Modulasi Amplitudo Pertemuan 3

1. Matakuliah : H0122 / Dasar Telekomunikasi Tahun : 2008 Modulasi AmplitudoPertemuan 3

2. Learning Outcomes Mahasiswa dapat menjelaskan tujuan modulasi, proses dan jenis modulasi amplitudo.

3. Outline Materi • Pengertian modulasi • Tujuan Modulasi • Proses Modulasi Amplitudo • Klasifikasi Modulasi Amplitudo

4. Pengertian • Modulasi ialah proses untuk mengkombinasikan suatu sinyal yang berisi informasi dengan sinyal lain. • Persamaan matematikanya: • ec(t) = Ec(t) cos (ωct + Φ(t)) dengan • ωc = 2f c • ec = sinyal termodulasi (gelombang carrier ) • Ec(t) = envelope dari ec(t) • Φ(t) = deviasi fasa dari ec(t) • f c = frekuensi carrier

5. Introduction • Amplitude Modulation is the process of changing the Amplitude of a relatively high frequency carrier signal in accordance with the amplitude of the modulating signal (Information). • AM Modulators are nonlinear devices with 2 inputs • A single, high frequency of carrier signal of constant-amplitude carrier signal • The low frequency information signal. • The Output of AM Modulator is called Modulated Wave and the shape of the Modulated Wave is called AM Envelope. • It is often used for commercial broadcasting of both audio and video signals.

6. AM Waveform • Generally. • Carrier Signal => VCsin(2fct) • Modulating Signal => Vmsin(2fmt) • Modulated Wave => Vam(t) • For Double sideband full carrier (DSBFC) AM waveform consists of: • DC voltage • The carrier frequency fc • Lower side frequency (fc - fm) • Upper side frequency (fc + fm)  Known as AM envelope

7. Indeks Modulasi • Indeks Modulasi : M = (modulated peak voltage-unmodulated carrier voltage)/(unmodulated carrier voltage), dengan 0<m<1

8. AM Waveform • The AM waveform reaches maximum value when the modulating signal amplitude is at maximum value. • The AM waveform reaches minimum value when the modulating signal amplitude is at maximum negative. • The repetition rate of the envelope is equal to the frequency of the modulating signal, and the shape of the envelope is identical to the shape of the modulating signal.

9. AM Waveform

10. Carrier Lower side band Upper side band Amplitude Lower side frequencies Upper side frequencies fc-fm(max) fc fc+fm(max) Frequency Frequency Spectrum • The Frequency Spectrum of AM DBSFC is shown below: • Frequencies between fc-fcmax and fc is called lower side band (LSB) • Frequencies between fc and fc+fmmax is called upper side band (USB) • The Bandwidth of AM DBSFC is FB = 2fm (max) • AM signal does not contain modulating signal frequency

11. Modulation Index • Modulation index is an indicator to describe the amount of amplitude change (modulation) present in an AM waveform (depth of modulation), basically stated in form of percentage. • It is defined mathematically as : m = Em / Ec • m = modulation coefficient (unitless) • Em = peak change in the amplitude of the output waveform voltage • Ec = peak change in the amplitude of the unmodulated carrier voltage • Percent modulation: M= m x 100%

12. Indeks Modulasi • Indeks modulasi dan daya sinyal : Pt = Pc [1+ {m2/2}] • dengan • Pt = daya total • Pc = daya carrier

13. Indeks Modulasi 0 < m < 1 m = 1 m > 1 mod 100 % (over-modulation)

14. e(t) e(t) t f 1/2T T fm t fc LSB USB t fc - fm fc + fm fc BT=2 fm  Klasifikasi AM • Klasifikasi modulasi amplitudo linier: • Double side band (DSB) • ec(DSB) = emEc(t) sin ωct • Bandwidth : BT = 2 fm

15. Klasifikasi AM • Single side band (SSB) • Bandwidth : BT = fm • Vestigial side band (VSB) • Bandwidth : BT = fm + α, where α < fm • Bandwidth DSB, SSB, and VSB: • BT(SSB) <BT(VSB) < BT(DSB)

16. Antenna AM modulator and output power amplifier Modulating signal source Carrier driver RF carrier oscillator AM Transmitter Diagram

17. RF section Bandpass Filter Mixer/ converter section Bandpass filter IF section Bandpass filter AM detector Bandpass filter Audio section AM Receiver Diagram

18. Summary • Telah dipelajari karakteristik modulasi amplitudo • Telah dipelajari peran sidebands.