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Modem Phone line filter

Discussion in 'Electronic Design' started by Bob Urz, Nov 16, 2006.

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  1. Bob Urz

    Bob Urz Guest

    Anybody got a good resource on how to build a phone line filter?
    I am near a AM radio tower and its really killing my dial up modem
    at times. I tried a DSL filter in line. but i think the AM band is
    not getting attenuated enough to do much good.

    Its hard to peg characteristics. Would a modem be considered a 200 ohm
    nominal load on the phone line?

    What type of filter for AM radio?

    Dual inline inductors?
    parallel capacitor with series resistor maybe?

    Any ideas or links?
  2. Meat Plow

    Meat Plow Guest
  3. Joerg

    Joerg Guest

    Hello Bob,

    If a series connection of DSL blockers doesn't work you might want to
    try a common mode choke. Either one off the shelf or take a #77 core and
    wind twisted pair through it as many times as it'll comfortably fit
    through there. In a pinch the core of a discarded flyback transformer or
    the big ferrite from a scrapped AT power supply can do. Mains
    transformer cores would not be so good here.
  4. Aratzio

    Aratzio Guest

  5. Meat Plow

    Meat Plow Guest

  6. A DSL filter should do an excellent job, knocking down the AM band by
    at least 25 decibels.

    Try plugging in a phone instead of the modem and listen carefully, with
    and without the DSL filter. The DSL filter should make a huge
    difference. If it helps, but you can still hear the radio station, put
    a second DSL filter inline.

    If the filter doesnt make much difference, then the audio detection is
    happening somewhere else, perhaps in another phone or your phone entry
    box signal limiters. Try unplugging the other phones one at a time and
    see if that makes any difference.

    If that doesnt find anything, you might have to put a dsl filter
    BEFORE the entry box or AFTER the entry box.
  7. Aratzio

    Aratzio Guest

    Nope, finger out my usual addy in here.

  8. Adrian C

    Adrian C Guest

    Does your telephone company have a duty in contract to provide you a
    clean phone line connection? Let them sort out filtering for you, and
    maybe boost the line perhaps?
  9. BOB URZ

    BOB URZ Guest

    I have tried some snap on split cores and not had much luck.
    As well as a single DSL type in line filter. This Am transmitter antenna is
    within 1000 feet of my house.

    After some research, i found most of the split cores are more for the MHZ
    region rather than the Kilo hertz AM radio band.

    I have some salvaged cores from my junk box. I would hate to use
    a flyback sized core. i think i might have a small input ac donut and
    maybe some inter stage stuff. I will have to poke around. Other than
    application, i wonder how you can ID the cores for there type or frequency

    I am curious how you do model this as a filter network. What load
    does a average modem put on the phone line? If you were trying to design a
    filter, you would have to know impedance's to be in the ballpark. It seems
    like a pie type with two inductors in line with
    the balanced modem leads with a cap/resistor across them would
    give a deeper cut in the unwanted frequencies.

  10. James Sweet

    James Sweet Guest

    Or as close to the computer as possible. I believe they should work fine
    in either direction, so you could plug it right into the computer then
    use a normal phone cord to the wall. The AM signal may be picked up
    right there in the cord into the modem.
  11. Joerg

    Joerg Guest

    Hello Bob,
    That's way too little inductance. You have to wind the twisted pair line
    through there many times. Also, after several uses the core halves often
    don't have a snug fit anymore and then the inductance drops rapidly.

    It's a matter of inductance. A split core snapped onto a line is only
    one turn. Not enough below 1.6MHz, usually.

    Take the largest one you find and wind twisted pair through there until
    it's hole is stuffed good.

    Unknown core ID: Wind ten turns around it, measure inductance, calculate
    Al value. Now measure I.D., O.D. and thickness, then go to the Amidon or
    Fair-Rite catalog to find which type matches. Most EMI ferrites will be
    #43 material. For AM band higher perm stuff works better but in a pinch
    you have to try to make do with what's there. BTW, don't use the
    Fair-Rite web catalog if you are on a slow internet connection. It's
    humongous PDF files.

    It's very little differential load but there is a DC current that can
    saturate inductor cores. That won't be much of a problem for a common
    mode choke.

    Another method is to notch out the frequency of that station if it gets
    in differentially (that is rare though).
  12. Meat Plow

    Meat Plow Guest

  13. Meat Plow

    Meat Plow Guest

    qba'g xabj jung lbhe hfhny nqql vf
  14. Hmm, funny that the DSL filter didnt help.

    The relevant frequencies are 100 x apart, so the filter is super non

    A couple of 2.5 mH chokes in series and two 0.01uf capacitors in
    parallel will make a dandy filter, at least 40dB down. That's
    roughly what's inside a DSL filter.

    But again, I'd follow the investigative steps from my previous msg. it
    won't do any good to place a good filter in a bad place.
  15. Eeyore

    Eeyore Guest

    I have here a BT RF2 filter.

    It simply consists of a 2.2mH choke in both lines.

    Like these

    I'll post a pic in abse.

    I'd expect a common mode choke to be better of course.

  16. Ken Weitzel

    Ken Weitzel Guest

    Hi Bob...

    Spent my working lifetime in broadcast, so can virtually guarantee
    that you don't have to do a thing :)

    Just call the station, ask for the Chief Engineer, and tell him of
    your problem.

    If your country is anything like Canada, he'll be there in a flash,
    and all of your problems will be gone :)

    If you want him there even faster'n a flash, just ask him if
    you're supposed to be telling him about the problem, or should
    it be the DOT/FCC/whatever country you're in :)

    Take care.

  17. Franc Zabkar

    Franc Zabkar Guest

    I think it's more like 600 ohm.

    - Franc Zabkar
  18. Don Bowey

    Don Bowey Guest

    That's bad advice.
    Just exactly why you think the radio station or FCC is going to care about
    RF getting into the premises wiring or telephone cable? The station
    engineer may make a filter recommendation, but is under no obligation to do
    any more than that.

    The issue is... What non-linear device is causing the RF to be rectified so
    it is audible. It could be a bad joint, a telephone, or whatever..


  19. I *hope* they use a more appropriate choke than that one-- if you read
    down a bit the specs say it resonates at around 1.35 MHz. That means
    it's more of a capacitor than anything else above that frequency, and
    not a very good inductor in the AM band. Although likely to be good
    enough at the lower DSL frequencies.

    A good 2.5mH RF choke is pie wound to minimize capacitance and is good
    up to 30MHz at least.

    Opening up a DSL filter here shows a somewhat better design -- a two
    solenoid-wound chokes and two mylar capacitors across each end.

    But that still is not quite optimum-- the DSL frequencies that filter
    tries to choke out are not quite as high as the AM BC band. SO a pair
    of *good* RF chokes will probably work better than the DSL filter type
    of chokes.
  20. Eeyore

    Eeyore Guest

    Well, they look like those parts. See the pic in a.b.s.e.

    No ? I thought they coincided with low end of AM quite accurately in fact.

    Probably so.

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