Conformity Magazine - December 2007 - (Page 30) Figure 4: Impact of resolution bandwidth setting on measured amplitude of broadband signal and an average measurement. Both measurements are preferably made with the same instrument settings, especially with an identical resolution bandwidth setting. If no amplitude changes are observed between the two measurements, a signal is considered narrowband. A signal is considered broadband if an amplitude change between the two measurements is observed, with the average measurement yielding the lower amplitude. In practice, EMI standards that call out this discrimination method (like CISPR 25) specify an amplitude difference of, for example, 6 dB which is used as a decision criterion. Per CISPR 25, a signal is considered to be narrowband if the amplitude difference between the peak and average detected signal is less than 6 dB. If the amplitude difference is greater than 6 dB, the signal is determined to be broadband. This approach is meaningful since the relative amplitude accuracy of the instrument is to be considered as well as other uncertainty factors that are introduced by different instrument settings between the two measurements (e.g., change of reference level setting). Figure 5 demonstrates the principle of this method by depicting the functionality of the peak and average detector. The peak detector will determine the envelope of the signal to be measured, which results in a low frequency signal at the detector output, or a DC signal in case the signal to be measured is a CW signal. Since the peak detector determines the amplitude envelope, it will provide the maximum signal amplitudes. The average detector is often implemented as a low pass filter that is placed after the peak detector in the signal processing chain. This low pass filter, often referred to as video filter, will be used as an integrator by setting the bandwidth value to either a predefined value, called out in a standard (e.g., CISPR 16-1-1, which specifies an integration time) or to a value that is smaller than the lowest spectral component Figure 5: Peak versus average detection Figure 6: Broadband signal detection of a scanning receiver 30 Conformity DeCember 2007
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