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probe attenuation

Discussion in 'Electronic Basics' started by Mike, Dec 24, 2004.

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  1. Mike

    Mike Guest

    Hello,

    I received my oscilloscope yesterday -- a tek 465b/dm44. Pretty nice
    machine, and I'm cluing myself in on how to get the best use out of it. I
    understand that probes can attenuate the input signal. The probe I got is a
    P6006 10X. I did a trivial experiment measuring the voltage on an AA
    battery, which was ~ 1.3 volts (confirmed with simpson meter and dm44). I
    put this on the scope and when the volt/div illumination is on .1, it
    occupies ~ 1.3 grids, which I expected. What I don't understand is that a
    book says the volt/div must be divided by the attenuation factor to get the
    actual reading. Shouldn't I have to multiple the attenuation by volt/div?
    Otherwise, my battery voltage would be .13! It doesn't make sense the other
    way around.

    Thanks!
    Mike
     
  2. If your probes don't have the hardware to tell the scope (it's a box
    near the scope end) then the probe will show 10x the actual signal. I
    have both kinds of probe for my 475, and the tek probe (P6106A) makes
    the adjustment. The tek manuals call this the 'readout' feature. See the
    following link for more information.

    http://www.tek.com/site/ps/0,,60-15109-INTRO_EN,00.html

    I didn't know this when I first got my scope either. It's annoying.
    Having a manual switch would be nicer.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  3. Yes. If the probe attenuates by 10 you have to multiply the reading with
    10 to get the real value. Continue to use your sense and do practical
    tests to find out how it works.

    Books are not always right. Authors often express themselves in less
    clear ways, or are simply wrong. When in doubt look at reality and trust
    the reality and your own intelligence more than the books.

    Your first oscilloscope is a great christmas gift to yourself, now you
    have something to play with for a long time.

    Get more than one book, so you can compare when in doubt.
    There are oscilloscope manuals for download on the web.
     
  4. Yes. If the probe attenuates by 10 you have to multiply the reading
    You can make your own special probes. You only need a few extra contacts
    like the one at the end of your 10X probe, most likely the standard BNC
    coaxial contact.

    With only a piece of coaxial cable you can make a 1X probe, so you can
    compare with the 10X probe.

    For radio work you can build an RF rectifying probe. It needs one or two
    diodes and a small capacitor. Look up the schematic on the web.

    Test your scope on signals and voltages you already know what they should
    look like, just like you did with the battery.
     
  5. Mike

    Mike Guest

    Thanks to everyone that posted. I realized shortly after my post that the
    author probably meant that a 10X probe actually divides the volt/div scale
    by 10.

    Cheers,
    Mike
     
  6. peterken

    peterken Guest

    it's just how you interpret it....
    using a 10x probe and interpreting the measurement means
    - multiply the displayed readout by 10
    OR
    - divide the voltage setting of the scope by 10

    both ways gets you the correct result
     
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