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Carrier-current intercom

Discussion in 'Electronic Design' started by PDRUNEN, Sep 11, 2004.

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  1. PDRUNEN

    PDRUNEN Guest

    Hi Group,

    I picked up some wireless intercoms to experiment with, these a older ones
    which generates a 200KHz FM signal and injects this onto the AC mains.

    Can anyone tell me how they interface the signal so that the prime power and
    this signal can share the copper? Any website that has projects using the
    MAINS as a method of transfering binary or analog data?

    Why do they use FM here? I thought 200KHz had alot of noise and would have
    expected SSB or AM.

    I saw a device from GE (at Target) called an instajack, this device can turn a
    AC wall outlet into a phone outlet. I have an application for using this with
    my computer to access the internet where I can not connect to a phone jack at
    that location , I can create one.

    Since I see them alot on ebay, I would suspect that the units don't work very
    well. Anyone with experience with them?

    Can the AC mains be used for UHF frequencies? I would place a ferrite core at
    the 5/8 wave point so that the UHF signal would not go any further. The goal
    here is just to listen to UHF frequency from which a message is broadcast. The
    problem I see is that the 60Hz would have to be really isolated. What if a
    made a coupling coil that would wrap around the rolex to create an inductive
    coupled transformer? A ferrite core would be placed around the Rolex at the
    5/8 watt distance.

    So, any thoughts?

    PDRUNEN
     
  2. Tim Shoppa

    Tim Shoppa Guest

    Two ways:

    1. If the unit is completely self-enclosed no exposed metal etc. then they
    are allowed, by some electrical codes, to capacitively couple the carrier
    into the AC power.

    2. More generally they can transformer-couple into the AC power.

    Most intercom/X10/line carrier gizmos are not properly isolated from
    the AC line, so use extreme caution when tinkering with their innards.
    Under no circumstances should you ever run a wire from their innards
    to anything that you value or touch. Optoisolation is acceptable *if*
    you know what you're doing.
    They use FM BECAUSE there is a lot of noise... SSB is not particularly
    cheap to do, and the keyword in the internal circuitry of these units
    is CHEAP.
    For voice-quality stuff they're acceptable. DO NOT expect to run at
    56 kbps over this! Maybe 1200 or 2400 baud, unreliably.

    If you're interested in data transmission over household AC wiring, look
    at the various X10 dissections on the net, such as:

    http://www.nysaes.cornell.edu/cc/staff/pool/homeauto/

    X10 uses a 120kHz carrier, and modulates only at zero-crossings (where
    hopefully there isn't a lot of switching noise), some of the references
    say they also transmit 120 and 240 degrees out of phase to handle 3-phase
    situations where the receiver is on a different phase. Very clever,
    but you may be disappointed at a data rate measured in a few bits per second.
    The power lines do about as well at UHF frequencies as any piece of
    Romex with all sorts of electrical noise being superimposed on it. That
    is, pretty lousy.

    Separating out 60Hz is easy, you're talking about 6 or 7 orders of
    magnitude in frequency between 60Hz and UHF. Separating out all the
    other electrical noise that's not at 60Hz will be the real issue.

    Again, use caution with the schematics of X10 and line-carrier intercoms
    you find, as they generally are not transformer isolated from the AC
    power. I know, I've said that three times already, but I cannot
    overemphasize this.

    Tim.
     
  3. Tim Shoppa

    Tim Shoppa Guest

    Two ways:

    1. If the unit is completely self-enclosed no exposed metal etc. then they
    are allowed, by some electrical codes, to capacitively couple the carrier
    into the AC power.

    2. More generally they can transformer-couple into the AC power.

    Most intercom/X10/line carrier gizmos are not properly isolated from
    the AC line, so use extreme caution when tinkering with their innards.
    Under no circumstances should you ever run a wire from their innards
    to anything that you value or touch. Optoisolation is acceptable *if*
    you know what you're doing.
    They use FM BECAUSE there is a lot of noise... SSB is not particularly
    cheap to do, and the keyword in the internal circuitry of these units
    is CHEAP.
    For voice-quality stuff they're acceptable. DO NOT expect to run at
    56 kbps over this! Maybe 1200 or 2400 baud, unreliably.

    If you're interested in data transmission over household AC wiring, look
    at the various X10 dissections on the net, such as:

    http://www.nysaes.cornell.edu/cc/staff/pool/homeauto/

    X10 uses a 120kHz carrier, and modulates only at zero-crossings (where
    hopefully there isn't a lot of switching noise), some of the references
    say they also transmit 120 and 240 degrees out of phase to handle 3-phase
    situations where the receiver is on a different phase. Very clever,
    but you may be disappointed at a data rate measured in a few bits per second.
    The power lines do about as well at UHF frequencies as any piece of
    Romex with all sorts of electrical noise being superimposed on it. That
    is, pretty lousy.

    Separating out 60Hz is easy, you're talking about 6 or 7 orders of
    magnitude in frequency between 60Hz and UHF. Separating out all the
    other electrical noise that's not at 60Hz will be the real issue.

    Again, use caution with the schematics of X10 and line-carrier intercoms
    you find, as they generally are not transformer isolated from the AC
    power. I know, I've said that three times already, but I cannot
    overemphasize this.

    Tim.
     
  4. Joerg

    Joerg Guest

    Hi PDRUNEN,

    Tim pretty much answered all the questions you had. Personally I
    wouldn't use powerline gear for anything where a malfuntion could become
    critical. Performance varies so much and can be influenced by things
    beyond your control because everybody in the house plugs and unplugs
    equipment.

    For example, someone plugs in a filtered powerstrip and suddenly your
    signal is in the noise. Or the opposite happens when communication only
    works while the dryer runs or a pizza bakes in the oven. These
    appliances operate on 240V and thus they bridge the phase. Turn them off
    and the coupling between phases drops substantially unless you have a
    protected cap somewhere that acts as an RF bridge.

    The same goes for "modern" appliances with switch mode supplies. Often
    manufacturers have to pinch every penny and they only provide enough EMC
    protection to just make the FCC limits, but no more. This can create a
    ton of wideband noise on the powerline and this noise can dwarf any
    signals. Switcher used to create certain spectral lines and if you were
    lucky your comm gear would be outside these. Nowadays dithering is the
    name of the game. Designers often try to spread the noise as evenly as
    they can so they can cut even more corners on EMC measures. As soon as
    the modulation scheme costs just a wee bit less than added RF filtering
    they'll do it.

    Regards, Joerg
     
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