Audio signal loss across a fiber optic link.

[jacktech]

I'm having a problem with an audio signal being degregated too much (-18db) transversing a pair of audio fiber modems and 150ft of multimode fiber optic cable. This application is a two-way radio system used for office personel to communicate with crews out in the field. I'm using the fiber to isolate the RF tower and its associated equipment from my inhouse wiring/communications equipment. The two rooms are 100ft apart and I didn't want a copper path between the two rooms. The RF room has the radio base station equipment and a 200ft antenna tower (lightning rod). The desktop radio remotes communicate to the base station via an audio signal on a single pair of copper cable. In a normal installation the single audio signal pair is bridge-tapped to all the radio remotes in the office. To provide electrical isolation I installed the audio fiber modems and the fiber between the base station and the bridge-tapped connection point for the office remotes. The fiber modems are set up as two wire transmit-only (always off-hook). Things work but the audio signal is getting 18 db of loss going through the two fiber modems which makes the audio quality poor.The radio technicians/engineers have no way to compensate for the signal loss. The fiber modem manufacturer has tried but according to his calculations the components in his modems should be producing this amount of loss. Does anyone have any experience in this type of application or know of a fiber optic modem that doesn't degrade the audio signal so much?

 

[a31ford]

If isloation of tower to controlhouse electroincs is the sole reason for going audio fiber modems, why not simply shorten the length of the fiber ? say, wack 130 or so feet off of it, and run copper from there ? Like there's still 20 ft. of isloation there, and if everything is grounded properly, the ground will take the lightening rod effect of the tower before it jumps 20 ft. of fiber. And at that point, IF the strike CAN jump 20ft, I don't think anything else would matter, cuz all else would be fried also..(people inluded).

Isolation is a relitive term, one person in a big city can feel isolated, how much isolation is needed, before it becomes "moot", 150ft, I feel is way to much for lighting strike effect.

Greg

 

[jacktech]

Thanks for your suggestion. The reason for 150ft of fiber is that the RF tower room is 120ft away from the inhouse telecommunications rack and by the time you add the lengths needed to get up into the cable tray and then back down you come up with 150ft. The problem is not with signal loss across the fiber, the problem is signal loss through the fiber optic modems at each end of the fiber. We've proven this by measuring the signal loss using a one foot fiber patch cable to connect the modems back-to-back. We had the same signal degredation no matter what length of fiber we had between the modems.
My hope is that some one may know of a audio fiber modem that may have adjustable attenuation/gain.

 

[robertjo24]

Try the following links:

http://www.commspecial.com

http://www.lascomm.com/data/lt8000.htm

The link to the Lascomm device will handle a large optic attenuation without voice degridation. You should not have fiber loss issues for such a short run.
Pay attention to coupling impedance between your radio system and control console, and the fiber modems, (ie. 600 ohm balanced 4wire vs unbalanced or two wire).
There are many devices on the net that should solve you issue.
Another hint, call a local Motorola (or similar) radio shop to inquire about a fiber remote control solution.

Good Luck

 

[jacktech]

Thanks for your suggestions, I've been working with the local Motorola radio distributor here and they've been trying to make adjustments on their equipment but aren't having any success in getting it fixed or offering any solutions other than a copper connection between the base station and the radio remotes in the office. They seem to think that lightning damage is something to live with, the customer takes a hit on the tower, the surge comes in and blows up a few cards and remotes and then they call the radio guys to come out and fix it. I've used Lascomm fiber modems before in a different application and they are still running. The units that I'm having all the problems with are Versitron units. The Versitron enegineer modified a pair of their two-wire analog modems to be always off-hook and transmitting for this application. Again, the issue is in the signal loss through the silicone components of the modem, not the fiber.

 

[robertjo24]

If you would like to email me a quick sketch of your set up, including radio & controller type (part numbers), I might know just the guy who could help.
This is a two wire, 600ohm input to the radio/controller?
Tone remote control?
Any particulars you list on the email would be helpful.
Send to joroberts24@earthlink.net