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Measuring RF output impedance

Discussion in 'Electronic Design' started by Paul Burridge, May 1, 2004.

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  1. Hi,

    I have a spare RF signal generator that has an unmarked output from a
    type of socket I've never seen before. What's the simplest way of
    establishing its output impedance? I've had a few ideas but no doubt
    someone out there will know of something better, since I always seem
    to end up making unnecessary work for myself.
    Any suggestions?

  2. W3JDR

    W3JDR Guest

    The most obvious method would be to measure the open-circuit RF output
    voltage, and then measure the output voltage with a known load. The output
    impedance is a simple calculation of the ratio of the voltages.
    However, depending on the vintage and quality of the generator, the
    impedance is likely to change with frequency, and possibly even with output
    level. For source-impedance-sensitive measurements, it's generally a good
    idea to put a small amount of fixed resistive attenuation at the output
    (6-10 dB) in order to "fix" the impedance.

  3. Load it with 50 Ohms, and measure output voltage, remove load and measure
    If it is half, it is 50, else do the math.
  4. Would it be a car-type output socket (deep recessed centre pin)? Would
    it be an old Advance generator with the rounded corners? If so, it's
    almost certainly 75R.
    If it has a resistive output attenuator with no DC blocking capacitor,
    you can switch to maximum attenuation (minimum output) and measure Zout
    with an ohm-meter.
  5. Ralph Mowery

    Ralph Mowery Guest

    The generators are more of a voltage source. They will deliver their rated
    output when loaded to the proper impedance. Load it with 50 ohms and see if
    it gives the rated output. If not try 70 ohms, or another value. One of
    the reasons for using a 6 db pad is that it helps isolate the impedance of
    the generator and receiver.
  6. John Fields

    John Fields Guest

  7. If it is half, you've got a funny generator.
  8. John Fields

    John Fields Guest

  9. Why that?
  10. I expect amplitudes to double or at least rise, after
    removing a load. Just nitpicking ;)
  11. Yes - sentence construction could have been more clear,
    it should have been inferred as 'If it is half *before you removed it*
    now that would be wrong too, 'If it WAS half' OK.
    No, actually it was correct, cause 'it' referred to the first measurement.
    Did you collect many nits while picking ;-)?
  12. Hello Paul,
    what is the brand name and model number of your RF signal generator.
    Can you descibe the socket to us.
    John Crighton
  13. Well, I also wondered what it means to Paul, once he has
    figured out that output impedance. Will it change his life
    dramatically... ;)
  14. Fred Bartoli

    Fred Bartoli Guest

    Sure. He now have a nice opportunity to ask the same question once more ;-)


    BTW, got some news of our goldmine order ?
    Not that I'm longing to have it, but they do seem to be waaaay slooooow....
  15. "Fred Bartoli"

    The last news was it got shipped on 21 april. I expect it 'real soon now'.
  16. Thanks John (and others).

    The model number ain't gonna mean a lot to anyone as this is a very
    old piece of kit (1950s) that I keep mainly out of a sense of
    reverence for the past. :) It's an old ex-RAF AVO. I have posted a
    picture of the socket to a.b.s.e under this same thread title...
    BTW, there's a 5p coin shown for scale, but since that won't mean much
    to anyone outside Britain, the outer of the socket is approx. 1" in
    diameter (which won't mean much to anyone in europe but it serves them
    right for adopting the metric system.:))
  17. My main sig gen states "output EMF using 6dB pad" next to the socket.
    WTF is a "6dB pad"?
  18. Or double, presumably. :)
    Well there you go; I knew there must be a more elegant solution to the
    one I dreamed up which basically involved taking a spread of 10 carbon
    resistors of from 10 - 1000 ohms and measuring the applied voltage
    across each, then arriving at power transferred in each by V^2/R;
    drawing a graph of the results and finding the point of maximum power
  19. John Fields

    John Fields Guest

  20. Ralph Mowery

    Ralph Mowery Guest

    My main sig gen states "output EMF using 6dB pad" next to the socket.
    A pad is usually 3 resistors in a small container. They can be a T or a Pi
    type. This is the way the resistors are configured in the pad. You may
    also see them referred to as an attenuator. The number 6 db is how much the
    pad reduces the signal in power. You can get them from about .5 db to 20
    db. They have to be used for the impedance they are rated at. They are for
    reducing the signal level and also to help isolate small differences in
    For the calibration to be accurate on your generator it sounds like the
    scale is calibrated so you need the 6 db pad after it. A 5 db pad will
    reduce the power by a factor of 4 or a voltage by a factor of 2.
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