Matakuliah : H0122 / Dasar Telekomunikasi Tahun : 2008 Modulasi FrekuensiPertemuan 4
Learning Outcomes Mahasiswa dapat menjelaskan teknik modulasi frekuensi dan karakteristiknya.
Outline Materi • Prinsip Modulasi Frekuensi • Bandwidth • Hubungan dengan Modulasi Fasa
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 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 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 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 • 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 Carrier Modulating signal FM PM
FM: PM: Jn (m) is the Bessel function of the first kind Spektrum
Spektrum • 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
Spektrum FM modulator: f = 10 kHz, fm = 10 kHz, Vc = 10 V, fc = 500 kHz, m=1
Bandwidth • Low-index modulation (narrowband FM) • m <1 (fm>>f), B = 2fm • High-index modulation (wideband FM) • m >10 (f >>> fm), B = 2fm • 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 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 Direct FM wave Vccos(2πfct) Kf is the deviation sensitivity Frequency Modulator
Summary • Telah dipelajari karakteristik modulasi sudut • Telah dipelajari modulasi frekuensi • Telah dipelajari peran sidebands.