Frequency reference device

Abstract

A local oscillator device for use with an application system is disclosed, wherein in a lock mode the local oscillator device provides a frequency reference signal and receives a frequency error value from the system, and wherein in a holdover mode the local oscillator device provides a frequency reference signal and an estimated frequency error value to the application system.

Description

Technical Field

[0001] This invention relates to equipment and techniques for obtaining accurate frequency references, clocks, and timing signals in networks deployed in various applications such as telecommunications, power grids, industrial automation, test and measurement, audio and video systems, and financial transactions. Background of the Invention

[0002] In modern networked systems, multiple clock signal sources are often present. These clock signal sources need to be synchronized. Several synchronization protocols are used to synchronize clocks in networked systems. For example, IEEE 1588 (also known as Precision Time Protocol (PTP)) is a standard for synchronizing clocks in networked systems. PTP is a master-slave protocol in which a single master clock provides a synchronization reference to all slave clocks in the network. PTP can achieve sub-microsecond synchronization accuracy. PTP works by exchanging timestamp packets between the master and slave clocks. These timestamp packets contain information about the time they were sent and received. The slave clocks use this information to calculate their offset relative to the master clock and adjust their own clocks accordingly.

[0003] When frequency reference devices, such as oven-controlled crystal oscillators (OCXOs), are used as frequency references from a clock, they must possess hardware and software for receiving and processing information received from the network, calculating offsets, and adjusting their frequency accordingly. This functionality requires hardware such as high-resolution digital-to-analog converters (DACs) and voltage-controlled (“pulled”) oscillators, which increases the complexity, size, and cost of the frequency reference device.

[0004] To ensure that the clock signal generated by the frequency reference device remains sufficiently accurate in "hold" mode—that is, when the network's master clock (such as a GPS clock, for example) is lost—additional requirements and needs for additional hardware and software resources arise. Summary of the Invention

[0005] As used in this specification and claims, the term "comprising" means "consisting at least in part of". When interpreting each statement in this specification and claims that includes the term "comprising", features other than those features referred to by that term may also exist. Related terms such as "comprise" and "comprises" should be interpreted in the same manner.

[0006] This invention provides a novel and inventive frequency reference device and method that, when an application system loses access to a higher-level reference signal, obtains a more accurate frequency reference and timing signal and achieves a longer hold period in hold mode.

[0007] The frequency reference device and method of the present invention can be used in a variety of networked systems, respectively as a local oscillator device and frequency and / or time reference in hold mode, and as a means of achieving accurate timing and frequency reference in hold mode.

[0008] In locked mode, when the application system has access to and utilizes higher-level clock signals, the application system uses one or more clock signals from the higher-level clock signals as frequency reference signals to evaluate the frequency of the local oscillator, calculates the frequency offset (error) value, and transmits the error value to the local oscillator device.

[0009] Over time, the local oscillator device of the present invention uses error information provided by the application system to characterize, model, or learn certain parameters related to its frequency stability. In some embodiments, this characterization, modeling, or learning is performed using both error information provided by the application system and data from one or more environmental sensors.

[0010] In hold mode, when the application system loses access to a higher-level clock signal, it switches to using the output signal of the local oscillator as its hold reference clock. In this mode, in addition to providing the clock signal, the local oscillator device of this invention also provides the application system with a frequency error estimate calculated within the local oscillator device based on the aforementioned characterization, modeling, or learning of its frequency stability parameters. This allows the application system to maintain its timing and frequency reference more accurately and for a longer hold period in hold mode compared to what might be achieved without the error information provided by the local oscillator device.

[0011] In some embodiments, to ensure acceptable performance levels "out of the box," the local oscillator device may use a model of characterization parameters or frequency stability parameters pre-loaded during its production process. Once deployed, the device can be configured to further refine the pre-loaded characterization or model using error information provided by the application system and optionally data from one or more environmental sensors.

[0012] On the other hand, the present invention provides a novel and inventive technique for maximizing hold-up capability / hold-up duration in a networked system using a local oscillator device. This is achieved by generating and evaluating parameters or a set of parameters characterizing the hold-up capability (“hold-up health”) of the local oscillator device.

[0013] In a broader sense, the present invention may also include portions, elements, and features individually or jointly mentioned or indicated in the description of this application, as well as any or all combinations of any two or more of the said portions, elements, or features, and where specific components mentioned herein have known equivalents in the field to which the invention relates are considered to be incorporated herein as if they were set forth separately. Attached Figure Description

[0014] The invention will now be further described with reference to the accompanying drawings, in which:

[0015] Figure 1 An example of an application system having the local oscillator device of the present invention is shown.

[0016] Figure 2 An example of the structure of a local oscillator device is shown.

[0017] Figure 3a , Figure 3b and Figure 3c Test data related to the performance testing of the local oscillator device and method of the present invention are presented. Detailed Implementation

[0018] Without limiting the scope of the invention, the invention is described herein by way of the following detailed description of the apparatus and methods thereof.

[0019] In the following description, specific details are set forth to provide a thorough understanding of the embodiments. However, those skilled in the art will understand that these embodiments can be practiced without these specific details. For example, modules (including modules in the form of software modules, functions, circuits, etc.) may be shown in block diagrams to avoid unnecessarily obscuring the embodiments. In other instances, well-known modules, structures, and techniques may not be shown in detail so as not to obscure the embodiments.

[0020] In one aspect, a frequency reference device is provided, which is intended or configured for use with an application system including an oscillator that generates a frequency reference signal and a digital communication interface, wherein the frequency reference device can be arranged or configured to operate in either of the following two modes:

[0021] In the first mode, the frequency reference device provides a frequency reference signal to the application system and receives information from the application system via a digital communication interface, with or without timestamp data, related to the frequency offset / error of the frequency reference signal.

[0022] as well as

[0023] In the second mode, the frequency reference device provides a frequency reference signal to the application system and provides an estimate of the frequency offset / error of the frequency reference signal via a digital communication interface.

[0024] Figure 1 An arrangement is shown in which the local oscillator device 1 of the present invention is used to provide a reference signal to an application system 2. The application system 2 may include a system CPU 3 running 1588 firmware, a PLL 4 for synthesizing an output frequency / system clock output 5, and a digital communication interface 6 for facilitating the exchange of digital data and control signals between the application system 2 and the local oscillator device 1.

[0025] Application system 2 and local oscillator device 1 can operate in one of two operating modes: lock-in mode and hold mode.

[0026] In locked mode, when application system 2 has access to and utilizes higher-level reference signal 7, application system 2 evaluates the frequency of frequency reference signal 8 provided by local oscillator device 1 (using one or more reference signals from higher-level reference signal 7 as frequency reference), calculates frequency and / or time error values, and sends the calculated error data to local oscillator device 1 via digital communication interface 6.

[0027] Over time, the local oscillator device 1 uses error information received from the application system 2, and in some embodiments, uses data from one or more sensors (such as, for example, temperature sensors, power level sensors, etc.) to characterize, model, or learn parameters of its frequency stability.

[0028] In hold mode, when application system 2 loses access to the higher-level reference signal 7, application system 2 switches to using the output signal (frequency reference signal 8) of local oscillator device 1 as its hold reference signal. In this mode, in addition to providing the frequency reference signal 8, local oscillator device 1 also transmits an estimate of the frequency offset or frequency error calculated within local oscillator device 1 based on the aforementioned characterization, modeling, or learning via digital communication interface 6. This allows application system 2 to maintain its timing and frequency reference more accurately and for a longer hold period in hold mode compared to what might be achieved without the error information transmitted by local oscillator device 1.

[0029] Local oscillator devices may include temperature-controlled (quartz crystal, silicon or other suitable material) oscillators, temperature-compensated (quartz crystal, silicon or other suitable material) oscillators, (quartz crystal, silicon or other suitable material) microelectromechanical systems (MEMS) oscillators, or any other oscillator device with a sufficiently stable output signal frequency.

[0030] Digital communication interfaces can be implemented as Serial Peripheral Interface (SPI) or Inter-Integrated Circuit (I2C) interfaces. 2 C) The interface or any other suitable digital interface form.

[0031] In locked mode, the higher-level reference signals used by the application system may include one or more of the following: Global Navigation Satellite System (GNSS) signals, Synchronous Ethernet (SyncE) signals, Pulses per Second (PPS) signals, Precision Time Protocol (PTP) signals, etc.

[0032] On the other hand, the present invention provides a method for obtaining accurate timing and frequency references in hold mode in an application system and thus obtaining a longer hold period, the method comprising the following steps:

[0033] - Configure the application system to generate offset / error information about the output frequency of the local oscillator device in the first (normal or "locked") operating mode, and send the offset / error information, with or without timestamp data, to the local oscillator device via a digital interface.

[0034] - The local oscillator device, operating in the first (normal or "locked") operating mode, is configured to receive the offset / error and timestamp information sent by the application system.

[0035] - Arrange the local oscillator device to characterize, model, or learn the dependence of the local oscillator device's output signal frequency on time and environmental conditions using received offset / error and timestamp information, as well as data optionally from one or more environmental / parameter sensors.

[0036] - The local oscillator device is configured to generate an estimate of its output frequency offset / error in the second (“hold”) operating mode, and the estimated offset / error information is sent to the application system via a digital interface.

[0037] The application system uses a digital communication interface to send and receive error data as needed in each operating mode, and also controls / switches the operating mode of the local oscillator device between the first and second modes.

[0038] In hold mode, the estimated offset / error information provided to the application system by the local oscillator device may include one or more of the following error components, either in combination or reported individually: frequency error caused by varying ambient temperature, frequency error dependent on the rate or direction of change of ambient temperature (also known as "hysteresis"), frequency error caused by time-related aging effects in the local oscillator elements, frequency error caused by varying power supply voltage, frequency error caused by varying output load conditions, frequency error caused by varying mechanical acceleration conditions, frequency error caused by varying pressure conditions, etc.

[0039] To obtain data related to the aforementioned changing conditions, the local oscillator device can utilize signals from one or more sensors, such as, for example, temperature sensors, power level sensors, output current sensors, acceleration sensors, and pressure sensors.

[0040] In some embodiments, the data sent to and received by the local oscillator in the first (“locked”) mode may include timestamp data, which can be useful for characterizing, modeling or learning the distribution of the aging effect of the local oscillator (i.e., the frequency dependence on time, also known as long-term frequency stability).

[0041] Figure 2 An example structure of a local oscillator device is shown. The local oscillator device includes an oscillator circuit 1, a central processing unit (CPU) 2, a digital communication bus 3, and a group 4 of one or more environmental sensors S1, S2, ..., Sn.

[0042] Oscillator circuit 1 includes a resonator 5, an oscillator sustaining amplifier circuit 6, and an output buffer 7. The oscillator circuit generates power via the output terminal F. R A frequency reference signal is provided to the application system. Advantageously, the oscillator circuit 1 in this embodiment is implemented as a free-running, non-pullable oscillator; this allows the use of a resonator with low pullability and eliminates the need for voltage control circuitry in the oscillator circuit—both factors result in lower complexity, lower cost, and smaller size, and allow for the implementation of the frequency reference signal F compared to other oscillator options such as voltage-controlled oscillators or temperature-compensated oscillators. R Higher frequency stability and lower noise level.

[0043] exist Figure 2 In the illustrated embodiment, resonator 5 is a quartz crystal resonator. In other embodiments, resonator 5 may be implemented as one of several other resonator types, such as, for example, a silicon MEMS resonator, a ceramic resonator, or a quartz MEMS resonator, such as XMEMS. ® Resonator.

[0044] CPU 2 can be any suitable microcontroller capable of performing the following: software required to serve communication via communication interface bus 3; and software required to characterize, model, or learn the dependence of the output frequency on environmental and time factors in the first (locked) mode and to generate estimates of frequency and time errors in the second (hold) mode.

[0045] Sensors S1, S2, ..., Sn may include one or more of the following: temperature sensor, power level sensor, output current sensor, acceleration sensor, pressure sensor, etc.

[0046] The local oscillator device can be implemented as a temperature-stabilized device, such as a thermostatically controlled oscillator, in which case at least some of the frequency-determining components (such as resonator 5, sustaining amplifier circuit 6, etc.) will be placed in a thermally stable environment (thermostatic bath).

[0047] Figure 3a , Figure 3b and Figure 3c Test data related to the performance evaluation of application systems using the frequency reference device of the present invention as their local oscillator device and the frequency and time correction method of the present invention are presented. The local oscillator device used in the test is a temperature-controlled crystal oscillator (OCXO). Figure 3a Data related to the accuracy of the OCXO aging model is shown. Time, in seconds, is plotted along the horizontal axis, showing that the test duration was just over 30,000 seconds. During this period, the OCXO operated with the simulated application system in two modes: locked mode and hold mode. In locked mode, the application system received GPS-based PPS signals, while in hold mode, PPS signals were unavailable to the application system. Figure 3a In the diagram, curve 1 indicates the periods of locked mode (curve 1 is high) and held mode (curve 1 is low). Curve 2 presents the real-time OCXO frequency measurement, and curve 3 shows the frequency value predicted by the aging model. The frequency values ​​are expressed as fractional frequency deviations relative to the OCXO's nominal frequency, in parts per billion (ppb), and are displayed along the vertical axis.

[0048] Figure 3bThe effectiveness of the device and calibration method of the present invention is illustrated. It shows the maximum time interval error (MTIE) value observed in an application system using an OCXO as a frequency reference in two scenarios: (1) no frequency error information from the OCXO is received in hold mode (curve 4); and (2) frequency error information from the OCXO is received and utilized in hold mode (curve 5). The MTIE value corresponding to curve 4 is plotted in nanoseconds (ns) along the left vertical axis, while the MTIE value corresponding to curve 5 is also plotted in nanoseconds (ns) along the right vertical axis. Figure 3b As can be seen, when the calibration method is applied, the application system benefits from a significant improvement in timing accuracy, reducing the MTIE to 64 ns compared to 2,664 ns without calibration.

[0049] Figure 3c A graph showing the PLL output frequency of the application system relative to time is presented. The PLL output frequency value is expressed as a fractional frequency deviation relative to the PLL's nominal output frequency, in parts per billion (ppb), and is plotted along the vertical axis. The graph demonstrates the PLL's ability to effectively self-calibrate and maintain the accuracy of its output frequency at sub-ppb levels without an external higher-level reference signal (i.e., in hold mode) by utilizing the device and method of the present invention.

[0050] In another aspect of the invention, the hold capability (“hold-up health”, “hold-up ready”, “hold-up state”, etc.) of the local oscillator device can be periodically evaluated / characterized, and the evaluation results can be used by the application system to make decisions related to local oscillator management and utilization when the application system needs to switch to hold-up operation. The hold-up health can be characterized in various formats, depending on the needs and requirements of the application system. For example, the application system may require the hold-up health of the local oscillator device to be reported as a simple binary value (“1” or “0”) indicating whether the accuracy of the frequency error estimate of the local oscillator device at the time of the hold-up health assessment is sufficient to obtain a certain hold-up period associated with the timing accuracy requirements of the application system: for example, the hold capability of the local oscillator device may indicate whether the frequency error estimate of the local oscillator device is accurate enough at the time of its evaluation so that a 4-hour hold-up period can be achieved with a maximum system time offset / time error of 1.5 µs (microseconds) if the application system and the local oscillator device need to switch to hold-up operation mode. Alternatively, maintainability can be assessed and characterized as the maximum maintainability period allowed by the accuracy of the local oscillator device's frequency error estimate if the application system and the local oscillator device were to switch to maintainability mode during maintainability assessment: for example, maintainability in this format could be reported as "510" minutes, indicating that the frequency stability estimate of the local oscillator device is sufficiently accurate at the time of reporting to facilitate a maintainability period not exceeding eight and a half hours. In yet another possible maintainability reporting format, maintainability can be reported as a matrix / table of timing error values ​​and their corresponding maximum possible maintainability periods: for example, a maintainability period of 4 hours for 1.5µs, 9 hours for 2µs, 24 hours for 3µs, etc. In yet another possible maintainability reporting format, maintainability can be characterized and reported as the maximum function curve fitting error relating the accuracy of the frequency error estimate to any of several varying environmental conditions and / or time factors. Maintainability reporting formats can be customized to the needs and requirements of the application system and are not necessarily limited to the example formats described above. Furthermore, health characterization can be based on reference frequency error data and environmental operating conditions corresponding to different time periods of past operations: for example, health characterization can be based on frequency error information and corresponding environmental conditions that occurred in the past 2 hours of operation, the past 24 hours of operation, or the past few days of operation.

[0051] Regardless of the format chosen for maintaining health characterization and reporting, and regardless of the time period of past operation selected for maintaining health characterization, the method / technique includes a method for evaluating the maintainability of a local oscillator device, comprising the following steps:

[0052] - Generate frequency error estimates corresponding to the time and / or environmental conditions that existed during a certain period of past operations.

[0053] - The accuracy of the generated frequency error estimate is evaluated using data correlated with the actual frequency error value within the same time period of past operations, and

[0054] - Generate parameters for maintaining health / maintaining ability based on the results of the aforementioned accuracy assessment.

[0055] Health assessment can be performed using the hardware resources of a local oscillator device and the software running on that device. Alternatively, health assessment can be performed using the hardware resources of an application system and the software running on that system. Another possibility is to implement the health assessment function in a distributed manner, utilizing the hardware and software resources of both the local oscillator device and the application system.

Claims

1. A frequency reference device intended for use with an application system, the frequency reference device comprising an oscillator for generating a frequency reference signal and a digital communication interface, wherein the frequency reference device can be configured to operate in either of the following two modes: In the first mode, the frequency reference device provides the frequency reference signal to the application system and receives, via the digital communication interface, information from the application system, with or without timestamp data, related to the frequency offset / error of the frequency reference signal. and In the second mode, the frequency reference device provides the frequency reference signal to the application system and provides an estimate of the frequency offset / error of the frequency reference signal via the digital communication interface.

2. The frequency reference device according to claim 1, wherein the oscillator is a quartz crystal oscillator.

3. The frequency reference device according to claim 1, wherein the oscillator is a silicon oscillator.

4. The frequency reference device according to any one of claims 2 and 3, wherein the oscillator is a temperature-compensated oscillator.

5. The frequency reference device according to any one of claims 2 and 3, wherein the oscillator is a temperature-controlled oscillator.

6. The frequency reference device according to any one of claims 2 to 5, wherein the oscillator is a microelectromechanical system (MEMS) oscillator.

7. The frequency reference device according to any one of claims 1 to 6, wherein the frequency reference device further comprises one or more of a temperature sensor, a power level sensor, an output current sensor, an acceleration sensor, and a pressure sensor.

8. The frequency reference device according to any one of claims 1 to 7, wherein the estimated value of the frequency offset / error of the frequency of the frequency reference signal includes one or more of the following components: frequency error caused by varying ambient temperature, frequency error depending on the rate or direction of change of ambient temperature (referred to as "hysteresis"), frequency error due to aging of the oscillator, frequency error caused by varying power supply voltage, frequency error caused by varying output load conditions, frequency error caused by varying mechanical acceleration conditions, and frequency error caused by varying pressure conditions.

9. A method for obtaining an accurate timing and frequency reference in an application system having a local oscillator device, the local oscillator device being capable of operating in a first operating mode or a second operating mode, the method comprising the following steps: - The application system is configured to generate offset / error information related to the output frequency of the local oscillator device in the first operating mode, and to send the offset / error information, with or without timestamp data, to the local oscillator device via a digital interface. - The local oscillator device is configured to receive the offset / error information, with or without timestamp data, sent by the application system. - The local oscillator device is configured to use received information and optionally data from one or more environmental sensors to characterize, model, or learn the dependence of the local oscillator device's output signal frequency on time data and one or more of the environmental data from the one or more sensors. - The local oscillator device is configured to generate an estimate of its output frequency offset / error in the second operating mode, and the estimate is sent to the application system via the digital interface.

10. The method for obtaining an accurate timing and frequency reference according to claim 9, wherein the estimated value of the frequency offset / error of the local oscillator generated by the local oscillator in the second operating mode includes one or more of the following components: frequency error caused by varying ambient temperature, frequency error depending on the rate or direction of change of ambient temperature ("hysteresis"), frequency error due to aging of the local oscillator device, frequency error caused by varying power supply voltage, frequency error caused by varying output load conditions, frequency error caused by varying mechanical acceleration conditions, and frequency error caused by varying pressure conditions.

11. The method for obtaining an accurate timing and frequency reference according to claim 9, wherein the information from the environmental sensor includes one or more of data obtained from a temperature sensor, data obtained from a power level sensor, data obtained from an output current sensor, data obtained from an acceleration sensor, and data obtained from a pressure sensor.

12. A method for evaluating the holding capability of a local oscillator device, the method comprising the steps of: - Generate frequency error estimates of time and / or environmental conditions that existed during a certain period of past operations. - The accuracy of the generated frequency error estimate is evaluated using data correlated with the actual frequency error value within the same time period of past operations, and - Generate retention capability parameters based on the results of the aforementioned accuracy assessment.

13. A local oscillator device capable of performing an assessment of its retention capability using the method of claim 12.

14. An application system for use with a local oscillator device, the system being able to perform an assessment of the holding capability of the local oscillator device using the assessment method according to claim 12.

15. An application system capable of operating with a local oscillator device and using the method of claim 12 to perform an assessment of the retention capability of the local oscillator device, wherein the assessment is performed using the hardware resources of both the application system and the local oscillator device.

16. The frequency reference device according to any one of claims 1 to 8, wherein the frequency reference device is capable of performing an assessment of its retention capability using the assessment method according to claim 12.

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