A frequency reference device

Abstract

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

Description

[0001] A FREQUENCY REFERENCE DEVICE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to equipment and techniques used to obtain accurate frequency reference, clock and timing signals in networks deployed in a variety of applications such as telecommunications, power grids, industrial automation, test and measurement, audiovisual systems, financial trading, and others.

[0004] BACKGROUND OF THE INVENTION

[0005] Multiple clock signal sources often exist in contemporary networked systems. The said clock signal sources need to be synchronized. Several synchronization protocols are used for synchronizing clocks over networked systems. For example, IEEE 1588, also known as the Precision Time Protocol (PTP), is a standard for synchronizing clocks over networked systems. PTP is a master-slave protocol, in which a single master clock provides a synchronization reference to all of the 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 packets contain information about the time at which they were sent and received. The slave clocks use this information to calculate their offset from the master clock and adjust their own clocks accordingly.

[0006] A frequency reference device such as, for example, an Oven-Controlled Crystal Oscillator (OCXO), when used as a frequency reference in a slave clock, would necessarily have the hardware and software for receiving and processing the information received from the network, calculate the offset, and adjust its frequency accordingly. This functionality requires the use of hardware such as high resolution d ig ita l-to-a na log converters (DACs), voltage- controlled ("pullable") oscillators, etc., which results in increased complexity, increased size, and higher cost of the frequency reference devices.

[0007] Additional requirements, and the need for additional hardware and software resources, arise to ensure that the clock signals generated by the frequency reference devices remain sufficiently accurate in "holdover" mode - this is when the network's master clock (such as, for example, a GPS clock) is lost.

[0008] SUMMARY OF THE INVENTION

[0009] The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting each statement in this specification and claims that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

[0010] The present invention offers novel and inventive frequency reference devices and methods that attain higher accuracy frequency reference and timing signals, and longer holdover periods in holdover mode when the application system has lost access to higher strata reference signals.

[0011] Frequency reference devices and methods of the present invention can be used in a variety of networked systems, as a local oscillator device and a frequency and / or time reference in holdover mode and as a way to achieve accurate timing and frequency references in holdover mode, respectively.

[0012] In locked mode, when the application system has access to, and utilises, higher strata clock signals, the application system evaluates the local oscillator's frequency, using one or more of higher strata clock signals as frequency reference signals, calculates the frequency offset (error) value, and communicates the error value to the local oscillator device.

[0013] Over time, the local oscillator device of the present invention uses error information provided by the application system to characterise, model, or learn certain parameters related to its frequency stability. In some embodiments, this characterisation, modelling, or learning is carried out using both the error information provided by the application system and data from one or more environmental sensors.

[0014] In holdover mode, when the application system loses access to higher strata clock signals, it switches to using the local oscillator's output signal as its holdover reference clock. In this mode, in addition to providing the clock signal, the local oscillator device of the present invention provides to the application system an estimate of frequency error calculated within the local oscillator device based on the aforementioned characterisation, modelling, or learning of its frequency stability parameters. This allows the application system to maintain, in holdover mode, its timing and frequency references more accurately and for longer holdover periods than it would be possible in absence of the error information provided by the local oscillator device. In some embodiments, in order to ensure an acceptable level of "out-of-the-box" holdover performance, the local oscillator device may use characterisation parameters or a model of its frequency stability parameters pre-loaded during its production process. Once deployed, the device may be configured to further refine the pre-loaded characterisation or model using error information provided by the application system and, optionally, data from one or more environmental sensors.

[0015] In yet another aspect, the present invention offers a novel and inventive technique of maximising holdover capabilities I holdover periods in networked systems utilising local oscillator devices. This is achieved through generating and assessing a parameter, or a set of parameters, characterising the holdover capability ("holdover health") of the local oscillator device.

[0016] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention is further described with reference to the accompanying figures in which, -

[0019] Figure 1 shows an example of an application system with a local oscillator device of the present invention.

[0020] Figure 2 shows an example of local oscillator device's structure.

[0021] Figures 3a, 3b, and 3c present test data related to performance testing of a local oscillator device and method of the present invention.

[0022] DETAILED DESCRIPTION OF THE INVENTION

[0023] Without limiting the scope of the present invention, the latter is illustrated herein by the following specific description of devices and methods of the present invention.

[0024] In the following description, specific details are given to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, modules, including those in the form of software modules, functions, circuits, etc., may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known modules, structures and techniques may not be shown in detail in order not to obscure the embodiments.

[0025] In one aspect a frequency reference device is provided, which is intended or configured for use with an application system, comprising an oscillator generating a frequency reference signal and a digital communications interface, wherein the frequency reference device can be arranged or configured to operate in any one of the following two modes: a first mode, wherein the frequency reference device provides to the application system the frequency reference signal, and receives from the application system, via the digital communications interface, information related to frequency offset I error of the frequency reference signal, with or without timestamp data, and a second mode, wherein the frequency reference device provides to the application system the frequency reference signal and, via the digital communications interface, an estimated value of frequency offset I error in the frequency of the frequency reference signal.

[0026] Fig. 1 shows an arrangement wherein a Local Oscillator Device 1 of the present invention is used to provide reference signals to an Application System 2. The Application System 2 may comprise a System CPU 3 running 1588 firmware, a PLL 4 to synthesize Output Frequency I System Clock Outputs 5, and a Digital Communications Interface 6 to facilitate digital data and control signal exchange between the Application System 2 and the Local Oscillator Device 1.

[0027] The Application System 2 and the Local Oscillator Device 1 can operate in one of two operation modes: locked mode and holdover mode.

[0028] In locked mode, when the Application System 2 has access to, and utilises, Higher Strata Reference Signals 7, the Application System 2 evaluates the frequency of the Frequency Reference Signal 8 provided by the Local Oscillator Device 1 (using one or more of the Higher Strata Reference Signals 7 as frequency reference), calculates the frequency and / or time error values, and sends the calculated error data to the Local Oscillator Device 1 via the Digital Communications Interface 6.

[0029] Over time, the Local Oscillator Device 1 utilises the error information received from the Application System 2 and, in some embodiments, data from one or more sensors (such as, for example, a temperature sensor, power supply level sensor, etc.) to characterise, model, or learn parameters of its frequency stability.

[0030] In holdover mode, when the Application System 2 loses access to Higher Strata Reference Signals 7, the Application System 2 switches to utilising the Local Oscillator Device's 1 output signal Frequency Reference Signal 8 as its holdover reference signal. In this mode, in addition to providing the Frequency Reference Signal 8, the Local Oscillator Device 1 sends, via the Digital Communications Interface 6, an estimated value of frequency offset or frequency error that is calculated within the Local Oscillator Device 1 based on the aforementioned characterisation, modelling, or learning. This allows the Application System 2 to maintain, in holdover mode, its timing and frequency references more accurately, and for longer holdover periods, than it would be possible in absence of the error information sent by the Local Oscillator Device 1.

[0031] The Local Oscillator Device can comprise an oven-controlled (quartz crystal-, silicon-, or other suitable material- based) oscillator, a temperature compensated (quartz crystal, silicon, or other suitable material based) oscillator, a (quartz crystal, silicon, or other suitable material based) Micro-Electro-Mechanical Systems (MEMS) oscillator, or any other oscillator device with sufficiently stable output signal frequency.

[0032] The Digital Communications Interface can be implemented as a Serial Peripheral Interface (SPI), or an Inter-Integrated Circuit (I2C) interface, or in a form of any other suitable digital interface.

[0033] The Higher Strata Reference Signals used by the Application System in locked mode can include one or more of Global Navigation Satellite System (GNSS) signals, Synchronous Ethernet (SyncE) signals, Pulse Per Second (PPS) signals, Precision Time Protocol (PTP) signals, and others.

[0034] In another aspect, the present invention offers a method of attaining accurate timing and frequency references in holdover mode and, consequently, longer holdover periods in an application system, the method comprising the steps of, arranging the application system to generate, in the first (normal, or "locked") mode of operation, the offset I error information in relation to the local oscillator device's output frequency, and send the offset I error information, with or without timestamp data, to the local oscillator device via a digital interface, arranging the local oscillator device, operating in the first (normal, or "locked") mode of operation to receive the said offset I error and timestamp information sent by the application system, arranging the local oscillator device to use the received offset I error and timestamp information and, optionally, data from one or more environmental I parametric sensors, to characterise, model, or learn the dependency of the local oscillator device's output signal frequency on temporal and environmental conditions, arranging the local oscillator device to generate, in the second ("holdover") mode of operation, estimates of its output frequency offset I error, and send the estimated offset I error information to the application system via the digital interface.

[0035] The application system uses the digital communication interface to send and receive error data as required in each mode of operation, and also to control I switch the operating mode of the local oscillator device between first mode and second mode.

[0036] The estimated offset I error information provided to the application system by the local oscillator device in holdover mode may comprise one or more of the following constituent error components, either combined or reported separately: frequency error caused by varying ambient temperature, frequency error that is dependent on the rate of change or direction of change of ambient temperature (also known as "hysteresis"), frequency error caused by time dependent ageing effects in the local oscillator elements, frequency error caused by varying power supply voltage, frequency error caused by varying output loading conditions, frequency error caused by varying mechanical acceleration conditions, frequency error caused by varying pressure conditions, and others.

[0037] In order to obtain data related to the aforementioned varying conditions, the local oscillator device may utilize signals from one or more sensors such as, for example, temperature sensors, power supply level sensors, output current sensors, acceleration sensors, and pressure sensors.

[0038] In some embodiments, the data sent to, and received by, the local oscillator in the first ("locked") mode can include timestamp data, which can be useful for characterizing, modelling, or learning the profile of local oscillator's ageing effects (i.e., frequency dependency on time, also known as long-term frequency stability).

[0039] Fig. 2 shows an example structure of the Local Oscillator Device. The Local Oscillator Device comprises an oscillator circuit 1, a central processing unit (CPU) 2, a digital communications bus 3, and a group 4 of one or more environmental sensors SI, S2, ..., Sn.

[0040] The oscillator circuit 1 comprises a resonator 5, an oscillator sustaining amplifier circuit 6, and an output buffer 7. The oscillator circuit generates a frequency reference signal provided to the application system via an output terminal FR. Advantageously, the oscillator circuit 1 of this embodiment is implemented as a free- running, non-pullable oscillator; this allows using a resonator with low pullability, and have no voltage control circuitry in the oscillator circuit - both of these factors not only result in lower complexity, lower cost, and smaller size, but also allow to achieve higher frequency stability of, and lower noise level in, the frequency reference signal FR, compared to other oscillator options such as voltage-controlled oscillators, or temperature compensated oscillators.

[0041] The resonator 5 in the embodiment shown in Fig. 2 is a quartz crystal resonator. In other embodiments, the resonator 5 can be implemented as one of several other resonator types, such as, for example, a silicon MEMS type resonator, a ceramic resonator, or a quartz MEMS resonator such as an XMEMS® resonator.

[0042] The CPU 2 can be any suitable microcontroller capable of executing software as required for servicing the communications over the communications interface bus 3, as well as software required to characterize, model, or learn, in first (locked) mode, the output frequency's dependency on environmental and temporal factors, and to generated estimates of frequency and time error in the second (holdover) mode.

[0043] Sensors SI, S2, ..., Sn may comprise one or more of temperature sensors, power supply level sensors, output current sensors, acceleration sensors, pressure sensors, and others.

[0044] The Local Oscillator Device can be implemented as a temperature stabilized device, such as, for example, an oven-controlled oscillator, in which case at least some of frequency determining components (such as the resonator 5, the sustaining amplifier circuit 6, and others) will be placed in a thermally stabilized environment (oven).

[0045] Fig. 3a, Fig. 3b, and Fig. 3c present test data related to the performance evaluation of an application system using a frequency reference device of the present invention as its local oscillator device and frequency and time correction methods of the present invention. The local oscillator device used in this test is an oven-controlled crystal oscillator (OCXO). Fig. 3a shows data related to the accuracy of the OCXO's ageing model. Time, in seconds, is indicated along the abscissa axis, showing that the test took just over 30,000 seconds. During that time period, the OCXO operated in conjunction with an emulated application system in two modes: locked mode and holdover mode. In locked mode, the application system received a GPSbased PPS signal, whereas in holdover mode the PPS signal was made unavailable to the application system. In Fig. 3a, graph 1 indicates periods of locked mode (graph 1 is HIGH) and holdover mode (graph 1 is LOW). Graph 2 presents real time OCXO frequency measurements, and graph 3 shows frequency values predicted by the ageing model. Frequency values are expressed as fractional frequency deviation values, in parts-per-billion (ppb), in relation to the OCXO's nominal frequency, and displayed along the ordinate axis.

[0046] Fig. 3b illustrates the efficacy of the device and correction method of the present invention. It shows Maximum Time Interval Error (MTIE) values observed in the application system using the OCXO as a frequency reference, in two scenarios: (1) without receiving the OCXO's frequency error information in holdover mode (graph 4) and (2) when receiving and utilizing the OCXO's frequency error information in holdover mode (graph 5). MTIE values corresponding to graph 4 are indicated, in nanoseconds (ns), along the left-hand ordinate axis, whereas MTIE values corresponding to graph 5 are indicated, also in nanoseconds (ns), along the right-hand ordinate axis. As can be seen from Fig. 3b, the application system benefits from a significant improvement in timing accuracy when the correction method is applied, reducing the MTIE to 64ns compared to 2,664ns without correction.

[0047] Fig. 3c shows a plot of the application system's PLL output frequency versus time. PLL output frequency values are expressed as fractional frequency deviation values, in parts-per-billion (ppb), relative to the PLL's nominal output frequency, and indicated along the ordinate axis. The graph demonstrates the PLL's ability to effectively self-correct and maintain the accuracy of its output frequency down to sub-ppb levels in absence of external higher strata reference signals (i.e., in holdover mode), by utilizing the device and method of the present invention.

[0048] In yet another aspect of the present invention, the holdover capability ("holdover health", "holdover readiness", "holdover status", etc.) of the Local Oscillator Device can be periodically assessed I characterized, and the assessment results can be used by the Application System for making decisions in relation to local oscillator management and utilization should the Application System need to switch to operating in holdover mode. The holdover health can be characterized in a number of different formats depending on the Application System's needs and requirements. For example, the Application System may require that the holdover health of the Local Oscillator Device is reported as a simple binary value ("1" or "0") indicating whether the accuracy of the Local Oscillator Device's frequency error estimates at the time of holdover health assessment would be sufficient to obtain a certain holdover period associated with the Application System's timing accuracy requirements: for instance, the Local Oscillator Device's holdover capability can indicate whether, at the time of its assessment, the Local Oscillator Device's frequency error estimates are sufficiently accurate to facilitate a holdover period of 4 hours at a maximum system time offset I time error of 1.5ps (microseconds) if the Application System and the Local Oscillator Device were to switch to holdover mode of operation. Alternatively, the holdover health can be assessed and characterized as the maximum holdover period that the Local Oscillator Device's accuracy of frequency error estimates would allow to achieve, if the Application System and the Local Oscillator Device were to switch to holdover mode at the time of holdover health assessment: for instance, the holdover health in this format could be reported as "510" minutes, indicating that the Local Oscillator Device's frequency stability estimates, at the time of reporting, are accurate enough to facilitate a holdover period of no longer than eight and a half hours. In yet another possible holdover health reporting format, the holdover health may be reported as a matrix I table of timing error values and their corresponding maximum possible holdover periods: e.g., 4 hours of holdover period at 1.5ps, 9 hours at 2ps, 24 hours at 3ps, etc. In yet another possible holdover health reporting format, the holdover health can be characterized and reported as the maximum function curve fit error related to the accuracy of frequency error estimates versus any of a number of varying environmental conditions and / or temporal factors. The holdover health reporting formats can be tailored to the Application System's needs and requirements, and are not necessarily limited to the example formats described above. Also, the holdover health characterization can be carried out based on reference frequency error data and environmental operating conditions corresponding to different time periods of the past operation: for example, holdover health characterization can be carried out based on data for frequency error information and corresponding environmental conditions that occurred over the last 2 hours of operation, last 24 hours of operation, or last several days of operation.

[0049] Regardless of which format is chosen for holdover health characterization and reporting, and regardless of what period of past operation is chosen for holdover health characterization, the method I technique includes a method of assessing holdover capability of a Local Oscillator Device, the method comprising the steps of, generating frequency error estimates corresponding to the temporal and / or environmental conditions present over a certain period of past operation, - assessing the accuracy of the generated frequency error estimates using data related to the actual frequency error values over the same period of past operation, and

[0050] - generating the holdover health I holdover capability parameters based on the results of the aforementioned accuracy assessment.

[0051] The holdover health assessment can be carried out using the hardware resources of, and software run in, the Local Oscillator Device. Alternatively, the holdover health assessment can be carried out using the hardware resources of, and software run in, the Application System. Yet another possibility is to implement the function of holdover health assessment in a distributed way, i.e. where hardware and software resources of both the Local Oscillator Device and the Application System are utilized for this purpose.

Claims

CLAIMS1. A frequency reference device, intended for use with an application system, comprising an oscillator generating a frequency reference signal and a digital communications interface, wherein the frequency reference device can be arranged to operate in any one of the following two modes: a first mode, wherein the frequency reference device provides to the application system the frequency reference signal, and receives from the application system, via the digital communications interface, information related to frequency offset I error of the frequency reference signal, with or without timestamp data, and a second mode, wherein the frequency reference device provides to the application system the frequency reference signal and, via the digital communications interface, an estimated value of frequency offset I error in the frequency of the frequency reference signal.

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

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

4. A frequency reference device according to any one of claims 2 and 3, wherein the oscillator is a Temperature Compensated Oscillator.

5. A frequency reference device according to any one of claims 2 and 3, wherein the oscillator is an Oven Controlled Oscillator.

6. A frequency reference device according to any one of claims 2 to 5, wherein the oscillator is a Micro-Electro-Mechanical-Systems (MEMS) Oscillator.

7. A frequency reference device according to any one of claims 1 to 6, further comprising one or more of temperature sensors, power supply level sensors, output current sensors, acceleration sensors, and pressure sensors.

8. A frequency reference device according to any one of claims 1 to 7, wherein the estimated value of frequency offset I error in the frequency of the frequency reference signal comprises one or more of the following components: frequency error caused by varying ambient temperature, frequency error that is dependent on rate of change or direction of change of ambient temperature (referred to as "hysteresis"), frequency error due to ageing of the oscillator, frequency error caused by varying power supply voltage,frequency error caused by varying output loading conditions, frequency error caused by varying mechanical acceleration conditions, and frequency error caused by varying pressure conditions.

9. A method of attaining accurate timing and frequency references in an application system with a local oscillator device, capable of operating in a first mode of operation or in a second mode of operation, the method comprising the steps of,- arranging the application system to generate, in the first mode of operation, offset I error information related to the local oscillator device's output frequency, and send the offset I error information, with or without timestamp data, to the local oscillator device via a digital interface, arranging the local oscillator device to receive the said offset I error information, with or without timestamp data, sent by the application system, arranging the local oscillator device to use the received information and, optionally, data from one or more of environmental sensors to characterise, model, or learn dependencies of the local oscillator device's output signal frequency on one or more of temporal data and environmental data from the one or more sensors, arranging the local oscillator device to generate, in the second mode of operation, estimates of its output frequency offset I error, and send the said estimates to the application system via the digital interface.

10. A method of attaining accurate timing and frequency references according to claim 9, wherein the estimates of the local oscillator's frequency offset I error generated by the local oscillator in the second mode of operation comprise one or more of the following components: frequency error caused by varying ambient temperature, frequency error that is dependent on rate of change or direction of change of ambient temperature ("hysteresis"), frequency error due to local oscillator device's ageing, frequency error caused by varying power supply voltage, frequency error caused by varying output loading conditions, frequency error caused by varying mechanical acceleration conditions, and frequency error caused by varying pressure conditions.

11. A method of attaining accurate timing and frequency references according to claim 9, wherein the information from the environmental sensors includes one or more of data obtained from temperature sensors, data obtained from power supply level sensors, data obtained from output current sensors, data obtained from acceleration sensors, and data obtained from pressure sensors.

12. A method of assessing holdover capability of a Local Oscillator Device, the method comprising the steps of, generating frequency error estimates corresponding to temporal and / or environmental conditions present over a certain period of past operation,- assessing the accuracy of the generated frequency error estimates using data related to actual frequency error values over the same period of past operation, and- generating holdover capability parameters based on the results of the aforementioned accuracy assessment.

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

14. An application system for use with a local oscillator device, the system being capable of carrying out an assessment of the local oscillator device's holdover capability using an assessment method according to claim 12.

15. An application system capable of operating with a local oscillator device and performing an assessment of the local oscillator device's holdover capability using the method according to claim 12, wherein the assessment is carried out using hardware resources of both the application system and the local oscillator device.

16. A frequency reference device according to any one of claims 1 to 8, capable of carrying out an assessment of its holdover capability using an assessment method according to claim 12.

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