Frequency Resolution of Spectrum Analyzer

Chapter Frequency Resolution

Guide to Spectrum and Signal Analysis
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Guide to Spectrum and Signal Analysis

  • | 9It should be noted however, that as the IF bandwidth is reduced so the time to sweep a given frequency range increases since the charge time of the IF filter increases. This means that the sweep time is increased to allow the IF filter to respond and therefore present an undistorted signal to the detector. These variables are generally taken into account automatically and are referred to as ‘coupling’. Beyond the detector can be more filtering known as Video Bandwidth and this can also be coupled to IF bandwidth and sweep time. These functions are coupled together since they are all inter dependent on each other, i.e. change one parameter setting and it affects the others.An additional facility available on most modern analyzers is a Zero Frequency Span mode. As mentioned earlier, most analyzers are based on the super heterodyne receiver design, where the local oscillator is swept continuously. If the local oscillator is manually tuned, the spectrum analyzer becomes a fixed tuned receiver whose frequency is determined by that of the local oscillator. In this mode the analyzer will display the time domain function since the frequency component is fixed even though the scan generator is still sweeping the display i.e. the display is now amplitude vs. time (Figure 8).Frequency ResolutionThe frequency resolution (typically called “resolution bandwidth”) of a spectrum/signal analyzer is its ability to separate and measure two signals in close proximity. This frequency resolution is determined by three primary factors:a) the IF filter bandwidth usedb) the shape of the IF filter andc) the sideband noise of the IF filterFigure 8