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

  • 4 | Guide to Spectrum and Signal AnalysisINTRODUCTIONEngineers and technicians involved in modern RF or microwave communications have many measuring instruments at their disposal, each designed for specific measurement tasks. Among those available are:a) The Oscilloscope – primarily developed for measuring and analyzing signal amplitudes in the time domain. (Voltage vs. time) Often 2, 4 or more channels of voltage vs. time can be viewed on the same display to show the relationships between signals. Extensive methods to trigger signals are often available to capture and display rare events.b) The Spectrum Analyzer – designed to measure the frequency and amplitude of electromagnetic signals in the frequency domain. (Frequency vs. time) Most modern analyzers also have the capability to demodulate analog modulated signals. Spectrum analyzers are the most versatile tools available to the RF engineer. This guide will describe the critical performance characteristics of spectrum and signal analyzers, the types of signals measured, and the measurements performedc) The Signal Analyzer – invaluable for measuring the modulation characteristics of complex signals. These units capture and process blocks of spectrum to reveal amplitude and phase relationships between signals. Newer models provide demodulation of digitally modulated signals used in most of today’s communications systems.d) The Signal Generator – an essential item of equipment for any communications test laboratory or workshop. The cost of a signal generator largely depends on the additional functions and facilities available as well as the type and quality of the frequency reference used.e) The Field Strength Meter (F.S.M.) – display the power density of an electrical signal incident on a calibrated antenna and thus give a direct reading of field strength in dBµV/m.f) The Frequency Counter – a digitally based instrument that measures and displays the frequency of incoming signals. Some models can also count ‘pulse’ and ‘burst’ signals.Frequency Domain / Time DomainAs mentioned in the introduction, electromagnetic signals can be displayed either in the time domain, by an oscilloscope, or in the frequency domain using a spectrum or signal analyzer. Traditionally, the time domain is used to recover the relative timing and phase information required to characterize electrical circuit behavior. Many circuit elements such as amplifiers, modulators, filters, mixers and oscillators are better characterized by their frequency response information. This frequency information is best obtained by analysis in the frequency domain. Modern Oscilloscopes provide frequency domain display modes and modern Spectrum and Signal analyzers provide time domain displays. One key difference between oscilloscopes and spectrum/signal analyzers is the resolution of the vertical axis. Oscilloscopes provide high resolution along the time axis but low (8 bit) amplitude resolution. Spectrum and signal analyzers provide high (16 bit or more) amplitude resolution to see small signals in the presence of large signals.