Conformity Magazine - December 2007 - (Page 29) When using spectrum analyzers or receivers for EMI troubleshooting measurements, no standard is to be applied that calls out a specific setting of the IF bandwidth. Therefore, it is mandatory to know if a measured signal is displayed as a narrowband or broadband signal in order to correctly determine the frequency of signals. Furthermore, some EMI standards like the older MIL-STD 461B provide two different limits for narrowband and broadband signals, which require a determination of the signal characteristic as part of the compliance measurement process. In both cases, suitable discrimination methods are necessary to determine a signal to be narrowband or broadband. Resolution Bandwidth Test As mentioned before, the reference for a signal to be broadband or narrowband is the resolution bandwidth setting of the test instrument used for the measurement. Some standards suggest the variation of the resolution bandwidth of the test instrument and observation of the resultant amplitude change of the signal under investigation. It is stated that an amplitude change, introduced by the variation of the resolution bandwidth, indicates the presence of a broadband signal. Conversely, if no amplitude change is observed, the signal is considered to be narrowband. Figure 4 depicts the measurement of an impulsive signal with a pulse repetition frequency (PRF) of 1 kHz and a pulse width of 7.7 μsec. If this signal is initially measured with a 100 Hz resolution bandwidth and the bandwidth is changed to 300 Hz, no change in amplitude is observed. Bandwidth settings that are lower than the PRF of the signal to be measured will result in the resolution of each individual spectral component. This will result in a narrowband measurement of the signal. A further increase in resolution bandwidth to 10 or 30 kHz will result in multiple spectral components located in the passband of the IF filter. A change in resolution bandwidth will result in an amplitude change of the measured signal, since wider IF bandwidths will encompass more spectral components and thus result in higher levels at the filter output. Using bandwidth settings that are wider than the PRF will indicate the presence of a broad band signal, since amplitude changes can be observed. Further increases of the resolution bandwidth to 1 MHz or greater will not yield changes in signal amplitude. This would indicate the presence of a narrowband signal, which is incorrect, in accordance with the definition. Large resolution bandwidths encompass the main spectral components of a signal (i.e., the main lobe and the first two side lobes of the spectrum), and do not lead to changes in the measured amplitude. Therefore, the variation of the resolution bandwidth as a means for determining the signal characteristic is of limited usefulness. Further information about the signal to be measured is required to avoid erroneous results. In addition, a change of bandwidth represents a change of the reference for the narrowband-broadband discrimination, which is very often neither permissible (by EMI standards) nor desirable for troubleshooting applications. It should be noted that this method provides conclusive results only when the signal under investigation is a CW signal. Peak vs. Average Detection Test A second discrimination for the determination of signal characteristics is the amplitude comparison between a peak Figure 3: Envelope detector December 2007 conformity 29 http://www.ddbunlimited.com http://www.ddbunlimited.com
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