Keysight N9030A Measurement Manual

Also see for x series: Manual Installation note Measurement guide Measurement guide Measurement guide

Contents

Keysight N9030A Measurement Manual Manual pdf 28 page image

28ConceptsFM ConceptsFM ConceptsFigure 2-3 FM waveformFM (Frequency Modulation) and PM (Phase Modulation) belong to anglemodulation. In FM, the instantaneous frequency deviation of the modulated carriersignal changed in proportion to the instantaneous amplitude of the modulatingsignal. And in PM, the instantaneous phase deviation of the modulated carrier withrespect to the phase of the unmodulated carrier is directly proportional to theinstantaneous amplitude of the modulating signal.The modulation index for angle modulation, β, is expressed by this equation:Equation 2-3Where Δfp is the peak frequency deviation, fm is the frequency of the modulatingsignal, and Δφp is the peak phase deviation.This expression tells us that the angle modulation index is really a function of phasedeviation, even in the FM case. Also, the definitions for frequency and phasemodulation do not include the modulating frequency. In each case, the modulatedproperty of the carrier, frequency or phase, deviates in proportion to theinstantaneous amplitude of the modulating signal, regardless of the rate at which theamplitude changes. However, the frequency of the modulating signal is important inFM and is included in the expression for the modulating index because it is the ratioof peak frequency deviation to modulation frequency that equates to peak phase.Unlike the modulation index for AM, there is no specific limit to the value of β,since there is no theoretical limit to the phase deviation; thus there is no equivalent of100% AM. However, in real world systems there are practical limits.Unlike AM, which is a linear process, angle modulation is nonlinear. This means thata single sine wave modulating signal, instead of producing only two sidebands,yields an infinite number of sidebands spaced by the modulating frequency.β = Δf p f m⁄ = Δφp