The subject invention concerns the field of RMS signal measurement in general, and specifically concerns a smart autoranging RMS measurement method and the apparatus to implement it.
Modern digital multimeters (DMMs) have the capability to measure the RMS (Root Mean Square) value of an AC signal. This capability is known variously as xe2x80x9cTrue RMSxe2x80x9d or xe2x80x9cAC RMSxe2x80x9d or xe2x80x9cAC+DC RMSxe2x80x9d.
Various different scales (i.e., ranges) are used on DMMs to measure signals of different amplitudes. For example, a DMM may have 500 mv, 5 v, 50 v, and 500 v ranges. Auto-ranging features which decide which range to select, based on the input signal, are well-known in the art. For example, a 35 v input signal would result in the selection of the 50 v range from the list of exemplary ranges given above.
It is important that the algorithm which is used to decide the correct range incorporate some form of hysteresis to prevent oscillation between two ranges. When making an auto-ranging decision in order to measure an AC signal, one must use the RMS value of the signal, as a guide. As noted above an input signal having a 35 v RMS value would auto-range to the 50 v range, regardless of other characteristics of the signal, such as peak amplitude.
A typical RMS-only auto-ranging algorithm would control the DMM to change to the next higher range for values of input signal exceeding 100% of the currently-selected range, and control the DMM to change to the next lower range for values of input signal smaller than 8% of the full scale value of the currently selected range. This algorithm provides a 2% hysteresis band, assuming that the ratio of the two adjacent ranges is 10:1.
Unfortunately, a DMM employing the auto-ranging algorithm described above may be ranged incorrectly, causing an incorrect reading to be displayed. It is herein recognized that this problem occurs when a signal is being measured which has large peaks, in comparison to its RMS value. A signal of this kind is said to exhibit a large crest factor. The large peaks may overdrive the analog conditioning circuits, or drive the analog to digital (A-to-D) converter beyond its limits (sometimes referred to as xe2x80x9crailingxe2x80x9d the A-to-D). For example, a 35 v RMS signal exhibiting a crest factor of 6 would have peaks (i.e., maximum signal excursions) of substantially 210 v. Such a condition greatly exceeds the dynamic range of the 50 v range of the DMM. As noted above, the typical auto-ranging algorithm would, nevertheless, cause the DMM to select the 50 v range, the measurement would be incorrect, and the user would not know it.
An apparatus and method for performing an auto-ranging function in electronics test equipment monitors both the RMS value and the peak value of the input signal to be measured. In the subject system, a peak signal measurement which exceeds the dynamic range of the currently-selected range will cause selection of the next higher range. A peak signal measurement which would exceed the dynamic range of the next lower measurement range, will prevent selection of the next lower range when the RMS value would otherwise have caused that selection. A peak signal value which does not exceed the dynamic range of the currently-selected measurement range, or the dynamic range of the next lower measurement range will allow control of measurement range selection to be determined by the RMS value of the input signal.