APPLICATIONS & Amplitude Modulation of Spectrum Analyzer


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

  • 18 | Guide to Spectrum and Signal AnalysisFigure 19 shows a signal analyzer display of QPSK modulation in polar display format. The polar display is called a constellation or vector diagram. APPLICATIONSAs stated in the introduction, spectrum analyzers are used to display the frequency and amplitude of signals in the frequency domain. Efficient transmission of information is accomplished by a technique known as modulation. This technique transforms the information signal, usually of low frequency, to a higher carrier frequency by using a third, modulation signal. But why modulate the original signal? The two primary reasons are:1) modulation techniques allow the simultaneous transmission of two or more low frequency, or base band signals onto a higher, carrier frequency and2) high frequency antenna are small in physical size and more electrically efficient.In this section we will consider three common modulation formats:• Amplitude Modulation or AM.• Frequency Modulation or FM.• Pulse Modulation or PM.Each modulation technique places emphasis on a particular area of the analyzer’s specification.Amplitude ModulationAs the name suggests, amplitude modulation is where the carrier signal amplitude is varied by an amount proportional to the amplitude of the signal wave and at the frequency of the modulation signal. The amplitude variation about the carrier is termed the modulation factor ‘m’. This is usually expressed as a percentage called the percent modulation, %M.The complex expression for an AM carrier shows that there are three signal elements.a) the unmodulated carrier.b) the upper sideband whose frequency is the sum of the carrier and the modulation frequency.c) the lower sideband whose frequency is the difference between the carrier and the modulation frequency.Q01001110IQPSK11 111110000000 00 000InformationBits:I-Data:Q-Data:TbT = 2Tb–A–AAAtttFigure 19