1. 1 Modulations/demodulations in Transmitters/Receivers Amplitude modulation (AM) Angle modulation – Frequency, phase

2. 2 AM modulation AM has the advantage of being usable with very simple modulators and demodulators Disadvantages include poor performance in the presence of noise and inefficient use of transmitter power Applications: broadcasting, aircraft communications in the VHF frequency range

3. 3 Full carrier AM V(t) = (Ec + em)sin(ωc x t) Example 3.1 Modulation index m = Em / Ec Over modulation

4. 4 Optical Carrier fc Modulating signal em DC bias AM modulation circuits RF modulation Optical modulation Modulating signal em

5. 5 Modulation index for multiple modulating frequencies m T = sqrt (m 1 2 + m 2 2 + …) Example 3.3

6. 6 Measurement of modulation index m = (Emax – Emin) / (Emax + Emin) Example 3.4

7. 7 Full carrier AM: frequency domain V(t) = Ec sin(ωc t) carrier + mEc/2 cos(ωc - ωm )t left sideband – m Ec/2 cos(ωc + ωm )tright sideband Example 3.5 Ec m/2 Ec

8. 8 Bandwidth and power relationships Bandwidth: B = 2 fm Power relationship: P lsb = m 2 /4 P c Pt = Pc (1 + m 2 /2) Ec m/2 Ec 2fm

9. 9 Some observations The total power in an AM signal increases with modulation, reaching a value 50% greater than that of the un-modulated carrier for 100% modulation The extra power with modulation goes into the sidebands: the carrier power does not change with modulation The useful power is rather small, reaching a maximum of 1/3 of the total signal power. For this reason, AM transmission is more efficient when the modulation index is as close to 1

10. 10 Measuring the modulation index in the frequency domain M = 2 x sqrt(P lsb / P c ) Example 3.11

11. 11 Quadrature AM AM modulator Phase shifter Cos Sin Demodulation is the reverse process

12. 12 QAM demodulation Carrier recovery Phase shifter Cos Sin To study the case when there exists phase shift from the carrier

13. 13 Suppressed-Carrier AM In normal AM, two-third of the transmitted power is found in the carrier Suppressed-carrier AM removes the carrier Psb = 0.5 Pc = 1/3 Pt Pc 1/6 Pt Pt/2 1/6 Pt Pt/2

14. 14 CSRZ RZ PW = 11.4 ps, ER = 13.7 dB PW = 9.2 ps, ER = 18.0 dB Streak camera trace Optical spectra Practical examples: RZ and CSRZ

15. 15 Single-sideband AM Two sidebands of an AM signal are mirror images Removing one sideband reduces the bandwidth, and improves the signal-to-noise ratio Pt/2 DSBSCSSB

16. 16 Power in suppressed- carrier signals Peak envelop power is used for suppressed- carrier signals: PEP = [Vp / sqrt(2)] 2 / R L Example 3.11

17. 17 Matlab simulation P128, example of AM modulation A simple AM modulator

18. 18 Angle modulation Angle modulation can be divided into frequency (FM) and phase modulation (PM) Both FM and PM are widely used in communication systems The most important advantage of FM or PM over AM is the possibility of improved signal to noise ratio

19. 19 Frequency modulation Frequency Implementation Amp VCO

20. 20 Frequency modulation Cos(ω c t + θ), ω c is the modulating signal Frequency deviation: f sig = fc + kf Em(t) Where kf is the modulator deviation constant

21. 21 Example 4.1 kf = 30 kHz/v, carrier frequency is 175 MHz, find out the frequency for an modulating signal equal to: 150 mV and –2V

22. 22 Frequency modulation index Peak frequency deviation δ = kf Em Fsig( fc + δ sin ω m t) Frequency modulation index m f = δ / f m

23. 23 Example 4.3 An FM transmitter operates at its maximum deviation of 75 kHz, find out the modulation index for a sine modulation signal with a frequency of 15 kHz and 50 Hz.

24. 24 Phase modulation kp = Φ/em Kp: phase modulator sensitivity Φ: phase deviation em: signal amplitude θ(t) = θc + kp em(t) in case of sin signal: θ(t) = θc + mp sinω m t

25. 25 Relationship between frequency modulation and phase modulation f Phase shift θ = ωt = Integral(ω dt)

26. 26 Implementation of a phase modulator Amp VCO

27. 27