Sweep Speed & Sensitivity and Noise Figure of Spectrum Analyzer

Chapter Sweep Speed

Guide to Spectrum and Signal Analysis
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Summary of Contents

Guide to Spectrum and Signal Analysis

  • 12 | Guide to Spectrum and Signal AnalysisA spectrum analyzer’s ability to resolve two closely spaced signals of unequal amplitude is not only dependent on the IF filter shape factor. Noise sidebands can reduce the resolution capabilities since they will appear above the skirt of the filter and so reduce the out of band rejection of the filter.Sweep SpeedSpectrum analyzers, incorporating swept local oscillators have the issue of needing to manage the sweep speed to prevent uncalibrated displays. Signal analyzers do not. Blocks of spectrum are processed together in a signal analyzer. See Figure 5. The sample rate of the A to D converter determines the span of spectrum that can be processed. The span is approximately ½ the A to D sample rate. The difference in sweep speed performance between the spectrum analyzer mode and signal analyzer mode is especially visible when very narrow resolution bandwidths are used. Most modern analyzers combine swept spectrum and signal analyzer technology. The spectrum analyzer mode offers very wide span views and the signal analyzer mode offers fast spectrum displays for narrow spans. The sweep speed for signal analyzer-based spectrum displays depends on the FFT computation speed. Dedicated FFT processing circuitry can speed up spectrum display rates to support searching for intermittent signals.Sensitivity and Noise FigureThe sensitivity of a spectrum analyzer is defined as its ability to detect signals of low amplitude. The maximum sensitivity of the analyzer is limited by the noise generated internally. This noise consists of thermal (or Johnson) and non-thermal noise. Thermal noise power is expressed by the following equation:wherePN = Noise power (in Watts)k = Boltzman’s constant (1.38 x 1023 JK–1) T = Absolute temperature (Kelvin)B = System Bandwidth (Hz)From this equation it can be seen that the noise level is directly proportional to the system bandwidth. Therefore, by decreasing the bandwidth by an order of 10 dB the system noise floor is also decreased by 10 dB (Figure 13).Figure 1310 kHz RBW1 kHz RBWPN = kTB