Pulse Response of Spectrum Analyzer

Chapter Pulse Response

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

  • 34 | Guide to Spectrum and Signal AnalysisWe know from single tone AM how the sidebands are produced above and below the carrier frequency. The idea is the same for a pulse, except that the pulse is made up of many tones, thereby producing multiple sidebands which are commonly referred to as spectral lines on the analyzer display. In fact, there will be twice as many sidebands (or spectral lines) as there are harmonics contained in the modulating pulse.The main lobe (in the center) and the side lobes are shown as groups of spectral lines extending above and below the baseline. For perfectly rectangular pulses and other functions whose derivatives are not continued at some point, the number of side lobes is infinite.The main lobe contains the carrier frequency and is represented by the longest spectral line in the center. The amplitude of the spectral lines forming the lobes varies as a function of frequency.Notice in Figure 39 how the spectral lines extend below the baseline as well as above. This corresponds to the harmonics in the modulating pulse having a phase relationship of 180° with respect to the fundamental of the modulating waveform. Since the spectrum analyzer can only detect amplitude and not phase, it will invert the negative going lines and display all amplitudes above the baseline.Because a pulsed RF signal has unique properties, care must be taken to interpret the display on a spectrum analyzer correctly. The response that the spectrum analyzer (or any swept receiver) can have to a periodically pulsed RF signal can be of two kinds, resulting in displays which are similar but of completely different significance. One response is called a line spectrum and the other is a pulse spectrum. We must keep in mind that these are both responses to the same periodically pulsed RF input signal and that line and pulse spectrum refer only to the response displayed on the spectrum analyzer.Pulse ResponseIf we increase the IF bandwidth in our example to 1 kHz, we get the display shown in Figure 40. Notice that the analyzer has lost the ability to resolve the spectral lines since B = PRF. The lines now displayed are generated in the time domain by the single pulses of the signal. We also see that the displayed amplitude of the spectrum envelope has increased. This is due to the fact that the IF filter is now sampling a broader section of the spectrum, thus collecting the power of several spectral lines.Figure 40