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Troubleshooting Transmitters & Receivers


Most fiber optic equipment is designed to be as simple as the application will allow. However, due to the complexity of some of the applications, things go wrong. In many cases, resolving a troubleshooting challenge can be as simple as properly cleaning the optical connectors. See article "Fiber Optic Connectors" for more information on cleaning optical connectors. In more complex scenarios, additional troubleshooting will be required. This article describes some of the more common problems that have been encountered.



Problems and Comments

1. PROBLEM: No optical power out of the transmitter or transceiver. A) Check the transmitter or transceiver power connection. If there is less than the specified supply voltage between power pins, a higher current power supply may be required. Be sure the power supply polarity is correct. B) Be sure that data input is present. Many data links put out no light if a logic "0" is input. Be sure that the input data is alternating between 0's and 1's, otherwise no output light may be present. 2. PROBLEM: No optical power out of the fiber at the optical input port. A) Check power at optical output port of the transmitter or transceiver. If optical power is present at optical output port, ensure that the proper fiber is connected at optical input port. Verify the integrity of the fiber. 3. PROBLEM: Receiver output electrical signal is noisy or intermittent. A) Check that optical loss does not exceed the rated value between transceivers or between the transmitter and receiver. If the loss is too high, reduce optical loss, or insert a repeater between transceivers or the transmitter and receiver. High loss may be caused by bad connectors, improperly seated connectors, or bad splices. See Fiber Optic Connectors for information on the proper use of connectors. B) Check the wavelength of the transceivers or transmitter and receiver. Detectors are typically optimized for one wavelength. Mixing 850 nm units and 1310 nm units, for instance, may result in poor or no performance. C) Be sure that the transmitter and receiver enclosures are grounded. It should be noted that 850 nm FM video links are generally bandwidth-limited at distances over 1.5 km. When this occurs, the receiver output will not be usable even when sufficient optical power is received. 4. PROBLEM: No signal out of the receiver. A) Verify signal input at transmitter. Be sure that an electrical input signal is present at the transmitter input. Also verify that the signal has proper amplitude, frequency, and impedance. For instance, a video signal must be 1.0 Volt peak-to-peak. Several high-speed data links can be configured for positive or negative ECL levels. Be sure that the power supply voltages and connections are correct. B) Check receiver power connection. If there is less than the specified voltage between the power supply pins, a higher current power supply may be required. Be sure that the power supply polarity is correct. 5. PROBLEM: Signal amplitude out of the fiber optic receiver is too large. A) Verify that the receiver output is terminated into the proper impedance. Many data links and most audio and video links require that a terminating resistor be added to the receiver output. If this resistor is omitted, the amplitude will be two times too large. If the value is incorrect, the receiver output level may be too large or too small depending on the value of the resistor. Video links typically require a 75 Ohm terminating resistor. Audio links typically require a 600 Ohm or 10 kOhm terminating resistor. Low-speed data links such as RS-422 and RS-485 typically require a 120 Ohm terminating resistor and high-speed ECL data links typically require a 50 Ohm terminating resistor. 6. PROBLEM: Signal out of the receiver is distorted. A) Verify the input signal at the transmitter. Must be 1.0 Volt peak-to-peak or less. A larger signal will cause distortion. B) Verify the fiber size. See Determining Fiber Size details. Fibers with larger core sizes may overload the receiver. Verify that receiver power is within specifications. 7. PROBLEM: Data errors occur. A) Be sure that the power supply voltage is correct and clean for both the transmitter and receiver or transceivers. B) Be sure that the enclosures are properly grounded, especially when using a wall-mount power supply. C) Be sure that the data inputs and outputs are properly terminated. D) Be sure that the input data levels are correct. E) Be sure that the optical input level to the receiver is within valid limits. 8. PROBLEM: Signal out of diplexer/demultiplexer is noisy. A) Check the copper or fiber optic link between the diplexer mux/demux pair. Ensure that the losses in the optical path do not exceed the loss budget of the transmitter/receiver pair used. 9. PROBLEM: Audio signal amplitude out of diplexer demultiplexer is too large or distorted. A) Verify the signal input at the multiplexer. B) Verify that the demultiplexer audio output has been properly terminated into the required impedance, usually 600 Ohm or 10 kOhm terminations. C) The audio input to the multiplexer must be 1.0 Volt RMS maximum. This translates to about 4 Volts peak-to-peak maximum.