Modulasi Frekuensi Pertemuan 4

1. Matakuliah : H0122 / Dasar Telekomunikasi Tahun : 2008 Modulasi FrekuensiPertemuan 4

2. Learning Outcomes Mahasiswa dapat menjelaskan teknik modulasi frekuensi dan karakteristiknya.

3. Outline Materi • Prinsip Modulasi Frekuensi • Bandwidth • Hubungan dengan Modulasi Fasa

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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

10. PM & FM Waveform Carrier Modulating signal FM PM

11. FM: PM: Jn (m) is the Bessel function of the first kind Spektrum

12. 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

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

14. Bandwidth • 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

15. 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

16. Modulating signal source Frequency modulator Direct FM wave Vccos(2πfct) Kf is the deviation sensitivity Frequency Modulator

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