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Bifilar Wound Balun Transformer

Discussion in 'Electronic Design' started by rickman, Nov 3, 2012.

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  1. rickman

    rickman Guest

    I am learning a bit about antenna design and one of the references I
    found talks about coupling the antenna to the feedline with a bifilar
    wound balun transformer. I dug a bit and although I found any number
    of references talking about bifilar wound baluns, none explained clearly
    why it is important to be bifilar wound.

    Any pointers?

  2. rickman

    rickman Guest

    Ah, a picture is worth a thousand words. I finally found a page that
    shows a bifilar balun in the application circuit I would be using it
    with and it makes perfect sense now. Well, mostly. The circuit is
    single ended to differential coupling.

    I get why the thing is wired up the way it is, I suppose it is important
    to use a bifilar winding to keep the two windings as identical as possible.

    Actually, I've looked at too many pages. I had two pages mixed up. I
    see the one that showed a toroidal core matching transformer is not the
    same page as the one that said to bifilar wind the balun. Seems the
    first one is a transformer like I'm used to seeing, but the bifilar
    wound balun is used in a different way that can't match impedance over
    the range I believe the toroid is doing.

    It's pretty amazing how many web pages there are that cover in such
    detail so many highly specialized topics! And most of these are hobby

  3. Bill Sloman

    Bill Sloman Guest

    A balun is actually a transmission line transformer.

    The twisted pair that constitutes the bifilar winding is a
    transmission line, with a particular characteristic impedance which
    depends on the diameter of the wire involved and the thickness and
    natire of its insulation.

    IIRR a twisted pair twisted out of enamel-insulated transformer wire
    has characteristic impedance in the ball-park of 120R.

    Google throws up a few tutorial papers

    Transmission line transformers keep on working to much higher
    frequencies than conventional transformers - the inter-winding
    capacitance becomes part of the transmission line rather than a simple
    parasitic load - and in fact only start falling over when the
    wavelength of the frequency being transmitted approaches the length of
    the winding.

    And - for John Larkin's benefit - this is electronics.
  4. Phil Allison

    Phil Allison Guest

    "Bill Sloman"
    A balun is actually a transmission line transformer.

    The twisted pair that constitutes the bifilar winding is a
    transmission line, with a particular characteristic impedance which
    depends on the diameter of the wire involved and the thickness and
    natire of its insulation.

    IIRR a twisted pair twisted out of enamel-insulated transformer wire
    has characteristic impedance in the ball-park of 120R.

    ** For clarity, it needs to be said that twisting of a pair of parallel
    wires in incidental to their operation as a transmission line. Twisting
    merely serves to eliminate radiation and pick up of external EM fields.

    A "bifilar wound " transformer may well have no twisting of the wires at
    all, but simply has them laid side by side in smooth layers.

    .... Phil
  5. Tim Williams

    Tim Williams Guest

    Not a necessary construction method; a balun is just a transformer with
    tapping such that it inverts one side.
    I got closer to 30 ohms last I measured a pair. Enamel is a whole lot
    thinner than extruded jacketing. It's going to be even lower in a
    piled-up winding due to the crowding.

    The low frequency way to think of it: your leakage inductance is almost
    exactly the inductance of the windings as a transmission line.

    If you take a piece of twisted pair 1m long, it'll have maybe 0.5uH
    inductance (measured at one end of the pair, shorting the far end, at a
    frequency well below the electrical length of the line). If you wind it
    up onto a form with an air core (making a bifilar solenoid, say), the
    self-inductance of each winding might be a few uH, while the inductance
    between wires remains the same (it's lower, if anything). Note that you
    can measure this leakage two ways: terminus shorted (as a transmission
    line) or secondary shorted (transformer leakage). The difference is, you
    test P1-S1 and short P2-S2, or test P1-P2 and short S1-S2.

    Now if you insert a permeable core, inductance goes way up (into the mH,
    perhaps), and coupling coefficient likewise goes up (some fraction less
    than 1.0). But leakage remains fairly constant.

    Leakage depends almost entirely on winding construction. Self-inductance
    depends on the windings and core. Coupling coefficient is the factor
    relating the two.

    (Yes, you can make a transformer that specifically depends on core
    geometry, not just winding construction. An example would be two coils at
    right angles, with a core snaked through each. Without the core, they
    have zero mutual inductance (infinite leakage). With the core, it's
    nonzero. I'm more interested in applications where you actually give a
    damn about performance in the first place. :) )

    The important thing about transmission line transformers is to forget
    about using them as transformers. Use them as transmission lines! If you
    put a few loops of coax on a core and drive the shield (calling the shield
    the primary, P1-P2), you can't expect any useful kind of behavior from
    that, because the shield carries all sorts of crazy currents, depending on
    how it's looped through, and which turns it's adjacent to, etc. If
    instead you drive the transmission line from one end (P1-S1), you'll get
    the same signal out (P2-S2), delayed, except the core allows you
    common-mode voltage. You could flip the terminal end around (S2-P2), and
    get an inverted signal!
    That's more or less what they do here. The shield necessarily does still
    carry a signal (the act of flipping the terminals forces the output
    voltage onto the shield anyway), but this occurs "after" the signal
    propagated through, and what you do with the shield is now an open
    variable -- you could loop it through a whole bunch of ferrite beads,
    damping out any oscillations.

    It follows that you can create any ratio by connecting transmission lines
    in parallel, looping them through a core (it doesn't even matter that the
    same core is used, it's just a common mode choke now!), and connecting any
    desired series-parallel combination on input and output sides to set the
    desired impedance and ratio.

    The dirty secret of transmission line transformers is, they aren't at all
    interested in reducing leakage inductance, or capacitance, or anything
    like that. It's just a big common-mode choke that lets you pipe signals
    from wherever to wherever else.

  6. Bill Sloman

    Bill Sloman Guest

    In the sense that the original source of the name was as a contraction
    of "balanced to unbalanced transformer".

    The wikipea article makes it fairly clear that one should understand
    it as a transmission line transformer. As Phil Alison correctly points
    out, you don't actually have to twist the wires together to make them
    into a transmission line, though twisting them is a mechanism which
    does keep the pair close together.
    How thick was the wire? The thickness of the enamel is more or less
    independent of the copper gauge, and the impedances is going to be
    appreciablyb higher for 40# gauge wire than for 10# gauge.
    Most of the field is confined between the two wires of the pair. I
    wouldn't think that adjacent wires would make much difference.

    This is wrong. I've certainly used them as 1:1 isolating transformers
    and they worked fine.
    There is some interesting literature on creating integer ratio
    transmission line transformers, and if you are clever enough I'm
    fairly sure that you can create non-integer ratios - I think there's a
    famous paper on the subject. There are also a lot of ways of getting
    it wrong.
    That's certainly one way of using them.
  7. Tim Williams

    Tim Williams Guest

    Except that, as I said, the leakage is not particularly low. One gets
    better performance in that regard from, say, copper foil pairs (which,
    ultimately, is still doing the same thing, but with a low impedance
    symmetrical stripline, not 50 ohm coax). Which is often done in power
    circuitry. But "very low leakage" is not what you're going for, so it's
    best not to claim that's what you're doing.

  8. rickman

    rickman Guest

    Wow, that's a lot of reading. Thanks.

  9. rickman

    rickman Guest

    See, this is the sort of stuff that, if I were a potential customer,
    would turn me off to doing business with you. Geeze, if I am talking to
    someone about what is going on in a system and they say to me, "but it
    works", I would think they didn't understand it at all.

    Do you not see how your posts make you look?

  10. Fred Abse

    Fred Abse Guest

    Anybody know how to accurately model a transmission line transformer in
    Spice, taking into account core properties?
  11. Fred Bartoli

    Fred Bartoli Guest

    Fred Abse a écrit :
    For a simple one, just as it is:
    use a TLine/RLC tline and between the 2 ""shield/ref plane" connections
    you just tie the magnetizing inductance, with maybe your core model
    (losses, non linearities, hysteresis,...)
  12. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    If you want to accurately model a coax cable you need two TLines. One
    modeling the center/shield transmission line, and a second one to model
    the shield WRT to "space".
    The "standard" perfect transformer is composed of a vcvs to transport
    voltage to the secondary and a CCCS to reflect the secondary current
    back to the primary, and a 0 voltage source to probe it.
    It is much less computationally demanding than the Tline which has to
    maintain history.
  13. John S

    John S Guest

    Well, apparently you're not.
    Oops! One potential customer lost! Damn, John, this will put you out of

    Geeze, if I am talking to
    Maybe the foot is on the other shoe. Maybe you didn't understand it at all.
    And you understand how your posts look? That's curious.
  14. Jamie

    Jamie Guest

    Those that don't know shit, should shit elsewhere ! Get it?

    The doctor made a mistake when you were born, they disposed the best
    part that came out of your mother, the afterbirth.

  15. rickman

    rickman Guest

    Amazing. There are times when a line is drawn and a designer says, "I
    understand this well enough", but the way you say it comes off like an
    amateur. I have spent a lot of time in my career fixing systems
    designed by people who obviously "only needed to make it work", but then
    it stopped working for some unknown reason.

    That is scary. I find a lot of people like that though. I just thought
    they were posers. I've never heard any of them brag about it.

    Yeah, that's what everyone does, but when they connect those parts,
    typically they understand everything about them and how to connect them
    that they need to.

    I'm talking about the statements you make that sound like they are from
    someone with no level of understanding.

    I shouldn't be posting about this. It is clear that you understand
    completely what you are saying and I expect you understand how it makes
    you appear. So sorry for bothering you with this.

  16. Jamie

    Jamie Guest

    At least you actually do something, not like a good many here that
    would like to make people think otherwise.

    I spend more time at actually experimenting with what works the best
    instead of fighting with PC software that only gets it close but not
    good enough.

    I just love those that talk shit and most likely hardly even touch a
    piece of equipment. When they do I am sure they're all thumbs and
    fingers with it and most likely end up getting some one else to do it
    for them and take all the credit for it.

    Those guilty of this need not to step forward, I already know who
    most of you are.

  17. I had a guy tell me a cheap scope (he always buys the most expensive
    equipment) I got for him was broken-- turns out the brightness control
    (or whatever you call it on a digital scope) was turned down. Same guy
    claimed an expensive SRS bridge with 0.1% accuracy was giving 10%
    error on a reading-- turned out he was using 100Hz to measure a
    tens-of-pF cap and it was performing well within spec according to the
    manual. He has written peer-reviewed papers on these things.. sad.
    It's a rare person that can get all the theory right and the practice
    right- they deserve to be well-rewarded.

    Best regards,
    Spehro Pefhany
  18. Tim Williams

    Tim Williams Guest

    As a physicist, I can affirm that. Others may vary.
    Doesn't count -- even the string theorists don't understand the stuff. ;-)
    And you don't?

    I do on every single board I make. Not closed-form, but open-form
    approximation, qualitative accuracy. Implemented in wetware, too. Works
    very well.
    Well, if you really wished, you'd buy the entire Ansoft suite and *do*
    it -- but I'm guessing that wish isn't as unconditional as it was phrased.
    In actuality, you don't care at all, and are more than happy enough
    guessing. Which again illustrates your inconsistent self-representation.

    Are you aware that ~20nH is ~86mm of 50 ohm, 0.67c coax?

    Assuming the headers pictured are 0.1" centers, the cores are roughly T37
    size ferrites, a bit thicker than usual. I get 14mm for the length of a
    single turn on a regular T37, so it might be closer to 18mm per turn,
    maybe 20mm with coax thickness. That's 60mm total length, or 14nH. The
    soldered connections and board traces have almost as much, depending on if
    there's a ground plane just out of sight or not. But by then it's not
    mutual, which is all the more reason it's not LL you're supposing about.

    Actual performance will show helical resonator action starting around
    1GHz, which is what the under-hump on your leading edge comes from. And
    probably other nasties if you tested it with a ps generator rather than
    the "sub-ns" this particular device produces.

  19. rickman

    rickman Guest

    I learned a little in chemistry classes... very little.

    My understanding is not that they don't understand it, it just doesn't
    predict anything different from existing quantum theory.

    Have you solved the Schrödinger wave equation for any of your systems.
    Only then will I call you a real engineer ;)

    Hey, want to help me design a LF shielded loop antenna from coax? It
    sounds like it would be right down your alley! I don't know nothing
    about birthin' no babies, Ms. Scarlett! But it looks like I'm going to
    have to learn...

  20. Tim Williams

    Tim Williams Guest

    Notwithstanding Feynman's quote, "nobody really understands QM", that's
    more accurately the problem, as I also understand it.
    Sure! My work is done:
    Well, maybe not *my* work, but... helpful nonetheless. Lots of excellent
    analysis on his website.

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