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Re: A Simple Lab Demo of Resistor Noise EMF.



At 05:52 PM 3/19/01 -0600, brian whatcott wrote:

John mentioned he tried this out with
a "Fluke AC true-RMS measuring DVM". I would have thought that
the "AC true RMS value" of a DC 1.6 volt signal was 1.6 volts.
i.e. sqrt(1.6^2) = 1.6

No. This illustrates the distinction:
-- DC-coupled true-RMS, versus
-- AC true-RMS (which is short for AC-coupled true RMS).

I *want* a meter where the AC scale throws away the DC component and tells
me the true-RMS of what's left. That's what I intended to buy, and that's
what John Fluke intended to build. And that's exactly what I've got, to
one part in 10^3 or 10^4. No problem.

...or perhaps it's true RMS of each polarity, hence
sqrt[(1.6^2 + 0^2)/2] = 1.131

I have no idea what that means.

How can the True RMS value of a DC signal be zero, I wonder?

That rather depends on whether it's DC coupled or AC coupled.

I tried this check on two DVMs, first the Radio Shack 22-188,
then an el cheapo $10 special, "M-830B".

RS on DC 1.592 volts (Rayovac alkaline cell)
M-803B on DC 1.603 volts

RS on AC 1.2 first sample, then decreasing,
either polarity on 2 V AC autorange.

M-803B on AC 2.8 volts constant for one polarity,
0.0 volts for the other, 200 V AC range.

I don't think anybody suggests either of these is
AC true RMS reading, for sure!

I hereby suggest that the RS meter may be indicating AC true-RMS. Or it
may not. No evidence that bears on this question has been adduced.

The M803B does not appear to be a very fine measuring instrument.