Topband: Light fiber question

[Bill Wichers]

Um, no... Not really -- you do NOT need modulators and demodulators to use fiber for *analog* applications. If you want to run your signal over a commercial transport network you will (to digitize the signal and deal with framing for SONET, Ethernet etc.). Cable TV systems run their RF over fiber with very wide frequency ranges (50-700+MHz). I don't know dynamic range off hand since that's not a parameter that comes up with the digital systems I work with (we use extinction ratio instead). The downside is that the equipment to do that is not cheap. This is what I do all day at work (well, digital only, but optical) :-)

Anyway, if you only want to run a topband signal it shouldn't be difficult to run the entire band over fiber. You'd need to use a small laser (although an LED would probably work fine since 2MHz is very slow compared to the data rates used on most digital transport systems these days). The important part would be that you'd need to run a feedback loop with the laser/LED and a photodiode monitoring the output. The combined system would be your fiber driver. The receiver would be simpler -- just a photodiode and an amplifier. If you used a decent laser in the 1310nm band and singlemode fiber (the most common kind) you should be able to send your signal at least 5 miles or so without really doing anything special. 

If I was to build a system like this, I'd use a small 1310nm laser driving an optical splitter. One output from the splitter would be the "output", the other would go to a photodiode that would be part of a feedback loop with the laser. The receiver would be a photodiode and an amplifier to convert the optical signal back into an electrical signal. Note that optical splitters are specified in percentages, not decibels, so a 50/50 is "3dB down" on both outputs, a 90/10 is "10dB down" on one output, etc.

It would be an interesting project, although I think it would be a lot simpler to just use low-loss coax unless you run is REALLY long or you have some very special requirements. 

Fiber is lightning proof though -- just plastic and glass :-)

  -Bill

> People forget, or don't know, fiber cables require modulators and
> demodulators. The modulators and demodulators are not simple, and have
> horrible dynamic range compared to a simple piece of coax.
> 
> I can't think of a good Ham shack application using fiberoptics for coupling RF
> signals. First, the dynamic range stinks compared to coax. Second, common
> mode is very easily completely cured with extreme measures.