Ceyear 4051 Series, User Manual

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4. Operating Instruction
4.2 Advanced operation guide
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Fig. 4.10 Spectrogram when resolution bandwidth (10 kHz) is smaller than signal frequency bandwidth
4.2.1.4 Distinguish two signals with unequal amplitude
1) Basic concepts of filter selectivity
When distinguishing two signals with unequal amplitude but similar frequency, the filter shape in the Series
Signal Analyzer, i.e. 3 dB bandwidth should be taken in to account. The filter shape is defined by filter
selectivity, which the ratio of 60 dB bandwidth against 3 dB bandwidth. Filter bandwidth and selectivity are
shown in Fig. 4.11. If one small signal is located very close to a large signal, then such small signal may be
overwhelmed by the large signal.
Fig. 4.11 Diagram of filter bandwidth and selectivity
Fig. 4.12 shows the diagram of relationship between two close signals, in which a indicates frequency interval
of two signals and k indicates half of the filter bandwidth of the large signal. A should be larger than k in
order to measure level of the small signal.
Digital filter bandwidth of the analyzer is about 1/3 of typical analog resolution bandwidth filter, which means
when distinguishing a nearby signal, a wider resolution bandwidth can be used to cut sweep time.