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ADSL cable simulation

Discussion in 'Electronic Design' started by Myauk, Nov 10, 2008.

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  1. Myauk

    Myauk Guest

    Dear All,

    I need to design an atteuator to simulate 9000ft of ADSL cable loss.
    Could anybody please let me know what are are the do's and don't's on
    practical design of it. Based on my studies, it should give 22.76 dB
    of attenuation at 150 kHz and 50.7455dB attenuation at 772 kHz.
    What else should i considered more?

    Best Regards
     
  2. Phil Allison

    Phil Allison Guest

    "Myauk"

    ** Is this a REAL " flesh and blood": attenuator ??

    Or a ** software simulation ** of one ??

    I should NOT have had to ask that

    - you PITA twit.


    BTW:

    " 50.7455 dB " ??????????

    Wot fucking planet is this prick on.



    ....... Phil
     
  3. Guest

    test -------------------------------------------
     
  4. I assume you are talking about an actual hardware attenuator, and not just a
    software simulation?

    With practical transmission line simulation like this I've always found it's
    prudent to actually use the real cable in a real system test. It would not
    be out of the bounds of practicality to actually use 9000ft of cable (I've
    done this sort of length before a few times), or test a shorter length and
    extrapolate your results. You can also test the performance of a real cable
    and match it with any "simulated" attenuator.

    Dave.
     
  5. This reminds me of something I made in the late '90s. I had to test HDSL
    cards and we didn't like the idea of heavy cable drums in our
    manufacturing test setup. So I made some equivalent circuits out of
    discrete R,L and C components on a board that could plug into our racks.

    Steps:
    1. Get the RLGC parameters for the line you want to simulate. These
    will be a function of frequency. Bellcore used to publish technical
    reports with cable parameters measured on a representative sample of
    actual subscriber lines.

    2. Work out how accurate your simulation needs to be, in terms of dB
    error as a function of frequency. There will probably be some upper
    frequency, beyond which you don't care about the accuracy at all (which
    is good, as most simulations based on lumped elements have an upper
    frequency limit beyond which the accuracy gets really bad).
    BTW, your use of so many decimal places in 22.76 and 50.7455 dB is rather
    naive; you are unlikely to get an accuracy of better than +/- a few dB
    over any non-trivial frequency range.

    3. Create a lumped model of the line (see the links (below) for ideas),
    and check it in your preferred version of Spice. Increase the number of
    sections (which reduces the ripple and increses the upper frequency
    limit) until you get the required accuracy with practical component
    values. Check both the attenuation and impedance.
    The inductors and (to a lesser extent) the capacitors will not behave
    like their ideal models. You will need to include this in your
    simulation. BTW, avoid ceramic caps here, unless they are NP0.

    The loss in these reactive components actually helps with the simulation.
    Remember, we are trying to recreate the behaviour of something that has
    both skin effect and dielectric losses.

    4. Lay it out on a PCB. There will probably be a few hundred sections,
    so a PCB is the only practical way. You must be very careful with the
    layout, as crosstalk will ruin the attenuation characteristics. (BTW,
    crosstalk is also a problem for cable drums.)

    5. Build and test.


    Some links to similar posts:
    http://groups.google.com/group/sci.electronics.design/browse_thread/threa
    d/ec84b4ce3d5a07c3

    http://groups.google.com/group/comp.dsp/browse_thread/thread/90590c5fe7e9
    fb52/

    Regards,
    Allan
     
  6. What are you trying to prove by this simulation?
    The main problem is the direct capacitive coupling between the input and
    the output. Because of that, the frequency/phase response can be very
    peculiar albeit have nothing to do with the reality. Do not use a piece
    of actual cable cable winded on a bobbin; you will have to unwind it.


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
  7. Bob

    Bob Guest

    My thoughts on this may be hopelessly naieve but I'd
    think about doing it as two 10dB H-pads and a low pass
    filter that rolls off at about 13dB per octave.

    I suspect that it needs to be more or less symetrical
    as you want to communicate both ways.

    Inductor and capacitor filter in the middle of the
    circuit, 10dB pads at both ends. Having the pads at
    the ends will make the source impedance look more like
    the characteristic impedance of the line which is
    probably around 110ohms but you need to look that
    up yourself.

    There are a few h-pad calculators on the web eg
    http://www.nu9n.com/tpad-calculator.html

    I'd exepct you could come up with a filter design with
    about eight elements that would give a suitable rolloff.
    Might need quite large inductors due to the low frequency.

    Bob
     
  8. Guest

  9. Myauk

    Myauk Guest

    Dear all,,

    Thanks for your kind suggestions.

    Now I have better confidence for starting up.

    By the way... Mr Allan...I cannot reach the links you described
    there..somehow they are not complete...could you please provide the
    title of the discussions??

    Best Regards..
     
  10. Myauk

    Myauk Guest


    Yes Mr Dave..

    I am talking about an actual hardware attenuator.

    From practical measurement I found out that the series resistance
    value is 250 ohms in each wire. So I am thinking of adding series
    resistance of 250 ohms to simulate the resistance of the wire.


    ---/\/\/\-----


    .../\/\/\....

    This will give an attenuation of -3dB for all frequencies.

    Then I am thinking of creating a T network with two series inductors
    and one parallel capacitor which will give attenuation of around -20dB
    at 150kHz.

    Then I will addjust values acordingly to have the attenutaion around
    -51dB at 772 kHz.

    Hopefully, this can be used instead of actual 9000 ft cable drum for
    ADSL link test...

    Please correct me if you find any practically impossible things in my
    idea...

    I am new in this kind of things..

    Thank you so much for everything..


    Regards
     

  11. Think of it as an aptitude test.

    Hint: find my post at groups.google.com, click on "more options" then click
    on "show original".

    Allan
     
  12. Myauk

    Myauk Guest

    Thank you so much.

    Now I am able to find it.

    Regards
     
  13. Bob Smither

    Bob Smither Guest

    One way to design such a simulator is to use optimization. The program
    CCICAP has a genetic algorithm based optimization feature that is well
    suited to this type of design.

    I have included Myauk's specifications in an example that can be studied
    here:

    http://www.c-c-i.com/node/24

    I used two identical sections of RLC low pass filters. Using guessed
    values for the RLC components results in an initial error at the
    specified frequencies of 37.6 dB. After optimization the error is
    reduced to .08 dB.

    The example could be improved upon with more sections if additional
    attenuation specifications are available.

    Regards,
    Bob Smither
     
  14. Steve

    Steve Guest

    They do sell DSL loop simulators by the way.
    Different models, some with and without noise sources and
    programmable loop lengths to allow automated testing.
     
  15. Myauk

    Myauk Guest

    I think they have more features than it is necessary for us.
    Do you know any good source where I could check the price?
    If the cost is less than the cost of developing it myself, I'd better
    buy it.
    :p
     
  16. Myauk

    Myauk Guest

    Dear Allan..

     Bellcore used to publish technical
    Is there any link to this? I could not find it.

     BTW, avoid ceramic caps here, unless they are NP0.

    Could you please explain to me why I should avoid using ceramic caps?

    Regards
     
  17. In one of the posts I directed you to, I mentioned these standards:

    ETSI ETR-152 (HDSL standard)
    ETSI ETR-080 (ISDN U interface standard)
    ANSI T1.601 (ISDN U interface standard)

    Google will help you to find copies.

    Ceramic caps have a "voltage coefficient" that describes their change in
    capacitance with the applied voltage. This will produce non-linear
    distortion of your signals, and reduce the accuracy of your smimulation.
    The wires you are trying to model typically use some sort of plastic
    insulation (e.g. polyethylene) which does not have this effect.

    BTW, you should also make sure the inductors are used well below
    saturation, for similar reasons.

    Regards,
    Allan
     
  18. Ross Herbert

    Ross Herbert Guest

    :Dear All,
    :
    :I need to design an atteuator to simulate 9000ft of ADSL cable loss.
    :Could anybody please let me know what are are the do's and don't's on
    :practical design of it. Based on my studies, it should give 22.76 dB
    :eek:f attenuation at 150 kHz and 50.7455dB attenuation at 772 kHz.
    :What else should i considered more?
    :
    :Best Regards


    I remembered seeing some artificial line section values in a document I was
    researching recently. You can ind this document here
    http://www.commsalliance.com.au/documents/standards/S002_2005

    See Fig. 26 on p.63
     
  19. Myauk

    Myauk Guest


    Thank you so much

    That one also is of some help to my work.

    Best Regards
     
  20. JosephKK

    JosephKK Guest

    I once built one for pe-39 6 pair 22 and found for a frequency range
    of 200 to 20,000 Hz i had to use "segments" of not much more than
    1000 feet. At your frequency range you may need to do 100 foot
    lengths or less. You should probably do 1000 foot groups in separate
    boards on shielded boxes.
     
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