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Re: Getting matching transformer from telephone

Discussion in 'Electrical Engineering' started by Tomi Holger Engdahl, Dec 29, 2008.

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  1. Here are some of my experiences on making mu own such devices:
    Good quality transformers seem to cost considerable amoutn of money.
    600 ohms 1-to-1 matching transformers are quite rare in telephones.
    Modern normal telephones are normally "floating" line powered
    devices where electronics connect directly to line. The whole
    small device is "floating" isolted from everythign else
    so that gives good balance.

    You can find 600 ohms 1-to-1 matching transformers most often
    on modems. And those are also in some telephones that use
    external power...
    Propably not any transformer in a modern phone at all.
    And in older ones where there was a transformer that is most
    propably not a type of transformer you are looking for
    (for details on transformers used at beginning of
    Modern normal telephones are normally "floating" line powered
    devices where electronics connect directly to line.
  2. Phil Allison

    Phil Allison Guest

    "Salmon Egg"

    ** Errr - yep.

    Impedance ratings matter very much in relation to the drive and load
    impedances that are bests for a particular transformer.

    Get them wrong like that and the overall frequency response will not be flat
    across the audio band.

    ....... Phil
  3. Eeyore

    Eeyore Guest

    Twisted pair cable as used for telecoms has a nominal 100-110 ohm
    inpedance. See the ADSL specs.

    600 ohms is an irrelevant historical nonsense from the days when they used
    telegraph wires for phone circuits.

  4. Bill Janssen

    Bill Janssen Guest

    Impedance for an un-loaded pair varies with frequency. So at DSL
    frequencies the impedance
    is 100 to 110 Ohms. But for telephone use They preferred to use loaded
    pairs and they were
    designed to be 900 Ohms. And the phone should be a reasonable match to
    the line to minimize refections which bother the users as echoes

    The transformers in some sets was not an isolation transformer but a
    hybrid and matching transformer. The carbon mic. used in the old phones
    was powered from the line. And many
    of the newer phones derive power from the line. So isolation can't be used.

    Bill K7NOM
  5. Phil Allison

    Phil Allison Guest

    "Bill Janssen"

    ** Huh ??

    Audible echoes on a few miles of twisted pair ???

    The speed of electrical signals must be a * hell of a lot * slower than C
    where you live - pal.

    ** At least you do know what an "isolation " transformer is.

    Unlike some here.

    ...... Phil
  6. Phil Allison

    Phil Allison Guest


    ** **** OFF -

  7. Phil Allison

    Phil Allison Guest


    ** **** OFF -

  8. Phil Allison

    Phil Allison Guest


    ** Oxygen thieves like this anencephalic turd need shooting.

    .... Phil
  9. The twisted pair cable as used for telecoms has a nominal 100-120 ohm
    impedance for high frequecies the ADSL system uses. The ADSL
    system uses frequencies between 138 kHz and 1.1 MHz for downstream
    data (and 25 kHz to 138 kHz ofr upstream). This 100-120 ohm
    impedance holds pretty well for those frequencies above 100 kHz.

    For lower frequencies the the impedance of the telephone cable
    is not anymore that 100-120 ohm, but something else.
    For voice frequencies used on on normal telephone (300-3400 Hz)
    the impedance is normally considerable higher than 120 ohms.

    Normal telephone subscriber lines in USA (0.4-0,6mm subscriber PE
    insulated vaseline filled cable) are 770 ohm resistor (with 2uf series
    capacitor) and 47nF parallel capacity.

    2 uF
    | || |
    | | |
    --- | | 770 ohms
    --- 47 nF | |
    | |

    This diagram is referred to 800Hz, but impedance is rather complex,
    and varies from high value at low frequency and drops to ca. 150 ohm
    on 10kHz and 120-125 ohm above 100kHz.

    Some telephone lines can have higher impedance (typically 1100 ohms in
    lines with loading coils or telephone air cables).

    In european specifications (for Finland etc..) I have seen this
    that complex reference impedance Z = 270 + (750 //150 nF)

    750 ohm
    270 ohm +--|_____|--+
    _____ | |
    --|_____|---+ +-----
    | || |
    150 nF

    Typical cable used in for subscriber lines has following
    characteristics: 0.5 mm diameter wire, loop
    resistance 182 ohm/km and pair capacitance 39 nf/km.

    Telephone Laboratories (1971) gives the followign information on
    typical cable characteristics:

    "The primary constants of twisted pair cables are subject to
    manufacturing deviations, and change with the physical environment
    such as temperature, moisture, and mechanical stress. The inductance,
    L, is of the order 1 mH/mile for low frequencies and the capacitance,
    C, has two standard values of 0.066 and 0.083 uF per mile although
    lower capacitance cables are under development.

    Of the primary constants, only C is relatively independent of
    frequency; L decreases to about 70 percent of its initial value as
    frequency increases from 50 kHZ to 1 MHz and is stable beyond; G is
    very small for PIC (polyethylene insulated cables) and roughly
    proportional to frequency for pulp insulation; and R, approximately
    constant over the voiceband, is proportional to the square root of
    frequency at higher frequencies where skin effect and proximity effect

    600 Ohms is somewhat of a compromise between different real-life
    impedances that could be seen. Normal telephone line connections are
    theoreticallydesigned to be 600 ohm resistive impedance. This 600
    ohm is kept as international reference for designing telephone line
    equipment (typically the signal powers are measured to 600 ohm load).
    In practice the telephone line does lot look like pure 600 ohm

    Telephone equipment which is designed to operate with 600 ohm loads
    will operate with those real-life lines, but it's performance is worse
    than in ideal situation. Typically the modems are designed for 600 ohm
    reference impedance because they can handle the sidetone.

    The return loss of the terminal equipment must be greater than 10 dB
    when compared to 600 ohm reference. This measurement applies to
    telephones, modems and other terminal equipments. NET4 technical specs
    are European specs and they are used in many European countries (NET4
    is actually a collection of different specs in use in different countries).

    For best performance the telephones are designed to the exact line
    impedance. Matching the hybrid circuit to the real line impedance
    (instead of 600 ohm) will improve the feedback typically by
    3-6dB. 20dB sidetone is easy to achieve, but 30dB is also not too
    difficult provided you can measure the line impedance and take steps
    to build a correct balancing network.
  10. Hey! Floyd doesn't have an attitude, and is pretty darned good at
    accepting other standards and practices, once they are brought to his
    attention. I am sure he will concur with you once he realizes from your
    post, that the geography and era were different than that he had his
    mindset in.
  11. Roy

    Roy Guest

    Floyd and You, Archneedsmeds' ?
    No Shit' Sherlock...gotcha beat don't it? One day, soon, I'll look back
    on this and I'll dump all the arrogant self deluded trolls and
    filerkillers in the crap block.

  13. **** off, Roy. There is not a damned thing happening in this group
    that has a fucking thing to do with you.

    Grow the **** up, punk.
  14. Roy

    Roy Guest

    From: Archimedes' Lover
    On Tue, 30 Dec 2008 08:35:24 -0500, (Roy) wrote:
    Floyd and You, Archneedsmeds' ?
    No Shit' Sherlock...gotcha beat don't it? One day, soon, I'll look back
    on this and I'll dump all the arrogant self deluded trolls and
    filerkillers in the crap block.
        **** off, Roy. There is not a damned thing happening in this
    group that has a fucking thing to do with you.
        Grow the **** up, punk.
    **** you, that's what's eating you all up inside, that I am wall to wall
    in here.
    Give it up already, you're headed for self destruction, Faggot!

    Roy Q.T.
    [have tools, will travel]
  15. Eeyore

    Eeyore Guest

    WRONG ! For all practical cables used on the 'local run' for telecoms it's
    100-100 ohms.

    Utter tripe.

    You haven't a FUCKING CLUE what you're talking about.

  16. Eeyore

    Eeyore Guest

    They weren't using something that resembles a single pair of Cat 5 back then.

    As I said, it's HISTORICAL only and of no practical value.

  17. Eeyore

    Eeyore Guest

    General Post Office.
  18. Eeyore

    Eeyore Guest

    Which is why it's 100 ohms or so just like Cat 5 etc.

  19. Eeyore

    Eeyore Guest

    Unfortunately has has NO CLUE about the characteristic impedance of twisted pair
    cable as used for telecoms.

    "UTP is also finding increasing use in video applications, primarily in security
    cameras. Many middle to high-end cameras include a UTP output with setscrew
    terminals. This is made possible by the fact that UTP cable bandwidth has
    improved to match the baseband of television signals. While the video recorder
    most likely still has unbalanced BNC connectors for standard coaxial cable, a
    balun is used to convert from 100-ohm balanced UTP to 75-ohm unbalanced."

    *** 100-ohm balanced UTP ***

  20. Eeyore

    Eeyore Guest

    Quite so !

    Absolutely not. About 100 ohms.

    Via 'Post Office Telecommunications'.

    He has made that evidently apparent.

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