CATV analog modulation video over fiber has a typical 50dB dynamic range. See
e.g. http://www.emcore.com/wp-content/uploads/Medallion-6000-Series.pdf
Tim N3QE
-----Original Message-----
From: Bill Wichers [mailto:billw@waveform.net]
Sent: Thursday, October 24, 2013 12:10 PM
To: Tom W8JI; Shoppa, Tim; Bruce; topband@contesting.com
Subject: RE: Topband: Light fiber question
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.
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Topband Reflector
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