irig_accuracy_and_connection_requirements

ARBITERIRIG-B Time Code Accuracy and Connection RequirementsSYSTEMSwith comments on IED and system design considerationsBill Dickerson,P.Eng.Basic Accuracy Capability of theIRIG-B Time Codetypically generated using similar drivers.This meansthat the IRIG-B unmodulated signal can easily be gener-The original use of the IRIG time codes was as a 'timeated with the fundamental accuracy of digital logic:a fewtrack'to be recorded along with test data on multichanneltens of nanoseconds.instrumentation tape recorders (ITRs)at missile testranges.The IRIG-B code,with a 100 bit per secondMost substation 'intelligent electronic devices'(IEDs)signalling rate and a 1 kHz sine wave carrier,was idealthat accept the unmodulated IRIG time code use anfor recording on voice-grade channels.Due to bandwidthoptically-isolated input.This breaks ground loops,mak-limitations of these voice-grade channels (delay anding possible direct connection throughout a control roomphase shifts),the IRIG-B code was guaranteed only towithout excessive concem for grounding and potentialprovide one millisecond resolution and accuracy,equaldifferences.Such optocouplers only require a few milli-to one period of the(1 kHz)carrier signal.amperes of input current,making it possible to connectmany loads to a single IRIG-B driver.The optocouplerWhen the modulated IRIG-B code is transmitted over aoutput is normally connected via a pulse-conditioningdirect connection(i.e.no ITR).channel bandwidth limita-circuit to a logic input (a timer-counter,for example)tions are far less significant.It is relatively easy to buildwhich measures the time of amrival and width of each IRIGIRIG-B generation hardware with accuracy of one micro-pulse.The accuracy of this process is also quite good:second,and to demodulate the signal at an accuracyeasily better than one microsecond,and potentially a fewlevel of tens of microseconds.The challenge is totens of nanoseconds.accurately generate and detect the carrier phase angle.Most decoders use simple zero-crossing detectors,Unmodulated(Level-Shift)IRIG-B TimeCodeWiringwhich are generally adequate although the signal slewrate has an inflection point(caused by the change inUnmodulated or level-shift IRIG time code is generallysignal level)at the on-time mark.For most applicationsdeveloped by a system clock at a level of approximatelyusing the modulated IRIG signal,however,this level of5 volts peak,i.e.the 'high'levelis approximately+5V andperformance (tens of microseconds)has proved ad-the 'low'level approximately zero volts.This signal isequate.normally distributed using copper wiring,which may beeither coaxial(typically the common RG-58 types)orSince the modulated IRIG signal is basically an audioshielded twisted pair.Most drivers are unbalanced andsignal,like a telephone signal,similar techniques may bethe clock outputs are often coaxial(typically BNC).used for distribution.The input impedance of a typicaldecoder is several kilohms,so alow source impedanceFor applicat