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Tie phone lines

Discussion in 'Electronic Design' started by ScadaEng, May 3, 2008.

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  1. ScadaEng

    ScadaEng Guest

    I have aproject that needs to "tie" a VOIP modem phone line to a local pots
    line. The device works in most locations as designed, but not all. In one
    particular area the lines are poor, with low gain. Because of this the DTMF
    signals do not get to the device. I am using two 600:600 ohm transformers to
    connect the two lines now. The primary of T1 goes to L1 through a
    non-polarized 10µf series cap. T1 secondary connects to T2 secondary. and T2
    secondary is connected to L2 through another 10µf (np) cap. This is a
    simplified explanation, I have line protection as well as two girator
    circuits to sieze the line at the proper impedance. The problem is the
    losses in this circuit. I'm loosing around 3 dbm through both transformers
    plus the losses accross each cap adds to the problem. I'm wondering if I can
    tie the lines after a bridge rectifier by tieing the bridge negatives and
    coupling the positives with my 10µf cap? I know that different line
    impedances will play into this, but i'm hopping that won't be an issue. The
    VOIP modem being isolated should not create any common ground problems I
    would think. Any thoughts?
     
  2. Rewire the transformers as a hybrid
    http://www.lundahl.se/pdfs/datash/6702.pdf



    martin
     
  3. ScadaEng

    ScadaEng Guest

    Thanks Martin,
    I did do a circuit with LTSpice using two hybrids and two amplifiers (to
    compensate for inductive losses). Problem with that is the receive to
    transmit isolation (and vice vers) changes quite a bit with frequency.
    Because of the nature of the transformer hybrids they allow for a bit of
    feedback (sidetone) which is unwanted for my application. I tried making a
    hybrid electronically, using two op amps and the existing two transformers,
    however I encountered similar results to the passive transformer hybrid
    circuit. I'm basically looking for an easier solution, design time is short.
    Thanks for your input.
     
  4. ScadaEng

    ScadaEng Guest

    Both Pri & Sec are center tapped, but not seperated.
    The girator circuit allows me to couple the line at a desired dc load
    without affecting the impeadance,
    I believe I set that around 20ma. It offers a high impeadance to the line to
    allow the ac component to pass. Therefore the impeadance is
    the series cap and transf primary (close to 600?). The transf cannot connect
    to the line directly
    being it is of the "dry" type, and would saturate by any line current.

    I have never uploaded a schematic to the news group. Do I send it here? I
    created it in "Eagle" (.sch). Are there
    file size limitations?
     
  5. ScadaEng

    ScadaEng Guest

    I did not use a passive hybrid because the losses were to great. I created
    an electronic hybrid,
    that seems to be the way that all the telephone manufactures are going with.
    The physical size of
    two passive hybrids are not an option. The elect hybrid reacts the same,
    only I can add amplification easily.
    I have two amps in a spice model now, both at a gain of 3x. The seperation
    is good with 500HZ into one line,
    and 1KZ into the second. I can achive around 22 db seperation! I can live
    with that, but trying other
    freqs yielded seperation results as bad as 3 db! I am using 300-3000 HZ as
    my goal.

    I've looked at IC's designed for telephones. One option is to use two
    MC34014's back to back crossing
    the rec & trans lines. The problem with that is no one uses transformers
    anymore! All of these chips are designed around a
    bridge rectifier on the line. If I use that, my isolation goes away! I tried
    to find out how these conference phones tie the lines,
    but came up dry. Do they just parallel the lines I wonder! I don't have a
    conf phone to play with, might be worth for me to get one.
     
  6. ScadaEng

    ScadaEng Guest

    Don,
    I posted a partial shot of my schematic. The circuit was to big to put the
    entire thing into a screen shot.
    I also posted a copy of the spice hybrid I've been modeling.
    Thanks...
     
  7. ScadaEng

    ScadaEng Guest

    Martin,
    Do you know of a supplier for the LL6702? I've been unsuccessfull in trying
    to locate anyone who sells hybrid transformers anymore. Any links would be
    appreciated.
     
  8. ScadaEng

    ScadaEng Guest

    I found a circuit on line that is equivilant to a telco 22 repeater. This
    fits my needs exactly!
    It uses a readily available hybrid transformer, TM-028 available from
    mouser.
    Thanks to all for your replys!
     
  9. Shame on you, you didn't give a link to th cct



    martin
     
  10. ScadaEng

    ScadaEng Guest

  11. ScadaEng

    ScadaEng Guest

    Good question! I thought I would insert an opto isolator (led-to NPN) led in
    series with
    the "dummy line" resistor (which I will use a pot - for line imp matching).
    That way I can
    monitor line current for the hang up "wink" pulse. That will work on the
    VOIP end, but
    another problem is the POTS line. Where I'm using it, they don't use "wink"
    pulse for hang up.
    I think I'm going to look for 20 seconds of silence, then hang up.

    Anyway, the repeater offers new chalanges. I think I will need two
    independent bi polar
    DC supplies to individually power each op amp. Otherwise I will ground out
    the hybrid!
    Maybe I can steal power from each line, I'll have to chew on that.
    It's not the easy solution I was hoping for, but it's the best I have so
    far.

    Steve...
     
  12. ScadaEng

    ScadaEng Guest

    Thinking more about your question, I did not explain myself very well. I do
    not need to extend the DC
    supervisory. The only signal I will need to use is the above mentioned
    "wink" hang up pulse. My device will
    disconnet each line (see relays in my schem.) I have a PIC chip that does
    the logic.
     
  13. ScadaEng

    ScadaEng Guest

    The unit will not be used in the US, although I did stick closely to the
    guidelines.. It is a one - off, I have no plans to market it.
     
  14. ScadaEng

    ScadaEng Guest

    The 3-4 db loss in my present transformer design is not a problem here in
    the US, the problem was with a
    country with a poor phone system. It's not like the US, you can't tell them
    they have poor gain, line imbalance, etc...
    If the phone works (can talk) they don't want to know about it.

    I have no imeadiate plans to market the device, but I might be talked into
    selling my plans
    to someone who wants to venture further.

    Steve...
     
  15. Mark

    Mark Guest

    consider boosting the forward gain only when the dialing touch tones
    are being sent... the tones are normally at a level that is higher
    than the voice..

    Mark
     
  16. ScadaEng

    ScadaEng Guest

    consider boosting the forward gain only when the dialing touch tones
    are being sent... the tones are normally at a level that is higher
    than the voice..

    Mark

    I'm not actualy creating additional gain (don't want to piss off the phone
    co)
    I just want to compensate for the losses in the passive hybrid, unity gain.
     
  17. JosephKK

    JosephKK Guest

    Quit fooling around and get proper line hybrids, transformer or
    otherwise. Then, solving your remaining problems will be greatly
    simplified.
     
  18. JosephKK

    JosephKK Guest

    At this point my advice is just buy a proper passive hybrid or "DAA".
    Unless you are used to dealing with that "black magic" just buy a
    licensed one. With my mild acquaintance with the subject, that is
    what i do.

    Hmmph. Once you are on the 4-wire side of the hybrid it is very easy
    to add gain.
    Yes, it completely hides the hybrid (or 2- wire to 4-wire conversion).
    Sound powered systems. I think the military still uses them.
     
  19. JosephKK

    JosephKK Guest

    You might try one of these:

    http://www.audiotheater.com/phone/phone.html
     
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