Improve testability of opto PCBs

By including testability "features" at the start of a design, you simplify test setups and procedures later on, whether you test equipment during R&D, production, or field use. I've compiled a list of 10 design-for-test (DFT) guidelines that will help not only design engineers, but all members of a design team who can influence how products undergo testing.

Before you get into the details of a design, you can use a general technique to locate places at which added test points or components can increase testability. First, draw a diagram that includes the major blocks in an opto-electronic product, and consider how each block might fail—the point of failure. Then, determine what someone troubleshooting this product might do to locate the defective block. Next, consider what added elements would help a technician who needed to find this failure in the final design.

If possible, break each block into sub-blocks—or actual circuits—and repeat the process described above. Then, consider again any additional design modifications that would help a technician find failures in the sub-block.

Figure 1 A remote feedback loop uses an optical splitter to supply a switch at the receiver with raw output from a local trnsmitter. Moving the switch-- under processor control-- to its B position completes the local feedback loop so firmware can check the local operation of the receiver and transmitter.

1. Provide direct access to fiber-optic receivers and transmitters. Be sure to provide access to the electrical signal immediately after an optical-to-electrical (O/E) converter. You'll need to test the raw electronic signal from the receiver before it undergoes any processing. If possible, add a level detector at the O/E converter's output and provide a way for the processor to read the detector's output. The level detector can be a simple R-C integrator followed by an ADC. In the case of an optical transmitter, such as a laser diode, provide electrical access immediately before the electrical-to-optical (E/O) converter.

Route the O/E and E/O detectors' outputs to the card edge and to the backplane. Telecommunication systems route many test points to the backplane because technicians spend a lot of time behind systems. A technician requires extra time to work his or her way around a rack of equipment to connect instruments to test signals at the edge of a PCB. So, put signals in both places for easy access.

2. Include external or internal optical feedback loops. When a board includes both O/E and E/O converters, provide internal firmware—on the board or in the overall system—that can perform a loopback test. This sort of test involves transmitting known data from the E/O converter, feeding the optical signal immediately back to the optical receiver (the O/E converter), and comparing the transmitted and received data. A short external fiber-optic cable can make the connection between the transmitter and the receiver. The board or the system must provide a connection, perhaps through a serial port or a LAN, that can control the operation of such tests.

Remote systems that require testing may require onboard optical feedback loops (Figure 1). In normal operation, the optical switch remains in its A position to accept external optical signals. In test mode, firmware moves the switch to its B position to provide a feedback loop from the transmitter to the receiver. Again, the test firmware must perform the transmitter-to-receiver error checking.

If you must use this sort of built-in loop, the transmitter's optical "budget" must include sufficient power to allow for about a 2-dB loss in the loop—a loss of about 1 dB in the switch and 1 dB in the optical splitter. Your design budget must include the cost of the switch and the splitter.

Your design also should include circuits that open the electrical part of the loops that bias lasers or cool laser diodes. Opening an electrical loop almost always proves easier than opening a loop in the optical parts of a circuit. Open loops prove easier to troubleshoot and test than closed loops because you can test individual circuit elements without the influence of feedback. Design your electronic circuits and optical systems so you can open feedback and other loops by either electrical or mechanical means. Thus, if an electronic component fails, you can still open a loop mechanically.

3. Route internal optical-data triggers to high-frequency connectors. The availability of these trigger signals on the board can save the expense of adding a clock-recovery module to the typical data-communication analyzer or communication system analyzer. These instruments make eye-diagram measurements and analyze bit-error rates.

4. Use connectors to link onboard optical components. When possible, use connectors instead of fusion splices to link optical components on a board. Standard connectors provide easy access to intermediate points in optical paths during troubleshooting. Connectors can complicate a design, though, because they contribute insertion loss, cause reflections, produce polarization effects, and so on. Thus, you must ensure any added connectors will not significantly affect the design's performance.

Not all optical paths can accommodate a connector. When you connect the output of a laser transmitter to an optical isolator, for example, you must use a fusion splice to reduce reflections back into the laser. A pair of connectors used in place of the fusion splice would reflect too much light back into the laser and defeat the purpose of the isolator.

5. Use optical switches that include electrical outputs. Some fiber-optic switch modules link an optical switch to an electrical switch so the two change state simultaneously. Firmware can monitor an electrical switch's signal to confirm that the corresponding optical switch operated successfully.

Set up such switches to provide a logic level that you can monitor. If the switch provides only a simple open-closed contact, connect the contact going to the processor to power through a suitable pull-up resistor and ground the other contact. This arrangement provides a logic level that corresponds to the switch's position. A few commercial fiber-optic switches provide built-in logic circuits that output a logic signal for each optical switch.

Figure 2 Placing fiber-optic connectors at a downward angle will produce gentle bends that do not disturb fiver-optic signals. The angle also reduces the risk that the output of a laser will cause eye injury.

6.Mount optical connectors at a downward angle. By placing fiber-optic connectors at a downward angle, you reduce problems caused by a tight fiber-bend radius (Figure 2). Tight bends can attenuate optical signals, and the attenuation can vary significantly as the optical fibers move. The angular mounting can also reduce the possibility of accidental eye injury from stray—often-invisible—laser radiation. Horizontally mounted connectors make it all too easy for an engineer or technician to get hit in the eye with a high-power laser beam.

You can also improve eye safety by using laser-connector plugs or caps made of a pliable material. It's easy to cover or block connectors with these devices, and they're easy to remove prior to connecting cables to transmitters. If your design places a laser or transmitter away from a board edge, ensure that the optical output will not encounter any shiny surfaces that could reflect eye-damaging light off the board.

7. Include short fiber loops that will accommodate clamp-on meters.A clamp-on meter (Figure 3) produces a temporary "macro bend" in an optical fiber. A specific bending radius

Figure 3 A live-fiber detector slightly bends a fiber so some light escapes into the cladding. A built-in detector senses the light so the user knows whether or not the fiber is in use. Courtesy of EXFO.

Because the macro-bending technique causes a loss of a few decibels in the optical signal, you can use it to cause small power changes in an optical signal. Power measurements further down-stream from the meter should confirm that the meter has attenuated the signal. This sort of technique can help you trace an optical signal's path. If you introduce a macro bend in a fiber and observe no downstream attenuation, you'll have to do more troubleshooting to find out where the optical signal you're observing comes from.

8. Put spare fibers in multiple splice trays. A splice tray (Figure 4) holds excess optical fiber and cable as well as fiber-optic splices and connectors. Some fiber-optic systems can have hundreds of connecting fibers and cables that, due to the nature of their connections, have excess length that you can't simply cut off. Instead of placing all the excess fiber and cable in one tray, which can get messy, use several trays and group excess fibers in a logical arrangement that makes sense for your system. If someone suspects a fault in one part of a fiber-optic circuit and needs access to a specific fiber, he or she will have fewer fibers to sort through.

9. Ensure that you can disable forward error correction. When your design includes forward error correction (FEC) circuitry, make sure that a firmware diagnostic command will disable both the encoding operation at the transmitter and the decoding operation at the receiver. Otherwise, the FEC operation can mask bit errors. FEC adds two or more bits per byte of data to implement error detection and error correction. Thus, a data link could produce many errors, but if the FEC circuits correct them all, it will prove difficult to locate the source of the errors. You'll find FEC used in many optical-communication systems because it improves system perfor

Figure 4 A fiber-optic splice tray can hold connectors (left), splices (right), and excess fiber lengths. Courtesy of ADC.

10. Use lasers or FO transmitters that supply power data. Some optical sources use a simple comparator circuit or a power-monitor output to indicate the go/no-go state of the transmitter. Unfortunately, these outputs simply indicate that an optical source has failed. By specifying optical sources that supply an internal ADC, you'll obtain more useful diagnostic information. Firmware built into your application can check the ADC's output value and display it or compare it with previously stored information to indicate power-output trends and changes.

These 10 DFT tips cannot cover every aspect of DFT techniques for fiber-optic and opto-electric systems, but they should help you improve the testability for the products you design. The key to success is convincing designers to include test as part of the design, not as an afterthought.