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models for simulating a dsl link

Discussion in 'Electronic Design' started by Eeyore, May 25, 2007.

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  1. Eeyore

    Eeyore Guest

    A couple of recent threads about the effects of DSL (micro)filters gave me the
    idea of analysing their effect under various conditions in a simulation package.

    I've got several examples of filters of varying designs here and I'd really like
    to know how much difference and indeed what kind of differenc(es) they can make.

    As some know from previous threads I'm a bit sceptical about how much they
    affect the DSL signal itself.

    Before I can do this I need some suitable 'models' for the various bits of
    telecom gear involved. Can anyone point me in the right direction ?

    Graham
     
  2. Unlike cable (DOCSIS) modems, DSL comes in many flavors with widely
    varying performance specifications. Take a look at the Wikipedia article
    for a start (http://en.wikipedia.org/wiki/DSL). I don't think there is
    an industry standard like there is for cable broadband. Each
    manufacturer has their own specs.

    As far as the DSL filters go, since they are intended to be placed on
    all of the other (non DSL) equipment on the shared line, their
    performance might also depend on the nature of the telephone equipment
    connected downstream of them.
     
  3. Eeyore

    Eeyore Guest

    That won't affect the basic physics though, which is all I want to model.

    That's exactly one of the things I'd like to experiment with.

    Graham
     
  4. You can do a simple test by unplugging the filter and putting a lead from
    the ADSL modem straight into the phone socket.
    Although the different quality of the cable might be as important as the
    filter.

    They are bound to attenuate slightly.
    Even a straight through extensions lead three inches long would have some
    attenuation.
    Whether it makes enough of a difference is the issue.


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  5. Eeyore

    Eeyore Guest

    Yes I know this kind of thing. It misses the point though. I want to *quantify*
    in decibels of loss, frequency response etc the effect of such things using a
    modern electronics simulation package.

    Absolutely not. If a cable wasn't twisted pair that might make a difference
    though. I wouldn't call that 'quality' myself. I like to see things called by
    their correct names.

    Graham
     
  6. Alec

    Alec Guest

    If you wish to model the filters you will need to measure the inductance and
    capacitance of the elements of the filter.

    If you can do this you probably have the equipment to measure the real
    performance so why not simply measure the performance under the conditions
    you are interested in?

    Alec
     
  7. Eeyore

    Eeyore Guest

    Yes. The Capacitance values are printed on the caps btw.

    Not from the exchange end. I don't have access to that for one thing ! The whole
    entity of the line from exhange to subscriber is what I intend to simulalte.
    It's quite simple as long as I can get the numbers to punch in for various
    pieces of kit such as phones.

    Let me put it another way. Right now I'm looking for phone schematics so I can
    make a model of them. Can you point me in the direction of any ?

    Because I want to simulate any number of generalised (and specific) conditions
    and measure the effect of individual changes.

    Measuring performance the wayyou suggest is so passe these days and hoplessly
    time consuming.

    Graham
     
  8. John

    John Guest

    Try looking on http://www.telephonesuk.co.uk/ There's wiring diagrams and
    schematics on there, I believe (not looked myself so don't shout at me if
    there isn't :eek:)

    John
     
  9. Guest

    You can measure the physical parameters R, L and C and put these
    into an analysis package to get the passband, attenuation, phase shift
    and other parameters, expressed in compatible units such as db.

    This won't be much help because the filter assumes input and
    output impedance parameters that are laboratory design concepts,
    that don't match real life, so your analysis package graphs won't
    match real life. In fact you may find that what appears to be a
    cheapo sub-standard filter gives better results because it creates
    a better match between the incoming lines impedance, and the
    modems input impedance.

    Like a lot of HF electronic design you can impedance match (often with
    difficulty) at a specific frequency, but the match rarely holds over
    a range of frequencies where the source and load are complex,
    and frequency dependent, numbers.
    Complex numbers in this sense are numbers/vectors with real and
    imaginary components, where you get into hyperbolic functions.

    Like your TV antenna should be 75 ohms across its frequency range,
    and your TV's antenna socket should be 75 ohms across the
    full range of TV channel frequencies, it's a pie in the sky concept.
    In the heyday of CB radio it was all about matching, many and
    various, were the weird and wonderful combinations of coils and
    capacitors, pie, half pie, etc.
     
  10. Eeyore

    Eeyore Guest

    But *does* the filter (did the filter designer) assume that ? It certainly doesn't
    have to be that way. That's why I fancy simulating some.

    At the lowish frequencies used for ADSL I'm not clear how much that 'matching' is
    relevant. It certainly won't be a big issue at ~ 30kHz for example, rather more of
    one at 300kHz and 1MHz..

    Graham
     
  11. Joel Kolstad

    Joel Kolstad Guest

    All of that is still around... it's quite visible with the HF amateur radio
    guys, and of course in something like a cell phone it's all there but just
    much, much tinier.

    When you're talking about transmitters, not only is proper matching desirable
    from the point of view of getting as much signal into the air as possible,
    it's also important in that serious mismatches can create voltages that simply
    blow up the transmitter's finals. :) (At least with solid state devices --
    tubes apparently are much more immune to this.)
     
  12. Rich Grise

    Rich Grise Guest

    Find someone nearby with DSL, and ask them if they have a spare filter. My
    DSL provider gave me a handful of filters - just see if you can get
    somebody to give you one, and hack it and find out what's actually inside
    the thing.

    By Occam's razor, it's probably just an ordinary LC low-pass filter,
    probably balanced and impedance-matched to the line(s).

    Good Luck!
    Rich
     
  13. Guest

    What you are up against is that an ADSL filter designer has to
    start from a set of R, L, C parameters that create the impedence
    of an incoming exchange line.
    The problem is that someone a quarter mile from the exchange
    sees a different incoming line impedence to someone 2 miles
    from the exchange.
    In the early days of telephony loading coils were used to level
    the frequency response of lines over the speech band, don't
    know if they are still used. In essence twisted pair street
    cables are very capacitive, and loading coils were inductive
    to compensate.
    You have to assume what comprises a typical ADSL line.
    Then you have to hope that the typical ADSL modem matches
    the line, and would be a good match when plugged in without
    a filter.
    Having made these sweeping assumptions you now decide
    how sharp the filter needs to be, and what configuration to
    use.
    You end up with a filter that hopefully does as good a job
    as possible in most situations.
    Matching is always important because the launch energy at
    the source has to go somewhere, it can't just be destroyed and
    disappear, "conservation of energy" and if it's not absorbed
    it is likely in 200 pair street cables, to appear as noise in
    adjacent (more than likely) ADSL pairs.

    Unfortunately a lot of street cables in the UK are immediate
    post war, though there is still a bit of paper insulated, lead
    overall, still around. Never designed for ADSL.

    These cables were never speced for Near End Cross Talk
    (NEXT), if we were starting again they would be, but then
    if we were starting again fibre to the kerb would be a better
    model.

    But if you wan't to dig deeper
    http://whatis.techtarget.com/definition/0,,sid9_gci1059969,00.html
    is a good starting point for understanding what is involved.
     

  14. Paul, some of the filters have two output phone jacks. Filtered, for
    a telephone, and unfiltered for the DSL modem.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  15. DSL modems aren't supposed to be used downstream of DSL filters. DSL
    filters are supposed to be put in line with everything _except_ the
    modem.
     
  16. Eeyore

    Eeyore Guest

    I talk to myself and find I have 3 spare filters ! :~)

    2 of them are basically simple 2nd order LC filters feeding the POTS socket. The
    L consists of 2 windings on one core to retain line symmetry. They also include
    the 'ring capacitor' too, a UK thing AFAIK. The bell circuit runs as a 3rd wire
    inside the house here. Doing it that way avoided 'bell tinkle' in the old days
    of pulse dialling.

    See above. The third is stuffed full of bits. 3 x 2-section inductors, 5 caps,
    and 2 Rs.

    Not so sure about impedance matched. What impedance is a phone anyway, never
    mind 4 in parallel ( the max load for a line and a single filter is *supposed*
    to cope ) ?

    Graham

    p.s. *what impedance is a phone anyway* ? Please see thread title ! Off-hook,
    on-hook I need data !
     
  17. Eeyore

    Eeyore Guest

    'Create the impedance'. Can you elaborate on that ? I don't think it's that at all
    unless I'm very mistaken. In any case the impedance of a filter will be all over the
    place with frequency.

    More like a different resistance actually. At *audio* frequencies the short run of cable
    from the exchange doesn't really impose any characteristic impedance value.

    At the DSL frequencies the issue will come into play but the DSL filter's not in that
    path so the DSL modem / router has to deal with that one just for itself.

    I've heard of them but know almost nothing more about them. I gather DSL won't work if
    there's one on the line. I suspect also they may have been there to help with the volume
    level on long lines.

    OK. Easy to simulate again if so inclined.

    Yup. Or several examples of same from 100m from the exchange to several km.

    They are apparently.

    I'm not making that decision. I have some examples here and they vary considerably.
    That's the objective of this exercise. To see just how much difference a DSL filter can
    make and in what ways.

    Only when you're talking about a transmission line. POTS telephone local loop circuits
    are *NOT* transmision lines.

    Can't take all that into account too. I'm interested in what's likely to have been
    around for say 30-40 yrs.

    Thanks.

    Graham
     
  18. Eeyore

    Eeyore Guest

    Yes. The modem connection is simply a pass-through.

    Graham
     
  19. Guest

    Although it may be a pass through, what is shunted across the line
    is important. Like a recent post here, where sync only occured when
    the phone was lifted. What happens on the phone side will be reflected
    onto the ADSL side.

    Tlelephone twisted pair has a characteristic impedence, for
    high spec cables the mfr usually quotes it, along with figures
    for NEXT and other line characteristics.
    You can do it the hard way and start from the dielectric
    constant of the insulation, the wire spacing (thickness of
    the insulation), and the wire diameter.
    Old timers will remember the Royal Core of Signals
    bible The Handbook of Line Transmission, and the
    various BT books.
    There are also test sets for measuring line parameters.

    You can measure anything, it's really a matter of
    determining what you need to measure, and how it
    relates to the real world.
     
  20. Eeyore

    Eeyore Guest

    But not at all frequencies. I don't deny it must have an effect of some kind of
    course.

    Not at the lengths used for the local loop at audio frequencies. This is a total
    fallacy.

    Twisted pair stuff has a characteristic impedance of around 100 ohms (when it
    becomes relevant which it isn't at audio frequencies in the local loop).

    That is indeed what matters. As I said, it's about 100 ohms for LLU wiring.

    Graham
     
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