WO2007086780A1 - Digitally controlled crystal oscillator device and method for controlling such a device - Google Patents

Digitally controlled crystal oscillator device and method for controlling such a device Download PDF

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Publication number
WO2007086780A1
WO2007086780A1 PCT/SE2006/000106 SE2006000106W WO2007086780A1 WO 2007086780 A1 WO2007086780 A1 WO 2007086780A1 SE 2006000106 W SE2006000106 W SE 2006000106W WO 2007086780 A1 WO2007086780 A1 WO 2007086780A1
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WIPO (PCT)
Prior art keywords
oscillation frequency
frequency value
capacitance
actual
digitally controlled
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PCT/SE2006/000106
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French (fr)
Inventor
Duan Mou
Pontus Svens
Jesper Engvall
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Infineon Technologies Ag
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Priority to PCT/SE2006/000106 priority Critical patent/WO2007086780A1/en
Publication of WO2007086780A1 publication Critical patent/WO2007086780A1/en

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Classifications

    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
    • H03B5/00—Generation of oscillations using amplifier with regenerative feedback from output to input
    • H03B5/30—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element being electromechanical resonator
    • H03B5/32—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element being electromechanical resonator being a piezoelectric resonator
    • H03B5/36—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element being electromechanical resonator being a piezoelectric resonator active element in amplifier being semiconductor device
    • H03B5/366—Generation of oscillations using amplifier with regenerative feedback from output to input with frequency-determining element being electromechanical resonator being a piezoelectric resonator active element in amplifier being semiconductor device and comprising means for varying the frequency by a variable voltage or current
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
    • H03B2200/00—Indexing scheme relating to details of oscillators covered by H03B
    • H03B2200/003—Circuit elements of oscillators
    • H03B2200/005—Circuit elements of oscillators including measures to switch a capacitor
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03B—GENERATION OF OSCILLATIONS, DIRECTLY OR BY FREQUENCY-CHANGING, BY CIRCUITS EMPLOYING ACTIVE ELEMENTS WHICH OPERATE IN A NON-SWITCHING MANNER; GENERATION OF NOISE BY SUCH CIRCUITS
    • H03B2201/00—Aspects of oscillators relating to varying the frequency of the oscillations
    • H03B2201/02—Varying the frequency of the oscillations by electronic means
    • H03B2201/0275—Varying the frequency of the oscillations by electronic means the means delivering several selected voltages or currents
    • H03B2201/0283—Varying the frequency of the oscillations by electronic means the means delivering several selected voltages or currents the means functioning digitally
    • H03B2201/0291—Varying the frequency of the oscillations by electronic means the means delivering several selected voltages or currents the means functioning digitally and being controlled by a processing device, e.g. a microprocessor
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
    • H03J2200/00—Indexing scheme relating to tuning resonant circuits and selecting resonant circuits
    • H03J2200/10—Tuning of a resonator by means of digitally controlled capacitor bank

Definitions

  • This invention relates to the field of crystal oscillators .
  • this invention is drawn to precise tuning control for crystal oscillators.
  • the frequency of a crystal controlled oscillator tends to remain constant with a high degree of accuracy.
  • temperature variations between the sites result in sufficient frequency mismatch between both ends of the communication channel to render the data unrecoverable.
  • process variation in the manufacture of the crystals typically results in mismatch on the same order of magnitude as that introduced by the temperature variations.
  • a hardware based frequency tracking system can be used to ensure synchronization between the ends of a communications channel.
  • Digitally controlled crystal oscillators DCXO: s
  • a common design incorporates a binary weighted array of selectable tuning capacitors, which may be used to achieve the necessary capacitances for the oscillator.
  • Each capacitor of e.g. an n-bit binary weighted array of selectable tuning capacitors has an associated switch. The switches enable selectively placing capacitors in parallel with each other.
  • the capacitor associated with the least significant bit has a value of C B .
  • the next significant bit associated with a capacitor has a value of 2C B .
  • Each next significant input signal bit is associated with a capacitor having a value twice as large as the previous input signal bit.
  • the binary tuning array can be used to converge very quickly on the appropriate capacitance value.
  • the output capacitance value is a monotonic function of the input code. Only n capacitors, switches and control lines are required for an n-bit input code in order to realize 2 n distinct values.
  • thermometer coded i.e. a fully coded
  • Each capacitor of an m-bit thermometer coded switched capacitor network has an associated switch to enable selectively placing capacitors in parallel with each other. Although the capacitors have each a selectable switch, once a capacitor is selected by increasing (or decreasing) input code, further increases (or decreases) do not switch that capacitor out (or in) again.
  • a thermometer coded switched capacitor network requires decode logic. The output capacitance is inherently a monotonic function of the input code even if the values of the switched capacitors are not identical.
  • the switched capacitor network may be made nonlinear by using different capacitance values for each switch.
  • thermometer coded switched capacitor network can be fabricated on an integrated circuit die
  • one disadvantage of the thermometer coded switched m-bit capacitor network is that a very large number of capacitors and switches are required to realize the distinct capacitance values for an input code.
  • the thermometer coded switched capacitor network may consume considerably more die area to achieve the same dynamic range as that achieved by the binary weighted switched capacitor network.
  • Patent No. 6,747,522 may be used to achieve both coarse and fine tuning of the DCXO.
  • the segmented network includes a binary weighted switched capacitor network portion for coarse tuning and a thermometer coded switched capacitor network portion for fine tuning.
  • the binary weighted switched capacitor network and the thermometer coded switched capacitor network are coupled so that their respective contributing output capacitances are in parallel.
  • segmented switched capacitor network may be advantageous in some applications, it may in other applications such as those requiring large dynamic range and high resolution result in poor tuning and/or the use of a large number of capacitors .
  • a further limitation of the above hard-wired capacitor network based designs is that they severely limit the flexibility of choosing crystal suppliers since they require the use of a crystal with specified crystal parameters such as e.g. motional capacitance and nominal load capacitance.
  • DCXO digitally controlled crystal oscillator
  • a DCXO device comprising a crystal resonator and a binary weighted switched capacitor network with a switchable capacitance.
  • a microprocessor is provided for controlling the switched capacitor network to obtain a desired oscillation frequency of the DCXO device.
  • the microprocessor receives (i) an actual or current oscillation frequency value of the DCXO device and (ii) a desired oscillation frequency value, e.g. from a radio base station.
  • the processor is further arranged to calculate a capacitance or load capacitance difference required for tuning the DCXO device from the ⁇ actual oscillation frequency value to the desired oscillation frequency value based on static and motional capacitances of the crystal resonator, on the actual or current capacitance, i.e. load capacitance, of the DCXO device, and on the actual and desired oscillation frequency values, and to switch the switched capacitor network in response to the calculated capacitance difference.
  • a binary weighted switched capacitor network may be used also for applications requiring a large dynamic tuning range.
  • the nonlinearity between the oscillator frequency and the capacitance is taken care of in the software .
  • the capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value is calculated as:
  • ⁇ f is the difference between the desired oscillation frequency value and the actual oscillation frequency value
  • F nom is the nominal oscillator frequency
  • C 0 is the static capacitance
  • C 1 is the actual capacitance or load capacitance
  • C n is the motional capacitance.
  • the invention is particularly suitable for compensation for frequency mismatch due to temperature variations and/or due to process variation in the manufacture of the crystals.
  • the present invention provides for tuning of a DCXO device even if the crystal is exchanged for another crystal of another type having another oscillation frequency-capacitance dependency.
  • a method for controlling a DCXO device which comprises a crystal resonator, a binary weighted switched capacitor network with a switchable capacitance, and a microprocessor.
  • the method is performed in the microprocessor and comprises the steps of: receiving an actual oscillation frequency value of the DCXO device, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value based on static and motional capacitances of the crystal resonator, on the actual capacitance of the DCXO device, and on the actual and desired oscillation frequency values, and switching the binary weighted switched capacitor network in response to the calculated capacitance difference.
  • a computer program product loadable into the internal memory of a computer, and comprising software code portions for carrying out the method as described above when the computer program product is run on the computer.
  • Fig. 1 is a schematic block diagram of a DCXO device according to an embodiment of the invention.
  • An embodiment of a DCXO device comprises a DCXO 11 and a microprocessor or computer 12.
  • the DCXO includes a crystal resonator 13 and a binary weighted switched capacitor network 14 with a switchable capacitance known per se in the art.
  • the DCXO 11 may further comprise active and passive components such as an amplifier, a current source and resistors and capacitors known per se in the art (not explicitly illustrated in Fig. 1).
  • the microprocessor 12 is provided for controlling the binary weighted switched capacitor network 14 to obtain a desired oscillation frequency of the DCXO device.
  • the microprocessor 12 comprises a first input connected to the output of the DCXO device via a feedback circuit 15 to receive an actual oscillation frequency value 16 of the DCXO device, and a second input connected to receive a desired oscillation frequency value 17, e.g. from a communication unit such as a radio base station.
  • the microprocessor 12 comprises one or several outputs for outputting control signals to the binary weighted switched capacitor network 14.
  • the microprocessor 12 is arranged to calculate a capacitance or load capacitance difference ⁇ C required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value based on static C 0 and motional C m capacitances of the crystal resonator, on the actual capacitance C 1 of the DCXO device, and on the actual and desired oscillation frequency values. Finally, the microprocessor 12 switches the binary weighted switched capacitor network in response to the calculated capacitance difference by means of outputting the control signals.
  • the DCXO device is efficiently tuned to the desired oscillation frequency.
  • the capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value is calculated as :
  • AC 1 2( ⁇ f/F nom )(C 0 + C 1 ) /C 1 .
  • ⁇ f is the difference between the desired oscillation frequency value and the actual oscillation frequency value
  • F nom is the nominal oscillator frequency
  • C 0 is the static capacitance
  • C 1 is the actual capacitance
  • C m is the motional capacitance
  • the microprocessor 12 is provided for repeating the steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value, and switching the binary weighted switched capacitor network in response to the calculated capacitance difference. This procedure may be performed continuously during use, at regular times, or when the frequency difference ⁇ f has become larger than a given threshold value.
  • the tuning capabilities of the inventive DCXO device may be used for temperature compensation and/or for tuning a low quality resonator with large frequency tolerance during manufacture.
  • the inventive DCXO device may be capable of tuning the oscillator frequency at least ⁇ 20 ppm, more preferably at least ⁇ 40 ppm, and most preferably at least ⁇ 60 ppm with a tuning resolution of at least ⁇ 1 ppm, more preferably at least ⁇ 0.1 ppm, and most preferably at least ⁇ 0.01 ppm.
  • the crystal resonator 13 which has a given oscillation frequency-capacitance dependency, is exchangeable for another crystal resonator of another type having another oscillation frequency-capacitance dependency.
  • the microprocessor 12 repeats the steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the digitally controlled crystal oscillator device from the actual oscillation frequency value to the desired oscillation frequency value, and switching the binary weighted switched capacitor network in response to the calculated capacitance difference.
  • the invention is implemented in software and the oscillation frequency is tuned using the nonlinear dependence between oscillation frequency and capacitance.
  • the segmented switched capacitor network may be fabricated on the same integrated circuit as other parts of the DCXO device.
  • the DCXO may be based on any kind of oscillator architecture such as e.g. Pierce, Colpitts, Clapp, Butler, Modified Butler, and Gate oscillator circuits.

Abstract

A DCXO device comprises a crystal resonator (13), and a binary weighted switched capacitor network (14) with a switchable capacitance. A microprocessor (12) is provided for controlling the switched capacitor network to obtain a desired oscillation frequency of the DCXO device. The microprocessor receives (i) an actual oscillation frequency value (16) of the DCXO device and (ii) a desired oscillation frequency value (17), e.g. from a radio base station. The processor is further arranged to calculate a capacitance difference (ΔC1) required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value based on static (C0) and motional (Cm) capacitances of the crystal resonator, on the capacitance (C1) of the DCXO device, and on the actual and desired oscillation frequency values, and to switch the switched capacitor network in response to the calculated capacitance difference.

Description

DIGITALLY CONTROLLED CRYSTAL OSCILLATOR DEVICE AND METHOD FOR CONTROLLING SUCH A DEVICE
TECHNICAL FIELD OF THE INVENTION
This invention relates to the field of crystal oscillators . In particular, this invention is drawn to precise tuning control for crystal oscillators.
DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION
Many communication systems require precise timing to permit synchronization of a receiver clock signal with a transmitter clock signal. Sophisticated communication algorithms require precise synchronization between the near and far end terminals for maximum data throughput on a communication channel.
The frequency of a crystal controlled oscillator tends to remain constant with a high degree of accuracy. However, even if substantially identical crystals with the same vibrational characteristics are used at the transmitter and receiver, temperature variations between the sites result in sufficient frequency mismatch between both ends of the communication channel to render the data unrecoverable. Further, process variation in the manufacture of the crystals typically results in mismatch on the same order of magnitude as that introduced by the temperature variations.
A hardware based frequency tracking system can be used to ensure synchronization between the ends of a communications channel. Digitally controlled crystal oscillators (DCXO: s) are sometimes used as components in a frequency tracking system. A common design incorporates a binary weighted array of selectable tuning capacitors, which may be used to achieve the necessary capacitances for the oscillator. Each capacitor of e.g. an n-bit binary weighted array of selectable tuning capacitors has an associated switch. The switches enable selectively placing capacitors in parallel with each other.
As a result of the binary weighting of capacitor values, no decode circuitry is required for the input signals that control the switches. The input signals serve as switch controls for their associated switch. The capacitor associated with the least significant bit has a value of CB. The next significant bit associated with a capacitor has a value of 2CB. Each next significant input signal bit is associated with a capacitor having a value twice as large as the previous input signal bit. Thus for an n-bit array, the capacitor associated with the i'th input has a value 21Cg where i = {0, . . . n-1} where CB is the capacitor value associated with the input signal of the least significant bit.
The binary tuning array can be used to converge very quickly on the appropriate capacitance value. The output capacitance value is a monotonic function of the input code. Only n capacitors, switches and control lines are required for an n-bit input code in order to realize 2n distinct values.
One disadvantage of this architecture, however, is that it is less suitable if a large dynamic range, such as e.g. ± 60 ppm, of the tuning frequency is required. The nonlinear relationship between the oscillation frequency and the capacitance puts restrictions on the dynamic range. The use of the architecture for applications, where a large dynamic range is required, will simply result in poor tuning.
Another design utilizes a thermometer coded (i.e. a fully coded) approach. Each capacitor of an m-bit thermometer coded switched capacitor network, has an associated switch to enable selectively placing capacitors in parallel with each other. Although the capacitors have each a selectable switch, once a capacitor is selected by increasing (or decreasing) input code, further increases (or decreases) do not switch that capacitor out (or in) again. A thermometer coded switched capacitor network requires decode logic. The output capacitance is inherently a monotonic function of the input code even if the values of the switched capacitors are not identical. The switched capacitor network may be made nonlinear by using different capacitance values for each switch.
Although a thermometer coded switched capacitor network can be fabricated on an integrated circuit die, one disadvantage of the thermometer coded switched m-bit capacitor network is that a very large number of capacitors and switches are required to realize the distinct capacitance values for an input code. Thus the thermometer coded switched capacitor network may consume considerably more die area to achieve the same dynamic range as that achieved by the binary weighted switched capacitor network.
A segmented switched capacitor network as described in U.S.
Patent No. 6,747,522 may be used to achieve both coarse and fine tuning of the DCXO. The segmented network includes a binary weighted switched capacitor network portion for coarse tuning and a thermometer coded switched capacitor network portion for fine tuning. The binary weighted switched capacitor network and the thermometer coded switched capacitor network are coupled so that their respective contributing output capacitances are in parallel.
While such segmented switched capacitor network may be advantageous in some applications, it may in other applications such as those requiring large dynamic range and high resolution result in poor tuning and/or the use of a large number of capacitors .
A further limitation of the above hard-wired capacitor network based designs is that they severely limit the flexibility of choosing crystal suppliers since they require the use of a crystal with specified crystal parameters such as e.g. motional capacitance and nominal load capacitance.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a digitally controlled crystal oscillator (DCXO) device, which lacks the above shortcomings and drawbacks.
It is in this respect a particular object of the invention to provide such a device, which may be tuned over a large dynamic range and with a high resolution.
It is a further object of the invention to provide such a device, which does not require a large number of capacitors.
It is a yet further object of the invention to provide such a device, in which the crystal may be replaced by another crystal having different crystal parameters.
It is a still a further object of the invention -to provide a method for controlling a digitally controlled crystal oscillator device, which fulfills any of the objects given above.
These objects are according to the present invention attained by electronic devices and methods as claimed in the appended patent claims.
According to an aspect of the invention there is provided a DCXO device comprising a crystal resonator and a binary weighted switched capacitor network with a switchable capacitance. A microprocessor is provided for controlling the switched capacitor network to obtain a desired oscillation frequency of the DCXO device. The microprocessor receives (i) an actual or current oscillation frequency value of the DCXO device and (ii) a desired oscillation frequency value, e.g. from a radio base station. The processor is further arranged to calculate a capacitance or load capacitance difference required for tuning the DCXO device from the ■ actual oscillation frequency value to the desired oscillation frequency value based on static and motional capacitances of the crystal resonator, on the actual or current capacitance, i.e. load capacitance, of the DCXO device, and on the actual and desired oscillation frequency values, and to switch the switched capacitor network in response to the calculated capacitance difference.
By implementing the control of the tuning of the DCXO device in software of a microprocessor a binary weighted switched capacitor network may be used also for applications requiring a large dynamic tuning range. The nonlinearity between the oscillator frequency and the capacitance is taken care of in the software .
Preferably, the capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value is calculated as:
AC1 = 2 (Δf /F110J (C0 + C1) /Cn
where Δf is the difference between the desired oscillation frequency value and the actual oscillation frequency value, Fnom is the nominal oscillator frequency, C0 is the static capacitance, C1 is the actual capacitance or load capacitance, and Cn. is the motional capacitance. The invention is particularly suitable for compensation for frequency mismatch due to temperature variations and/or due to process variation in the manufacture of the crystals.
Furthermore, the present invention provides for tuning of a DCXO device even if the crystal is exchanged for another crystal of another type having another oscillation frequency-capacitance dependency.
According to a further aspect of the invention there is provided a method for controlling a DCXO device, which comprises a crystal resonator, a binary weighted switched capacitor network with a switchable capacitance, and a microprocessor. The method is performed in the microprocessor and comprises the steps of: receiving an actual oscillation frequency value of the DCXO device, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value based on static and motional capacitances of the crystal resonator, on the actual capacitance of the DCXO device, and on the actual and desired oscillation frequency values, and switching the binary weighted switched capacitor network in response to the calculated capacitance difference.
According to yet a further aspect of the invention there is provided a computer program product loadable into the internal memory of a computer, and comprising software code portions for carrying out the method as described above when the computer program product is run on the computer.
Further characteristics of the invention and advantages thereof will be evident from the detailed description of embodiments of the present invention given hereinafter and the accompanying Fig. 1, which is given by way of illustration only, and is thus not limitative of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic block diagram of a DCXO device according to an embodiment of the invention.
DETAILED DESCRIPTION OF EMBODIMENTS
An embodiment of a DCXO device comprises a DCXO 11 and a microprocessor or computer 12. The DCXO includes a crystal resonator 13 and a binary weighted switched capacitor network 14 with a switchable capacitance known per se in the art. The DCXO 11 may further comprise active and passive components such as an amplifier, a current source and resistors and capacitors known per se in the art (not explicitly illustrated in Fig. 1).
For instance, the above cited U.S. Patent No. 6,747,522 as well as U.S. Patent No. 6,798,301 B2 describe the use of binary weighted switched capacitor networks for crystal oscillators . Various oscillator designs are disclosed. The contents of the above cited patents as well as of references therein are hereby incorporated by reference.
The microprocessor 12 is provided for controlling the binary weighted switched capacitor network 14 to obtain a desired oscillation frequency of the DCXO device. The microprocessor 12 comprises a first input connected to the output of the DCXO device via a feedback circuit 15 to receive an actual oscillation frequency value 16 of the DCXO device, and a second input connected to receive a desired oscillation frequency value 17, e.g. from a communication unit such as a radio base station. The microprocessor 12 comprises one or several outputs for outputting control signals to the binary weighted switched capacitor network 14. To this end the microprocessor 12 is arranged to calculate a capacitance or load capacitance difference ΔC required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value based on static C0 and motional Cm capacitances of the crystal resonator, on the actual capacitance C1 of the DCXO device, and on the actual and desired oscillation frequency values. Finally, the microprocessor 12 switches the binary weighted switched capacitor network in response to the calculated capacitance difference by means of outputting the control signals. Hereby, the DCXO device is efficiently tuned to the desired oscillation frequency.
Preferably, the capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value is calculated as :
AC1 = 2(Δf/Fnom)(C0 + C1) /C1.
where Δf is the difference between the desired oscillation frequency value and the actual oscillation frequency value, Fnom is the nominal oscillator frequency, C0 is the static capacitance, C1 is the actual capacitance, and Cm is the motional capacitance.
In one embodiment the microprocessor 12 is provided for repeating the steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the DCXO device from the actual oscillation frequency value to the desired oscillation frequency value, and switching the binary weighted switched capacitor network in response to the calculated capacitance difference. This procedure may be performed continuously during use, at regular times, or when the frequency difference Δf has become larger than a given threshold value.
The tuning capabilities of the inventive DCXO device may be used for temperature compensation and/or for tuning a low quality resonator with large frequency tolerance during manufacture. For instance, the inventive DCXO device may be capable of tuning the oscillator frequency at least ± 20 ppm, more preferably at least ± 40 ppm, and most preferably at least ± 60 ppm with a tuning resolution of at least ± 1 ppm, more preferably at least ± 0.1 ppm, and most preferably at least ± 0.01 ppm.
In another embodiment the crystal resonator 13, which has a given oscillation frequency-capacitance dependency, is exchangeable for another crystal resonator of another type having another oscillation frequency-capacitance dependency. After such change the microprocessor 12 repeats the steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the digitally controlled crystal oscillator device from the actual oscillation frequency value to the desired oscillation frequency value, and switching the binary weighted switched capacitor network in response to the calculated capacitance difference.
The invention is implemented in software and the oscillation frequency is tuned using the nonlinear dependence between oscillation frequency and capacitance.
An integrated circuit DCXO device and methods for tuning the DCXO are provided. The segmented switched capacitor network may be fabricated on the same integrated circuit as other parts of the DCXO device. The DCXO may be based on any kind of oscillator architecture such as e.g. Pierce, Colpitts, Clapp, Butler, Modified Butler, and Gate oscillator circuits.
In the preceding detailed description, the invention is described with reference to specific exemplary embodiments thereof . Various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

Claims

1. A digitally controlled crystal oscillator device comprising:
- a crystal resonator (13) and
- a binary weighted switched capacitor network (14) with a switchable capacitance, characterized in that
- said digitally controlled crystal oscillator device comprises a microprocessor (12) for controlling said binary weighted switched capacitor network to obtain a desired oscillation frequency of said digitally controlled crystal oscillator device, said microprocessor being provided for:
- receiving an actual oscillation frequency value (16) of said digitally controlled crystal oscillator device,
- receiving a desired oscillation frequency value (17),
- calculating a capacitance difference (AC1) required for tuning the digitally controlled crystal oscillator device from said actual oscillation frequency value to said desired oscillation frequency value based on static (C0) and motional (Cm) capacitances of the crystal resonator, on the actual capacitance (C1) of the digitally controlled crystal oscillator device, and on said actual and desired oscillation frequency values, and
- switching said binary weighted switched capacitor network in response to said calculated capacitance difference.
2. The device of claim 1, wherein said crystal resonator (13) is a low quality resonator having a large tolerance on the frequency of the crystal resonator.
3. The device of claim 1 or 2, wherein said microprocessor (12) is provided for repeating the steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the digitally controlled crystal oscillator device from said actual oscillation frequency value to said desired oscillation frequency value, and switching said binary weighted switched capacitor network in response to said calculated capacitance difference.
4. The device of any of claims 1-3, wherein
- said crystal resonator (13), which has a given oscillation frequency-capacitance dependency, is exchangeable for another crystal resonator of another type having another oscillation frequency-capacitance dependency, and
- said microprocessor (12) is provided for repeating the steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the digitally controlled crystal oscillator device from said actual oscillation frequency value to said desired oscillation frequency value, and switching said binary weighted switched capacitor network in response to said calculated capacitance difference if said crystal resonator is exchanged for said another crystal resonator of another type.
5. The device of any of claims 1-4, wherein said device has a tuning range of at least ± 20 ppm, more preferably at least ± 40 ppm, and most preferably at least ± 60 ppm.
6. The device of any of claims 1-5, wherein said device has a tuning resolution of at least ± 1 ppm, more preferably at least ± 0.1 ppm, and most preferably at least ± 0.01 ppm.
7. A method for controlling a digitally controlled crystal oscillator device, which comprises a crystal resonator (13) and a binary weighted switched capacitor network (14) with a switchable capacitance, the method being characterized by the steps of :
- receiving an actual oscillation frequency value (16) of said digitally controlled crystal oscillator device,
- receiving a desired oscillation frequency value (17),
- calculating a capacitance, difference (AC1) required for tuning the digitally controlled crystal oscillator device from said actual oscillation frequency value to said desired oscillation frequency value based on static (C0) and motional (Cm) capacitances of the crystal resonator, on the actual capacitance (C1) of the digitally controlled crystal oscillator device, and on said actual and desired oscillation frequency values, and
- switching said binary weighted switched capacitor network in response to said calculated capacitance difference.
8. The method of claim 7, wherein the steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the digitally controlled crystal oscillator device from said actual oscillation frequency value to said desired oscillation frequency value, and switching said binary weighted switched capacitor network in response to said calculated capacitance difference are repeated.
9. The method of claim 7, wherein
- said crystal resonator (13), which has a given oscillation frequency-capacitance dependency, is exchanged for another crystal resonator of another type having another oscillation frequency-capacitance dependency, and
- said steps of receiving an actual oscillation frequency value, receiving a desired oscillation frequency value, calculating a capacitance difference required for tuning the digitally controlled crystal oscillator device from said actual oscillation frequency value to said desired oscillation frequency value, and switching said binary weighted switched capacitor network in response to said calculated capacitance difference are repeated after that said crystal resonator has been exchanged for said another crystal resonator of another type.
10. The method of any of claims 7-9, wherein said method is performed in a microprocessor (12).
11. A computer program product loadable into the internal memory of a computer, and comprising software code portions for carrying out the method as claimed in any of claims 7-9 when said computer program product is run on said computer.
PCT/SE2006/000106 2006-01-25 2006-01-25 Digitally controlled crystal oscillator device and method for controlling such a device WO2007086780A1 (en)

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Publication number Priority date Publication date Assignee Title
JP2016106481A (en) * 2009-05-07 2016-06-16 クゥアルコム・インコーポレイテッドＱｕａｌｃｏｍｍ Ｉｎｃｏｒｐｏｒａｔｅｄ Overlapping, two-segment capacitor bank for vco frequency tuning

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GB2320628A (en) * 1996-12-18 1998-06-24 Nec Technologies Hybrid reference frequency correction system
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US6172576B1 (en) * 1998-04-02 2001-01-09 Seiko Epson Corporation Capacitor array unit connected to oscillation circuit with a piezoelectric reasonator, capacitor array unit controller oscillation frequency adjusting system and oscillation frequency adjusting method
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JP2016106481A (en) * 2009-05-07 2016-06-16 クゥアルコム・インコーポレイテッドＱｕａｌｃｏｍｍ Ｉｎｃｏｒｐｏｒａｔｅｄ Overlapping, two-segment capacitor bank for vco frequency tuning

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