Pulse Spectrum & MEASUREMENT EXAMPLES & Intermodulation Distortion of Spectrum Analyzer

Chapter Pulse Spectrum

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

  • www.anritsu.com | 35A pulse repetition rate equal to the resolution bandwidth is the demarcation line between a true Fourier series spectrum, where each line is a response representing the energy contained in that harmonic and a pulse of the Fourier transform response.Pulse Spectrum A pulse spectrum occurs when the bandwidth B of the spectrum analyzer is equal to or greater than the PRF. The spectrum analyzer in this case cannot resolve the actual individual Fourier frequency domain components, since several lines are within its bandwidth. However, if the bandwidth is narrow compared to the spectrum envelope, then the envelope can be resolved. The resultant display is not a true frequency domain display, but a combination of time and frequency domains. It is a time domain display of the pulse lines, since each line is displayed when a pulse occurs, regardless of the frequency within the pulse spectrum to which the analyzer is tuned at that moment. It is a frequency domain display of the spectrum envelope.MEASUREMENT EXAMPLESThe measurements described in this section are generally available ‘one button’ functions on modern, high performance spectrum analyzers as but may not appear on all the available models.Intermodulation DistortionSignals generated by intermodulation distortion appear as signals that are separated from the original signals by the frequency difference of the original signals. The level of this intermodulation distortion depends on the levels and frequencies of the input signals. When two signals are input, the distortion is observed as 3rd order distortion, and when the input signal level is decreased by 10 dB, the distortion decreases by 30 dB. Figure 41 shows this relationship and the point (where the input signal meets the distortion component) is called the intercept point.Carrier2nd order distortion3rd order distortionIntercept pointOutput level (dB)Input level (dB)0–10–20–30–40–50–50–40–30–20–10–0Figure 41