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Fiber optics newbie

T

Tim Shoppa

Jan 1, 1970
0
OK, I've done optoisolation stuff before, but now for RFI reasons I want
to locate a receiver far away from a computer.

Isolation transformers were tried, but the capacitance between the windings
(something like 100pF) let way too much noise through in the 3-30MHz
region.

My bandwidth requirements are minimal (audio and a 1200 baud serial data
link). I really want to minimize the amount of digital noise (and stuff)
at the receiver end... putting a PC there is a complete no-no.

I see that Digi-Key has some fiber-optic receivers and transmitters for
very reasonable prices. For example, the Sharp GP1FA511TZ transmitter
and GP1FA511RZ receiver, which are a couple dollars for a pair
and have TTL-level in and out. But I have some really basic questions:

1. I can just turn the audio into PWM and run it over these, right? No
need for 50% duty cycle to the LED? In my head I'm thinking of these just
like the two sides of an optoisolator... am I too far off?

2. What's the optical interface of these parts? I see there are several-
hundred-dollar receivers and transmitters that use the same type of fiber
connectors that we use on network equipment at work. But these cheap fiber
parts seem to use a different interface - maybe one used in consumer digital
audio? Maybe they don't have any at all?

It looks like I can rig up a little fiber optic receiver/transmitter that'll
let me locate the receiver away from the computers for a cost of less than
$20 in parts... that'd be perfect, if I can pull it off.

Tim.
 
J

John Larkin

Jan 1, 1970
0
OK, I've done optoisolation stuff before, but now for RFI reasons I want
to locate a receiver far away from a computer.

Isolation transformers were tried, but the capacitance between the windings
(something like 100pF) let way too much noise through in the 3-30MHz
region.

My bandwidth requirements are minimal (audio and a 1200 baud serial data
link). I really want to minimize the amount of digital noise (and stuff)
at the receiver end... putting a PC there is a complete no-no.

I see that Digi-Key has some fiber-optic receivers and transmitters for
very reasonable prices. For example, the Sharp GP1FA511TZ transmitter
and GP1FA511RZ receiver, which are a couple dollars for a pair
and have TTL-level in and out. But I have some really basic questions:

1. I can just turn the audio into PWM and run it over these, right? No
need for 50% duty cycle to the LED? In my head I'm thinking of these just
like the two sides of an optoisolator... am I too far off?

2. What's the optical interface of these parts? I see there are several-
hundred-dollar receivers and transmitters that use the same type of fiber
connectors that we use on network equipment at work. But these cheap fiber
parts seem to use a different interface - maybe one used in consumer digital
audio? Maybe they don't have any at all?

It looks like I can rig up a little fiber optic receiver/transmitter that'll
let me locate the receiver away from the computers for a cost of less than
$20 in parts... that'd be perfect, if I can pull it off.

Tim.


How far do you need to run? If it's short, you can use the cheap
led/plastic fiber/integrated silicon receiver links. Long stuff needs
lasers, glass fiber, and more expensive receivers.

Everything else you suggest sounds fine.

John
 
U

Uns Lider

Jan 1, 1970
0
OK, I've done optoisolation stuff before, but now for RFI reasons I want
to locate a receiver far away from a computer.

This might not help you, but what about putting SPDIF audio through a
pulse transformer (salvage from an Ethernet card maybe) and running it over
Cat 5? Not certain whether that'd result in minimal or maximal emissions.
2. What's the optical interface of these parts? I see there are several-
hundred-dollar receivers and transmitters that use the same type of fiber
connectors that we use on network equipment at work. But these cheap fiber
parts seem to use a different interface - maybe one used in consumer digital
audio? Maybe they don't have any at all?

"Toslink" maybe? That's what the consumer digital audio optical interface is
called. I believe that Toslink is actually the name of one of the
manufacturers, not really the connector type.

-- uns
 
I

Ian Buckner

Jan 1, 1970
0
Tim Shoppa said:
This might not help you, but what about putting SPDIF audio through a
pulse transformer (salvage from an Ethernet card maybe) and running it over
Cat 5? Not certain whether that'd result in minimal or maximal
emissions.

I've already tried isolation transformers. The capacitive couplings
between the windings (measured to be about 100pF) lets through way too
much RF.
"Toslink" maybe? That's what the consumer digital audio optical interface is
called. I believe that Toslink is actually the name of one of the
manufacturers, not really the connector type.

Yes, "Toslink" seems to be the operative word. Maybe the reason it's not
mentioned by name on any of the data sheets is trademark-related, since you
say it's the name of a manufacturer, not a standard? In any event, the
transmitter/receivers are a few bucks a pair and the fiber is about a
dollar a foot. It seems that max range is 15 ft or so, and that's a
start!

Tim.[/QUOTE]

Well, "Toslink" is not actually the company name, it is Toshiba.

There are a number of off the shelf fibre optic solutions, you could
go to:

http://www.semiconductor.agilent.com/cgi-bin/morpheus/home/home.jsp?pr
odCat=Fiber+Optics&pSection=Networking&subSection=Fiber+Optics

(sorry about the length, it will no doubt wrap). From there, click on
the
"Low Cost Fibre Solutions" link, which will take you to an index with
more info than you can shake a stick at.

Note: I work for the company.

Regards
Ian
 
T

Tim Shoppa

Jan 1, 1970
0
Ian Buckner said:
Well, "Toslink" is not actually the company name, it is Toshiba.

There are a number of off the shelf fibre optic solutions, you could
go to:

http://www.semiconductor.agilent.com/cgi-bin/morpheus/home/home.jsp?pr
odCat=Fiber+Optics&pSection=Networking&subSection=Fiber+Optics

(sorry about the length, it will no doubt wrap). From there, click on
the
"Low Cost Fibre Solutions" link, which will take you to an index with
more info than you can shake a stick at.

Interesting stuff. I had done some stuff with relatively expensive
Fiber Optic ethernet before, and the consumer/low-end side of the
spectrum is surprisingly affordable.

What I would like to see for this particular project is a transmitter
module with integrated A/D and PCM abilities. This would let me start
with analog and go straight to optical. And then I could do the reverse
on the other end with some integrated receiver. I can use
Sharp/Toshiba/Agilent transmitters/receivers along with some TI or NatSemi
A/D and D/A and PCM chips all in combination but for a one-off low-fidelity
application like mine it's easier to just go with PWM or FM modulation
made out of jellybean parts.

I've done a little bit of research and it looks like a fair number of
PC-clone sound cards have copper SPDIF ports that are easily converted
to optical. If I had a way to easily go from optical SPDIF back to analog
at the far end, without using a PC but with a single tiny (hopefully low-RF
noise) module, that would be so perfect.

Tim.
 
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