Temperature compensating circuit for a crystal oscillator

Abstract: A temperature compensating crystal oscillator (TCXO) includes a temperature compensating circuit that provides a temperature compensated signal to a voltage tuned crystal oscillator (VCXO) to generate an output signal having a stable frequency over a desired temperature range. The TCXO is operable over a temperature range that includes a hot temperature region, a middle temperature region and a cold temperature region. The voltage tuned oscillator includes a crystal having a resonant frequency that varies over the temperature regions which is defined by a cubic curve. The compensating circuit includes three compensating means that generate respective control signals in response to respective voltages and the ambient temperature of the crystal oscillator. Each control signal is proportional to a respective portion of the cubic crystal curve defined by the hot, cold and middle temperature regions. Each respective voltage is provided by an independent voltage source. Each of which are adjustable to tune independently each respective control signal to compensate the VCXO whereby the output frequency of the TCXO is substantially stable over the operating temperature range.

Description: 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. The fixed resistors in the network are trimmed or changed to adjust the network output voltage to match the characteristics of the crystal curve. It is possible with this technique to achieve frequency stability on the order of +/- 1.0 ppm (parts per million) over a temperature range of -40° C to +85° C. To achieve this frequency stability, it is necessary to use precision, hght tolerance resistors that are uniquely calculated for each oscillator. In addition, the thermistors must also be selected so that the thermistor slopes and ratios match the crystal being used. The calculations and measurements necessary to select these components result in a time consuming process of iteratively testing, changing components, and retesting the oscillators until they have been "massaged" to meet the specifications. 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. 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 non-volatile 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. US Patent Serial No 5,041,799 (Pirez) discloses a further digital compensation system in which a digital "word" is generated from a number of input parameters relating to a specific category of crystal, the word representing certain response characteristics of the crystal. That word is then used in a compensating circuit that adjusts the values of control signals relating to respective low, middle and high temperature regions of the temperature/frequency response characteristic of the crystal, in conjunction with an ambient temperature signal, to generate a combined control signal to effect compensation. One of the specific response characteristics is the inflection point of the crystal's characteristic cubic curve. 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 a 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 According to a first aspect of the present invention, there is provided a temperature compensating circuit for adjusting the output frequency of a voltage tunable crystal oscillator operable over a predetermined temperature range that includes a hot temperature region, a middle temperature region and a cold temperature region. the oscillator having a crystal with a resonant frequency that varies over the temperature regions as a function of temperature according to a characteristic cubic curve, said temperature compensating circuit comprising: first, second and third compensating means for generating respective first, second and third control signals in response to voltages from respective first, second and third voltage sources and the ambient temperature of the crystal oscillator, the first, second and third control signals being substantially inversely proportional to respective portions of the curve of the crystal over respective ones of the temperature regions; and said first, second and third compensating means (18-20) comprising first, second and third means (32, 42, 50) respectively, receiving the voltages from the respective first, second and third voltage sources (28-30) for adjustably and independently providing a fourth, a fifth and a sixth voltages to adjust the respective first, second and third control signals (24-26) for tuning the voltage tunable crystal oscillator, characterised in that: each of the first, second and third means comprises a voltage divider comprising a thermistor and a resistor connected in series with the respective voltage source; and summing means for combining the first, second and third control signals to generate a summed control signal for tuning the voltage tunable crystal oscillator, whereby the output frequency of the oscillator is substantially stable over the corresponding temperature regions. According to a second aspect of the present invention, there is provided a method of adjusting a voltage tunable crystal oscillator operable over a predetermined temperature range that includes a hot temperature region, a middle temperature region and a cold temperature region, the oscillator having a crystal with a resonant frequency that varies over the temperature regions as a function of temperature according to a characteristic cubic curve, the method comprising the steps of: generating first, second and third control signals in response to voltages from respective first, second and third voltage sources and the ambient temperature of the crystal oscillator, the control signals being substantially inversely proportional to respective portions of the characteristic curve over respective ones of the temperature regions; providing the said control signals to the crystal oscillator; and independently adjusting the voltages from the respective voltage sources to tune the crystal oscillator to provide a substantially stable output frequency over the temperature regions, characterised in that: the step of adjusting the voltages from the respective sources comprises deriving said control signals from respective voltage dividers connected in series with the respective voltage sources, each said divider comprising a thermistor and a resistor; and combining the first, second and third control signals to generate a summed control signal for tuning the voltage tunable crystal oscillator, whereby the output frequency of the oscillator is substantially stable over the corresponding temperature regions. Preferably, the voltage divider in the second compensating means comprises a said resistor and a said thermistor connected in series from the second voltage source to ground, the resistor having a terminal connected to ground. The voltage dividers in each of the first and third compensating means preferably comprise a said resistor and a said thermistor connected in series from the first and third voltage sources to ground, the thermistor in each case having a terminal connected to ground. The invention also provides, in a third aspect, the combination of a temperature compensating circuit according to any of the preceding paragraphs and a voltage tunable crystal oscillator including a crystal having a resonant frequency that varies over the temperature regions, and voltage tuning means for varying the frequency of the crystal oscillator. In each aspect of the invention, the temperature compensating circuit generates a composite control signal that tunes the crystal oscillator to provide a substantially stable output frequency over the entire temperature range. 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 output voltages at 24-26 of each of the networks 18-20 are inversely proportional to the frequency of the crystal for each 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 of 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 of the crystal over the temperature range. Three independently adjustable 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 to tune the output voltages at 24-26 respectively so that the summed control signal compensates for the frequency change of 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 to match the individual crystal eliminating the need to accurately adjust 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. The cold temperature voltage source 28 is connected to a 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 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 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 a 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. 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 adjusted by microcomputer 92 which is used only during calibration. 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 represent 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. 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 scope of the invention.

Claims: A temperature compensating circuit for adjusting the output frequency of a voltage tunable crystal oscillator (16) operable over a predetermined temperature range that includes a hot temperature region, a middle temperature region and a cold temperature region, the oscillator having a crystal with a resonant frequency that varies over the temperature regions as a function of temperature according to a characteristic cubic curve (6), said temperature compensating circuit comprising: first, second and third compensating means (18-20) for generating respective first, second and third control signals (24-26) in response to voltages from respective first, second and third voltage sources (28-30) and the ambient temperature of the crystal oscillator, the first, second and third control signals (24-26) being substantially inversely proportional to respective portions of the curve (6) of the crystal over respective ones of the temperature regions; and said first, second and third compensating means (18-20) comprising first, second and third means (32, 42, 50) respectively, receiving the voltages from the respective first, second and third voltage sources (28-30) for adjustably and independently providing a fourth, a fifth and a sixth voltages to adjust the respective first, second and third control signals (24-26) for tuning the voltage tunable crystal oscillator, characterised in that : each of the first, second and third means (32, 42, 50) comprises a voltage divider comprising a thermistor (36, 46, 54) and a resistor (34,44, 52) connected in series with the respective voltage source (28-30); and summing means (68-70) for combining the first, second and third control signals to generate a summed control signal for tuning the voltage tunable crystal oscillator, whereby the output frequency of the oscillator is substantially stable over the corresponding temperature regions. A temperature compensating circuit according to Claim 1, wherein the second control signal (25) is substantially inversely proportional to the portion of said characteristic curve over the middle temperature region. A temperature compensating circuit according to Claim 2, wherein the third control signal (26) is substantially inversely proportional to the portion of said characteristic curve over the hot temperature region. A temperature compensating circuit according to any preceding claim, further including an amplifying means (76) for increasing the gain of the summed control signal and providing a voltage offset (84). A temperature compensating circuit according to any preceding claim, further including buffer means (74) means for electrically buffering the first, second and third compensating means (18-20) from the oscillator (16). A temperature compensating circuit according to any of Claims 1 to 5, wherein the compensating means (18-20) each includes respective buffer means (60-64) for electrically isolating the respective control signals (24-26). A temperature compensating circuit according to any of Claims 1 to 6, wherein the voltage divider (42) in the second compensating means (19) comprises a said resistor (44) and a said thermistor (46) connected in series from the second voltage source (29) to ground (40), the resistor having a terminal (48) connected to ground (40). A temperature compensating circuit according to Claim 7, wherein the voltage divider (50) in the third compensating means (20) comprises a said resistor (52) and a said thermistor (54) connected in series from the third voltage source (30) to ground (40), the thermistor (54) having a terminal (55) connected to ground (40). A temperature compensating circuit according to Claim 8, wherein the voltage divider (23) in the first compensating means (18) comprises a said resistor (34) and a said thermistor (36) connected in series from the first voltage source (28) to ground (40), the thermistor (36) having a terminal (38) connected to ground (40). In combination, a temperature compensating circuit (14) according to any preceding claim and a voltage tunable crystal oscillator (16) including a crystal (22) having a resonant frequency that varies over the temperature regions, and voltage tuning means (114) for varying the frequency of the crystal oscillator. A combination according to Claim 10, wherein the voltage tuning means is a varactor (114). A method of adjusting a voltage tunable crystal oscillator (16) operable over a predetermined temperature range that includes a hot temperature region, a middle temperature region and a cold temperature region, the oscillator having a crystal with a resonant frequency that varies over the temperature regions as a function of temperature according to a characteristic cubic curve (6), the method comprising the steps of: generating first, second and third control signals (24-26) in response to voltages from respective first, second and third voltage sources (28-30) and the ambient temperature of the crystal oscillator, the control signals (24-26) being substantially inversely proportional to respective portions of the characteristic curve over respective ones of the temperature regions; providing the said control signals (24-26) to the crystal oscillator; and independently adjusting (32, 42, 50) the voltages from the respective voltage sources (28-30) to tune the crystal oscillator to provide a substantially stable output frequency over the temperature regions, characterised in that : the step of adjusting the voltages from the respective sources comprises deriving said control signals from respective voltage dividers (32, 42, 50) connected in series with the respective voltage sources (28-30), each said divider comprising a thermistor (36, 46, 54) and a resistor (34, 44, 52); and combining (68-70) the first, second and third control signals (24-26) to generate a summed control signal for tuning the voltage tunable crystal oscillator, whereby the output frequency of the oscillator is substantially stable over the corresponding temperature regions.

Inventors: FRY STEVEN, FRY, STEVEN
Applicants: CTS CORP, CTS CORPORATION
Application year: 1998
Main classification: H03L1
IPC codes: H03L1, H03B5