ArticleThe Effects Of Signal Clipping
by Jack Frarey
This paper describes the many types of signal clipping and the resultant spectrum displays. The introduction prefaces one possibility of signal clipping that can occur is when the operator disables the analyzer’s auto-ranging feature. Both symmetric & asymmetric clipping causing A/D convertor overflow are described with both time and spectrum graphs showing resultant displays. Accelerometer overload, clipping prior to the anti-aliasing filters, transient clipping and other forms of clipping are also covered in this paper. Multiple graphs are included displaying the results of various forms of clipping.
Most vibration analysts are aware that they should avoid signal clipping since it generates a set of harmonics that could be misdiagnosed as machine looseness rather than poor FFT setup. Signal clipping typically occurs when we become frustrated when the analyzer decides to auto-range at just the time we want to take data. To overcome this we disable the auto-ranging feature. With auto-ranging disabled, it is possible to over-range the A/D converter (analog to digital converter) at a time when the analyst is too busy to note the overload light.
A careful analyst should always double check data that shows a harmonic family in the FFT to ensure that clipping did not occur. If this were the only effect of clipping and it could always be detected by examining the time data, then signal clipping would not be a big problem. Unfortunately, signal clipping can produce other effects and the signal may be clipped in a portion of the instrumentation system where it is not easily detected. Symmetric and asymmetric clipping produce different effects. Asymmetric clipping can introduce DC and low frequency components in the data. It can also produce modulation products for signals with more than one signal present. Symmetric clipping produces less errors in the data but must still be guarded against to preserve data quality. This paper will discuss ways that the data can be clipped and the various effects on the processed data.
An A/D converter is very unforgiving of any situation where an input signal has a larger amplitude than the A/D range. Most A/D’s will simply clamp the signal to the max range of the converter. In Figure 1, the proper operation of the A/D is shown. The signal in the time domain is a sin wave whose amplitude is below the A/D input range. The second trace shows the spectrum. The third trace shows a log plot of the spectrum. The only harmonics that are visible are over 60 dB down from the peak. Figure 2 shows the case where the signal amplitude exceeds the A/D range. The time domain signal is squared off at the top and bottom. The log plot of the spectrum shows many harmonics. As we will see in the next section, pure symmetrical clipping would produce only odd harmonics. Since pure symmetrical clipping is difficult to achieve, one can observe some even numbered harmonics in the spectrum.”
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