Voltage Controlled Oscillator

Abstract

A voltage controlled oscillator implements optimising its effective frequency versus voltage transfer function by generating and applying a frequency control 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 voltage controlled oscillator, comprising an electronic circuit arranged to produce a regenerated frequency control signal comprising at least one signal generated as a function of an initial frequency control signal, 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 voltage controlled oscillator according to claim 1, wherein the regenerated frequency control signal comprises a combination of two or more signals generated as functions of an initial frequency control signal, 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 voltage controlled oscillator according to claim 1, wherein said function comprises a sigmoid function, or said functions comprise a sigmoid function.

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

5. A voltage controlled oscillator according to claim 3, wherein said sigmoid function comprises an Arctangent (arctan) function.

6. A voltage controlled oscillator according to claim 2, wherein two or more of said functions satisfy said requirements.

7. A voltage controlled 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 voltage controlled oscillator, comprising an electronic circuit arranged to produce a regenerated frequency control signal comprising at least one signal generated as a function of an initial frequency control signal, 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 regenerated frequency control signal comprises a combination of two or more signals generated as functions of the initial frequency control signal, 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 comprises a sigmoid function, or said functions comprise 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 voltage controlled crystal oscillator according to claim 1.

16. A method of manufacturing voltage controlled oscillators which each comprise an electronic circuit arranged to produce a regenerated frequency control signal comprising at least one signal generated as a function of an initial frequency control signal, which comprises the steps of, characterizing each oscillator's frequency versus voltage transfer function; 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 optimize the regenerated control signal for the individual oscillator.

17. A method according to claim 18 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 voltage controlled oscillator, comprising an electronic circuit arranged to produce a regenerated frequency control signal comprising at least one signal generated as a function of an initial frequency control signal, 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 DRAWINGS

[0036] The invention is further described with reference to the accompanying figures in which:

[0037] FIG. 1 shows the structure of an analog VCTCXO (prior art).

[0038] FIG. 2 shows example plot of a Tanh function.

[0039] FIGS. 3, 3a, and 3b show an example of two Tanh functions (FIGS. 3 and 3a) that are formed using different values of adjustable coefficients, and the sum of these two functions (FIG. 3b).

[0040] FIG. 4 shows six individual Tanh function signals combined to form a desired shape function.

[0041] FIG. 5 shows an example of an electronic circuit arranged to generate a Tanh function signal.

[0042] FIGS. 6, 6a, and 6b illustrate the adjustments of a generated Tanh function.

[0043] FIG. 7 shows measurement data of a VCXO linearized using a technique of the present invention.

[0044] FIG. 8 shows an example structure of a VCTCXO Integrated Circuit C deploying a VCXO linearization technique of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0045] Further herein, the present invention is illustrated by an embodiment where it is used to improve the frequency stability in VCTCXO devices by reducing the compensation tilt errors.

[0046] In order to minimize a VCTCXO's frequency stability errors, the VCXO control signal is generated using the techniques of the present invention and applied to the VCXO (the latter comprises part of a VCTCXO, as shown in FIG. 1 (prior art)). Depending on the shape of the VCXO's frequency versus voltage transfer function, the VCXO control signal is formed as one, or a combination of two or more, signals generated as functions of the initial frequency control signal, with said functions satisfying the set of specific requirements mentioned above.

[0047] An example of such a function is the Hyperbolic Tangent (Tanh) function. An example plot of a Tanh function is shown in FIG. 2. Advantageously, the Hyperbolic Tangent (Tanh) function generates a smooth, rounded and bounded analog curve which is highly suitable for minimizing VCTCXOs' compensation tilt errors.

[0048] 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.

[0049] The Tanh function can be mathematically expressed in a number of different ways. The following equation lends itself to convenient manipulation:

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

[0050] 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:

[00002]$y=\begin{array}{c}\\ \\ a\\ \underset{\begin{array}{c}\mathrm{Vertical}\\ \mathrm{Gain}\end{array}}{/}\end{array}.Math.\frac{e^{2.Math.b\left(x-c\right)}-1}{\begin{array}{c}{e}^{2.Math.b\left(x-c\right)}+1\\ \underset{\mathrm{Slope}.Math..Math.\mathrm{Inflection}}{/.Math.\backslash }\end{array}}$

[0051] Furthermore, by generating and combining multiple Tanh curves it is possible to form a highly variable smooth curve that closely matches a given VCXO's transfer function curve. Plots shown in FIG. 3 present an example of two Tanh functions that are formed using different sets of a, b, and c coefficients, and the sum of these two functions. The first of the three plots shows the first Tanh function, formed with a=1, b=5, and c=1.5. The second plot shows the second Tanh function, formed with a=1.5, b=1, and c=2.5. The third plot shows the sum of the two Tanh functions.

[0052] By generating one or more Tanh signals (voltages or currents) and combining them, one can form a VCXO control signal that is suitable to reduce the VCXO's effective transfer function non-linearity and therefore the VCTCXO's compensation tilt frequency errors. The plot shown in FIG. 4 shows six individual Tanh function signals combined to form a desired shape function.

[0053] An example of an electronic circuit arranged to generate a Tanh function signal is shown in FIG. 5. In this circuit, an appropriately sloped VCXO frequency control voltage named Linear VCO is used as one of the input voltages and the Tanh function argument; the tail current of the differential amplifier QN5, QN6, defined by binary switched current mirrors comprising transistors QN0 to QN4 and switches SW1 to SW5, sets the gain of the Tanh function; and Vset Tanh Inflection voltage sets the horizontal position of the Tanh function. This circuit will generate one Tanh function signal (current Tanh IouT) that is can be combined with other, similarly generated, Tanh function signals to match the desired shape of the target VCXO transfer function.

[0054] The three graphs shown in FIGS. 6, 6a, and 6b illustrate how peak-to-peak amplitude (the function value change magnitude over the substantially higher slope region), the slope value in the substantially higher slope region, and the inflection point (horizontal position of the substantially higher slope region) of the Tanh function generated by the circuit shown in FIG. 5 can be adjusted by changing the corresponding input signals.

[0055] Possible Tanh function generating circuit implementations are not limited to the example shown in FIG. 5, and a person skilled in the art of electronic circuit design will be able to come up with alternative circuits to generate Tanh function signals.

[0056] Several instances of the circuit shown in FIG. 5, or of an alternative circuit generating a chosen suitable function signal, are likely to be used in a typical embodiment of this invention to generate and combine a number of generated function signals and form a VCXO control signal to reduce the VCTCXO's compensation tilt errors. Such VCXO linearization technique allows to further reduce the residual effective frequency versus voltage transfer function non-linearity from about 1% (current state of the art) to values that are at least an order of magnitude smaller (i.e., about 0.1% or less).

[0057] It will be appreciated by persons skilled in the art that the number of function signals generated to form a frequency control signal according to the present invention depends on the shape of a given VCXO's transfer function. 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 graph in FIG. 7 shows measured data of a VCXO linearized with a linearization circuit combining six generated Tanh function signals. The upper dotted line 1 shows the initial non-linear frequency versus voltage transfer function of the VCXO circuit. The lower dotted line 2 shows the required control voltage to be generated by the Tanh linearization circuit in order to achieve a linear effective VCXO transfer curve. The continuous curved line 3 shows the regenerated frequency control signal. As can be seen from the graph, the required and the actual control voltage curves are very close. The achieved modified effective VCXO transfer function is shown as the middle line 4 in the plot. A linear fit done to this line shows that the residual non-linearity of the resulting effective VCXO transfer curve is about 0.01%.

[0059] The VCXO effective transfer function linearization technique of the invention has a number of advantages over approximation with polynomial functions or over piecewise approximation, such as, [0060] all function generating circuits deployed in a VCTCXO device of the invention are active across the entire frequency control voltage range, thus causing no discontinuities in the resulting effective frequency versus voltage curve; [0061] the individual function curves can be adjusted in order to form a VCXO control signal to fit any or almost any individual VCXO transfer function curve.

[0062] The VCXO linearization technique of the present invention can be implemented as part of a VCTCXO Integrated Circuit (IC). An example structure of a VCTCXO IC deploying the VCXO linearization technique of the present invention is shown in FIG. 8. In this example, the temperature compensation voltage V.sub.COMP generated by the Temperature Compensation Function Generator is combined with the externally applied frequency control voltage VCO, and the combined signal is used to form a VCXO frequency control voltage as either a single, or as a combination of two or more, signals generated as Tanh functions in the Tanh Linearization Circuit block as per the present invention, and the thus formed control signal is applied to the VCXO circuit. In this example, the Tanh Linearization Circuit generates Tanh function signals; as pointed out before, other suitable functions can be used, as long as the selected functions satisfy the following requirements: [0063] the function must have a region approximating a plateau, and a substantially higher slope region; and [0064] the said two regions are connecting continuously (i.e., without discontinuities) and smoothly; and [0065] 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.

[0066] 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. 5), or by digital circuits (such as a microcontroller). In either case, the signals are generated as functions of the initial frequency control signal, which can be an analog signal (for example, a user-controlled analog tuning signal), or a digital signal (for example, a user-controlled digital tuning signal, or a digitized analog tuning signal).

[0067] 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.

[0068] The higher frequency stability of VCTCXO 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.