( 206 ), the G-force is continued to be measured at ( 202 ). If the G-force is strong enough to shift the frequency of the oscillator ( 206 ), it is then determined what temperature of the crystal is needed to counteract the frequency shift ( 208 ). As discussed above, in embodiments of the present invention, this is done with use of G-force Vs. frequency and temperature Vs frequency tables. Once the temperature is determined ( 208 ), the crystal oven is adjusted accordingly ( 210 ). This shifts the frequency back to a desired frequency. 
   The methods and techniques described here may be implemented in digital electronic circuitry, or with a programmable processor (for example, a special-purpose processor or a general-purpose processor such as a computer) firmware, software, or in combinations of them generally defined as modules. Apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. A process embodying these techniques may be performed by a programmable processor executing a program of instructions to perform desired functions by operating on input data and generating appropriate output. The techniques may advantageously be implemented in one or more programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and DVD disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASICs). 
   Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.