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Strange high frequency push pull transformer action

Discussion in 'Electronic Design' started by mook johnson, Nov 4, 2012.

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  1. mook johnson

    mook johnson Guest

    As mentioned before I'm working with a design that has a 10MHz push-pull
    transmitter to drive a 100 ohm cat6 cable.

    The cable is terminated with 100 ohms on both ends and the length is
    ~200 ft. This is a multi-drop implementation much like RS485 but
    needed custom drivers due to the environment.

    The driver is configured as a push pull with 5V on the center tap. The
    2n7000 transistors are on each leg of the driver and are driven by high
    speed logic 5V chips. What I expected to see during transmission on
    the centertapped "driver" side of the transformer is a waveform like
    this when measured across Q2.

    + _________
    + | | 2X Vin
    + | |
    + | |
    + | Q1 |
    + | on |
    + | |
    + | |
    +----------| |------------| |----------- Vin
    + | |
    + | |
    + | |
    + | Q2 |
    + | on |
    + | |
    + | |
    + | |
    + |--------|
    +====================================================

    instead it looks like this where the drain voltage of Q2 drops to ~
    1/2Vin when Q1 turns on.
    +
    +
    +
    +
    +
    +----------| |------------| |----------- Vin
    + | | | |
    + | | | |
    + | Q1 | | |
    + | on | | Q2 |
    + |_______| ~1/2 Vin | on |
    + | |
    + | |
    + | |
    + |--------|
    +====================================================


    This of course reduces the amplitude of the "bus" side signal since it
    is the difference between Q1 drain and Q2 drain.

    Pulses are 30 - 60nS wide.

    So instead of the expected +/- 8 - 10V on the bus I am getting +/- ~2

    What I've already looked at.

    1) the signals look like a current driven push pull push pull power
    supply. So I removed the transformer and shorted the two drain
    connections together and measured inductance. (singe nH range. < 1 ohm
    XL). I measure the CT voltage with a scope and it is rock steady, 10uF
    ceramic directly from there to ground to insure that.


    2) Changed the frequency down the 5MHz to see if it is a reflection of
    the pulse. Signals look identical just 2x wide was expected. :(

    3) checked insertion loss, with an impedance analyzer and it looked
    good. < 1dB


    4) Q1 was removed and replaced with a ohm ranger (semi flyback style).
    and the lower the resistance got, the higher the amplitude seen on the
    bus side. until transformer saturation.


    There is something going on with that transformer I'm sure of it. It
    was designed by a 3rd party that has RF experience and they are
    convinced that it is some kind of RF/transmission line matching issue.
    I tend to think its more simple than that.



    Any thoughts?
     
  2. Jeroen

    Jeroen Guest

    It looks as if the transformer connections got mixed up, with 5V
    applied to one end of the primary and the transistor drains on
    the center tap and the other end of the primary.

    Jeroen Belleman
     
  3. What's the open-circuit primary inductance from either side to CT AND
    across both primary windings? (3 numbers).



    Best regards,
    Spehro Pefhany
     
  4. Joerg

    Joerg Guest

    Like Jeroen I believe the most likely cause is that one of the primary
    windings is flipped. IOW, instead of connecting a dot side with non-dot
    they may have connected two dot sides or two non-dot sides.

    Hang a 100ohms resistive load to the output. If still looking as in you
    2nd pic I could almost bet it's indeed a half primary flip. As Forrest
    Gump said, ..it happens :)
     
  5. mook johnson

    mook johnson Guest


    Its a 1:1 transformer with a CT on the driver side.

    Bus side with primary open: 200uH
    CT to leg with bus side open: 50uH (either leg)

    CT to both legs tied together with bus side open: ~ 5nH

    Nothing obviously wrong. Any fancy RF/transmission line stuff could be
    taking place here?
     
  6. mook johnson

    mook johnson Guest

    Ahh that was my first though as well. Forgot the mention I checked for
    that.

    I guess i should have added more waveform.

    If I probe Q1 drain instead of Q2, I get the same result. where Q1 goes
    from Vin to ground and goes from Vin to ~ 1/2Vin when Q2 turns on.

    Could it be some strange kind of saturation? I don't think so since it
    the waveforms immediately look that way. Typically when i have
    saturated a core the first part of the waveform looks right then when
    the inductance falls the waveform drops off at the end. This one doesn't
    do that unless it is at the very start and I can see it.

    The wave shape looks pretty good with sharp edges and flat tops so it
    doesn't look too bad but your never know.
     
  7. Schematic of your source termination?



    Best regards,
    Spehro Pefhany
     
  8. Joerg

    Joerg Guest

    Yup, I'd double, triple and quadruple-check that. One method is to
    ground the center tap, feed a square wave from a function generator into
    the secondary and see if opposite polarities come out the primaries.

    If it was saturation the current intake would be high and the 2N7002
    FETs would attempt to unsolder themselves.
     
  9. Mmmh. There are several things that do not fit my first thought:

    One is that if indeed one of the FETs connects to the CT instead
    of either end, the remaining end goes below GND, which will
    forward-bias the other FET's body diode. He wouldn't have missed
    that, I presume.

    Second, his inductance measurements appear to make good sense
    for a correctly connected transformer. (50uH from CT to either
    end and 200uH on the secondary with primary open. 5nH with
    both ends of the primary shorted together. Although that would
    imply a coupling factor of .9999, which is suspiciously
    good.)

    Third, in the first waveform diagram, one cannot possibly have
    pulse widths of 60ns at a repetition rate of 10MHz. 30ns would
    be OK though.

    Maybe it's one of those cases where what you think you see is
    not actually what is there.

    You mentioned your twisted pair cable is doubly terminated.
    Do you actually see about 50 Ohms between the wires?

    Jeroen Belleman
     
  10. Joerg

    Joerg Guest

    You are old enough to remember the 2N3055, some of them had their TO3
    cans adopt a blue tint from gross overheating, like the exhaust pipes of
    a well-seasoned Harley-Davidson. Yet many of those were still good for
    more rock sessions.
     
  11. Joerg

    Joerg Guest


    That can also be the case if one of the primary halves is flipped.

    That doesn't sound possible. If it is true then we have a viable Nobel
    prize candidate in our midst :)

    That might have just been a notation error. The plot he described looks
    suspiciously like flipped transformer connection. I've had that happen
    on the bench when I whipped up a transformer by hand a bit too fast, or
    the phone rang while wiring it up. So now I alwats keep a Sharpie in the
    bench drawer for marking, same as the Edding pens in Europe.

    [...]
     
  12. Jeroen

    Jeroen Guest

    It's not impossible. I have this 1:4 impedance ratio transformer
    with a 6-decade bandwidth, which corresponds to a coupling factor
    of some 1e-6 below unity. Granted, it's a Guanella transmission
    line affair, using ferrite and Vitrovac loaded UT47 coax; not a
    wire-wound thing like I suppose we have here.

    Jeroen Belleman
     
  13. Guest

    .------------->
    | .--->
    | |
    '-'-'-'-'-'
    =============
    (A) .-.-.-.-.-.-. (B)
    | | |
    ||--' +5v '--||
    ||<-. .->|| Q2
    _||--+ Q1 +--||__
    | |
    === ===

    Not possible. If the transformer's right, the drain of the "off" FET
    has to rise when the other turns on, there's simply no other choice.

    If the "off" FET's drain didn't rise to 10v, or spiked then decayed,
    etc.--that would be different. For Vds not to rise at all, but fall
    instead--that's not possible.

    The transformer's wrong. Has to be.
     
  14. Guest

    It's pretty safe to assume Mook's off trying to untangle those darn
    primary connections. Happens to the best of us.
     
  15. Jamie

    Jamie Guest

    yeah well, that would help out in the replacement process :)

    Jamie
     
  16. Guest

    keep seeing those old schematics for transistorized ignition using a
    2n3055 pop up now and then, surprised any of them work since it is
    only rather for 60V

    -Lasse
     
  17. Guest

    Betcha he finds this:

    .------------->
    | .--->
    | |
    '-'-'-'-'-'
    =============
    (A) .-.-.-.-.-.-. (B)
    | | |
    +5v-' ||--' '--||
    ||<-. .->|| Q2
    __||--+ Q1 +--||__
    | |
    === ===

    That would explain the first waveforms, anyhow.
     
  18. Joerg

    Joerg Guest

    Not always: One of the guys in our army unit was building a hobby
    electronics project. He sat at the desk in his socks. Something
    unsoldered itself, fell down, melted through a sock, lodged itself
    between his toes ... phsssss ...
     
  19. Jamie

    Jamie Guest

    Shot in the dark:

    It could be miller effect due to the capacitance
    between the drain and gate..
    The opposite side is generating a pulse in an instance and thus the
    cap located there is pushing a pulse over to the gate and forces it to
    bias on. Of course this is short and shouldn't last long however, at the
    frequency you are operating at, it maybe showing the effects if you are
    not driving the gate low and hard enough in the off state.

    You should probe at the gate of Q1 when Q2 turns on.

    Jamie
     
  20. Joerg

    Joerg Guest

    My bet would be this (asterisk is winding orientation dot):

    .------------->
    | .--->
    |* |
    '-'-'-'-'-'
    =============
    (A) .-.-.-.-.-.-. (B)
    |* | *|
    | | |
    ||--' +5v '--||
    ||<-. .->|| Q2
    __||--+ Q1 +--||__
    | |
    === ===
     
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