User interface design for the HP 8146A OTDR - optical time-domain reflectometer - Technical

Hewlett-Packard Journal, Feb, 1993 by Robert Jahn, Harald Seeger

To enable engineers to download help text to the instrument in the field, flash EPROMs are used to store software and text. With flash EPROMs the permanent software in the instrument is reduced to a software loader that reads software from an external device like a flexible disk, a memory card, or a host system (via the HP4B) to program the flash EPROMs. This makes it easy to make software enhancements and support new laser modules. Native language help text can be implemented on demand and distributed by flexible disk or even electronic mail.

Scantrace

When users want to analyze their fiber links this typically means searching for nonreflective and reflective events* and the start and end of the fiber under test. They want to know the loss or gain resulting from splices and gain elements and the through loss and return loss of connectors. Scantrace is an automatic trace analysis and fault detection algorithm provided in the HP 8146A that does all these measurements in less than 10 seconds.

To understand the advantage of the Scantrace algorithm, consider the steps involved in manually making a splice measurement from just the plots that appear on the instrument display. The first step would be to position line segments on each side of the event on the trace (see Fig. 3). The line segments are least-squares approximations of the backscatter on either side of the splice. The line segment lengths can be adjusted by changing the two marker positions. The difference in the signal strength of the two line segments gives the splice loss. The correct position of the splice is the the last point on the backscatter just before the event. In the same manner the through loss of reflective events can be measured and positioned. Three markers are necessary to measure the return loss of reflective events. Finally, a least-squares approximation measurement can be used to calculate the attenuation between the two events.

If we apply the above procedure to an 80-km fiber with splices every two kilometers, 40 splices would have to be calculated and 40 sections would have to be measured for attenuation. Also, the reflective event of the OTDR connector and the reflective event at the fiber end must be measured. All these measurements would take more than half an hour. With Scantrace these measurements can be done in less than 10 seconds. Fig. 4 shows a printout of a complete analysis of the 80-km fiber mentioned here.

Result Documentation

The HP 8146A provides two ways to document measurement results: hard copy or storage on external devices. To identify a measurement, an identification code, an operator name, and comments can be added before printing or storing. Hard copies can be made by an internal printer, an external printer, or a plotter. The internal printer is a thermal printer offering a resolution of 640 dots and a 4-inch printing width. The display resolution of 500 x 320 pixels is easily transferable to the printer's resolution after rotating the picture 90 degrees. Printing on the internal printer takes about 20 seconds. External devices are accessible via the HP-IB or RS-232 interfaces. When one interface is configured as the dump output port, the other interface is configured as a remote control port. All PCL-II compatible printers like ThinkJet, Deskjet, and others are supported by the OTDR. Plotters must be standard HP-GL-compatible. A series of traces stored on a flexible disk or a memory card can be printed to the internal or external printer.