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Che cosa significano i numeri mostrati dall'OTDR
Articoli sulla pratica della misura: che cosa misurare, come leggere il risultato e perché strumento e software a volte non concordano. Scritti a partire dalle domande che arrivano dal cantiere.
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Come leggere una traccia OTDR e misurare un tratto
Che cosa fa ogni pulsante, come posizionare i cursori A e B e perché lo strumento mostra tre valori di perdita per un solo tratto: 2P, LSA e 5P.
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Cos'è questo sulla traccia 5
- The splice that seems to gain: where the minus comes from and why the reverse shot fixes it Why an OTDR reports negative loss at a splice, what the backscatter coefficient has to do with it, why the mean of two directions removes the error, and how to pair two opposing traces of one fiber.
- The OTDR put the end of fibre in the wrong place The end of fibre sits short of the real length: the instrument did not see the line to its end. What it looks like on a real trace, why range and averaging do it, and what to shoot again.
- Noise at the end of an OTDR trace The far end of a trace turns ragged when the returning light is no stronger than the receiver noise. What it looks like on a real trace, and what to shoot again.
- A step that is bigger at 1550 nm than at 1310 nm is a bend A step with no spike can be a splice or a bend. How a second wavelength tells them apart, shown on a real trace at 1310, 1550 and 1625 nm, and what to do about a bend.
- Short fibre on an OTDR: why the start of the trace shows nothing Can an OTDR test a fibre under 100 m? That depends on the pulse width and the launch cable. One real cable shot at four pulse widths shows how much of the start each pulse hides.
Misura 19
- What is an OTDR and how does it work How an optical time domain reflectometer finds splices, connectors and breaks from the light that comes back out of the fiber, what it measures, and how an OTDR differs from an OLTS.
- How to use an OTDR The order of work with an optical time domain reflectometer, from a clean connector to a saved file: launch cable, refractive index, wavelengths, range, pulse width, averaging, and what to check on the trace before you leave the site.
- Why there are three losses and only one span Three ways of measuring loss between the cursors: two-point, least-squares and five-point. How they differ, when they disagree, and what the deviation is telling you.
- Index of refraction: the one number the whole length rests on What IOR in a .sor file is, where the instrument gets it, why it moves the distance to a splice closure by metres, and how to recompute a trace to the datasheet value.
- PON trace: splitter loss, three pulse widths, and many fiber ends How many decibels a splitter takes, with a table from 1×2 to 1×128, why the step on the trace is larger than the calculation, and where the fiber ends come from.
- Dead zone: why the OTDR cannot see an event that is there What an OTDR dead zone is, how the event dead zone differs from the attenuation one, and what a file looks like when two events were not separated.
- OTDR dead zone calculator How many metres after a connector your OTDR cannot resolve at your pulse width, whether it will see two close events apart, and whether your launch cable is long enough. Zones measured on files from real instruments.
- Launch cable: what it is for and what goes unmeasured without it Why an OTDR cannot measure the first and last connector of a link without a launch cable, what a receive cable adds, what lengths are used, and why a trace file sometimes holds two different lengths.
- Three lengths of one OTDR trace: acquisition, span and fiber How acquisition length differs from span and fiber length, which one an OTDR divides by for loss per kilometre, and how that puts too low a figure into a record.
- OTDR pulse width: what the instrument picks for you A short pulse resolves close events, a long one reaches the far end. What the instrument sets on each range and behind a PON splitter, transcribed from its own table.
- How to use an EXFO OTDR and read its report The order of work on an EXFO OTDR: cleaning and connecting, refractive index, first connector check, range, pulse and time. And how to read the instrument's PDF report: verdict, span results, markers, event table.
- How to use a VIAVI OTDR and read its report The two modes of a VIAVI OTDR, Smart TEST and Expert OTDR, the order of acquisition, the three alarm levels, the SmartLink view and what goes into the PDF report the instrument builds.
- How to use a Fluke OptiFiber Pro OTDR and read its report The order of work on a Fluke OptiFiber Pro: Auto OTDR, fiber type and test limit, launch and tail cord compensation, EventMap, saving by cable ID. And what LinkWare PC does with the result: the report, your own logo, export to .sor.
- Embedded OTDR: how to read baseline and current traces Traces from an embedded OTDR or a fiber monitoring unit: service wavelengths, empty headers, what the long trace misses, and overlaying current on baseline.
- How to use a mini OTDR Low-cost handheld OTDRs from NOVKER, Joinwit, Komshine, Grandway, TriBrer, SNR, NOYAFA, COMPTYCO AUA and other brands: Auto and Expert modes, acquisition settings, how to get the .sor file off the unit and open it without the software from the disc.
- How to use a Grandway OTDR and get the files off it Grandway FHO5000 and Mini1000 OTDRs: what the menu items mean, the acquisition settings, where the instrument keeps .sor files, and how to copy them to a computer or a phone.
- How to use a Tribrer OTDR and get the files off it Tribrer TB-700, TB-720 and AOR500 OTDRs: what the menu and the keys mean, the acquisition settings, where the .sor files are stored, and how to copy them to a computer.
- How to use an OPWILL OTDR and get the files off it OPWILL FTS510 OTDR: what the home screen and the buttons mean, the acquisition settings, saving a trace as .sor, and copying traces and reports to a USB stick.
- How to use a NOVKER OTDR and get the files off it NOVKER NK3200 and NK3200D mini OTDR: what the eight menu tiles and the keys mean, the acquisition settings, where the .sor files are saved, and how to get them onto a computer.
Eventi 4
- What the events on an OTDR trace mean What each kind of event on an OTDR trace means: reflective and non-reflective, the fiber end, false ghost reflections, and why the instrument's list and ours differ.
- Optical return loss (ORL): what the number is, and why everyone's differs How return loss differs from the reflectance of one event, why an OTDR and an ORL meter disagree, and what our own figure shows when the instrument wrote none.
- Why your splice is missing from the OTDR event table: the detection threshold A detection threshold and an acceptance limit are two things under one word. The first decides whether an event reaches the table at all, and where it sits in the file.
- Why a connector reflects so much A large reflection at a connector: what it means, why the end face is usually to blame, and how to check that with traces taken before and after cleaning.
Soglie e file 10
- OTDR viewer download: what people install, and when you don't have to What people download to open a .sor: the instrument maker's software, general-purpose Windows viewers, and why the same job is done in a browser with nothing installed.
- How to correct a .sor header without the vendor software Cable, fiber, locations A and B, operator, comment, index of refraction: .sor fields are edited in the browser, and the edit stays visible inside the file.
- Where "within limits" comes from: acceptance thresholds The verdict column appears once a limit is set, and several figures are held to limits. Which ones they are, and why no normative values are given here.
- How to read an OTDR report An OTDR fiber report has four parts, and every number in it belongs to the instrument. What each event-table column means, how reports differ between makes, where the pass mark comes from, and why the same file gives a different verdict in another program.
- Cable mode: checking each fiber against the factory data How Cable mode checks each fiber of a cable against its factory data: where to enter length, tolerance and attenuation per wavelength, what a red cell and the Outside factory data mark mean, and what the mode leaves to you.
- How to read an optical loss test (OLTS) result What an EXFO .olts loss test result and a Fluke LinkWare .flw database hold: loss in each direction and the average, ORL, the reference and the limits. How to read the margin and where the verdict comes from.
- Fluke fiber test results explained A LinkWare report for a Fluke fibre test: what Test Summary, loss, limit and margin mean, where the limit comes from, what Reference is, and why a test with negative loss fails on a healthy margin.
- How much loss is allowed Allowed loss is coefficient times length plus splice allowance times the number of splices. Where each term comes from, and where the file keeps length and splice count.
- How to open a WTK file from a Wavetek MTS 5000 OTDR A .wtk file from a Wavetek MTS 5000 opens in the browser: the fiber trace, the event table, the acquisition settings and a save to .sor. No vendor software licence needed.
- How to open an .iolm file from an EXFO instrument An .iolm file from the EXFO iOLM application opens in the browser: the fiber trace, the table of link elements, length, loss and ORL. PDF report and save to .sor.
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