Temperature Compensated Crystal Oscillator

Abstract

A temperature compensated crystal oscillator implements temperature compensation by generating and applying a temperature compensation signal via a function having a plateau region and a higher slope region, where a horizontal position of the higher slope region, a slope value in the higher slope region, and a function value change magnitude over the higher slope region are adjustable.

Claims

1. A temperature compensated crystal oscillator, comprising an electronic circuit arranged to generate a temperature compensation signal comprising at least one signal generated as a function of a signal carrying temperature information, and wherein the said function satisfies the following requirements: the function has a plateau region, and a continuously and smoothly connected substantially higher slope region; and a horizontal position of the substantially higher slope region, a slope value in the substantially higher slope region, and a function value change magnitude over the substantially higher slope region are adjustable.

2. A temperature compensated crystal oscillator according to claim 1, wherein the temperature compensation signal comprises a combination of two or more signals generated as functions of a signal carrying temperature information, and wherein at least one of said functions satisfies the following requirements: the function has a plateau region, and a continuously and smoothly connected substantially higher slope region; and a horizontal position of the substantially higher slope region, a slope value in the substantially higher slope region, and a function value change magnitude over the substantially higher slope region are adjustable.

3. A temperature compensated crystal oscillator according to claim 1, wherein said function or functions comprise(s) a sigmoid function.

4. A temperature compensated crystal oscillator according to claim 3, wherein said sigmoid function comprises a Hyperbolic Tangent (Tanh) function.

5. A temperature compensated crystal oscillator according to claim 3, wherein said sigmoid function comprises an Arctangent (arctan) function.

6. A temperature compensated crystal oscillator according to claim 2, wherein two or more of said functions satisfy said requirements.

7. A temperature compensated crystal oscillator according to claim 6, wherein two or more of said functions comprise a sigmoid function.

8. An integrated circuit suitable to construct a temperature compensated crystal oscillator or for use as an auxiliary IC to improve the frequency stability of TCXO devices, comprising an electronic circuit arranged to generate a temperature compensation signal that comprises at least one signal generated as a function of a signal carrying temperature information, and wherein the said function satisfies the following requirements: the function has a plateau region, and a continuously and smoothly connected substantially higher slope region; and a horizontal position of the substantially higher slope region, a slope value in the substantially higher slope region, and a function value change magnitude over the substantially higher slope region are adjustable.

9. An integrated circuit according to claim 8, wherein the temperature compensation signal comprises a combination of two or more signals generated as functions of a signal carrying temperature information, and wherein at least one of said functions satisfies the following requirements: the function has a plateau region, and a continuously and smoothly connected substantially higher slope region; and a horizontal position of the substantially higher slope region, a slope value in the substantially higher slope region, and a function value change magnitude over the substantially higher slope region are adjustable.

10. An integrated circuit according to claim 8, wherein said function or functions comprise(s) a sigmoid function.

11. An integrated circuit according to claim 10, wherein said sigmoid function comprises a Hyperbolic Tangent (Tanh) function.

12. An integrated circuit according to claim 10, wherein said sigmoid function comprises an Arctangent (arctan) function.

13. An integrated circuit according to claim 9, wherein two or more of said functions satisfy said requirements.

14. An integrated circuit according to claim 13, wherein two or more of said functions comprise a sigmoid function.

15. An electronic apparatus comprising the temperature compensated crystal oscillator according to claim 1.

16. A method of manufacturing temperature compensated crystal oscillators which each comprise an electronic circuit arranged to generate a temperature compensation signal comprising at least one signal generated as a function of a signal carrying temperature information, which comprises the steps of, characterizing each oscillator's residual frequency versus temperature error; causing said at least one signal to comprise a plateau region, and a continuously and smoothly connected substantially higher slope region; and for each oscillator individually adjusting any one or more of a horizontal position of the substantially higher slope region, a slope value in the substantially higher slope region, and a function value change magnitude over the substantially higher slope region, to optimise said temperature compensation signal for the individual oscillator.

17. A method according to claim 16 including for each or at least some oscillators individually adjusting all of a horizontal position of the substantially higher slope region, a slope value in the substantially higher slope region, and a function value change magnitude over the substantially higher slope region, to optimise said temperature compensation signal for the individual oscillator.

18. A temperature compensated crystal oscillator, comprising an electronic circuit arranged to generate a temperature compensation signal comprising at least one signal generated as a function of a signal carrying temperature information, which function comprises a plateau region and a continuously and smoothly connected substantially higher slope region, and wherein any one or more of a horizontal position of the substantially higher slope region, a slope value in the substantially higher slope region, and a function value change magnitude over the substantially higher slope region are adjustable.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0035] The invention is further described with reference to the accompanying figures, in which:

[0036] FIG. 1 shows the structure of an analog TCXO (prior art).

[0037] FIG. 2 and FIG. 2a show examples of two real-life TCXOs' residual temperature compensation error curves.

[0038] FIG. 3 shows a plot of a Hyperbolic Tangent (Tanh) function.

[0039] FIGS. 4, 4a, and 4b present an example of two signals generated as Tanh functions of temperature (FIGS. 4 and 4a), and the result of combining the two functions to form a temperature compensation signal suitable to correct residual compensation errors in a TCXO (FIG. 4b).

[0040] FIG. 5 illustrates slope adjustment in a generated Tanh function.

[0041] FIG. 6 shows an example of an electronic circuit that generates a Tanh voltage.

[0042] FIG. 7 illustrates how the horizontal position, slope and magnitude of the Tanh function generated by the circuit shown in FIG. 6 change when the V Set Inflection voltage, RSLOPE resistor value and RGAIN resistor values are changed.

[0043] FIG. 8 shows an example structure of a TCXO IC deploying the temperature compensation technique of the invention.

[0044] FIG. 9 shows an example structure of a stand-alone auxiliary IC used to post-compensate a conventional (prior art) TCXO.

DETAILED DESCRIPTION OF EMBODIMENTS

[0045] In order to further reduce TCXOs' frequency versus temperature stability errors, additional (post-compensation) signals are generated and applied to the VCXO (the latter comprises part of a TCXO, as shown in FIG. 1 (prior art)). Depending on the shape of a given TCXO's residual frequency error curve, the post-compensation signal is generated as one, or a combination of multiple, signals, each such signal formed as a function satisfying the aforementioned requirements, such as, for example, the hyperbolic tangent Tanh function.

[0046] Advantageously, the hyperbolic tangent function Tanh generates a smooth, rounded and bounded analog curve which can be used effectively when reducing TCXOs' residual frequency errors. An example plot of a Tanh function is shown in FIG. 3.

[0047] Another advantage of the Tanh function is that Tanh function signals can be readily generated using bipolar electronic circuits, as a bipolar differential transistor pair has a Tanh response.

[0048] The Tanh function can be mathematically expressed in a number of different ways. The following equation lends itself to convenient manipulation:

[00001]$y=\frac{e^{2.Math.x}-1}{e^{2.Math.x}+1}$

[0049] By adding adjustable coefficients to the above equation, the magnitude (vertical gain), slope, and horizontal position (inflection point) of the Tanh curve can be adjusted:

[0050] Furthermore, by generating and combining multiple Tanh curves it is possible to form a highly variable smooth curve that closely matches a given TCXO's residual frequency error curve. Plots shown in FIGS. 4, 4a and 4b present an example of two signals generated as Tanh functions of temperature (FIGS. 4 and 4a), and the result of combining these two functions to form a temperature compensation signal suitable to correct residual compensation errors in a TCXO (FIG. 4b). The two Tanh signals are generated using different sets of a, b, and c coefficients. The first of the three plots shows the first Tanh function, formed with a=1, b=0.1, and c=-5. The second plot shows the second Tanh function, formed with a=-0.8, b=0.03, and c=25. The third plot shows the combination (sum) of the two Tanh functions.

[0051] By generating one or more Tanh signals (voltages or currents) and combining them, one can form a temperature compensation signal that is suitable to further reduce a TCXO's residual frequency versus temperature errors.

[0052] For each of the Tanh functions generated, the magnitude (i.e., the function value change magnitude over the substantially higher slope region) is set by adjusting the value of coefficient a, the slope is set by adjusting the value of coefficient b, and the horizontal position is set by adjusting the value of coefficient c. The plot in FIG. 5 illustrates how the slope of a Tanh function can be adjusted within a range from 0(ppm/ C.) when the coefficient b is set to zero, to (1 ppm/ C.) when b=3.

[0053] An example of an electronic circuit that generates a Tanh voltage is shown in FIG. 6. In this circuit, a signal carrying temperature information (temperature sensor output voltage V Temp Sensor) is used as one of the input voltages and the Tanh function argument. The value of the RGAIN resistor connected between emitters of transistors QP26 and QP28 determines the magnitude of the generated Tanh function (corresponds to coefficient a in the equation above); the value of the resistor RSLOPE connected between emitters of transistors QN1 and QN2 determines the slope of the generated Tanh function (corresponds to coefficient b in the equation above); another circuit inputat terminal V Set Inflectionaccepts a voltage that determines the horizontal position of the generated Tanh function curve (corresponds to coefficient c in the equation above).

[0054] The graph shown in FIG. 7 illustrates how the horizontal position, slope and magnitude of the Tanh function generated by the circuit shown in FIG. 6 change when the V Set Inflection voltage, RSLOPE resistor value and RGAIN resistor values are changed.

[0055] While FIG. 6 shows an implementation example of a circuit generating a hyperbolic tangent Tanh voltage, the Tanh function generating circuitry is not limited to the example shown in FIG. 6, and a person skilled in the art of electronic circuit design will be able to come up with alternative circuits to generate a Tanh function signal.

[0056] Several instances of the circuit shown in FIG. 6, or of an alternative circuit generating a suitably chosen function signal, are likely to be used in a typical embodiment of this invention, to generate and combine a number of signals as functions of a signal carrying temperature information and thus form a post-compensation signal to reduce the residual TCXO error. Such post-compensation technique allows to reduce the residual frequency versus temperature instability from 100 parts per billion (PPB) or higher (current state of the art) to around 5 PPB.

[0057] It will be appreciated by persons skilled in the art that the number of function signals generated to form a post-compensation signal according to the present invention depends on the shape of a given TCXO's residual frequency versus temperature error. In at least some embodiments of the invention, several (two or more) function signals will be generated. While there are a number of functions satisfying the aforementioned requirements, in at least some embodiments several signals of the same function type will be generated, with the choice of specific function type defined largely by the practicalities of generating the function in the electronics hardware deployed.

[0058] The temperature compensation technique of the present invention has a number of advantages over digital post-compensation or over piecewise analog post-compensation, such as, [0059] All function generating circuits deployed in a TCXO device of the invention are active across the intended compensation temperature range, thus causing no discontinuities in the frequency versus temperature post-compensation error curve. [0060] The operation of the TCXO device of the invention can be purely analog, i.e. no digital activity takes place during normal operation of the device and no digital noise due to clocking or digital signal transitions is generated. [0061] The individual function curves can be adjusted in order to form a compensation signal to fit any or almost any TCXO residual error curve.

[0062] The temperature compensation technique can be implemented as part of a complete TCXO integrated circuit (IC), or as a stand-alone auxiliary IC that allows to use the temperature compensation technique to improve the frequency stability of prior art TCXOs, such as, for example, the TCXO shown in FIG. 1 (prior art).

[0063] A structure example of a complete TCXO IC deploying the compensation technique of the present invention is shown in FIG. 8. In this example, the V.sub.COMP voltage generated by the Temperature Compensation Function Generator (as in FIG. 1 (prior art)) is added to a post-compensation voltage V.sub.PCOMP generated by the Post-Compensation Voltage Generator as per the present invention, and the sum of the voltages V.sub.COMP and V.sub.PCOMP is applied to compensate the VCXO and thus achieve higher stability of frequency F.sub.OUT. The Tanh Post-Compensation Voltage Generator produces the additional compensation signal V.sub.PCOMP as either a single, or as a combination of two or more, signals generated as Tanh functions of the Temperature Sensor's output signal; as pointed out before, other suitable functions can be used, depending on the semiconductor process technology used to implement the device, as long as the selected functions satisfy the following requirements: [0064] the function has a region approximating a plateau, and a substantially higher slope region;

[0065] and [0066] the said two regions are connecting continuously (i.e., without discontinuities) and smoothly; and [0067] the horizontal position of the substantially higher slope region, the slope value in the substantially higher slope region, and the function value change magnitude over the substantially higher slope region are adjustable.

[0068] It is best if the functions chosen to be used for generating the one or more signals to form the temperature compensation signal are such that the horizontal position of the substantially higher slope region, the slope value in the substantially higher slope region, and the function value change magnitude over the substantially higher slope region are adjustable independently from each other, although functions with interactions between these parameters can be also deployed, as the interactions can be accounted for when optimal parameter values are being determined.

[0069] Although the invention is illustrated herein by showing how it can be used to effect additional temperature compensation (post-compensation) in TCXO devices where primary temperature compensation is done by other techniques (such as, for example, a polynomial function), the present invention can be used, without deviating from its concept, to form the primary temperature compensation signal too, if the frequency versus temperature characteristic of the uncompensated oscillator lends itself to a close enough approximation by either a single, or by a combination of several, Tanh functions, or any other functions satisfying the aforementioned requirements.

[0070] Without deviating from the concept of the present invention, the signals generated as Tanh functions, or as any other functions satisfying the aforementioned requirements, can be generated either by analog circuits (such as, for example, the circuit shown in FIG. 6), or by digital circuits (such as a microcontroller). In either case, the signals are generated as functions of a signal carrying temperature information, which can be an analog signal (produced, for example, by an analog temperature sensor), or a digital signal (produced, for example, as a result of digitizing the output signal of a temperature sensor).

[0071] In the embodiments described above all of the horizontal position of the higher slope region, slope value in the higher slope region, and the function value change magnitude over the higher slope region are adjustable. However in other embodiments any one only or two only of the horizontal position of the higher slope region, slope value in the higher slope region, and function value change magnitude over the higher slope region may be adjustable. For example it may be sufficient in some applications for only the horizontal position and function value change magnitude to be adjustable.

[0072] As an alternative embodiment of the invention, a stand-alone auxiliary IC can be implemented and used to post-compensate prior art TCXOs. The structure of such an auxiliary IC is shown in FIG. 9. In the auxiliary IC (encompassed by the dashed line in FIG. 9), the CONTROL VOLTAGE that is usually applied to the VCO input of the PRIOR ART TCXO, is combined with the post-compensation voltage V.sub.PCOMP generated as per the techniques of the present invention, for example as a single, or a combination of a two or more, Tanh function voltages. The output of the summing block is the new control signal that is now applied to the VCO input of the PRIOR ART TCXO to post-compensate the PRIOR ART TCXO.

[0073] The high frequency stability of TCXO devices implemented using the techniques of the present invention will benefit the performance of any electronic apparatus where stable reference frequency versus temperature characteristics are required. Such apparatus include, but are not limited to, portable and stationary telecommunication equipment, high speed networking equipment, radio communication equipment, and navigation equipment.