Abstract:
An electrical device to compensate for crystal oscillator frequency shifts occurring over a temperature range includes a voltage divider for generating a temperature variable, compensation voltage at an output. The output of the voltage divider is to be electrically coupled to the oscillator so that the compensation voltage compensates for the crystal oscillator frequency shifts otherwise occurring over the temperature range. A voltage source is to be coupled to an input of the voltage divider for inputting a generally fixed voltage during normal crystal oscillator operation, and providing for multiple and repeatable adjustments to the fixed voltage before beginning the normal crystal oscillator operation.

Description:
This application is a C-I-P of Ser. No. 08/961,689 filed Oct. 31, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to crystal oscillators and more specifically to a temperature compensated crystal oscillator having a compensating circuit that stabilizes the output frequency of the crystal oscillator over a desired temperature range. 
     BACKGROUND OF THE INVENTION 
     Crystal oscillators are commonly used for a number of applications that require a stable output frequency. The output frequency, however, varies as a function of the ambient temperature of the oscillator. FIG. 1 shows a graphical representation of the frequency of a typical uncompensated AT cut quartz crystal versus the ambient temperature. As shown the curve  6  has a generally cubic curve shape that can be characterized by three temperature regions. The curve in the cold temperature region (−35° C. to approximately +10° C.) has a linear portion having a positive slope and a nonlinear portion wherein the slope of the curve changes polarity. The curve in the middle temperature region (+10° C. to +50° C.) has a linear portion having a negative slope. The curve in the hot temperature region (+50° C. to +90° C.) has a linear portion having a positive slope and a nonlinear portion wherein the slope of the curve changes polarity. The point of inflection  8  is in the middle temperature region at approximately +28° C. 
     A number of techniques to compensate for this frequency variation of the crystal includes the use of analog circuitry. One such analog compensation technique uses a resistor/thermistor network. For temperature range applications that extend into the non-linear portions of the AT cut crystal curve, at least three thermistors are necessary to compensate for each temperature region. Negative temperature coefficient thermistors are put into a network with a number of fixed resistors. The network is then supplied with a stable, fixed voltage source. By selecting the proper thermistors (for nominal value and temperature slope) and the value of the fixed resistors in the network, it is possible to match a variety of “AT” cut crystals and cancel the frequency vs. temperature drift over a wide temperature range. Stabilities of better than 0.5 ppm can be achieved with this method over a temperature range of −40° C. to +85° C. 
     While this technique is well suited to some applications, there are some disadvantages which limit wider usage. First, a wide range of precision, tight tolerance resistors (usually 1% or better) must be stocked. Second, a set of resistor values unique to each oscillator must be selected and manually installed. Third, the calculations and measurements necessary to select these components result in a time consuming process of iteratively testing, changing components, and re-testing the oscillators until they have been “massaged” to meet the specifications. Fourth, the thermistors must also be selected so that the thermistor slopes and ratios match the crystal being used. Fifth, interactions between the thermistors in the combined network limit the precision of the compensation that can be achieved. Sixth, because of the simple voltage divider action of the network, the output voltage has a limited dynamic range making operation at low voltages impractical. 
     Some attempts at automating the “massaging” process by trimming the resistors and matching the crystal have been successful for specific applications with moderate stabilities, however full automation has proven to be very difficult. Further, it is impossible to reverse the trimming process in order to decrease the resistance of the trimmed resistor. Resistors which are trimmed are typically screen printed onto the circuit substrate, and therefore cannot be simply replaced. 
     One method for tuning a crystal oscillator is shown in U.S. Pat. No. 5,473,289. A single linear temperature sensor is used as implemented by one or more diodes. This combination produces a straight line function of voltage vs. temperature which is then applied to a plurality of voltage function generator circuits that generate a series of straight line segments of varying slopes and intercepts. A switching circuit then controls which segment is active at a given temperature, in effect summing all of the segments over the operating temperature range to give an approximation of the crystal curve made up of a series of straight lines. A drawback with this approach is that the compensation voltage does not generate a smooth match to the cubic crystal curve, but rather employs discrete, distinct segments with crossover points to approximate the cubic crystal curve. 
     Another analog method uses thermistor/capacitor networks in a similar manner as the resistor/thermistor networks by adjusting the effective reactance of one or more fixed capacitors as the temperature varies. This method is very cost effective and has been produced for consumer applications requiring moderate stabilities of +/−2.5 ppm over a tighter temperature range. For applications that require operation in a wider temperature range, tighter matching of the components is required which becomes increasingly difficult due to the inability to match the crystal slopes and limitations in component values, tolerances and stabilities. 
     Another analog method uses multipliers that multiply a voltage which is linearly proportional to temperature which then generates a square and cubic term. These signals are then scaled appropriately and added together to produce a third order polynomial which matches the crystal curve to be compensated. This process still requires a set of resistors to be selected or trimmed and therefore, requires subsequent corrections which requires physical replacement or modification of one or more resistors. 
     A digital compensation technique includes the use of look-up tables. The frequency differential crystal curve between a selected temperature range is stored in a look-up table. The binary data stored at each memory location of the look-up table contains a compensation value that corresponds to each temperature increment. The output of a linear temperature sensor is digitized over the operating temperature range by an analog-to-digital (A/D) converter. The output from the A/D converter addresses the look-up table stored in nonvolatile ROM. The selected binary compensation value that corresponds to the ambient temperature of the oscillator is converted to a voltage by a digital-to-analog (D/A) converter which is used to tune the frequency of the crystal oscillator. 
     The ultimate stability obtainable by this approach is determined by the resolution of the A/D and D/A converters. Stabilities better than the hysteresis and repeatability of an AT cut crystal (about 0.05 ppm) are achievable over some temperature ranges with the proper system design. 
     All digital compensation systems, however, exhibit some degree of quantization noise, caused by the discrete steps of the conversion process. This is seen as a discrete jump in the output frequency as the compensation is updated. This effect can be minimized by increasing the resolution of the converters and filtering of the output, but it is very difficult to reduce it below the tolerance threshold of some systems. Spurious noise caused by feedthrough and coupling of digital switching components can also be a severe problem. 
     Another digital compensation technique used a microcomputer. This greatly reduced the amount of non-volatile programmable memory that is needed since interpolation or curve-fitting routines required much less stored data. Some success at Application Specific Integrated Circuit (ASIC) implementation has been achieved, but various issues have prevented these oscillators from being widely used. 
     The latest approaches for microcomputer compensation have used crystal self-temperature sensing techniques for best accuracy and repeatability. This method operates the crystal on both the fundamental and third overtone modes simultaneously. This is usually done with an SC cut crystal, but it is also possible with AT cut crystals. The apparent angle shift between the modes produces a signal which is very accurately proportional to temperature when the difference between the third overtone frequency and the fundamental multiplied by three (3) is compared. Since the frequency of these oscillators cannot be adjusted or tuned without affecting the calibration of the thermometer signal, external means for generating the stable output frequency must be employed. 
     This type of microcomputer controlled TCXO utilizing crystal self-temperature sensing has achieved the best stability of any compensated oscillator. Overall stabilities of better than 0.05 ppm over −55° C. to +85° C. have been reported. The complexities of these oscillators, however, make them relatively expensive, and they still suffer from some of the noise problems which are inherent in digital compensation systems. 
     Accordingly, it is an object of the present invention to provide a cost effective crystal oscillator that has a highly stable output frequency over a desired temperature range. 
     It is another object to provide a crystal oscillator wherein the tuning of the output frequency may be easily adjusted. 
     It is a further object to provide temperature compensated crystal oscillator that eliminates the need to trim or select the proper component value. 
     The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, an electrical device to compensate for crystal oscillator frequency shifts occurring over a temperature range includes a voltage divider for generating a temperature variable, compensation voltage at an output. The output of the voltage divider is to be electrically coupled to the oscillator so that the compensation voltage compensates for the crystal oscillator frequency shifts otherwise occurring over the temperature range. A voltage source is to be coupled to an input of the voltage divider for inputting a generally fixed voltage during normal crystal oscillator operation, and providing for multiple and repeatable adjustments to the fixed voltage before beginning the normal crystal oscillator operation. 
     In another aspect of the present invention, a method for tuning a crystal oscillator that has a temperature compensating circuit and a voltage source for inputting a tuning voltage to the temperature compensating circuit includes the steps of a) providing a tuning voltage from the voltage source to the temperature compensating circuit; b) determining a frequency variation range of the crystal oscillator over a temperature range; c) comparing the frequency variation range to a desired frequency operating range; d) setting the voltage source to permanently generate the voltage if the frequency variation range is equal to or less than the desired frequency operating range; and e) adjusting the voltage source to generate a different tuning voltage if the variation range is greater than the desired frequency operating range and repeating steps a-e with the different tuning voltage. 
     In a further aspect of the present invention, a temperature compensated crystal oscillator includes a voltage tunable crystal oscillator having a crystal with a resonant frequency that varies over a temperature range. A voltage divider generates a temperature variable, compensation voltage at an output. The output of the voltage divider is to be electrically coupled to the oscillator so that the compensation voltage compensates for the crystal oscillator frequency shifts otherwise occurring over the temperature range. A voltage source is to be coupled to an input of the voltage divider for inputting a generally fixed voltage during normal crystal oscillator operation, and providing for multiple and repeatable adjustments to the fixed voltage before beginning the normal crystal oscillator operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical representation of the cubic shape curve of the frequency of an uncompensated AT crystal as it varies over temperature. 
     FIG. 2 is a circuit diagram of temperature compensated crystal oscillator including a temperature compensating circuit embodying the preferred embodiment of this invention. 
     FIG. 3 is a circuit diagram showing an alternative embodiment of a temperature compensated crystal oscillator of the present invention. 
     FIG. 4 is a top plan view of a temperature compensated crystal oscillator of FIG.  3 . 
     FIG. 5 is a top plan view of a common board including a plurality of temperature compensated crystal oscillators of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 illustrates a temperature compensated crystal oscillator (TCXO)  10  embodying the present invention that includes a compensating circuit  14  and a voltage tunable crystal oscillator (VCXO)  16 . The compensating circuit generates a voltage control signal at  17  to the voltage tunable crystal oscillator to compensate for the variations in frequency of the crystal over an operating temperature range as shown in FIG.  1 . The voltage tunable oscillator  16  is known in the art and therefore, not discussed in great detail. 
     The compensating circuit  14  includes three networks  18 ,  19 ,  20  to compensate for the frequency variation of the crystal  22  over hot, cold and middle temperature regions. The cold temperature network  18  primarily compensates for the frequency variation within the cold temperature region (approximately −35° C. to +10° C.). The middle temperature network  19  primarily compensates for the frequency variation within the medium temperature region (approximately +10° C. to +50° C.). The hot temperature network  20  primarily compensates for the frequency variation within the hot temperature region (approximately +50° C. to +90° C.). The change in output voltages at  24 - 26  of each of the networks  18 - 20  are inversely proportional to the change in frequency of the crystal associated with a change in temperature within a corresponding temperature region of the crystal curve. For example, the output voltages at  24 ,  26  have a negative slope that is inversely proportional to the slope of the frequency change of the crystal in the hot and cold temperature regions, respectively, of the crystal curve of FIG.  1 . The output voltage at  25  has a positive slope which is inversely proportional to the slope of the frequency change of the crystal in the middle temperature region of the crystal curve. The combination of these three signals  24 - 26  in the proper proportions will therefore cancel and compensate for the characteristic drift in frequency of the crystal over the temperature range that includes the cold, middle and hot temperature regions. 
     Three independently adjustable analog voltage sources  28 ,  29 ,  30  are provided to the corresponding cold temperature compensating network  18 , middle temperature compensating network  19  and a hot temperature compensating network  20 . Contrary to the prior art, each of the voltage sources  28 - 30  are set independently of one another to tune the output voltages at  24 - 26  respectively so that the summed control signal compensates for the frequency change of the oscillator generated by the crystal over the entire temperature range. This ability to independently adjust the voltages of the output sources  28 - 30  enables the compensating networks  18 - 20  to be easily tuned by multiple and repeatable adjustments to the voltages of the output sources to match the individual crystal so as to eliminate the need for accurately adjusting the compensating networks using resistors having tight tolerances. This method of tuning the networks  18 - 20  also eliminates the need to trim the resistors or iteratively change resistors to obtain the desired compensating voltage and matching the crystal as described hereinbefore. Once the voltages from the output sources  28 - 30  are set, the voltages are held constant throughout the operating life of the oscillator. 
     The cold temperature voltage source  28  is connected to a first voltage divider  32  comprising resistor  34  and thermistor  36 . The thermistor  36  is connected to the lower leg of the voltage divider  32  having one terminal  38  connected to ground  40 . The thermistor  36  has a negative temperature coefficient and thus, its resistance decreases as the ambient temperature increases. Consequently, the voltage at junction  56  decreases as the temperature increases. The middle temperature voltage source  29  is connected to a second voltage divider  42  comprising resistor  44  and thermistor  46 , which has a negative temperature coefficient. The thermistor  46  is connected to the upper leg and the resistor  44  is connected to the lower leg of the voltage divider  42  having one terminal  48  connected to ground  40 . Consequently the voltage at junction  57  increases as the ambient temperature increases. The hot temperature voltage source  30  is connected to a third voltage divider  50  comprising resistor  52  and thermistor  54 , which also has a negative temperature coefficient. The thermistor  54  is connected to the lower leg of the voltage divider  50  having one terminal  55  connected to ground  40 . Consequently, the voltage at junction  58  decreases as the ambient temperature increases. 
     The junctions  56 - 58  of each of the voltage dividers  32 ,  42 ,  50  are connected to a respective voltage follower  60 ,  62 ,  64  which buffer the attenuated signals to eliminate any interaction between each of the compensating networks  18 - 20 . The output of each voltage followers  60 ,  62 ,  64  is summed at junction  66  through respective resistors  68 - 70 . Resistor  72  is connected between junction  66  and ground  40 . The summed voltage is provided to a voltage follower  74  to provide additional buffering from the oscillator  16 . The output voltage of voltage follower  74  is amplified and offset by amplifier stage  76 . The output of voltage follower  74  is provided to the inverting input of amplifier  78  through resistor  80 . Resistor  82  is connected between the output and inverting input of amplifier  78 . An offset voltage  84  is connected to the non-inverting input of amplifier  78 . 
     The resulting composite voltage control signal at  17  of the compensating circuit  14  is inversely proportional to the crystal curve of the FIG. 1 which adjusts the frequency of the voltage tunable oscillator  16  to compensate for the frequency variation of the crystal over a desired temperature range. The voltage dividers  32 ,  42 ,  50  generate a logarithmic voltage vs. temperature function in response to the temperature range affecting the crystal oscillator to compensate for frequency shifts otherwise occurring over the temperature range. The composite control signal is consequently a smooth blend of the various logarithmic thermistor characteristics which produces an accurate match to the cubic crystal curves. 
     Turning to FIG. 3, an embodiment of the present invention is implemented using an application specific integrated circuit (ASIC)  90 . The reference numbers for the components common with the embodiment of FIG. 2 are the same. As shown, the voltage sources  28 - 30  for each of the compensating networks  18 - 20  are repeatedly adjustable by programmable controller or microcomputer  92  which is used only during calibration. Once the oscillator is tuned, the compensation voltages generated by the voltage sources  28 - 30  are generally held at fixed values throughout the operating life of the oscillator. Microcomputer  92  provides, using a serial link, a clock signal at port  93  and a data signal at port  94  to each respective shift register  96  and memory means  98  disposed on the ASIC  90  which independently generate a digital signal representative of a desired voltage for tuning the respective compensating network  18 - 20  as described hereinbefore. Each digital signal is then converted to an analog signal by digital-to-analog converters (DACs)  100  which respectively provide the cold, middle and hot temperature voltages of the voltage sources  28 - 30  (see FIG.  2 ). Each shift register  96  and memory means  98  are enabled by a digital signal at ports  101 ,  102 ,  103 ,  104 ,  105 , so that the proper data may be loaded into the respective shift registers during calibration. 
     Similarly, the offset voltage  84  provided to amplifier  78  and reference voltage  106  is generated by microcomputer  92  in the same manner as the compensating voltages  28 - 30 . 
     The voltage control signal at  21  of the compensating circuit  14  is provided to a switching means  108  that can switch inverting amplifier  110  in series with the compensating circuit  14  to switch the polarity of the output voltage of the compensating circuit  14 . The output voltage signal is then provided to a voltage controlled oscillator circuit  16  through resistor  112 . The voltage control oscillator circuit, which includes a crystal  22  and varactor  114  to control the resonant frequency of the oscillator, is known in the art. The output signal of the oscillator  16  at  116  is connected to capacitor  119  to remove the DC component of the output signal. The output of the oscillator  16  may be connected to one input  118  of a nand gate  120 , and the other input  121  is provided to disable the output of the oscillator  16  by pulling the input  121  low. 
     The ASIC  90  includes the components that-are generic to all oscillators  10  of the type embodying this invention, and the components that vary in accordance with the crystal frequency and type, such as the voltage dividers  32 ,  42 ,  50  and the crystal  22 , are disposed on a circuit board  124  adjacent the ASIC  90  as shown in FIG.  4 . This configuration allows a single ASIC to be used for all oscillators irrespective of its frequency. In addition, the use of the ASIC  90  permits the oscillators  10  to be produced at reduced costs. As shown in FIG. 5, a plurality of oscillators having an ASIC that may be assembled and tested on a common board  126  before separation therefrom. The common board  126  includes a connector  128  having terminals  130  interconnected to each ASIC. The common board is interconnected to a test bed (not shown) which tests each ASIC. 
     A typical calibration procedure includes providing tuning voltages from the voltage sources  28 - 30 . The frequency variation range of the crystal oscillator over a temperature range is then determined. The frequency variation range is then compared to a desired frequency operating range. The voltage sources are permanently set to generate the source voltages if the frequency variation range is equal to or less than the desired frequency operating range. If, however, the variation range is greater than the desired frequency operating range, the voltage sources are adjusted to generate a different tuning voltage and the calibration steps are repeated. 
     In sum, the invention shown in the above-described embodiment permits all of the oscillators to be assembled with identical components. By employing conventional digital to analog (DAC) converters for the variable voltage sources, the compensation process can be completely automated. The DACs provide much better resolution in making adjustments (approximately 0.03% tolerance vs. the conventional 1% tolerance for resistor selection). By eliminating the interactions between the sections of the network, matching to the crystal within a few tenths of a ppm may be achieved. 
     Although the invention has been shown and described with respect to an exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.