4-20 mA Signal on a Fiber Optic Transmitter

[Nardie]

I am having trouble with transmitting a 4-20 mA signal via a fiber optic cable. This a new installation that transmit a 4-20 mA signal from a Signet 9950 dual channel transmitter to a RLH 420-01-1 fiber optic transmitter. The 4-20 mA signal is converted from a fiber optic signal to a normal 4-20 mA analog signal via a RLH 420-01-2 fiber optic receiver. From the fiber optic receiver, the 4-20 mA signal is sent to a Omega DP25B-E level indicator to display the tank level. During start-up testing the 4-20 mA signal is distorted at the level indicator. The overall length of the fiber optic cable is about 2200 ft with a 5.106 dB loss based on the OTDR report. I not very familiar with fiber optic applications, so I am asking does anyone have a similar application that could provide some help with the 4-20 mA signal distortion. Also, the signet transmitter can only transmit the 4-20 mA signal over a maximum 1000 ft. So, if the fiber option is not available what type, if any, 4-20 mA signal booster/amplifier is available. I appreciate any responses.

 

[synchro]

You could set one of the output channels of the fiber optic receiver for a 0-10V voltage output to verify that you're getting a linear response there. If it's distorted it would verify the problem is earlier than that point in the chain. What type of distortion are you seeing? Is it clipping or compression at one end of the range?

 

[Lost_RFTech]

Time for some test equipment and perhaps a better description of your distortion. Things to try: 1.) Measure the current output of the 9950 and verify that it matches whatever stimulus you are applying. 2.) Substitute your own 4-20 mA source in place of the 9950 and verify expected results out of the fiber conversion system and/or level indicator. 3.) connect your own source directly to the Omega device and verify proper operation. One of these tests should at least allow you to isolate the source of the problem.

 

[Jraef]

Chances are about 99.99% that the problem is not in the fiber optic transmission system, it's in the electrical signals at one end or the other. All the fiber optic transmitter and receiver do is convert the analog electrical signals to pulses of light and back again to electrical signals and one of the main benefits to that is almost total immunity to noise and distortion of the light signals.

Distortion in a 4-20mA signal is usually the sign of noise getting into the conductors. Did you use shielded wire and ground one end?

 

[Nardie]

Thanks for the responses. We have tried our setup with a new fiber optic transmitter and receiver with much better results. This was done with only a short patch fiber. We will test the new equipment with our full installation next week. I will post the results. Unfortunately, we did not use a TWSH pair for some reason. Again thanks for your responses.