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Interesting experiment with ADSL splitter

Discussion in 'General Electronics Discussion' started by Jeniffer Chris, Nov 26, 2013.

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  1. Jeniffer Chris

    Jeniffer Chris

    Nov 25, 2013
    As we know, the ADSL splitter box has 1 input and 2 outputs, one for phone signal line, and the other for internet signal line
    Plug the telecommunication line into the input. Then in the output lines, I make the exchange. I plug the phone line to the internet output, and the internet line to the phone output
    Then I dial the phone to call to a fax number
    Surprisingly, I heard the busy tone, instead of fax tone.
    Note: When I connect in normal way, plug the phone line to the phone output, and internet line to the internet output, this effect does not happen. That means I heard the fax tone as usual

    Second experiment

    I have been considering what happens if I make the feedback from out put to input of a ADSL splitter, as well as change input to output and vice versa
    Below are my steps:
    - Take the first splitter, plug the phone line to the Net output (NOT the input) of it
    - Plug the input of the first splitter to the input of the second splitter
    - Plug the Phone output of the second splitter to the phone output of the first one
    - Plug the Net output of the second splitter to Modem, and connect to laptop

    See diagram in my attached screenshot

    Here I made some abnormal things:
    - Telephone signal transmits into the output, not the input of the splitter
    The output of second splitter is connected back to the first one

    Surprisingly, my laptop can access internet normally. Nothing wrong happens
    Why my extraordinary actions do not affect the operation of the splitters?

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    This would have worked even without the feedback from phone to phone output.
    The splitter consists of two filters. A low pass filter for the telephone signal and a high pass filter for the DSL signal. The filters need to be bi-directional to let pass the relevant signals in both directions. In your setup you simply have tw DSL filetrs in series. These will let pass the DSL signal, so you have internet access.
    Only thing is that each filter attenuates the signal a bit. Two filters in series attenuate twice as much as one filter. The distance that can be achieved via DSL is thus reduced, or the data rate. As long as you are near your outdoor DSLAM you won't notice any difference. If you life afrther away from the DSLAM, you will notice these effects.
  3. Jeniffer Chris

    Jeniffer Chris

    Nov 25, 2013
    My expectation that the feedback will cause system working incorrectly. However, it works well as usual. It's my surprise
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Look at the internal circuit of the splitter:
    Net and input are directly connected.
    Phone is decoupled by a low pass filter.

    Therefore your "circuit" doesn't create any feedback, just a short circuit between phone line and Modem.
  5. Jeniffer Chris

    Jeniffer Chris

    Nov 25, 2013
    What will happen if I create a feedback loop by cutting into the cables and splice the red to the green and other green to the other red
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You will cause the device to fail to operate correctly.

    Surely you don't think that the manufacturer supplies a device that you can make better by simply shorting out wires?
  7. Jeniffer Chris

    Jeniffer Chris

    Nov 25, 2013
    Finally I can cut into the cable to make the real connection, as seen in my attached screenshot

    In a), I loop the net output to input, and connect phone output to a telephone machine. No signal at all

    In b), I loop the phone output to input, and connect net output to a modem, this time I can access internet normally

    Is it working correctly?

    Attached Files:

  8. davenn

    davenn Moderator

    Sep 5, 2009
    You quoted Steve's last comment
    yet you ignored his advice, do you not understand what he was implying ?

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