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Electronic Measurements

Signal Parameters

Last Modification: February 4 2013

For complex waveforms, it's often inadequately to specify only the effective or average value. Sometimes there are limits on parameters such as ringing, overshoot and rise and fall times, and must also be specified.

Defining parameters.

Figure 1 shows a not ideal square wave with the definitions in the time and amplitude domain.
On the right side of the square wave the corresponding histogram is shown. This histogam shows how often a certain value appears. The histogram consists of a number of containers (bins) that the amplitude levels represents. From the voltage that is analysed samples are taken with a regular time interval. The value of the sample is measured and the counter of the corresponding bin is increased. The two "bins" that have the highest values are defined as the top and base.

square wave with histogram
Fig. 1: Square wave with histogram.

Below are some common definitions for a square wave shaped voltage. (Where voltage is read, it can also be replaced by current.)

The above definitions can also be applied to other waveforms.


The term amplitude, which is used in measuring and monitoring, differs from the official definition as used in maths and physics: The amplitude is the maximum value of a harmonic vibration relative to its resting state.

Manufacturers of measuring instruments use most often the peak to peak value of a signal as the definition of the amplitude; The value of the output of function generators is identified as: "Amplitude ... Vpkpk." The term amplitude is also used in general to indicate a quantity: "Amplitude accuracy:... %."

In practice, the official definition is of little importance because pure harmonic waveforms are very rare. In this article the general definition is used: The peak to peak value of a clean signal.


A number of measuring instruments, such as digital oscilloscopes, uses the histogram to determinate the base and top. The amplitude is the difference between the top and base level. Caution should be taken, not all signals has an easy identifiable base and top as shown below.

Noise influence

(noise)sine with histogram
Fig. 2: Sine wave with and without noise and the corresponding histograms.

The graph on the right shows a clean sinewave (red) and a sinewave with noise (blue) with the related histograms in corresponding colours.
The histogram shows that the clean sinewave has the most common values on the minimum and maximum levels. The histogram has a sharp boundary at the ends. The two peaks in the histogram correspond to the peaks of the sinewave.
The sinus with noise has less sharp boundaries in the histogram. Also, it is noticeable that the peaks in the histogram are slightly shifted towards the center.
The histograms show evidently that a signal with noise measures a smaller amplitude compared with the signal without noise. This situation can be avoided by removing the noise prior the measuring by using a low-pass filter. The real amplitude is equal for both (with and without noise) signals.

Incomplete references

Full wave rectified sinewave with histogram
Fig. 3: Full wave rectified sine with histogram.

The following example shows a full wave rectified sine. The histogram shows now only a single peak. The top level is well defined, but the base can not recognised due to the absence of a lower peak in the histogram. A good instrument will therefore use other methods to find the base and top.
The figure shows also that by doing a full wave rectifying a signal is created that has twice the number of periods. The measured frequency has doubled.

Flat histogram

triangle wave with histogram
Fig. 4: Triangle voltage with histogram.

As a final example, a saw tooth or triangle waveform. With this shape, all voltages are equally common. The histogram is therefor completely flat. Again, a good instrument will choose a different method to define the top and base. Often, the base and top will be made equal to the edges of the histogram. This has the disadvantage that the top and base are not well defined if a signal is noisy. The best practice is to remove the noise before the signal is measured.

A note about the definitions in the time domain: The rise and fall times are exactly defined. The instrument will do the measurement between the 10...90% or 20...80% boundaries. The measured time will therefore be smaller.

What is measured?

The examples above shows already that measured values depend on the measurement method and signal conditions. Check always the method the instrument uses.

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