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Fiber optics newbie

Discussion in 'Electronic Design' started by Tim Shoppa, Sep 15, 2003.

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  1. Tim Shoppa

    Tim Shoppa Guest

    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

    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.

  2. John Larkin

    John Larkin Guest

    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.

  3. Uns Lider

    Uns Lider Guest

    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.
    "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
  4. Ian Buckner

    Ian Buckner Guest


    I've already tried isolation transformers. The capacitive couplings
    between the windings (measured to be about 100pF) lets through way too
    much RF.
    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


    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:

    (sorry about the length, it will no doubt wrap). From there, click on
    "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.

  5. Tim Shoppa

    Tim Shoppa Guest

    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.

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