I have a TRUE RMS DVM. Hooked it in parallel with a digital scope to the function generator at 60 Hz. At zero offset, the DVM reading tracks the scope RMS calculation for sine/square/triangle waveforms.

The scope knows how to measure the RMS value of a sinewave with voltage offset.

4V DC is 4V RMS on the scope, but zero on the DMM.

A look at the DVM specs shows that the AC RMS measurement has a low frequency of 45 Hz. That explains the observation. BUT That does not sound like a TRUE RMS measurement????

Older AC meters would measure the peak-peak AC value or the average of the absolute value of the AC, then scale the result so that it was the same as the RMS value for a sine wave -- but it would be wrong for square, triangle, etc.

So, it was "truer" RMS.

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My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.

"True" RMS meters block the DC, then run the signal through a circuit that actually takes the square of the absolute value of the AC, filters it, then takes the square root.

So, it is "truer" RMS than what came before.

If you know the DC component and you know the RMS value of the AC component, then you can calculate the overall RMS value of the wave, which is pretty cool.

Try measuring the RMS value of a signal that goes off of your oscilloscope screen, or that has rare big spikes and lots of little crud

-- see how "true" your RMS is then.

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My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.

Try using the DC range (on the DVM) when measuring DC voltages.

For a superimposed waveform or noise or PARD, the AC range will read zero, unless the AC oscillations are within the range set on the DVM. At a very fine range setting, you might read the ripple if it is slow enough. Most meters won't sample too fast. There are a lot of nice ones though that do read fast.

You have to use the millivolt scale and the meter has to be able to read at the frequency of the noise/ripple you are wanting a voltage of.

The scope is better for examining such signals.

What the meter maker means by "True RMS" is that the meter is better than the old, cheap "RMS" meters which simply averaged the value from the peaks. The new meters actually analyze the waveform from moment to moment.

Many true-RMS meters use AC coupling to remove the DC component, but = good ones also have an AC+DC option, which is simply DC coupled. The AC = coupling is very useful for things such as ripple on a DC signal. But the AC+DC measurement is necessary for things like variable duty-cycle rectangular waveforms and PWM signals. Generally you can determine the actual = true-RMS value of a signal with DC offset by using Vrms =3D sqrt( Vac^2 + Vdc^2).

In my work, I need to measure the true-RMS value of a short pulse of = power line frequency AC. If you do a computation of the value based on = samples, the value will vary widely for short pulses but then converge eventually = to the steady-state true-RMS value. I have found that it is better to use a sampling rate that is an integer multiple of the line frequency, so 1200 = Hz works well for both 50 Hz and 60 Hz, with 24 or 20 samples per cycle. = And when reading the RMS value, a sample of 100 or 200 mSec shows the least amount of "jitter" since it comprises exactly 5 or 6 complete cycles.

This is also SOP for many DC digital meters, so that AC line noise is essentially canceled out, resulting in relatively flicker-free readings.

Some discussion of this can be found in an article I wrote on Circuit Breaker Testing Technology:

** DMMs ( hand held or bench types) with "true rms" ranges date back to the early 1980s when Analog Devices introduced their " true rms to DC" converter ICs.

A DMM is essentially an analogue meter with a numerical display based on a pulse counter and dual slope integrator. This arrangement can only measure DC voltages. In order to read AC ones, there is an AC to DC converter - either a precision rectifier or a true rms to DC converter inside the DMM. This is engaged when an AC range is used.

It is the performance of the latter circuits that determines the AC performance of the meter - plus the design and bandwidth of the input attenuator.

The effective sampling rate of most DMMs is only 3 or 4 times a second - ie the same as the display update rate.

The early TrueRMS and wide bandwidth meters even today are essentially (RF)dummy loads with a thermometer.

It is interesting to note that English people talk about RMS (Root Mean Square) values, while in many European languages something equivalent to effective value (e.g. Effektivwert) is used, i.e. the DC level that would produce the same heating effect in a resistive load as the AC/DC signal being measured.

I don't know about that. True RMS meters theoretically don't make a lot of assumptions about their input signals, whereas there's a very long history of measurements based on peak- or average-reading AC meters with their scale dorked to read the correct RMS value, assuming that the input waveform is sinusoidal.

For noise waveforms, average-reading meters read 1 dB low--a little matter of 10% error.

"Electrical equivalent" would leave me wondering what the underlying assumptions are. "True RMS" is a lot clearer.

Look at the HP link at page 17, what this circuit actually does, could be described with the following analogy.

Two identical 50 ohm dummy loads with identical thermometers are used. The (modulated) RF signal to be measured, is fed into the other dummy load and the temperature is observed.

A DC supply is connected to the other dummy load through a potentiometer and the potentiometer is adjusted, until the other thermometer reads the same as the first thermometer. The potentiometer output voltage is measured using any ordinary DC volt meter. The DC voltage reading is the effective value of the modulated RF waveform.

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