Checking Crest Factor and Bandwidth Before Trusting a True RMS Reading

A true RMS meter can measure nonsinusoidal signals, but the label does not imply unlimited peak handling or bandwidth. A narrow pulse waveform can have a modest RMS value while its peaks overdrive the selected range. A filtered or clipped input then produces an inaccurate number even though the display never indicates an obvious overload.

Describe the waveform before choosing the range

Crest factor is peak magnitude divided by RMS magnitude. Fluke’s 287/289 manual defines the measurement, while individual accuracy tables specify the supported conditions. Check whether the selected function measures AC only or AC plus DC, and whether crest-factor allowances depend on the fraction of full scale.

For an ideal pulse train switching between zero and 5 V at ten-percent duty cycle, total RMS voltage is 5 times the square root of 0.1, approximately 1.581 V. The total-waveform crest factor is about 3.16. Its DC component is 0.5 V; the AC-only RMS component is 1.5 V. These are different valid quantities, so a disagreement may reflect the chosen function rather than a defective instrument.

Check peaks and frequency content separately

Use a suitable oscilloscope measurement to verify amplitude, duty cycle, and unexpected overshoot. Then compare the meter’s bandwidth with the significant harmonic content. Raising the meter range may prevent peak clipping but reduce resolution. Lowering repetition frequency may expose low-frequency coupling limits. Neither change guarantees that all pulse harmonics are measured accurately.

Validate against a known waveform

Generate a low-voltage waveform with independently verified amplitude and duty cycle, then sweep the intended operating range. Keep the generator’s output-loading convention explicit. If the meter is used to infer resistor heating, use total RMS voltage across that resistor and verify that the waveform is within the measurement specification.

Relate the setup to generator termination and amplitude and oscilloscope acquisition settings. Report the waveform definition and measurement mode with the result; the word RMS alone is not enough to reproduce the test.