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Fluke 189 AC over DC function

Discussion in 'Electronic Basics' started by Chris W, Dec 29, 2005.

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  1. Chris W

    Chris W Guest

    I got a Fluke 189 for Christmas and am wondering about this one feature,
    it has an option when you are in the DC volts mode where it will tell
    you the amount of AC in the DC. My question is, if I were to hook the
    same signal to a scope, would the value I see on the meter be the same
    amount of the ripple I see with the scope? I'm pretty sure that is the
    way it works but I ask because every PS I have measured so far all have
    the same value of 15 to 17 millivolts. Only one of those was under load
    at the time and even then it wasn't much of a load (25% of rated). Also
    it is supposed to have a DBm but I can't figure out how to get anything
    but DBV. I'm not even sure how it is going to read DBm since that is a
    power level and it would have to measure voltage and current to get a
    power level. I don't think it can do that but maybe it can.

    --
    Chris W
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  2. I'm pretty sure the manual is online at fluke.com. If you put it on AC only
    (rather than AC+DC), do you still read the same amount of ripple? On the AC
    scales it will filter out the DC component. I don't have my 187 in front of
    me right now, but my recollection is that AC over DC is just giving you the
    the AC-only and DC-only readings at the same time. Which is certainly
    handy!

    It won't read true dBm. What it gives you is more properly termed dBu,
    which is the same as dBm assuming a 600 ohm load. Basically it's just
    taking a voltage reading and scaling it logarithmically, with 0dBu = 0.775V.
    I think you have to go into the setup screens to switch the dB reading to
    dBm instead of dBV. But my recollection about that might be wrong - maybe
    it doesn't do it at all. (It's easy to convert; dBu = dBV + 2.22.)
     
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