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Matakuliah : H0122 / Dasar Telekomunikasi Tahun : 2008. Modulasi Frekuensi Pertemuan 4. Learning Outcomes. Mahasiswa dapat menjelaskan teknik modulasi frekuensi dan karakteristiknya. Outline Materi. Prinsip Modulasi Frekuensi Bandwidth Hubungan dengan Modulasi Fasa. Angle Modulation.

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Learning Outcomes

Mahasiswa dapat menjelaskan teknik modulasi frekuensi dan karakteristiknya.


Outline Materi

  • Prinsip Modulasi Frekuensi
  • Bandwidth
  • Hubungan dengan Modulasi Fasa
angle modulation
Angle Modulation
  • The angle modulation can be expressed mathematically as:

m(t) = Vccos [ωct + (t)]

    • m(t) = angle modulated wave
    • Vc = peak carrier amplitude (Volt)
    • c = carrier radian frequency (rad/sec)
    • (t ) = instantaneous phase deviation (radians)
  • The magnitude of the frequency (f) and phase deviation ()is proportional to the amplitude of the modulating signal, Vm and the rate at which the changes are occurring is equal to the modulating signal frequency, fm.
angle modulation1
Angle Modulation
  • Frequency & Phase Modulation (FM & PM) are both forms of Angle Modulation.
  • Because of its superior performance than AM, it is used extensively for commercial broadcasting radio broadcasting, television sound transmission, 2-way mobile radio, cellular radio, microwave and satellite communications systems.
frequency modulation
Frequency Modulation
  • Frequency Modulation is the process of changing carrier frequency by the modulating signal while the carrier amplitude remains constant.
  • As the modulating signal amplitude increases, the carrier frequency increases and vice versa.
  • The amount of change in carrier frequency produced by the modulating signal is called Frequency Deviation (f). Meanwhile, the change in phase is called Phase Deviation ()
  • The deviation is proportional to the amplitude of the modulating signal.
frequency modulation1
Frequency Modulation
  • FM produces pairs of sidebands spaced from the carrier in multiples of the modulating frequency.
  • The modulation index m of FM signal is the ratio of the frequency deviation fd to the modulating frequency, fm (m = fd / fm)
  • The modulation index determines the number of significant pairs of sidebands in FM signals.

Kfis the frequency deviation sensitivity

Frequency Modulation

The frequency of a harmonic carrier signal is varied in such a way that the instantaneous frequency deviation i.e. the difference between the instantaneous frequency and the carrier frequency is linearly related to the size of the modulating signal at a given instant of time.

modulation index
Modulation Index
  • Frequency modulation index is defined as

m = Kf Vm/ωm

  • Frequency deviation which is the change in carrier when acted on by a modulating signal frequency is given by:
    • Peak frequency shift in hertz
    • Peak-to-peak frequency deviation of carrier swing
  • Therefore m can be rewritten as

m=Δf /fm


PM & FM Waveform









Jn (m) is the Bessel function of the first kind




  • m = modulation index
  • Vc = peak amplitude of the unmodulated carrier
  • J0(m) = carrier component
  • J1(m) = first set of side frequencies displaced from

the carrier by ωm

  • J2(m) = second set of side frequencies displaced

from the carrier by 2ωm

  • Jn(m) = nth set of side frequencies displaced from

the carrier by nωm



FM modulator: f = 10 kHz, fm = 10 kHz, Vc = 10 V, fc = 500 kHz, m=1



  • Low-index modulation (narrowband FM)
    • m <1 (fm>>f), B = 2fm
    • High-index modulation (wideband FM)
    • m >10 (f >>> fm), B = 2fm
  • Actual bandwidth
  • B = 2nfm
  • (use Bessel table, n = number of significant sidebands)
  • Carson’s rule (approx 98% of power)
    • B = 2 (f + fm)
    • Δf = peak frequency deviation
    • fm = modulating frequency
fm pm modulator
FM & PM Modulator
  • FM modulator = integrator followed by a PM Modulator
  • FM Demodulator = PM demodulator followed by a differentiator
  • PM Modulator = Differentiator followed by an FM Modulator
  • PM Demodulator = FM demodulator followed by an integrator

Modulating signal source

Frequency modulator


FM wave


Kf is the deviation sensitivity

Frequency Modulator



  • Telah dipelajari karakteristik modulasi sudut
  • Telah dipelajari modulasi frekuensi
  • Telah dipelajari peran sidebands.