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Fluke 189 AC over DC function

C

Chris W

Jan 1, 1970
0
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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W

Walter Harley

Jan 1, 1970
0
Chris W said:
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.

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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