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Homemade Wideband Current Transformer

Discussion in 'Electronic Design' started by John Devereux, Apr 10, 2013.

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  1. Hi,

    Since it seems a useful thing to have, I am making a wideband current
    probe, ~10-1000MHz, like one of these:

    <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure42.jpg>

    Every home should have one. It's from the article:

    <http://www.interferencetechnology.com/the-hf-current-probe-theory-and-application/>

    1) They suggest an electrostatic shield made from copper tape, "with a
    small gap around the inside of the toroid". Well this turns out to be a
    PITA, I have shredded finger tips from copper cuts and am pausing for
    thought.

    What about just using very thin coax instead, and using it's shield as
    the - uh - shield?


    2) The output frequency response of a commercial probe is supposed to
    look nice and smooth like this:

    <http://www.fischercc.com/ViewProductGroup.aspx?productgroupid=127>
    <http://www.fischercc.com/productfiles/DS F-51 Rev--RLSE_cd51.pdf>

    But he is measuring more like this for both the home-made and
    commercial:

    <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure9.jpg>

    Is there a better way to do this measurement?


    Needless to say I am a neophyte at RF, but do have some kit available
    (thanks, ebay).
     
  2. I am hoping to use this for checking cable emissions for EMC, so it
    needs to be reasonably sensitive.
     
  3. Tim Williams

    Tim Williams Guest

    Making a CT is easy beans. A hundred turns on ferrite into a suitable
    resistor (a terminated transmission line, in this case) will easily do
    MHz, maybe 20 if it's somewhat small. Amazingly, commercial CTs can suck
    eggs: a Triad CST306 (I think) has its lowest resonant mode at something
    like 300kHz!

    It gets better if you wrap it with aluminum or copper tape, because this
    reduces the leakage inductance and coupling between winding segments,
    suppressing the helicotoroidial resonator modes. Probably, if lossy tape
    were used, it would help dampen as well as shield the resonances. A high
    burden resistance also discourages resonances, forcing the feedpoint to
    not be a good current antinode.

    It helps a lot to use fewer turns, raising the resonant frequency.
    Segmented windings, arranged carefully, may help to discourage certain
    resonant modes. If a large ratio is required, multiple stages can be
    used.

    Past maybe 100MHz, with any approach, you're pretty much working in black
    magic territory. Good luck.

    Tim
     
  4. I think you just need to stop the current from flowing, so a layer of
    copper tape with some other insulating tape between the overlapping
    layer section should work.

    George H.
     
  5. To get a response curve that makes sense for an RF current transformer,
    you have to be able to excite it with a known current. The usual way
    to do so is by inserting the transformer into a suitably sized
    and carefully terminated coaxial transmission line. The line's
    characteristic impedance ensures that the current is flat over
    frequency (to within a few dB or so). The 'suitable size' bit is
    to make sure that the insertion of the transformer does not make
    a discontinuity in its impedance. I check this through reflectometry:
    The line with the transformer in place should look like a smooth
    transmission line all the way through.

    That's the way I test beam current transformers here. In its final
    application, the primary is not a wire; It's a particle beam. I get
    bandwidths of about 3GHz. OK, my kit is upwards of 35k$ a piece.
    Then again, it's all custom made, has to hold a vacuum too, be
    radiation resistant, etc. That has a way of driving up the cost...
    (Just the ferrite cores are 1k$ a piece, and I use five.)

    Come to think of it, the thing is warped such that many wouldn't
    even recognize it as a transformer. We call them "wall current
    monitors". Here's a picture, if anyone is interested:
    <http://cern.ch/psring/psring/showpicture.php?section=03>

    Jeroen Belleman
     
  6. Ha! I had a thought last night it might be something like that. Be a big
    coax for my one though. I did wonder if a sort of giant microstrip
    (megastrip?) would also work :)

    It looks like that same company I linked to also does calibration
    fixtures:


    Cool, thanks!
     
  7. Word of the week ^^^^^^^^^^^^^^^
     
  8. Yes I must try to use that one more often.

    But, Success!

    I built the transformer, along with the finger-shredding copper tape
    electrostatic shield.

    Seems to work well, as far as I have been able to test.

    Transimpedance is 20 ohms.

    Looks beautifully flat response when you look at the first 20MHz.

    Generally flat response continues to about 200MHz, with some +/- ~10dB
    wiggles up to 1GHz+. Don't know how much of this is the response, and
    how much the excitation. I might try my megastrip idea to test it
    further.

    Will try to post some pics later.
     
  9. OK, here it is:

    <http://ee.devereux.me.uk/hf-current-probe.jpg>

    I made a mistake, it is more like 7 or 8 ohms transimpedance. About 1
    ohm impedance on the "primary",as could be expected from the 7 turns and
    50 ohm secondary "burden".

    <http://ee.devereux.me.uk/plot0003.png>

    That is with 1mA from the tracking generator, so the 7.87mV display is
    7.87 Ohms.
     
  10. I love the display and output of HP stuff from that time.

    my scope has some silly parallel port printer module, and there's some
    painfully slow windows software that runs over a 9600 baud serial port to
    do screep captures like you posted. the fonts seem optimized for use with
    a plotter or something like that.
     
  11. Yes it's nice - picture a bit blurry since was just my mobile phone.
    The spectrum analyzer expects to plot to a plotter, so that plot was
    acquired over GPIB, then run through a HPGL to PNG filter. You can tweak
    options to get different resolution and colours / line thicknesses for
    the various elements of the plot.
     
  12. Have you used any GPIB to USB converters that function and didn't cost
    $500?

    It seems silly to run out and buy a PCI card for this and be chained to
    one desktop forever.
     
  13. No, actually.
    Yes, I use a old SFF PC just for that, running a linux distribution. It
    does all the "GPIB stuff", has scripts to automate various
    "experiments". It's headless, just a box, but I can remote in to it of
    course. For the rare times I go "on the road" when I need GPIB I do have
    a NI USB interface, but it is not quite as reliable.

    The thing that causes most problems is handling a "screen dump" from the
    various instruments. I think they all want to be controlling a HP
    plotter then, instead of being controlled by a GPIB master. I never
    really got to the bottom of it, but I needed the PCI card to get it to
    work reliably.
     
  14. josephkk

    josephkk Guest

    Personally i use the Prologix, LLC devices; nicely better price. Seem to
    work just fine.
    Back in the day i used to be a bit if a GPIB guru. Ask me by PM to
    discuss the instrument to plotter things. We may need to trick the
    interface a bit to read the control words that activate the plotter.
    However they are usually settable in the instrument GPIB menus. Some
    configurations do not need the control words (promiscuous transmit and
    receive).

    ?-)
     
  15. Robert Baer

    Robert Baer Guest

    * Takes forever in load attempt; i gave up.
    * Should work fine; ground ONLT one end; the other leave open.
    * Takes forever in load attempt; i gave up.
     
  16. Robert Baer

    Robert Baer Guest

    Sadly, the link to the drawing fails.
     
  17. Yes, I'm not very surprised. It's a php script putting out a
    TIFF picture. It has 'header("Content-type: image/tif");' as
    the first line producing output. That works fine for Mozilla-
    type browsers, but IE doesn't seem to understand it. So I put
    in a quick hack that makes it work in-house at least. Of course,
    you are outside.

    If you have a suggestion towards a good fix, I'm all ears.

    Jeroen Belleman
     
  18. I have used that too, worked fine for simple things, but ISTR it seemed
    to get lost when doing sequences of commands.

    Or I did.
    Thanks Joseph that is a generous offer :) I would love to take you up on
    it sometime but I am up to my neck in other projects at the moment,
    would not be able to do it justice!

    Thanks,
     
  19. JW

    JW Guest

    I've heard good things about this adapter that is alleged to be 100%
    compatible with Agilent's controller. And the price can't be beat.

    http://bmjd.biz/
     
  20. mike

    mike Guest

    GPIB is exceedingly complex.
    BUT
    If you have a modern instrument that's programmed in ASCII, it takes
    zero understanding
    of the protocol.
    A PIC processor can easily copy ASCII between RS-232 and GPIB.
    And you can plug on a RS-232 to USB or Bluetooth adapter and get
    anywhere you want.

    Doesn't meet ANY standards. Doesn't look pretty. It ain't fast.
    But it's cheap, it works and one chip fits in a gender-changer dongle.
     
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