Transmitting linear 4-20mA d.c. signal over fibre optic cable

Start by asking the supplier if the 3km length limit in their data sheet is
for their multimode optics unit or if it applies to their singlemode
offering too.

The reason I suggest this is that they quote an optical link budget of 12dB.
That might be consumed by the product of bandwidth and loss on multimode due
to speckle and plain multipath attentuation/pulse spread. If the
transmission path is attentuation or spread limited then 12dB on singlemode
is going to be a whole lot further, provided the launch is good into the
fibre. The downside might be that in the digital coding they have assumed a
maximum turn around time that length limits at 3km - I would guess
unlikely - they shouldn't be cloccking the link that fast for the quoted
performance and this is a single ended application so there are no
collisions to worry about unlike a true network.

If your link is 3 or 4 miles through duct and the like, you want to be in
single-mode anyway. Yes the investment in splicing equipment for the fibre
is greater, but you are then firmly in the telecoms pool of expertise rather
than the jumped up "long-LAN" pool of expertise. Frost and water damage to
multi-mode is a greater problem than with singlemode in my bitter
experience. Single mode splicing professionals tend to do the job once and
right.

As an aside, how often does your panel meter need to update? Have you
considered low power radio or is that out of the question for some reason?
That would be the utility/scada/rtu way to go...

Top hit on a Google for "4 20mA low power radio":

http://www.omniinstruments.co.uk/radiotel/tx7120rx7120.pdf

That would seem to do the job, if the pointer on the metercan tolerate
moving once per second - you also have a back channel for control purposes
and some digitals for heartbeat and the like.

Hope this helps.

Dave Slee
Riding Mill Energy Contracts Ltd.

denbigh1974@boltblue.com says...

I looked at the datasheet for that device, and it isn't clear what the
resolution of the converted signal is.  The specs list "960
samples/sec", which implies an A/D converter, but the only accuracy spec
is "100 uA deviation"...

A bit of googling returned Weed Instruments, who offer analog to fiber
multiplexers capable of 40 km:
http://weedinstrument.com/product_groups/fiber/multi.html

--Gene