HP PN83475 1_Communication Waveform Measurements

8/14/2019 HP PN83475 1_Communication Waveform Measurements

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Commu nicat ion w aveformmeasurements

Product Note 83475-1


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In t roduct ionThe H P 83475 provides a com-prehen sive set of measu remen tcapabilities specifically gear edfor digital comm un icat ionsapplicat ions. Th is product n otewill review some of the ba sicmeasur ements performed by theHP 83475. Specific exam plesfor a na lysis and conform an cewaveform measurements areprovided.

Communica t ionsm e a s u r e m e n t sDevelopment a nd ma nu facturingof digital commu nicat ions com-ponents and equipment r equireextensive test ing of performan cechara cteristics. A subset of th esechara cteristics can be testedan d monitored th rough physicallayer waveform measurements.These measurements serve tochara cterize th e t ime-doma inresponse of a communicationsystem or component an d play

an importan t role in assuringsufficient operational margin andtr an smission qu ality. Commu ni-cations standa rds, such as SDHand SONET, require severa lwaveform mea suremen ts toassu re equipment compliance tosta nda rd specified performa ncelimits. The HP 83475 measur esma ny of the t ime-domain char-acteristics of digita l data an dperforms several of the sta ndar drequired waveform

measurements.

Commun ica t i o nss tandardsTelecommun ications a nd Data -commun ications stan dar ds, suchas SDH, SONE T, and FDDI,requ i re s t r ingen t ope ra t ingcharacteristics from compliantequipm ent . This allows for com-pat ibility between equipmen tfrom different ma nu factur ers.Intensive testing and analysisar e required t o gain conform an ceto the st an dar d specificationsand to eventua l ly obta in th erequired equipment performance.

Sta nda rds are typically specifiedin accordance with th e ISO OSINet work Model which dividesdifferent network tasks intolayered modules. Each layer re-quires different types of testing.The lowest layer in th e OSINetwork Model is referred t o asth e ph ysical layer. The layerspecifications are essentiallyconcerned with issues involvedin bit tra nsm ission. For example,

physical layer st an dar ds specifysignal levels and sha pe, cableconn ectors, an d cable. Several of

the physical layer parameterscan be m easur ed or verified witha digital sa mpling oscilloscope a nd

a wide-ban d, DC-coupled optical-to-electrical (O/E) converter.

Conformanceme a s u r e men t sTypically done in componen t a ndequipment man ufacturing, astanda rds-driven measur ementis mask an d/or templat e wave-form compliance. Stan dar dssuch as SDH, SONET and F ibreChann el require transmittertime-domain response complianceto specified geomet ries k nownas waveform templates or masks.A pulse or eye-diagram mask test combines ma ny of th e pulseparam eters in a single test andin some cases m ay come closerto specifying wha t is n eeded forproper circuit operat ion th an t heindividua l pulse or eye par am eter.Such tests are done at the t rans-mitter outpu t an d provide a finalcheck to assur e that the system

or component ha s su fficient m ar-gin for p roper operation un derreal operat ing conditions.

Figure 1. Eye-diagram m askdefinit ion for SDH/SONET.

1+Y1

1.0Y2

0.5

Y10.0

Y10.0 X1 X2 X3

STM-1(155.52 Mb/s)

STM-4(622.08 Mb/s)

X1/X4 0.15/0.85 0.25/0.75

X2/X3 0.35/0.65 0.40/0.60

Y1/Y2 0.20/0.80 0.20/0.80

STM-16(2.48832 Gb/s)

X3-X2 0.2

Y1/Y2 0.25/0.75

Amplitude

Unit Interval

X4 1.0 0.25 1.25


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A standard defined O/E referencereceiver is required, by stand ardssuch as SDH a nd SONET, when

performing a ma sk or templatetest on l ightwave tr ansmitters.The frequency response of thisreferen ce receiver is tight ly con-tr olled a nd is sp ecified as t ha tof a fourt h-order Bes sel-Thomsonfilter with a 3 dB frequen cy at0.75 times th e appropriate bitra te. This hea vy filtering servesto int egrate t he avera ge powerwithin t he bit period, reducingthe effects of overshoot and noise.Thus, more consistent measure-ment resul ts can be typical lyobtained.

Wave form ana lysisBeyond st an dar d-defined mea-suremen ts , fur t her an alys is i stypically perform ed by usin gphysical layer waveform measu re-ment s. The time-domain response,usu ally in t he form of an eyediagram , provides valuable in-sight int o the per forman ce of a

commu nication tr an sceiver orsystem. In development, wave-form ana lysis through para metricmeasurement s can be very usefulas a first step in u nderstandingsystem or component performa nce.As an exam ple, eye-diagra mwaveform para meters, such asnoise, jitter, an d rise t ime, canprovide insight into system orcomponent perform an ce limita-tions. In order to obtain maximumaccur acy, these para meter s are

often cha racter ized u s ing thehistogram stat istical functions of a sampling oscilloscope and areperformed at the highest possiblebandwidth (BW).

A nominal extinction ratio onl ightwave t ra nsmit ters i s a lsorequired by standar ds such asSDH, SONET and Fibre Chann el.This measurement can be conduc-ted on th e eye-diagra m r esponseof the tr ansmitter th rough theuse of a sampling oscilloscopean d a DC-coupled r eference O/Ereceiver. The average opticalpower of an S DH/SONET tr an s-mi t t e r i s ano the r impor t a n tpa ram ete r t ha t mus t complywith s t an dar d specif ica t ions .Other physical layer measur e-ment s related to the time-doma inresponse of communicationsequipment include jitter an d BitEr ror Rat io (BER). BER is thefundamental performance char-acteristic of a communicationssystem or componen t an d istypical ly measured wi th a Bi tEr ror Ra tio Test (BERT) system.

Figure 2. Eye-diagramcharacterization

The HP 83475 Lightwa veComm u n i c a t i onsAnalyzerThe HP 83475 Lightwave Com-munications Analyzer introducesa convenient an d cost effectivesolution th at fits into man y of th e applicat ions wh ere a n oscil-loscope an d extern al O/E ha ve

been tr aditionally employed. Theinstrument is based on a porta -ble fra me with an an alog look and feel. It provides very fastupdate rates at all sweep speedsto give immediate feed-back whenthe waveform under analysischanges. Within t his platform, t heHP 83475 introduces an integra -ted a nd calibrated optical chann eltha t allows for peak and averageoptical power m easur ements.

Rise Time Fall Time

Overshoot

80% Level 50% Level

20% Level

Voltage Histograms

TimeHistograms

v vv v

v

v


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In a ddition, two 500 MHz elec-tr ical chan nels ar e available forgeneral pur pose circuit testing

and ana lys is . The HP 83475provides comprehensive commun-ications waveform analysis cap-abilities with bu ilt-in aut omatichistogram m easurements suchas noise an d jitter. Furt herm ore,the ins t ru ment a lso has a l l of th e capabili t ies of a gener al-purpose 500 MHz oscilloscope.

of the int erna l O/E a nd t he elec-trical measur ement chann el, acalibrat ed vert ical Watts scale

ha s been pr ovided to view theinsta nta neous power levels of th e optical wa veform. The demod-ulated output of the integratedO/E receiver is accessible on th efront pan el of the ins t ru mentan d can be conn ected to th eelectr ical input t hr ough a BNC

jum per or a filter.

Besides allowing for calibrat edpower measurement s, the inte-grat ed optical chann el can fun c-

tion as a fully complian t SDH / SONET or Fibre Chan nel refer-ence receiver syst em. An optionalcalibrat ion of the combin edresponse of the integrat ed O/E,a h ar dware filter, and t he elec-tr ical chann el can provide mea-sur ed compliance to SDH/SONETan d/or Fibre Cha nn el referencereceiver specifications. Thereferen ce receiver specificat ionsar e provided in th e form of afrequency response window abouta fourth-order Bessel-Thomsonresponse with a 3 dB frequencyat th ree four th s th e specified bitra te. Optical cha nn el referencereceiver options are availablefor S DH/SONE T OC-1 (51.840Mb/s), STM-1/OC-3 (155.52Mb/s), an d Fibre Ch an nel 133MBau d. A referen ce receiver isrequired for ma sk testing to theabove standa rds. It is a lso recom-men ded for optical extin ctionrat io measurements.

Figure 3. The HP 83475convenien t ly measuresboth op t ica l and e lec-tr ical signals.

The lightwave technology integra-ted within the HP 83475 consistsof a low-distort ion, P IN-based1 GHz O/E converter. The opticalinput provides a 62.5/125 mmultimode fiber interface. Aninnovative calibration procedureis implemented on th e HP 83475optical chan nel to provide, witha single connection, both averagean d instan tan eous optical powermeasurements. By monitoringth e photodiode curr ent, a built-in a verage optical power meterhas been implemented. In addi-tion, through char acterization

Figure 4. SDH/SONETreference rece iverf requency respons e

OscilloscopeMainframe

External Trigger

1 GHz O/E Converter

OpticalSection

Optical Input

Demodulated Output

Jumper/Filter

Average Power Monitor

Channel 1

Channel 2

SDH/SONET reference receiver specificationsFourth-Order Bessel-Thompson Response

0 0.5 1 1.5 2

20

15

10

5

0

Fraction of Bit Rate

Nominal Attenuation (dB)

Standard

Window

vv


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S e t t in g u p t h ei n s t r u m e n t

In operating the instrum ent, therear e ha rdk eys an d softkeys.Har dkeys reside on th e instr umentfront pan el and th eir fun ctionsar e printed directly on t he keys.These keys are denoted wi thbold type such a s A u t o s c a l e .Softkeys a re th ose tha t a relocated across the bottom of thedisplay and wh ose fun ctions a regiven on th e instru ment display.Softkeys can cha nge dependingon th e selected h ar dkey. They

are n oted in brackets such as[Rise Time]. The instrument hassoftk eys th at offer a choice of settings or functions. An exampleis [Opt P mtr On Off] . In t h iscase, you ca n ch oose On bypressing t he softkey u nt il Onis h ighlight ed, or choose Offby pressing th e softkey u ntil Off is h ighlighted. Softk ey choicesar e noted as: [Opt P mtr On Off]On .

Caution: Extreme ca re shou ldbe t aken to avo id damag ingthe in s t rumen t connec to r

ferrules. Damaged con nectorsreduce me asurement integrityand are n ot user serviceable .

Please r efer t o the conn ector carechapt er in the instr ument User sGuide for conn ector cleanin g andcare informa tion.

Con n ec t i ng de mo do u tp u t t o ch a nne l 2To make optical chan nel measu re-ments, th e demodulated outputof th e integrat ed O/E m ust bephysically connected to theCha nn el 2 electr ical input . Forunf il t e r ed measuremen t s , a50 Ohm BN C jumper is pr ovidedto connect the Demod outpu tand t he Chann el 2 input. Thenominal impedance of the O/Edemodulated output is 50 Ohm.Thu s, in order t o achieve animpedance mat ch, the Ch ann el

2 input sh ould be set to 50 Ohm.To set Ch an nel 2 to a 50 Ohminpu t p res s : O p t i c a l / E l e c 2 ,[Inpu t 50 Ohm 1 MOhm] 50 Ohm.

Clean f iber end s andins t rume nt t es t por t sIt is a good measu remen t pr acticeto clean fiber in ter faces beforemaking measurements and per-forming calibrations. Clean fiberends an d conn ector ports ar eessential for good mea sur ement s.The optical chann el port an d allfiber ends should be kept clean.Dirty conn ectors can result inredu ced sensitivity. To clean th eoptical chann el input, t he con-nector ada pter is removed fromthe ins t ru men t f ron t pa ne l ,exposing the cable ferrule.

Opt ica l o r e lec t r ica lThe HP 83475 can be configuredwith either two electrical channelsor one electr ical a nd one opticalchann el . Cha nn el 1 i s s t r ic t lyfor e lect r ica l measu remen ts .Chan nel 2 can be set up a s eitheran electrical or an optical mea -sur ement chann el. To define th eChan ne l 2 fun c t ion , p re s s :Optical/Elec 2 , [Chan 2 Off OpE2] Op or E2. Op sets Chann el2 as an optical measur ementchannel. Note that the verticalscale is given in wa tt s. E2 setsChan nel 2 as an electrical chan -

nel and the vertical scale is givenin volts.

When Op is selected, t he p eak power of the waveform can bemeasu red to the vertical accur acyspecified for t he optical chan nel.Vert ical accura cies a re specifiedfor 780, 1310, an d 1550 nm . Forth is an d other speci fica t ions ,refer to th e HP 83475 Techn icalSpecificat ions docum ent ( P/N5962-7935E ).

Note: The optical chan nel canbe ac t ivated ( Op se lecte d)on ly i f t he Channe l 2 inpu timpedance i s s e t a t 50 Ohm.

If th e HP 83475 includes instru -ment options 050, 051, 052 and/or053, the demodulated outpu t a ndCha nn el 2 can also be conn ectedwith a reference receiver h ar d-war e filter. Referen ce receiverfilters ar e required for standa rddefined optical eye-diagram

testing. For proper filter perfor-man ce , t he Chann e l 2 inpu tshould be set at 50 Ohm.


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S e l ec t ope r a ti ngwa v e l eng t hWhen using th e HP 83475 opti-ca l chann el , th e appropr ia tewavelength calibration sh ouldbe selected. The wavelengthcalibrat ion activates t he correctvertical scale and power m etercalibration for the optical signalat th e input . For example, if a1310 nm s igna l is being an alyzed,the 1310 nm ver t ica l channelan d power met er calibrat ionshould be activated. To activateth e 1310 nm factory calibration,

press: Optical Cal , [Show DefltUser] Deflt, 1310 nm. Sta nda rdcalibrat ions ar e specified for20 nm from the calibrated wa ve-length.

The H P 83475 allows for u sercalibrations at wavelengths 20 nm from t he factory calibra -tion wavelength s of 780, 1310,and 1550 nm. Up to three usercalibra tions can be per formedan d saved. Refer to the H P 83475

User s Gu ide for th e us er cali-brat ion pr ocedure.

When you pr ess the Autoscalekey, th e an alyzer scales all chan -nels tha t ha ve signals applied

an d selects a t ime-base ra ngebased on th e trigger source. Thetr igger source selected is t hehighest numbered input t hatha s a signal ap plied. If a signalis connected t o the externa ltrigger input on the HP 83475,then it is selected as the t riggersource.

S e t t i n g t he s am p l e s i z efor s ta t is t icalme a s u r e men t sSeveral of the m easurement sma de by the H P 83475 are basedon values obtained from a st atis-tical distribut ion of dat a a ccum -ulat ed at a par ticular wa veformlocation. The data can be arranged,for st at istical an alysis, into whatis called a histogram. A histogramis a graphical representation of the data distribution. Para meterss u ch a s m e a n a n d s t a n d a r ddeviation can be obta ined from

a h istogram. These parameterscan be used a s a ba sis for wave-form a na lysis. For exam ple, th estan dard deviation of a time his-togram taken at the eye-diagramcrossing point is t ypically iden-t i f ied as th e s ignal j it t e r. TheHP 83475 also uses histogram sto obtain eye-diagram parameterswhich ar e used for m ask position-ing and scaling. The eye-diagramcrossing points an d m ean 1 an d0 levels ar e sta tist ically deter -

mined an d used to scale an d posi-t ion a mask to an eye diagram.

Autosca leAfter conn ecting eith er an opticalsignal or an electrical signal toan H P 83475 input chann el, thesignal display can be au tomati-cally optimized by pressing theAutoscale front pan el key. Usingau toscale requires signals witha frequency range greater t hanor equal to 50 Hz and a du tycycle great er th an 1 percent.

Se l ec t i ng t h emeasu reme n t ty p e

The HP 83475 can m ake au to-mat ic measur ements e i ther oneye diagrams or pu lse waveforms.The instr um ent m ust be config-ured to make th e desired measur e-ment t ype. The measurementalgorithm s ar e different depend-ing on t he t ype of waveformtha t is to be ana lyzed. If an eyediagram is to be analyzed, accessthe eye-diagram measurementsby pressing Power /Vol t age ,[Meas Type Genl Wvfm Eye:NRZ]

Eye:NRZ or Time , [Meas TypeGenl Wvfm Eye:NRZ] Eye:NRZ.If a pulse measurement n eedsto be made, un der th e [Meas TypeGenl Wvfm Eye:NRZ] softkey,mak e sure t ha t Genl Wvfm ha sbeen selected.

Only one type of measu remen tcan be made a t a t ime. If mea-sur ement s of a given type areactive, and th e measurement typeof the ac t ive measuremen t i s

chan ged, the measurements ar eautomatically deactivated andth e resul ts sect ion displaysGen l off or Eye off.

For eye-diagram measurements,th e HP 83475 allows the user t oset th e num ber of h is togramsamples used in th e determina-tion of sta tistical measu remen tsan d mask scaling. There ar e twotypes of histograms acquired bythe HP 83475, time and a mplitudehist ogra ms. You can indepen -dent ly set sam ple sizes for both

types of histogram s. Figure 5illustra tes t he locations of thetime and amplitude histogramsrelative to th e waveform.

N o t e : T h e H P 8 3 4 75 u s e shistograms to make several of the waveform measurements .Howeve r, t he h i s tog rams a renot shown on the ins t rumentdisplay.


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Figure 5. Time and

ampl i tude h i s togramson an eye d iagram

You can set t he n um ber of am -plitude histogram samples bypressing: Power/Voltage , [More1/4], [More 2/4], [More 3/4],[Ampl Sam p]. Use th e general-pur pose knob to set th e num berof am plitu de sam ples to be ac-quired in the determ inat ion of th e eye-diagram vertical para -met ers. Typically, a sa mple size

of around 1000 is sufficient to getgood accuracy and scaling time.

L a rg e s a m p l e s i z e s l e a d t ov e r y a c c u r a t e r e s u l t s . H o w -e v e r, m e a s u r e m e n t t i m e i ssignificantly increase d. Typi-ca l ly, a t ime s ample s i ze o f a round 300 i s su ffi c i en t fo rgood accuracy in the determin-ation of eye-diagram crossingpo in t s .

str eam is similar to re-timingth e data because th e signa l ordata j it ter is greatly attenuat ed

ar ound th e triggering point. Inaddition, when a na lyzing eyediagrams, triggering on the da tadoes not pr ovide a worst case ortrue representation of the signaleye patter n. When t riggering ondata, only a subset of all of thepossible tran sitions is representedby the eye diagram. This su bsetis represen tat ive of the sectionson th e data str eam tha t provideth e requ ired condit ions for atr igger; nam ely a rising or fallingedge with sufficient signal en ergy.If a rising edge is selected as t hetr igger condition, for exa mple,sections of the dat a pa tter n con-ta ining long st rings of high a nd/orlow levels, such as 11110000, arenot captur ed on t he eye pattern.

You can set th e nu mber of timehistogram sa mples by pressing:Time , [More 1/4], [More 2/4],[More 3/4], [Time Sa mp]. Use t hegeneral-purpose knob to set th enu mber of time sam ples to beacquired in the determ ination of th e eye-diagram crossing point s.Typically, a sample size of around300 is su fficient to get good accur-acy and scaling time.

Note: Statistical measurem entt ime i s ve ry sens i t i ve to thenumber of se lec ted t ime sam-ples . This i s due to the ve ryna r row h i s tog ram window srequired for the acquis i t ionof t ime or horizontal samples.

Triggering

The t r igger source for theHP 83475 can be specified asChann el 1, Chann el 2, ExternalTrigger, or Line. Triggering ona signal applied to Chan nel 1 or2 allows for triggering on a datastream whenever a reference clock is unavailable. The externaltrigger allows for triggering ona referen ce or r ecovered clock signal. Line triggering selects anintern al signal, of the sam e fre-quency as th e power line signal

(50 to 60 Hz), as th e tr igger source.

Triggering on th e data , by usingthe Cha nnel 1 or 2 internal tr ig-ger, is both a practical and com-mon pra ctice. Depending on t hetest objectives, useful inform a-tion is available thr ough thismeth od. However, un der th iscondition, accurat e a na lysis of timing informa tion can not beobtained. Triggering on the data

To analyze signal jitter and/orobtain a m ore accurat e represen-ta tion of the signal-eye patter n,it is recommen ded tha t an exter-nal reference signal be utilizedas the trigger. The referencesignal can be obtained from t hedat a source or thr ough a clock recovery device. The data signalcan be connected t o one of thetwo data channels and t he triggersignal can be conn ected t o theother available data chann el orto the external t r igger input .The HP 83475 has a viewableexternal t r igger input . Whenboth Ch ann el 1 and 2 are beingut ilized, the r eference tr iggersignal can be conn ected t o theexterna l trigger. This signal canbe displayed for ana lysis an dt iming measurement s betweendat a chan nel and t rigger. Toview the extern al trigger signa lpress: Source , [Ext], [Menu],[Ext Trig View On Off] On.

Time Histograms

Time Histogram Window

VoltageHistogramWindow

Voltage HistogramsMean "1" Level

Mean "0" Level

RMS Jitter

v

v

v

v

v

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Transceiverconfo rmance

m e a s u r e m e n t sMeasur ing averageop t i c a l po w e rAverage power is a funda ment almeasu remen t for an y lightwavecommu nications system or tra ns-mitter. I t is a param eter tha t isdirectly related to BER perfor-man ce. In order t o assure a nom-inal system BE R performa nce,s t anda rds such a s SDH andSONET require tha t the nominalmean laun ched power of a com-pliant tran smitter be within acertain r ange. There are severalmean launched power ran gesdepending on tr ansmitter appli-ca t ion. The SDH a nd SONE Tsta nda rds def ine two types of appl ica t ions . These ar e in t r a-office an d int er-office. Th e in ter -office application is sub-dividedinto short -ha ul and long-ha ulimplement at ions . Tran smit terand sys t em pa ramete r s a respecified for all of these.

In cont ra st, for int er-office long-ha ul 1310 nm applications, thetypical repea ter link distan ce is

ar ound 40 km a nd, consequen tly,the mean launched power ran geincreases to between 5 dBman d 0 dBm . Typically, in commer-cially available tra nsm itters, amargin is included in tran smit-ter launch power accounting forend of life per form an ce. In effect,short hau l tran smitters could beman ufactur ed for a t least 8 dBmlaun ch power. This pr ovides a7 dB end-of-life m ar gin.

externa l attenu ator should beused in conjunction with t he in-tern al att enua tion correction of

th e HP 83475. First , you mu stdetermine the att enuat ion valueof the att enuator and ma ke surethat you have the optical channelactivated. Then, to correct theex te rna l a t t en ua t ion , p re s s :Optical/Elec2 , [More 1/2], [ExtAttn]. Use the genera l-pur poseknob below t he Cursors/Masksfront pan el key to d ia l in th eexternal attenuation. This willcompensa te for th e external at ten-ua t ion to provide an accur at ereading of the average p ower.

Note: In addition to the pow ermete r r ead ing , t he ex te rna la t t enua t ion co r rec t ion a l socor rec t s t he op t i ca l channe lver t ica l sca le and a l l auto-ma t i c ampl itude pow er mea-su remen t s .

Caut ion: To avo id dam age tothe an alyzer, make su re theave rage op t i ca l power l eve l

of the s ignal into the OPTICALINPUT is less than +4 dBm(2.5 m W).

The mean launched power ran gesar e provided such th at systemBER performa nce is maint ainedover th e physical distan ce tar -geted for the given application.For e xam ple, for in tr a-office,155 Mb/s, 1310 nm applications,th e typical link dista nce is lessthan 2 Km and the mean launchedpower r ange r equired for 10 12 BERperformance is 15 to 8 dBm.

N o t e : F i x e d s i n g l e m o d e( 9 /1 2 5 m ) a t t e n u a t o r s c a nprov ide unexpec ted powe rreadings when interfaced withthe 62.5/125 m mu lti-mod eHP 83475 input . Some s inglemode attenuators achieve sig-nal dampening through leak-a g e i n t o t h e 1 2 5 m f ib e rc ladding. This s ignal conte ntc a n b e d e t e c t e d b y a m u l t i -mode 62.5 m f iber core .

By monitoring the photodiodecurrent on th e integrated P INphotodiode, the H P 83475 mea -sur es average optical power atth e push of a but ton. After select-ing th e correct operat ing wave-length calibration an d conn ectinga signal to th e HP 83475 opticalinput, you can measure averagepower by pr essing: P o w e r / Voltage , [Opt P mt r Off On] On.

At 1310 and 1550 nm, the

HP 83475 optical power meterha s a dyna mic ran ge of 24 to3 dBm. To accur at ely measur epower levels above 3 dBm, a n

Figure 6. HP 83475 opticalpower measurement


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Lightwave t ransmi t te rma sk t e s tFiber-optic communicationsstanda rds such as SDH, SONET,and F ibre Chann el require thatthe t ran smitter t ime-domainresponse, usua lly in t he form of an NRZ uni-polar eye diagra m,have a particular sh ape. Thissha pe is specified in ter ms of ama sk or template which dictat esregions where th e wa ve-formshould not reside. In th e manu -factu ring of sta nda rd compliantequipment, optical transmitter

ports must be tested for mask complian ce as a fina l check of proper operat ion. Typically, ea chrat e with in a stan dards hierarchyha s its own ma sk geometr icalspecifications.

The following st eps sh ow you howto use th e an alyzer to performan SDH/SONET STM-1/OC-3

(155 Mb/s) mask test:

1) Obtain a st able eye-diagra mdisplay. Adjust th e vertical scaleso tha t th e displayed eye diagramis around four divisions pea k t opeak. Adjust the vertical offsetso th at t he eye diagra m is cen-tered a bout th e middle of the ver-tical scale. Horizontally, ma ke suretha t a t least two crossing pointsar e displayed. For a 155 Mb/ssigna l, the h orizont al sweep time

should be set t o around 1 ns/div.Adjust t he delay to center th eeye diagram on th e display.

[Margin], Press this key anduse th e genera l-pur pose knobto adjust t he test ma rgin. The

test m ar gin ma kes th e confor-man ce test either more diffi-cult or ea sier to pass. It is fun -damen tally, a linear expansionor r eduction of the ma sk. Apositive margin can be u sed toset a t est performa nce limitfor process cont rol. A negat ivemar gin can be u sed to betteran alyze the perform an ce limi-ta tions of non-conform ingtransmit ters .

[Max Hits], Press this key anduse th e genera l-pur pose knobto set the ma ximum allowednum ber of mask hits for a pa sscondit ion t o tak e place. Typi-cally, this par amet er is set toa va lue of zero hits.The HP 83475 can aut omatically

test waveform compliance to avariety of optical ma sk sta nda rdspeci fica t ions . In add i t ion,th rough an externa l controller,custom ma sks can be down-loaded

into the HP 83475. This allowsfor complian ce testing to user -defined ma sk specificat ions. Th eavailable optical stan dar d mask tests are:

SONET/SDH(ITU-T G.957 an dBellcore GR-253-CORE):

OC-1, 51.84 Mb/sOC-3/STM-1, 155.52 Mb/s

ANSI X3.88:Fibre Cha nn el 133 MBdFibre Cha nn el 266 MBd

2) Select t he stan dard m ask t estby pressing: Curso r s /Masks ,[Select Mask], [OC3/STM1],[done].

3) Define the t est par ameters bypressing: [Define Test]. In thissoftkey m enu , you can definetest para meters by pressing thefollowing softkeys:

[Test for X sec], Pr ess th is keyand use the general -purposeknob to set th e test time. Thetest t ime is directly related toth e nu mber of waveforms wh ichar e tested to the optical ma sk.Typically, 2 seconds is sufficientto obtain a good repr esenta tionof the transmitter eye diagram.

[On Pa ss CONT PRINT STOP],Pr ess this key to toggle throughthr ee action choices which a retriggered on a pass t est condi-tion. CONT causes t he a na lyzer

to contin ue n ormal a cquisition,PRINT causes th e ana lyzer t osend a ha rd copy of the testresults to an externa l printer,and STOP causes the a nalyzerto stop dat a acquisition a ndfreeze the test resu lts on th edisplay.

[On Fa il CONT PRINT STOP],Pr ess this key to toggle throughthr ee action choices which a retriggered on a fail test condi-tion. CONT causes t he a na lyzerto continu e n ormal a cquisition,PRINT causes th e ana lyzer t osend a ha rd copy of the testresults to an externa l printer,and STOP causes the a nalyzerto stop dat a acquisition a ndfreeze the test resu lts on th edisplay.


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4) Once test para meters ha vebeen se t , t he ma sk t e s t can beperformed by pres sing: [Pr evious

Menu], [Do Test].

The analyzer will scale and posi-tion th e mask on th e eye diagraman d th en go into infinite persis-tence mode while acquiring dat apoint s for t he specified test time.Once the dat a h as been acquiredand tested against th e mask, thetest resu lt will flash on th e lowerpart of the screen an d th e speci-fied tes t a ction will be performed.The test result is also provided

below th e [Do Test ] softk ey.

The second option is to press [ShowMask On ly Once Cont inuousOff] Continuous. In other words,

toggle the key until Continuousis displayed.

In Continu ous mode, th e ana lyzerwill continuously acquire histo-gram data to scale the mask tothe eye diagram . If th e eye dia-gram changes or th e instrumentsett ings are chan ged, the ma sk will automatically be scaled tothe displayed eye diagra m. Thescaling will work only if both eye-diagram crossing points a nd th e

dat a h igh and low levels are

Cus to m m a sk t e s t i n gBeyond th e aut omatic standar dmask t es ts , the HP 83475 canmeasure compliance to user-defined mask geometries. ThroughHP -IB or RS-232 rem ote contr ol,up t o two cust om masks can bedown-loaded int o the H P 83475non-volatile mem ory. Ea ch ma sk can be defined t o have up to eight

separa te geometrical regions ora t otal of 120 points, wh icheverlimit is reached first. The cust ommask s a lso suppor t a l l of theau tomat ic tes t ing a lgor i thm s,such a s scaling and positioning,employed with th e built-in sta n-dard masks. For details on creat-ing and p erform ing cust ommask tes ts , p lease refer to th eHP 83475 User s Guide (RevisionNovember 1995).

Figure 7. STM-1/OC-3tes t resu l t s

If th e eye-diagram sha pe cha nges(ie., laser bia s is chan ged) or if the vert ical or horizontal instr u-ment sett ings ar e chan ged, themask will no longer be scaled tothe eye diagram. Two optionsexist t o rescale th e mask . One isto perform another test th roughthe [Do Test] softkey. This cantake some time depending on thesett ings of the test param eters.

shown in the instru ment display.The scaling time will depend u ponth e selected number of amp litu de

and time histogram sa mples.

Figure 8. RescaledSTM-1/OC-3 opticale y e m a s k


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11

Elec t r ica l t e lecompu l se m as k t e s t i ngTypically, lightwave transmittersignal shape is dictat ed by stan-dard specified eye-diagram masks.Electr ical telecom t ra nsmitt ershave s imi l a r r equ i remen t s .However, th e signal sh ape forelectrically tran smitted signalsis typically specified in t erm s of a pu lse mask or template. Thepu l se t empla t e , li ke an eye -diagra m ma sk, seeks to combineman y pu l se pa ramete r s in asingle test. The specified shape

is geared to provide proper circuitopera tion an d t o allow for multi-vendor ne t work equ ipmen tmatching at different network interfaces.

eye-diagram mask s ar e providedfor versatility and convenience.As will be explained, an eye-

diagram test needs less sophisti-cated mea sur ing technology asit ut ilizes th e entire signal.

Signal character is t ics int ime d iv i s ion m ul t ip lex ing(TDM) equipmen tThere a re two types of dat aencoding techniques typicallyencoun tered in e lec t r ica l t e le-comm un icat ions t r an smiss ion.T h e s e a r e A lt e r n a t e M a r k Inversion (AMI) and Coded Mark Inver s ion (CMI). The m ostcomm only used is AMI. CMI istypically utilized for th e higherd igi t a l h i e ra r chy l ine r a t e s ,such as t he PDH 139.264 Mb/sra te (commonly known as E4).Electrical, unipolar signals ar econverted to either AMI or CMIbefore tr an smission.

AMI is a 3-level non-retur n-tozero (NRZ) code for which, in its

Figure 9. Unipolarb inary sequence and i t sequiva len t AMI sequence

Pu lse matching cha ra cteristicsare defined by industry stan dardsand recommendations such asITU-T Recommendation G.703,ITU-T Recommendation I.430,ANSI T1.102-1987, the currentrevision of T1.102-199X, an d

th e T1.403-1988. The ITU-T(formerly CCITT) recommenda-tions refer to wha t is t ypicallyknown a s th e PDH (Plesiochr o-nous Digita l Hierar chy) network.These standards are typicallyused in most of th e world withthe exception of North America.The ANSI standards deal withthe specifications for the NorthAmerican network.

Though m ost of th e electr icalsignal mask s/templates ar e forpulse shape specification, someof the ma sks a re for eye-diagra mshapes. The electrical side of theBellcore SONET sta nda rd, forth e 51.84 Mb/s (STS-1) an d 155Mb/s (STS-3) line rates, specifiesboth eye-diagram a nd pu lse masksto test signal char acteristics. The

simp lest form , logic 1s arerepresented by a l tern at ing the1s p olarit y a nd logic 0s are

represen ted by a zero amplitudespace. An example is shown inFigure 9.

AMI meets th e objective of mini-mizing the DC component in th esignal. The voltage sum ma tion of all th e pulses on a well-balan cedAMI coded line will be zero.

CMI is a 2-level, bipolar non-ret ur n-to-zero (NRZ) code in whichth e logic 0 is coded so th at both

amplitude levels, A1 and A2,are attained consecutively, eachfor ha lf th e unit t ime interval(T/2). Similar to AMI, the logic1s are coded such t ha t t heamplitude alternates, betweenA1 and A2, for consecutive logic1s. An exa mple is given inFigure 10. Note th at for the logic0 th ere is a lways a posit ivetr ansition at t he midpoint of thebinary unit t ime interval.

1 0 1 0 01 1 0

0 1 0 1 01 0 1

Unipolar(Binary)

Bipolar(AMI)

Binary to AMI Signal Conversion


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12

Figu re 10. CMIcoded b inary s igna l

There ar e several zero substitu -tion algorithms typically utilizedin the telecommu nications tr an s-mission. A very sim ple codewhich is typically used for thePDH 2, 8 and 34 Mb/s (E1, E2,E3) line rat es is High DensityBipolar of order 3, also kn ownas HDB3. In HDB3, each block of four consecut ive zeros is replacedby 000V or B00V, wh er e V is anAMI code violat ion a nd B r epre-sents a n insert ed pulse conform-ing to th e AMI rule. Th e choiceof 000V or B00V is ma de so th atthe n um ber of B pulses betweencons ecutive V pulses is odd. Inother words, su ccessive V pulsesare of altern at e polarity so that

no DC component is intr oduced.

I so la t ed one sAn isolated 1 pu lse is a pulseun affected by in tersymbol inter-

ference or corru pted by noise. Inmost cases, du e to l ine codingconst r a in ts , an ideal i sola tedpulse can only be approximat ed.Typically, an isolat ed pu lse isappr oximated by a pulse tha t ispreceded by a nd followed by zerosta tes. The ANSI an d Bellcorepulse mask st an dar ds, for AMIcoded signals, ar e writt en for apositive isolated pulse. The pulsemask will always span m ore th anone pulse time slot. It is impor-tan t tha t the pulse chara cteris-tics preceding an d following th eisolated pu lse be evalua ted. Thechara cterist ics of th e pulse aswell as t he sett l ing before andafter the pulse must be considered.

Zero su bst i tu t ionTiming informa t ion must beder ived from t ra nsmit ted da taby the r eceiving equipment .Receiving equipment must phaselock to the t ran smitted data inorder t o recover informa tion. Forthis reason, it is importa nt tha tth e data on a telecommunicationline not rema in in the zero sta tefor an extended period of time.Equipment is designed so that,for AMI signal tr an smission, t heextended st rings of zeros arerepla ced by 1s pulses. Receivingequipment, in tur n, must be ableto identify the zero subst itutionsan d remove th e insert ed 1s torestore t he origina l inform ation.

When zero substi tu tion is exe-cuted, t he AMI coding scheme isviolated. In other words, th e insert-ed pulse is of the same polar ityas th e pr evious logic 1 pu lse.This AMI code violation flagsreceiving equipmen t so tha t it canremove the zero substitut ions.

Given the coding scheme, CMIcodes do not requ ire an y zerosubstitu tion. There a re no spaces(or zero amplitude t ime interva ls)in a CMI encoded binar y sequenceand there ar e plenty of tra nsi-tions for receiving equipment torecover clock an d syn chronize.

The isolated 1s standar ds arewritten for leading and t ra ilingzero sta tes. Extended zero stat esdo not norm ally exist in l ivetraffic (random data) because of th e zero substitut ion algorith msdiscussed pr eviously. Leadingand trailing zero requirementsvary depending on th e mask inus e. The Bellcore S TS-1 51.84Mb/s pulse mask test r equires apulse preceded by at least twozeros an d followed by at leastone zero.

Amplitude

Unit Interval

0 0 1 0 1 1 1 0

Binary Sequence:

A2

0

A1

Time

v v


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Fixed vo l t age ve r susnorma l i zed ampl i tudetempla te s

The am plitude of pulse templat esis typically specified either infixed volts or in normalizedam plitu de. The North AmericanBel lcore a nd ANSI t emplatespecificat ions ar e t ypically givenin norma lized amplitude. In otherwords, the template amplitudespecificat ions a re pr ovided a s afraction of the pea k pu lse ampli-tu de. The ITU-T PDH a mplitudespecifications, on th e other ha nd,ar e t ypically given in fixed vol-ta ge levels. This seeks t o incor-porate a n am plitude test with th epulse shape test . The HP 83475accommodates both types of specifications. The Bellcore spec-ified pulse mask tests s cale to thenormalized pulse while the P DHtemplat es scale to the absolut eamplitudes given by th e stan dard.

Figure 12. PDH E3,8.448 Mb/s puls emask d ef in i t ion Nega t ive pu l se t e s t ing

For AMI-coded signals, only th epositive masks ha ve been stan -dar dized. However, user s t ypi-cally also test for n egat ive 1spulse performa nce. One way todo this is to use the chan nel in-verting function of the HP 83475an d apply the positive pulse mask to the inverted n egative pulse.The H P 83475 inversion is donethr ough software, not ha rdware.In o ther words , an inver t ingam plifier is n ot used. Th e soft-

ware inversion minimizes ampli-tu de un certa inties to provide fora good measurement for negativepulse t emplate conform an ce.

Figure 11. BellcoreDS -3, 44.736 Mb/spulse mask de f in i t ion

Anoth er way to test a n egativepulse is to invert the positive pulsemask and apply it t o a negativepulse. The HP 83475 also pro-vides th is option for fixed volta geamplitude templates.

Normalized Amplitude Equation Time Axis Range (UI)

Upper Curve Upper Curve

0.85< T < 0.68 0.03

0.68< T < 0.36 0.5{1+sin[(pi/2)(1+T/0.34)]}+0.30.36< T < 1.4 0.08+0.407exp{1.84(T-0.36)}Lower Curve Lower Curve

0.85< T < 0.68 0.03

0.68< T < 0.36 0.5{1+sin[(pi/2)(1+T/0.34)]}+0.3

0.36< T < 1.4 0.03

1.0

0.5

0.0

0.5 0.0 0.5 1.0

NormalizedAmplitude

Time, Unit Intervals

v

v

v

v

v

v v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v

v 17 ns

8.65 ns

14.55 ns

12.1 ns

24.5 ns

29.1 ns

Nominal pulse

0.2

0.2

0.1

0.1

0.1

0.10.2

0.1

0.10

0.5

1.0

V


14/26


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Making a pu l se mask t e s twi th th e HP 83475The HP 83475 can perform t ests

to a var iety of indu str y-specifiedelectrical interface mas ks. Theelectrical mask tests availablewith the HP 83475 are:

SONET, Bellcore GR-253-CORE: STS-1 eye, 51.84 Mb/s STS-3 eye, 155.52 Mb/s STS-1 pulse, 51.84 Mb/s STS-3 one pu lse, 155.52 Mb/s STS-3 zero pulse, 155.52 Mb/s

ANSI T1.102: DS-1, 1.544 Mb/s DS-1c, 3.152 Mb/s DS-2, 6.312 Mb/s DS-3, 44.736 Mb/s

2) A trigger signal mu st be a vail-able. Two conditions are possiblefor tr igger, depending on the

waveform u nder an alysis, whichprovide a stable sequence of pulses:

pattern , a tr igger signal must beobtained which is synchronizedwith th e repet i t ion r a te of the

pattern and th is signal must beused as the input to the HP 83475external tr igger channel. Forexample, if a 128 bit pat tern isthe dat a u nder an alysis, a t riggers ignal t hat repeats every 128bits (or a multiple of th is) mu stbe used as th e extern al trigger.

3) Once a s table trigger is achievedan d a stable sequence of bits isobtained on th e display, the h ori-zont al t ime/div an d the delay

must be adjus ted to obta in th edesired pulse to which t he pulsema sk can be scaled.

ITU-T G.703 + SDH: E1, 2.048 Mb/s E2, 8.448 Mb/s E3, 34.368 Mb/s E4 one pu lse, 139.264 Mb/s E4 zero pulse, 139.264 Mb/s STM-1 one pu lse, 155.52 Mb/s STM-1 zero pulse, 155.52 Mb/s

The funda mental requirementfor a pu lse mask test is a stablepu lse display. The conditionsrequired to obtain t his with theHP 83475 are:

1) The signal under a na lysis mustbe repetitive. For exam ple, thesignal could be a repeat ing PRBS,a r epeating section of an SDH/ SONET fram e, an a ll ones AMIpat tern , or just a r epetitive, NRZ1010 sequence.

a) If th e signa l under a na lysisprovides a r ising or falling edgeat a fixed rat e, (ie., an NRZ 1010sequence) the ins t ru ment canbe triggered from th e data . Inother words, the internal channeltr igger can be used t o obtain asta ble sequence of pulses on t heinstrument display.

b) If the waveform un der a na lysisis a r epeating long patter n, forexample, a repeating 2 231 PRBS

Figure 13 . S tab le s equenceof bits from an AMI signal(a). PDH E1 (2 Mb/s) pu lsemask tes t resu l t s (b).

a.

b.


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In a ddition, if th ere is externa la t t enua t ion a t t he e l ect r i ca lchann el inpu t , th is can be cor-

rected by press ing: E l e c 1 orOptical/Elec 2 , [More 1/2] [ExtAtn]. Use the genera l-pur poseknob to enter the correction. Adjustth e Volts/div setting so th at thepulse is ar ound 4 t o 5 divisionspeak to peak.

With th e desired pulse center edon the display, the pu lse mask test can be performed. The t estkey sequence is essentially iden-tical to the sequ ence used for

testing a l ightwave tran smitter.However, un der Cursors/Masks ,[Select Ma sk], the a ppropriateline rate pulse mask specificationmu st be selected. For th e step-by-step pr ocedur e, refer to theLightwave Transmitter Mask Test section of this d ocum ent .

Ligh twavet ransmi t te r ex t inc t ionra ti o measu rem e n t s

Defini t ionExtinction r at io is a lightwavetran smitter param eter controlledby communications standardssuch as SDH, SONET and F ibreChann e l . Th i s pa ramete r i sdirectly related to system powerbudget which in tu rn is relatedto BER performa nce. In order t oun dersta nd its definition, it isimportan t t o first r eview how atypical fiber-optic digita l link

tran smits and receives data.

level biases the laser nearly off,so that very little light is tran smit-ted. (For reasons we will describe

in a moment, lasers ar e almostnever biased fully off, so a s ma llam ount of light is tra nsm ittedeven d ur ing a logic 0 pulse.)

Extinction ra tio is defined a sth e ra tio of the avera ge opticalpower in a logical 1 signa l to theaver age optical power in a logical0 signal. This definition is usedbecau se it is represen ta tive of how an actua l operat ing systemworks. A typical photon-count ing

receiver int egrates th e r eceivedsignal over t he ent ire bit period.Values a bove a pr eset th resh oldar e identified as 1s, and valuesbelow th e thr eshold are ident ifiedas 0s. The gr eat er t he differ-ence in power levels, the easierit is to ma ke t he correct decisionin the presence of noise.

Isolated 1sThe ANSI T1 an d Bellcore st an -dards require that pulse mask

test ing be perform ed on isolated1 pu lses. An isolat ed 1 pu lsecan be captu red by the HP 83475as long as the condit ions explainedun der Making a pulse mask test with th e HP 83475 are met.If th e signa l under a na lysis isnot repetitive but ra ther r andom,th e HP 83475 can sti l l tr iggeron th e data a nd display an eyediagram, but a s t able sequenceof pulses will not be displayed.In effect, a pulse meetin g the

isolated 1 conditions will notbe obtained an d, thus, th e appro-priat e test cann ot be performed.

For a typical, directly-modulatedd igi t a l l a se r t r ansmi t t e r, t heincoming electronic digita l dat adirectly contr ols th e bias on alaser diode. A digita l 1 logiclevel cau ses th e laser t o be biasedfully on, resu lting in m aximumlight t ra nsm ission. A logic 0

Figure 14. Extinctionratio definit ion

P = Average optical power in logical 1P = Average optical power in logical 0

Amplitude

Time

1 0 1 0

P1 P1

P0 P0P DARK

ER = P1P0

v

v

1

0


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Why n ot inf in i teext inc t ion ra t io ?If the st rongest signal occurs with

ma ximum difference betweenth e 1 and 0 powers , why is th elaser not bia sed complet ely off dur ing the t r an smiss ion of alogic 0 pu lse? The problem is th atin this condition the transmittersuffers from several tu rn -on re-lat ed effects. Relaxa tion oscilla-t ions cause serious overshoot,r inging, and j i t te r. Frequen cychirp also causes significantbroadening of the spectra l line-width. When t aking int o accountth e comm on disper sive effects of standa rd tra nsmission media,th ese waveform char acteristicscan significantly impact systemBER per forma nce.

At the other extreme, too sma llan extinction ra tio is not goodeith er. For a BER of 10 12 , a nextinction r at io of 8.2 dB causes apower pena lty of 1.3 dB compa redto an inf in i te ext inct ion r a t io(ignoring ji t ter an d relaxat ionoscillation problems). CurrentSDH an d SONE T specificat ionsrequire a minimum of 10 dB ex-tinction ra tio for most a pplicat ions,with 8.2 dB being acceptable incertain instances.

Standa rds forext inct ion ra t ioExt inction ra tio is specified by

several telecommun ications a nddatacommun ications standa rddocum ent s. ITU-T G.957 and Bell-core G R-253-CORE define t heminimu m ext inct ion r a t iosrequ i red by SDH an d SONETsystem s wh ile ANSI X3.88 doesthe sa me for F ibre Chann el sys-tems. TIA/EIA OFSTP-4 recom-mends parameter measurementprocedures and makes severalkey recommen dat ions t ha t go along way t owar d rem oving mea-surement ambiguity.

produces different extinction ra-tios at different dat a r ates an dpattern s, so i t is importa nt t hat

the data r ate and patt ern of themeasurement be reported a longwith t he extinction r at io.

The OF STP-4 recommen ded opti-cal reference receiver m ust ha vea fourth-order Bessel-Thomsonresponse wi th a 3 dB cutoff frequency at 0.75 t imes the bitra te. A tight tolerance windowis specified aroun d th e idealresponse. The filtered responseapproximates th e integrating

technique u sed by photon-coun tingreceivers and , at th e center of th eeye, provides for a good appr ox-imat ion of th e avera ge opticalpower s of th e 1 an d 0 levels.

Extinction Ratio Equivalent Definitions:

XR (dB) = 10 log P1 /P0

XT (%) = 100 x P0 /P1

The SDH/SONET standards specifyextinction ratio in dB. The parameter isalso typically specified in %.

The key points of OFSTP-4 aretha t extinction r at io should bemeasured at the full bit r ate usingan oscilloscope a nd a r eferenceO/E receiver with a s pecified fil-tered response a nd, s econdly,tha t th e average one and zeropower levels should be measu redas th e most prevalent values overth e full bit per iod or eye wind ow.In essence, the measurement of the most prevalent one and zerolevels sh ould be done on an eyediagram which overlaps all pos-sible data tra nsitions. For bestresults, it is also recommen dedthat t he measurement be madeusing the sta tistical histogramcapabilities available on moderndigitizin g oscilloscopes.

One importa nt point to consideris tha t extinction rat io is oftenused to estimate th e tra nsmitter seffect on system power pen alt y.For th e estimat e to be realistic,the measu rement n eeds to bemade on a da ta pa t t e rn a t t h edata rat e representat ive of actualuse. As might be expected, a laser

Measuremen t a lgo ri thmIn order to calculate extinctionra tio, the average power of th elogic 1 and 0 levels m us t becalculat ed. If a referen ce receiv-er is u sed, the integrat ing effectof th e filter ing r eceiver will pro-vide for a good appr oxima tion of th e avera ge power in t he 1 an d0 levels a t t he cent er of theeye. Thus, for extin ction r at iocalculat ions, t he ideal appr oachis to determine t he 1 a nd 0 levelsat th e cent er of the filtered eye.Internal histogram capabilitiesin m an y digital oscil loscopesmake th e measuremen t f a i r lyeasy to do. There a re essent iallytwo problems wi th t h is idealappr oach . F i r s t , w ith in theindust ry, there is no agreementon th e width of the window.


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Second, an y misalignment of th ewindow with r espect t o the tr ueeye center would cause measure-

men t inconsist encies or differencesdue t o the different cont ributionsof the rising and falling edges.The OF STP-4 committee t ook a naltern ative appr oach t o recom-mend the measurement be madeacross the full eye. This elimin-ated the effect of any slight mis-alignment. In stead of finding asimple mean, they recommendedusing the most prevalent 1 and0 values. Most pr evalent values,in essence, represen t t he locat ionof th e 1 an d 0 level histogra mpeaks. These, quite often, do notcoincide with t he m ean of th e dis-tributions. However, in principle,th ese should be very close to thetr ue average 1 and 0 powers.

measur ing mean values a t eyecenter a nd most prevalent valuesacross th e full eye can be one-

ha l f dB or more . HP cur rent lyrecommends measur ing meanvalues whi le us ing a na rrowwindow, ar ound 20 % of the bitperiod, about th e eye cent er. TheHP 83475, however, can be setto measu re over an y desired por-tion of th e bit. The mea sur ementof mean 1 and 0 values at th ecenter of th e eye provides for a nextinction-ra tio measurem entth at can be more rea list icallyapplied to power pena lty calcu-lations. Fu nda ment ally, the inte-grating effect of the referencereceiver will provide a good appr ox-imation of the 1 and 0 levelaverage powers at the center of the eye. In addition, th e meanlevel calculat ions pr ovide for amore repeat able and ph ysicallymeaningful measur ement in rela-tion to the a ctua l extinction r atiodefintion.

DC-offset correctionAnother pra ct ica l pr oblem inmeasur ing extinction rat io is tha t

referen ce receivers a nd oscillo-scope input s ra rely have exactlyzero offset volta ges over all t em-perat ur e and operat ing conditions.To avoid error, th e measu rem entalgorith m should subt ra ct offsetvoltage from the mea sur ed 1 and0 volta ge values. Th is is done byblocking the light to the opticalreceiver and measuring the resid-ua l offset . Offset volta ge canchan ge rapidly with temperat ure,so re-calibration should be per-formed whenever t emperatur evaries more tha n a few degreesfrom th e pr evious calibrat ion.

In pr actice th e use of most preva-lent values a nd a full bit windowis somewha t less th an ideal. Typ-ically, th e differen ce between

Figure 15. Extinction-ra t io measurement onan opt ica l eye d iagram

Sources o f measuremen tunce r t a in tyThough extinction r atio has a sim-ple definition, it is a par amet ertha t ha s caused mu ch contr o-versy throughout th e indust r y.Differences in mea sur ementtechnique often cau se a las er orLED t o appear within sp ecifica-tions on one system an d out of specifications on another system.This is a par ticular problem whenone system is in t he tra nsmittersupplier s outgoing t est depa rt-men t an d the othe r i s i n thecustomer s incoming inspectiondepartment. I t ha s been deter-mined that the most prominentcause of measu remen t inconsis-ten cies is differences in th e fre-quency response of measu remen t

system s. In effect, differences inth e frequen cy response of oscil-loscope an d O/E system s providefor inconsistent an d, sometimes,incorr ect results.

ER = 10 log {V(1)-Voffset / V(0)-Voffset }

"1" Level

"0" Level

Voltage Histogram Window

DC Offset + Noise

X % of Unit Interval

v

v

v v


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As previously mentioned, whenmeasu ring extinction ratio, th eus e of a four th -order Bessel-

Thomson r eference receiver isrecommended. This ensures tha tth ere is some consistency in th efrequency response of measu re-ment sys tems. H owever, i t i simpor t a n t t ha t t he r e fe rencereceiver meet th e SDH/SONETor Fibre Channel bandwidth spec-ificat ion th roughout th e entirespecified bandwidth. Specifically,it is importan t to verify that thereceiver be compliant down toDC. Low frequen cy gain r iseeffects, which p rovide for AC an dDC gain differen ces, can signifi-cantly corrupt an extinction rat iomeasu rement . Figure 16 i l lus-t ra tes th e effect . In t he t imedomain, a low frequency gainrise a ppear s as a slow tail onth e pulse. In effect, the signalDC component s ar e am plifiedmore tha n t he AC components .Thus , the pulses measured a tthe outpu t of the receiver a renot an accurate representationof th e input signal.

Other sources of error can a lsocause m easurement uncertainty.I t i s importan t t o have a good

dat a sour ce which provides aclean eye diagram as t he r efer-ence signal. A well-designedreference receiver wh ich m ini-mizes retu rn loss between thephotodiode and filter is a lso im-porta nt . The oscilloscope sh ouldalso be very linear an d chann elcrosstalk s hould be n egligible.Finally, sensitivity should alsobe considered. The mea surem entsystem should have sufficientsensitivity to repeata bly calculatethe r equired signal par ametersfor measu ring extinction r at io.Taking int o accoun t t he systemsens itivity, sufficient hist ogramdata samples should be acquiredto obtain r epeata ble extinctionratio measurements and minimizeun certa inty due to system noise.

uses a h istogra m-based algorith mto calculate t he par ameter on aneye diagram . Before doing t he

measurement, however, a DCoffset calibrat ion sh ould be per-formed. F urt herm ore, if usingth e optical cha nnel, make sur eto connect the D e m o d O u t toth e electr ical Channe l 2 input .This can be achieved througheither a 50 Ohm BNC jumper or,if the instru ment ha s the appro-priat e option, with a four th -orderBessel-Thomson filter. The filterconn ection is recommen ded forbes t r e su l t s . Th is mak es theoptical input , depending on thefilter option, into a SONE T/SDHor Fiber Chan nel reference re-ceiver. For proper perform an ce of the optical input, make sur e thatthe Cha nnel 2 input impedanceis set to 50 Ohms. The pr ocedurefor execut ing an extinction ra tiois given below.

Figure 16. Slow tai l effectdue to low f requency ga inr i ses on re fe rence rece iverf requency response .

Measur ing ext inct ion ra t iowith th e HP 83475The HP 83475 aut omat ica l lymeasures extinction ratio at thepush of a button. The instrument

1) First , make su re you obtain asta ble eye-diagr am display. Toget the best r esults, the eye dia-gram should be obtained u singa r eference clock as th e externa ltrigger signal. Make sure thatth e eye diagram is around 4 to 6vertical divisions in amplitu de.Horizont ally, you m ust ha ve atleast two crossing points displayedan d, for best resu lts, display onlytwo crossing points.

SDH compliant receiver

V

Vbase

top

Receiver failing SDH spec due to low freq. gain rise

V

V

top

base


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2) Make sur e you h ave the mea -sur ement type set for E ye: NRZ,the correct measurement source

selected (E2, Op or E1), an dselect an appr opriate num ber of t ime and ampl i tude samples .Typically, 300 time sa mples a nd1000 amplitude sam ples is suffi-c ient for repeata ble measure-ment s on signals of greater t ha n17 dBm average signa l power.

3) Perform a DC offset channelcalibration. Press Opt ica l Cal ,[Ext Ra tio Offset Cal]. Thenselect th e appropriate chann el

for calibra tion. To calibra te t heoptical chan nel press [Meas Op].If you are u sing an exter na l O/Ewith electrical Chan nels 1 or 2press eith er [Meas E1] or [MeasE2].

6) To set th e measu remen tdisplay un its, pr ess [More 2/4],[More 3/4], [Select Un its]. You

can set the measur ement unitsto eith er linear, dB, or %. Linearis simply the r atio of th e one tozero levels, dB is th e decibelequivalent of th is rat io, and %is the inverse of the linear ra tiomultiplied by 100.

7) To define th e mea sur ementwindow, press [Pr evious Men u],[Define Eye Window]. Select[Measure Auto Man] Auto to setautomatic crossing point location.

Highlight the [Min] and [Max]limits by pressing the ap propriatekeys . Use th e general -purposeknob to adjust the minimum a ndma ximum limits to th e desiredlocations within the eye crossings.0% defines t he left m ost cross-ing an d 100 % the next crossingpoint t o the right. For best re-sults, when using a filtered refer-ence receiver, it is recommend-ed to se t the minimum l imi t to40% an d the m aximum to 60%.Pr ess [Pr evious Menu ] whendone.

8) To measu re extin ction r at io,press [More 4/4], [ExtinctionRat io]. The extinction r at io is

shown in th e result s section of the display.

Transceiverw avefo rm ana lys i sEye-diagram characterizationmeasurements are typically per-formed to analyze the time-domainresponse of communicationstransmitters and receivers. Dis-tort ion, j i t ter, and disper sioncharacter is t ics of t ran smit ted

dat a directly impact system per-forma nce and must be appropri-ately minimized and compensated.It is import an t to consider thatchar acterization measurementsmust be done un der appropriateband width conditions. The testsys tem m ust h ave sufficientbandwidth margin to minimizemeasurement uncertainty onhigh-frequency waveform charac-teristics. In addition, the systemshould h ave sufficient sensitivitysuch that m easurement integrityis not compromised due to sys-

Note: Before pe rforming a DCoffset calibration, i t is recom -mended tha t t he in s t rumen tbe wa rmed up (act ivated) fora t leas t on e ho ur. This mini-mize s DC o ffse t d r i f t a ft e r

calibration. If environmentaltemperature chang es s ignif i -can tly, a ne w o ffset ca librationshou ld be pe r fo rmed .

4) Follow the on-screen instruc-tions for performing an offsetcalibrat ion. When the calibrat ionis complete, pres s [PreviousMenu].

5) To activate measu remen t tr ack-ing, press Power/Voltage [More1/4], [Show Meas Off On] On.When th is is activated, mar kersappea r sh owing the measure -ment location on th e eye.

Figure 17. Eye-d iagram ext inc t ion-ra t io measurementresu l t s


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tem noise. Furt hermore, the testsystem sh ould have a very flatfrequency response in order to

provide optimum waveformfidelity. For lightwave measure-ments, bandwidth ma rgin, sen-sitivity, an d flatn ess a re factorswhich t ypically differ depend ingon the selected measu rementreceiver t echnology. It is impor-ta nt to consider t he a pplicat ionan d th e test objectives to makethe optimum selection for yourmeasurement system.

Figure 18. Syste m BWmargin versus rise timem e a s u r e m e n t e r r o r.S y s t e m B W m a rg i n =sys te m BW/DUT BW

The H P 83475 provides severalau t omat i c measur emen t s tochara cter ize the t ime-domainresponse of lightwave and elec-trical transceivers. Measurementscan be per formed on either eyed iag rams or gene ra l , pu l sedwaveform s. The complete list of measur ements a vai lable with

th e HP 83475 is given below forboth eye diagrams and generalpulsed waveforms.

Eye d ia g ram measur emen t s :Time: Rise/fall tim e, bit r at e, bitperiod, dut y cycle distort ion, jitt er(rm s an d pk-to-pk)

Volta ge/power: Ext inction r at io,amplitude, overshoot, eye height,noise(rm s a nd pk-to-pk), onelevel, zero level, max level, minlevel, eye crossing %

Genera l wa vefo rm measuremen t s :Time: Rise/fall time, frequency,period, +width, width , delay,phase, duty cycle

Volta ge/power: Amplitu de, m axlevel, min level, peak-to-peak,

top level, base level, average, rms,pres hoot, oversh oot

Eye d i a g r a m v e r s u sg e ne ra l w ave fo rm sTime-domain response character-ization is typically done throughs ta t i s t i ca l m easuremen t s of waveform par am eters. Typically,rise t ime, fall t ime, oversh oot,d is tor t ion, noise an d j i t te r a remeasured for t he t r ansmi t t e rtime-domain response. Jitter anddistortion a re t ypically measur edat th e receiver.

These measur ements a re typicallyconducted either on a n eye dia-gram or a pulse. Eye-diagrammeasu remen ts pr ovide a compo-site view of all possible wa veformvariat ions a nd, in effect, ta keinto account pattern-dependenteffects. In es sence, eye-diagr ammeasu remen ts can be consideredto be a represent a t ion of th eaverage or mean behavior of thedevice un der test . In addition,given t he overlap of tra nsitions,a qu alitative view of tran smitterperform an ce can be convenient ly

obtain ed by simply looking atth e vertical an d horizonta l eye-opening characterist ics. Pulsemeasu rement s , on the otherha nd, isolat e response t o a sp ecificpat tern (usually 1010) an d allowfor a ccur at e focus an d a na lysisof t r an s i t ion chara cter is t ics .Typically, for example, whenchara cterizing the r ise time of atr an smitter, it is best to utilizea high r ise time, low repetitionrat e (relative to transmitter band-

width) 1010 patter n. This a llowsfor a n a ccur ate char acterizationof device rise t ime per form an ce.

Rise Time error due to test system contribution

Assumes a Gaussian Response For the Device Under Test (DUT)and the Measurement System

1 2 3 5 100.2

0.5

125

1020

50100

System bandwidth margin

Percent Error


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Transmit terme as u r em e n t sTypical tran smitter measu rementstha t can be performed with th eHP 83475 include:

r ise t ime and fall t ime ji t ter noise overshoot undershoot distortion

sensitivity is required, there ar eother options. The combinationof an avalanche photodetector

an d a linear a mplifier can typi-cally provide a greater tha n 10 dBincrease in sensitivity. Such re-ceivers, however, may cont ribut ean increase in non-linear effects.These effects m ay be acceptabledepending on th e applicationand test objectives.

When m ore sensitivity is required,APD-based receivers are recom-mended. A good example is theBCP 310 from Br oadband Com-

mun icat ions Pr oducts, Inc. of Melbourne, Florida. Such receiv-ers m ay be conn ected to one of th e e lect r ica l chann els of theHP 83475.

For genera l, pulse waveforms,the waveform distortion can bechara cterized. This can be done

thr ough measur ements of thedut y cycle of a 1010 pat tern a nd/ormeasur ements of the pu lse +an d widths. Ideally, the du tycycle should be at 50% and t he +an d widths sh ould be identical.

It is import an t to keep in mindth at sensitivity is a very impor-tant consideration for receivermeasu remen ts. E specially foroptical receiver testing, typicalreceived avera ge power levels,

after a long link of fiber, can bequite low. It is important t hat thereceiver have sufficient sensitiv-ity such t ha t t he r eceiver s elec-t r ica l outpu t s ignal can haveenough S/N margin with respectto the measu rement system n oise.This minimizes th e measur ementunce r t a in ty a t t r ibu ted to theinfluence of system noise.

With th e HP 83475, all of th eabove measur ement s, except forun dersh oot, can be perform ed

on an eye diagram . The measure-ments are done using histogram-based a lgorithm s. In effect, t heyare s ta t i s t ica l measurementsbased on t he a cquisit ion of auser-defined set of waveformsamples.

For pu l sed wavefo rms , t heHP 83475 available measurementset is differen t. Of the a bovemeasu remen ts , r i se t ime, fa l ltime, overshoot, un dershoot and

distortion can a ll be aut omati-cally perform ed.

If you are t rying to cha ra cterizethe tra nsmitted data after h eavyat tenu at ion du e to, for examp le,a long link of fiber, the sen sitiv-ity of the HP 83475 optical chan-nel can become a limiting factor.As previously ment ioned, t heHP 83475 optical cha nn el consistsof a ver y linear receiver composedof a P IN ph otodetector an d a

tr an simpedance am plifier. Thiscombin at ion provides very goodwaveform fidelity an d good sen -sitivity. Typically, at 1300 nm,signals down t o 20 dBm can beviewed an d a na lyzed. If higher

Re ce i v e r m e a s u r e men t sTypical receiver characteriza-tion measu rements ma de by theHP 83475 include:

jitter (rms an d pk-to-pk) distortion eye height

Other measurement s, when ap-propriate, can be performed. Allof the a bove measur ements canbe perform ed on eye diagram s.The jitt er is measur ed by ta kinga histogram, with a user-definednu mber of sam ples, at th e cross-ing point. Distortion can be char-acterized through a m easurementof th e eye-diagr am crossing levelpercent relative to the mean highto low level am plitu de.

The minimum S/N ra tio requiredto conduct an accur ate mea sur e-

ment depends on th e measure-ment algorithm and type (Generalor Eye). For eye-diagra m m ea-sur ement s, one way to improvemeasurement repeatabili ty is toincrease the sta tistical sam plesize. Unfortunately, waveformaveraging cann ot be condu ctedon a two-level eye diagram. The


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result is t he avera ge of the highan d low eye-diagra m levels. If apulsed waveform is being an a-

lyzed, waveform averaging canbe activated. For th e HP 83475,a S/N rat io greater th an 12:1 isrecommended for most eye-dia-gram measur ements. For generalwaveform measu remen ts, giventha t averaging can be conducted,lower signal-to-noise ratios maybe tolerat ed.

3) To act ivate mea sur ementtra cking, press Time or P o w e r / Voltage , [More 1/4], [Show Mea s

Off On] On. When th e measu re-ment is performed, markers ap-pear showing the m easurementlocation on t he eye.

Highlight the [Min] and [Max]limits by pressing th e appr opri-at e keys. Use th e genera l-pur pose

knob to adjust the minimum an dma ximum limits to the desiredlocat ions within th e eye crossings.0% defines t he left most crossingan d 100% the next crossing pointto the right. For best resu lts, itis recommended to set th e mini-mu m limit to 40% an d th e maxi-mu m t o 60%. When done, press[Previous Menu].

Making eye -d iagramme as u r em en t s

To ma ke the mea sur ement s onan eye-diagram, follow the stepsoutlined below.

1) First , make su re you obtain asta ble eye-diagr am display. Toget the best resu lts, the eye dia-gram sh ould be obtained u singa r eference clock a s th e externa ltrigger signal. Make sure thatthe eye diagram is ar ound 3 to 6vertical divisions in a mplitude.Horizont ally, you must ha ve at

least two crossing points displayedan d, for bes t resul ts , d isplayonly two crossing point s.

2) To access t he a ut omat ic timeand am pl itude measurements ,p res s e i the r t h e fron t pan e lPower/Voltage key or th e Timekey. Make sure you have th e mea-sur ement type set for E ye:NRZ,the corr ect mea sur ement sourceselected (E2, Op or E1), andselect an appr opriate num ber of t ime and ampl i tude samples.Typically, 300 time sa mples a nd1000 amplitude samp les is suffi-cient for repea ta ble measu re-ment s on signals of greater t ha n17 dBm average signal power.

4) To set t he u pper a nd lowerthr esholds for th e rise time andfall time measurements you mustgo to th e Time measurementsmenu an d p r es s [More 1 /4 ],[More 2/4], [More 3/4], [DefineThresh ]. Once in this sub menu,you h ave thr ee options to set up

th e thr esholds . The fi rs t twooptions set t he lower an d upperth resh olds a t 10%/90% or a t20%/80% of the mea n 1 to 0 levelampl i tude. The t h i rd opt ion isto set th e thresh olds a t user-defined levels. To do this, pr ess[Variable], [Lower], and then usethe general-purpose knob belowthe Measure front pan el sectionto set th e lower th reshold to thedesired level. You can t hen do thesame to set th e upper t hreshold.When done, press [Pr eviousMenu].

4) You can define t he location,on th e eye-diagram high level,on which th e noise measur ementwill be ma de. To define th e mea -su remen t window, p res s th efront panel Power/Voltage keyan d t hen [More 1/4], [More 2/4],[More 3/4], [Define Eye Window].

5) If you a re u sing th e opticalinput of the H P 83475 and are

using the calibrated watts verti-cal scale, you can select t he u nitsfor a mplitude m easurements aseith er d Bm or W. To do th ispress [Select Un i ts ] an d th enselect eith er [uW] or [dBm].

5) To begin p erformin g am pli-tude measur ements , press theappr opriate softkey to activatethe des i red measur ement . Toaccess different measu remen ts,scroll thr ough the Time a n d

Power/Voltage menus by pressingth e [More x/x] softkeys. Th efollowing figur e shows the disp laywith some of the measur ementsactivated.


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Figure 19. Eye-d iagram waveformcharacterization

ments such as per iod, b i t ra t e ,and wid th . As p rev ious lyment ioned, to increase m easur e-ment repeat abili ty, waveformaveraging can be used with pu lsedwaveforms. Figure 20 sh ows th edisplay wi th some of the keymeasurements activated.

S ummaryThe HP 83475 performs m an yof the measu remen t s for t hean alys is and t es t ing of d igita lcommunications equipment andcomponents. Built-in algorith msperform st an dar d-required con-formance measurements suchas mask and ex t inc t ion - ra t iotesting. In addition, the HP 83475aut omat ica l ly measures wave-form para meters such as r i setime and jitter. A low distortion,integra ted O/E converter providesaccurat e param etric measure-

ment s and a llows for peak a ndaverage optical power measu re-ments. Beyond lightwave com-munications test capabilities, theHP 83475 fun ctions a s a genera l-purpose 500 MHz oscilloscope. Twoavailable electrical channels canmeasu re electrical signa l cha rac-teristics and can be used to ana-lyze electrical tributary signalson commu nicat ions equipment.

Genera l wave formme as u r em e n t sIf you wish to perform generalwavefom m easur ements , for ex-am ple on an NRZ squa re wavewaveform , press either Time orPower/Voltage an d select [MeasType Genl Wvfm E ye:NRZ] GenlWvfm. The procedure for making

measurements is essentially thesam e as t he pr evious one for eyed iag rams . I t i s impor t an t t oha ve a st able waveform displayan d to show a sufficient nu m-ber of tra nsit ions for m easur e-

Figure 20. Pulsewaveform para-met r ic ana lys i s


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Uni ted S ta tes :Hewlett-Packard CompanyTest and Measurement Organization5301 Stevens Creek Blvd.Bldg. 51L-SCSant a Clara, CA 95052-80591 800 452 4844

Canada:Hewlett-Packard Canada Ltd.5150 Spectru m WayMississauga, OntarioL4W 5G1(905) 206 4725

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Data Subjec t to ChangeCopyright 1995Hewle t t -Packard CompanyPrinte d in U.S.A. 9/95

HP PN83475 1_Communication Waveform Measurements

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Transcript of HP PN83475 1_Communication Waveform Measurements