Signal Display Range of Spectrum Analyzer

Chapter Signal Display Range

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

  • 14 | Guide to Spectrum and Signal AnalysisWhen the signal power is added to the average noise power, the resultant signal power displayed will be 3 dB greater (Figure 14). This 3 dB difference is sufficient for low level signal identification.Signal Display RangeThe signal display range of a spectrum/signal analyzer with no input attenuation is dependent on two key parameters.a) The minimum resolution bandwidth available and hence the average noise level of the analyzer andb) The maximum level delivered to the first mixer that does not introduce distortion or inflict permanent damage to the mixer performance.Typical values for these two factors are shown in Figure 15.As the input level to the first mixer increases so the detected output from the mixer will increase. However, since the mixer is a semiconductor diode the conversion of input level to output level is constant until saturation occurs. At this point the mixer begins to gain compress the input signal, and conversion reverts from linear to near logarithmic. This gain compression is not considered serious until it reaches 1 dB.Input levels that result in less than 1 dB gain compression are called linear input levels (Figure 16). Above 1 dB gain compression, the conversion law no longer applies and the analyzer is considered to be operating nonlinearly and the displayed signal amplitude is not an accurate measure of the input signal.Figure 14