Transmission and Multiplexing (TM); Generic requirements for synchronization networks; Part 5-1: Timing characteristics of slave clocks suitable for operation in Synchronous Digital Hierarchy (SDH) equipment

Stadardises SDH timing distribution architectures, synchronisation network level jitter and wander requirements, slave clock timing specifications and primary reference clock specifications.

Prenos in multipleksiranje (TM) – Generične zahteve za sinhronizacijska omrežja – 5-1. del: Časovne značilnosti podrejenih ur, primernih za obratovanje v opremi sinhrone digitalne hierarhije (SDH)

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Status
Published
Publication Date
30-Nov-2003
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
01-Dec-2003
Due Date
01-Dec-2003
Completion Date
01-Dec-2003

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Transmission and Multiplexing (TM); Generic requirements for synchronization networks; Part 5-1: Timing characteristics of slave clocks suitable for operation in Synchronous Digital Hierarchy (SDH) equipment33.040.20Prenosni sistemTransmission systemsICS:Ta slovenski standard je istoveten z:EN 300 462-5-1 Version 1.1.2SIST EN 300 462-5-1 V1.1.2:2003en01-december-2003SIST EN 300 462-5-1 V1.1.2:2003SLOVENSKI

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SIST EN 300 462-5-1 V1.1.2:2003

EN 300 462-5-1 V1.1.2 (1998-05)European Standard (Telecommunications series)Transmission and Multiplexing (TM);Generic requirements for synchronization networks;Part 5-1: Timing characteristics of slave clocks suitable foroperation in Synchronous Digital Hierarchy (SDH) equipmentSIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)2ReferenceREN/TM-03017-5-1 (4a199idc.PDF)Keywordssynchronization, timing, transmissionETSIPostal addressF-06921 Sophia Antipolis Cedex - FRANCEOffice address650 Route des Lucioles - Sophia AntipolisValbonne - FRANCETel.: +33 4 92 94 42 00

Fax: +33 4 93 65 47 16Siret N° 348 623 562 00017 - NAF 742 CAssociation à but non lucratif enregistrée à laSous-Préfecture de Grasse (06) N° 7803/88Internetsecretariat@etsi.frhttp://www.etsi.frhttp://www.etsi.orgCopyright NotificationNo part may be reproduced except as authorized by written permission.The copyright and the foregoing restriction extend to reproduction in all media.© European Telecommunications Standards Institute 1998.All rights reserved.SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)3ContentsIntellectual Property Rights................................................................................................................................5Foreword............................................................................................................................................................51Scope........................................................................................................................................................72References................................................................................................................................................73Definitions, symbols and abbreviations...................................................................................................83.1Definitions.........................................................................................................................................................83.2Abbreviations.....................................................................................................................................................84Frequency accuracy..................................................................................................................................85Pull-in and pull-out ranges.......................................................................................................................86Noise generation......................................................................................................................................86.1Wander in locked mode.....................................................................................................................................96.2Non-locked wander..........................................................................................................................................106.3Jitter.................................................................................................................................................................106.3.1Output jitter at a 2 048 kHz and 2 048 kbit/s interface...............................................................................106.3.2Output jitter at a Synchronous Transport Module N (STM-N) interface...................................................117Noise tolerance.......................................................................................................................................117.1Jitter tolerance..................................................................................................................................................117.2Wander tolerance.............................................................................................................................................128Transfer characteristic............................................................................................................................149Transient response and holdover performance......................................................................................149.1Short-term phase transient response.................................................................................................................149.2Long-term phase transient response (holdover)...............................................................................................159.3Phase response to input signal interruptions....................................................................................................169.4Phase discontinuity..........................................................................................................................................1610Interfaces................................................................................................................................................16Annex A (informative):Considerations on bandwidth requirements...............................................18A.1Introduction............................................................................................................................................18A.2Relevant network requirements and assumptions..................................................................................18A.2.1ITU-T Recommendation G.825 STM-N jitter acceptance...............................................................................18A.2.2Wander accumulation in a synchronization distribution chain.........................................................................18A.2.3Phase transients due to automatic timing restoration.......................................................................................19SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)4A.3Conclusion.............................................................................................................................................20Annex B (informative):Measurement methods for transfer characteristics...................................21B.1Phase step response................................................................................................................................21B.2Frequency step response........................................................................................................................22B.3Sinusoidal phase response......................................................................................................................23B.4White phase noise response (frequency domain)...................................................................................23B.5White phase noise response (time domain)...........................................................................................24Annex C (informative):Information on the SEC noise model...........................................................25Bibliography.....................................................................................................................................................26History..............................................................................................................................................................27SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)5Intellectual Property RightsIPRs essential or potentially essential to the present document may have been declared to ETSI. The informationpertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be foundin ETR 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in respect ofETSI standards", which is available free of charge from the ETSI Secretariat. Latest updates are available on the ETSIWeb server (http://www.etsi.fr/ipr or http://www.etsi.org/ipr).Pursuant to the ETSI Interim IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. Noguarantee can be given as to the existence of other IPRs not referenced in ETR 314 (or the updates on the ETSI Webserver) which are, or may be, or may become, essential to the present document.ForewordThis European Standard (Telecommunications series) has been produced by the Transmission and Multiplexing (TM)Technical Committee.The present document has been produced to provide requirements for synchronization networks that are compatible withthe performance requirements of digital networks. It is one of a family of documents covering various aspects ofsynchronization networks:Part 1-1:"Definitions and terminology for synchronization networks";Part 2-1:"Synchronization network architecture";Part 3-1:"The control of jitter and wander within synchronization networks";Part 4-1:"Timing characteristics of slave clocks suitable for synchronization supply to Synchronous DigitalHierarchy (SDH) and Plesiochronous Digital Hierarchy (PDH) equipment";Part 4-2:"Timing characteristics of slave clocks suitable for synchronization supply to Synchronous DigitalHierarchy (SDH) and Plesiochronous Digital Hierarchy (PDH) equipment ImplementationConformance (ICS) Statement";Part 5-1:"Timing characteristics of slave clocks suitable for operation in Synchronous DigitalHierarchy (SDH) equipment";Part 6-1:"Timing characteristics of primary reference clocks";Part 6-2:"Timing characteristics of primary reference clocks Implementation Conformance (ICS)Statement";Part 7-1:"Timing characteristics of slave clocks suitable for synchronization supply to equipment in localnode applications".Parts 1-1, 2-1, 3-1 and 5-1 have previously been published as ETS 300 462 Parts 1, 2, 3 and 5, respectively.Additionally, parts 4-1 and 6-1 completed the Voting phase of the Two Step Approval procedure as ETS 300 462 Parts4 and 6, respectively.It was decided to prepare ICS proformas for several of the parts and this necessitated a re-numbering of the individualdocument parts. It was also decided to create a new part 7-1.This in turn led to a need to re-publish new versions of all six parts of the original ETS. At the same time, theopportunity was taken to convert the document type to EN.This has involved no technical change to any of the documents. However part 5-1 has been modified, due to editorialerrors which appeared in ETS 300 462-5.SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)6Transposition datesDate of adoption of this ETS:16 August 1996Date of latest announcement of this ETS (doa):31 December 1996Date of latest publication of new National Standardor endorsement of this ETS (dop/e):30 June 1997Date of withdrawal of any conflicting National Standard (dow):30 June 1997NOTE:The above transposition table is the original table from ETS 300 462-5 (September 1996, see History).SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)71ScopeThis European Standard (Telecommunications series) outlines requirements for timing devices used in synchronisingnetwork equipment that operates according to the principles governed by the Synchronous Digital Hierarchy (SDH).These requirements apply under the normal environmental conditions specified for SDH equipment. Typical SDHequipment contains a slave clock linked to a master, or a primary reference clock. The logical function of the SEC isdescribed in figure 2 of EN 300 462-2-1 [3]. In general the SDH Equipment Clock (SEC) will have multiple referenceinputs. In the event that all links between the master and the slave clock fail, the equipment should be capable ofmaintaining operation (holdover) within the prescribed performance limits contained within the present document.Slave clocks used in SDH equipment shall meet specific requirements in order to comply with the jitter specificationsgiven in ETS 300 417-1-1 [9] for plesiochronous tributaries.The case where clock performance is required in SDH equipment is outside the scope of the present document, seeEN 300 462-4-1 [10].2ReferencesThe following documents contain provisions which, through reference in this text, constitute provisions of the presentdocument.· References are either specific (identified by date of publication, edition number, version number, etc.) ornon-specific.· For a specific reference, subsequent revisions do not apply.· For a non-specific reference, subsequent revisions do apply.· A non-specific reference to an ETS shall also be taken to refer to later versions published as an EN with the samenumber.[1]ETS 300 019: "Equipment Engineering (EE); Environmental conditions and environmental testsfor telecommunications equipment".[2]EN 300 462-1-1: "Transmission and Multiplexing (TM); Generic requirements for synchronizationnetworks; Part 1-1: Definitions and terminology for synchronization networks".[3]EN 300 462-2-1: "Transmission and Multiplexing (TM); Generic requirements for synchronizationnetworks; Part 2-1: Synchronisation network architecture".[4]ITU-T Recommendation G.823: "The control of jitter and wander within digital networks whichare based on the 2 048 kbit/s hierarchy".[5]EN 300 462-6-1: "Transmission and Multiplexing (TM); Generic requirements for synchronizationnetworks; Part 6-1: Timing characteristics of primary reference clocks".[6]ITU-T Recommendation G.703: "Physical/electrical characteristics of hierarchical digitalinterfaces".[7]ITU-T Recommendation G.825: "The control of jitter and wander within digital networks whichare based on the synchronous digital hierarchy (SDH)".[8]ITU-T Recommendation Q.551: "Transmission characteristics of digital exchanges".[9]ETS 300 417-1-1 (1996): "Transmission and Multiplexing (TM); Generic functional requirementsfor Synchronous Digital Hierarchy (SDH) equipment; Part 1-1: Generic processes andperformance".[10]EN 300 462-4-1: "Transmission and Multiplexing (TM); Generic requirements for synchronizationnetworks; Part 4-1: Timing characteristics of slave clocks suitable for synchronization supply toSynchronous Digital Hierarchy (SDH) and Plesiochronous Digital Hierarchy (PDH) equipment".SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)83Definitions, symbols and abbreviations3.1DefinitionsFor the purposes of the present document, the definitions given in EN 300 462-1-1 [2] apply.3.2AbbreviationsFor the purposes of the present document, the abbreviations given in EN 300 462-1-1 [2], together with the following,apply:MTIEMaximum Time Interval ErrorNENetwork ElementPDHPlesiochronous Digital HierarchyPLLPhase Locked Loopppmparts per millionPRCPrimary Reference ClockSDHSynchronous Digital HierarchySECSDH Equipment ClockSSUSynchronisation Supply UnitSTM-NSynchronous Transport Module NTDEVTime DeviationUIUnit IntervalUIppUnit Interval peak to peakVCOVoltage Controlled Oscillator4Frequency accuracyUnder free-running conditions, the SEC output frequency accuracy shall be within 4,6 ppm with regard to a referencetraceable to a clock as specified in EN 300 462-6-1 [5].NOTE:The time interval for this accuracy specification is for further study.5Pull-in and pull-out rangesThe minimum pull-in range shall be ± 4,6 ppm, whatever the internal oscillator frequency offset may be. The Pull-outrange is for further study. A value of ± 4,6 ppm has been proposed.6Noise generationThe noise generation of a SEC represents the amount of phase noise produced at the output when there is an idealinput reference signal or the clock is in holdover state. A suitable reference, for practical testing purposes, implies aperformance level at least 10 times more stable than the output requirements. The ability of the clock to limit this noiseis described by its frequency stability. The measures Maximum Time Interval Error (MTIE) and Time Deviation(TDEV) are useful for characterization of noise generation performance.MTIE and TDEV are measured through an equivalent 10 Hz, first order, low-pass measurement filter, at a maximumsampling time t0 of 1/30 second. The minimum measurement period for TDEV is twelve times the integration period(T = 12t).SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)96.1Wander in locked modeWhen the SEC is in the locked mode of operation, the MTIE and TDEV measured using the synchronised clockconfiguration defined in figure 1a of EN 300 462-1-1 [2] shall have the

limits in tables 1 and 2, if the temperature isconstant (± 1 K).Table 1: Wander in locked mode for constant temperature specified in MTIERequirementObservation interval40 ns0,1 < t £ 1 s40 t 0,1 ns1 < t £ 100 s25 t 0,2 ns100 < t £ 1 000 sTable 2: Wander in locked mode for constant temperature specified in TDEVRequirementObservation interval3,2 ns0,1 < t £ 25 s0,64 t 0,5 ns25 < t £ 100 s6,4 ns100 < t £ 1 000 sThe model used to derive these numbers is described in (informative) annex C. The resultant requirements are shown bythe thick solid lines in figures 1 and 2.101,0TDEV1025100(ns)(constant temperature)TDEVObservation Interval

(s)(log scale)(log scale)6,43,20,11,01 000tFigure 1: TDEV as a function of an observation period ttSIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)1010110001000.163401.0101001000113(ns)(constant temperature)MTIE(variable temperature)150Observation Interval(s)tMTIEMTIE(log scale)(log scale)Figure 2: MTIE as a function of an observation period tWhen temperature effects are included of which the limits and rate of change are defined in ETS 300 019 [1], theallowance for the total MTIE contribution of a single SEC increases by the

values in table 3.Table 3: Additional wander in locked mode for variable temperature specified in MTIERequirementObservation interval0,5 t nst £ 100 s50 nst > 100 sThe resultant requirement is shown by the upper solid line in figure 2.6.2Non-locked wanderWhen a clock is not locked to a synchronization reference, the random noise components are negligible compared todeterministic effects like initial frequency offset. Consequently the non-locked wander effects are included insubclause 9.1.6.3JitterWhile most specifications in the present document are independent of the output interface at which they are measured,this is not the case for jitter production; jitter generation specifications shall utilize existing specifications that arecurrently specified differently for different interface rates. These requirements are stated separately for the interfacesidentified in clause 10. To be consistent with other jitter requirements the specifications are in Unit Interval peak topeak (UIpp), where the Unit Interval (UI) corresponds to the reciprocal of the bit rate of the interface.Due to the stochastic nature of jitter, the peak-to-peak values given in this clause eventually are exceeded. Therequirements shall therefore be fulfilled with a probability of 99 %.6.3.1Output jitter at a 2 048 kHz and 2 048 kbit/s interfaceIn the absence of input jitter, the intrinsic jitter at a 2 048 kHz or 2 048 kbit/s output interface as measured over a 60seconds interval shall not exceed 0,05 UIpp when measured through a band-pass filter with corner frequencies at 20Hz and 100 kHz each with a first order 20 dB/decade roll-off characteristic.SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)116.3.2Output jitter at a Synchronous Transport Module N (STM-N)interfaceIn the absence of input jitter at the synchronization interface, the intrinsic jitter at optical STM-N output interfaces asmeasured over a 60 seconds interval shall not exceed the limits given in table 4. The allowed jitter on an STM-1electrical interface is for further study.Table 4: Output jitter requirements for STM-N optical interfacesInterfaceMeasuring filter(Hz)Peak-to-peak amplitude(UI)STM-1500 to 1,3 M0,5065 k to 1,3 M0,10STM-41 k to 5 M0,50250 k to 5 M0,10STM-165 k to 20 M0,501 M to 20 M0,10for STM-1:1 UI = 6,43 ns;for STM-4:1 UI = 1,61 ns;for STM-16:1 UI = 0,40 ns.7Noise toleranceNoise tolerance of an SEC indicates the minimum phase noise level at the input of the clock that should beaccommodated whilst:-maintaining the clock within prescribed performance limits. The exact performance limits are for further study;-not causing any alarms;-not causing the clock to switch reference;-not causing the clock to go into holdover.In general, the noise tolerance is the same as the network limit for the synchronization interface in order to maintainacceptable performance. The jitter and wander tolerances given in subclauses 7.1 and 7.2 represent the worst levels thata synchronization carrying interface should exhibit. The requirements in subclause 7.1 have been derived by combiningthe most stringent requirements from each specific data interface and presenting them as a single specification whichdefines the performance of the SDH Equipment Clock (SEC). It is not expected that every synchronization interfaceshould tolerate the full requirements in figure 3. Consequently when testing a specific interface (e.g. an STM-N), theinterface is also bound by the jitter and wander tolerance limits defined in ITU-T Recommendations G.823 [4] andG.825 [7].7.1Jitter toleranceThe lower limits of maximum tolerable input jitter for signals carrying synchronization to SECs is given in figure 3 andtable 5.Table 5: Lower limit of maximum tolerable input jitterRequirementFrequency interval250 ns1 < f £ 19 Hz4 900 f -1 ns19 < f £ 49 Hz100 ns49 < f £ 105 HzSIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)12101001k100k10k1949100250(ns)Peak-to-peakJitter AmplitudeJitter Frequency f(Hz)1M(log scale)(log scale)1,0Figure 3: Lower limit of maximum tolerable input jitter7.2Wander toleranceThe clock shall tolerate (i.e. shall give no indication of improper operation) input wander as specified in figures 4 and5 (and tables 6 and 7 respectively). The templates in these figures are intended to represent the cumulative networkwander at the SEC input, i.e. for synchronization inputs the required wander tolerance should at least be equal to thenetwork limit.MTIE and TDEV are measured through an equivalent 10 Hz, first order, low-pass measurement filter, at a maximumsampling time t0 of 1/30 second. The minimum measurement period for TDEV is twelve times the integration period(T = 12t).Table 6: Input wander tolerance specified in TDEVRequirementObservation interval12 ns0,1 < t £ 7 s1,7t ns7 < t £ 100 s170 ns100 < t £ 1 000 s10112TDEVObservation Interval

(s)(ns)1101001k1001707(log scale)(log scale)1 0000,1tNOTE:The TDEV signal used for a conformance test should be generated by adding white, gaussian noisesources, of which each has been filtered to obtain the proper type of noise process with the properamplitude. Annex C gives an example of a set-up to generate white and flicker phase noise.Figure 4: Input wander tolerance (TDEV)SIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)13Table 7: Lower limit of maximum tolerable input jitterRequirementObservation interval0,25 ms0,1 < t £ 2,5 s0,1 t ms2,5 < t £ 20 s2 ms20 < t £ 400 s0,005 t ms400 < t £ 1 000 sObservation Interval

(s)MTIE10,01010020s)m1k5E-91E-7400((log scale)(log scale)5,02,01,00,250,10,11,02,5tNOTE:The numbers next to the sloped regions indicate the relative frequency offset.Figure 5: Input wander tolerance (MTIE)While suitable test signals that check conformance to the masks in figure 5 are being studied, test signals with asinusoidal phase variation can be used, according to the levels in figure 6.Table 8: Input wander tolerance specified in sinusoidal input wanderRequirementFrequency interval0,0016 f -1 ms0,00032 < f £ 0,0008 Hz2 ms0,0008 < f £ 0,016 Hz0,032 f -1 ms0,016 < f £ 0,13 Hz0,25 ms0,13 < f £ 10 Hz1010Wander Frequency f (Hz)Peak-to-peakWanderms)((log scale)(log scale)5,02,01,00,250,10,1 m0,32 m0,8 m1 m10 m16 m0,10,131,0AmplitudeFigure 6: Lower limit of maximum tolerable sinusoidal input wanderSIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)148Transfer characteristicThe transfer characteristic of the synchronous equipment clock determines its properties with regard to the transfer ofexcursions of the input phase relative to the carrier phase. The SEC can be viewed as a low-pass filter for the differencesbetween the actual input phase and the ideal input phase of the reference. The minimum and maximum allowedbandwidths for this low-pass filter behaviour are based on the considerations described in annex A and are specified asfollows:a)the minimum bandwidth of an SEC is 1 Hz;b)the maximum bandwidth of an SEC is 10 Hz.In the pass band the phase gain of the SEC is smaller than 0,2 dB (2,3 %). The above applies to a linear SEC model.However, this model should not restrict implementation. Measurement methods to determine clock transfercharacteristics are described in annex B.9Transient response and holdover performanceThe specifications in this clause apply to situations where the input signal is affected by disturbances or transmissionfailures (e.g. short interruptions, switching between different synchronization signals, etc.) that result in phase transientsat the SEC output (see clause 10). The ability to withstand specified disturbances is necessary to avoid transmissiondefects or failures. Transmission failures and disturbances are common stress conditions in the transmissionenvironment.To ensure transmission integrity it is recommended that all the phase movements at the output of the SEC stay withinthe level described in the following clauses.9.1Short-term phase transient responseThis specification reflects the performance of the clock in cases when the (selected) input reference is lost due to afailure in the reference path and a second reference input signal, traceable to the same reference clock, is availablesimultaneously or shortly after the detection of the failure (e.g. in cases of autonomous restoration). In such cases thereference is lost for at most 15 s. The output phase variation, relative to the input reference before it was lost, isbounded by the following requirements:The phase error should not exceed DF + 5 x 10-8 x S seconds over any period S up to 15 s. DF represents two phasejumps that may occur during the transition into and out of the holdover state. Each phase jump should not exceed 120 nswith a temporary offset of no more than 7,5 ppm.The resultant overall specification is summarized in figure 7. Background information on the requirements that led tothis specification are provided in annex A.Phase Error(ns)15240 120

0 Time (s)1 0000,016Figure 7: Maximum phase transient at the output due to reference switchingSIST EN 300 462-5-1 V1.1.2:2003

ETSIEN 300 462-5-1 V1.1.2 (1998-05)15This figure is intended to depict the worst case phase movement attributable to an SEC reference clock switch. Clocksmay change state more quickly than is shown here.The figure shows two phase jumps in the clock switching transient. The first jump reflects the initial response to a lossof the synchronization reference source and subsequent entry into hold-over. The magnitude of this jump corresponds toa frequency offset less than 7,5 ppm for a duration less than 16 ms. After 16 ms, the phase movement is restricted to lieunderneath the line with a slope of 5 x 10-8 in order to constrain pointer activity. The second jump, which is to takeplace within 15 s after entering holdover, accounts for the switching to the secondary reference. The same requirementsare applicable for this jump. After the second jump the phase error should remain constant and smaller than 1 ms.NOTE:Output phase excursion, when switching between references which are not traceable to the same PrimaryReference Clock (PRC), are for further study.In cases where the input synchronization signal is lost for more than 15 s, the specifications in subclause 9.2 apply.9.2Long-term phase transient response (holdover)This specification bounds the maximum excursions in the output timing signal. Additionally, it restricts theaccumulation of the phase movement during input signal impairments or internal disturbances.When a SEC loses its reference, it is said to enter the hold-over state. The phase error, DT, at the output of the SECrelative to the input at the moment of loss of reference should not, over any period of S > 15 s, exceed the followinglimit:DT(S) = {(a1 + a2) S + 0,5 b S2 + c}where:a1 = 50 ns/s (see note 1);a2 = 2 000 ns/s (see note 2);b = 1,16 x 10-4 ns/s2 (see note 3);c = 120 ns (see note 4).NOTE 1:The frequency offset a1 represents an initial frequency offset corresponding to5 x 10-8 (0,05 ppm).NOTE 2:The frequency offset a2 accounts for temperature variations after the clock went into holdover andcorresponds to 2 x 10-6 (2 ppm). If there are no temperature variations, the term a2 S should not contributeto the phase error.NOTE 3:The drift b is caused by ageing: 1,16 x 10-4 ns/s2 corresponds to a frequency drift of 1 x 10-8 /day (0,01ppm/day). This value is derived from typical ageing characteristics after 10 days of continuous operation.It is not intended to measure this value on a per day basis as the temperature effect will dominate.NOTE 4:The phase offset c takes care of any additional phase shift that may arise during the transition at the entryof the holdover state.This limit is subject to a maximum frequency offset of ± 4,6 ppm. The behaviour for S < 15 s is defined in subclause9.1. The resultant overall requirement for constant temperature (i.e. the temperature effe

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