Chapter Four: Angle Modulation

ANGLE MODULATIONIntroductionThere are three parameters of a carrier that may carry information:AmplitudeFrequencyPhaseFrequency and Phase modulation are closely related and grouped together as angle modulationFrequency ModulationPower in an FM signal does not vary with modulationFM signals do not have an envelope that reproduces the modulationThe figure below shows a simplified FM generatorInsert fig. 4.2

Frequency DeviationFrequency deviation of the carrier is proportional to the amplitude of the modulating signal as illustrated

Frequency Modulation IndexAnother term common to FM is the modulation index, as determined by the formula:

Phase ModulationIn phase modulation, the phase shift is proportional to the instantaneous amplitude of the modulating signalRelationship Between FM and Phase ModulationFrequency is the derivative of phase, or, in other words, frequency is the rate of change of phaseThe modulation index is proportional to frequency deviation and inversely proportional to modulating frequencyConverting PM to FMAn integrator can be used as a means of converting phase modulation to frequency modulation

The Angle Modulation SpectrumAngle modulation produces an infinite number of sidebandsThese sidebands are separated from the carrier by multiples of fmFor practical purposes an angle-modulated signal can be considered to be band-limitedBessel FunctionsFM and PM signals have similar equations regarding compositionBessel functions represent normalized voltages for the various components of an AM or PM signal BandwidthFor FM, the bandwidth varies with both deviation and modulating frequencyIncreasing modulating frequency reduces modulation index so it reduces the number of sidebands with significant amplitudeOn the other hand, increasing modulating frequency increases the frequency separation between sidebandsBandwidth increases with modulation frequency but is not directly proportional to itCarsons RuleCalculating the bandwidth of an FM signal is simple, but tedious using Bessel functionsCarsons Rule provides an adequate approximation for determining FM signal bandwidth:

Variation of FM SignalInsert fig. 4.9

Narrowband and Wideband FMThere are no theoretical limits to the modulation index or the frequency deviation of an FM signalThe limits are a practical compromise between signal-to-noise ratio and bandwidthGovernment regulations limit the bandwidth of FM transmissions in terms of maximum frequency deviation and the maximum modulation frequencyNarrow- and Wideband SignalsNarrowband FM (NBFM) is used for voice transmissionsWideband FM (WBFM) is used for most other transmissionsStrict definition of the term narrowband FM refers to a signal with mf of less than 0.5FM and NoiseOne of the original reasons for developing FM was to give improved performance in the presence of noise, which is still one of the major advantages over AMInsert fig. 4.10FM StereoThe introduction of FM stereo in 1961 was accomplished in such a way so as to insure compatibility with existing FM monaural systemsThe mono FM receivers must be able to capture the L+R signal of a stereo transmitter

FM Broadcasting Spectra

FM MeasurementsThe maximum frequency deviation of an FM transmitter is restricted by law, not by any physical constraintTraditional oscilloscope displays are not useful in analyzing FM signalsA spectrum analyzer is much more useful in determining the qualities of an FM signalPreemphasis and DeemphasisHigh pitched sounds are generally of lower amplitude than bass. In FM lower amplitudes means lower frequency deviation hence lower SNR.Preemphasis is a technique where high frequency components are amplified before modulationDeemphasis network returns the baseband to its original formPre/Deemphasis Response500 Hz 2120 Hz 15KHz-17dB17dB+3dB-3dBpreemphasisdeemphasisDeemphasis circuitIs between the detectorAnd the audio amplifier