Outline

1. Outline • Analog-to-Digital Conversion - Sampling • Digital Modulation Schemes • Revisit Analog Modulation Schemes • Amplitude Modulation (AM) • Frequency Modulation (FM)

2. Modulation Process • Information-bearing signals (e.g., voice, video) are called baseband signals. Other terms for information-bearing signals are message signal and modulating wave. • Modulation is defined as the process by which some characteristics of a carrier signal (typically a cosine wave) is varied in accordance with a message signal. • Modulation process is required to shift the frequency content of our message signals to a range that is acceptable by the transmission medium. (e.g., above 30 KHz for wireless transmission).

3. Modulation Types Analog Modulation: Digital Modulation: Message signal is analog (a.k.a continuous-time). Message signal is digital (a.k.a discrete-time). • Amplitude Modulation (AM) • Frequency Modulation (FM) • Phase Modulation (PM) • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK)

4. Digital Modulation Schemes Digital Modulation Schemes

5. Figure 4-8 Amplitude Change The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

6. Figure 4-9 Frequency Change The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

7. Figure 4-10 Phase Change The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

8. Also known as Symbol Rate Figure 5-24 Amplitude Shift Keying The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

9. Figure 5-27 Frequency Shift Keying The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

10. Figure 5-29 Phase Shift Keying The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

11. Figure 5-30 PSK Constellation The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

12. Figure 5-31 Quadrature PSK - QPSK 4-PSK The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

13. Figure 5-32 QPSK Constellation The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

14. Figure 5-33 8-PSK Constellation The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

15. Figure 5-35 4-QAM and 8-QAM Constellations The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

16. Figure 5-36 8-QAM Signal The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

17. Figure 5-37 16-QAM Constellation The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

18. Figure 5.17Bit and baud

19. Table 5.1 Bit and baud rate comparison

20. Sampling – Pulse Amplitude Modulation (PAM)

21. Sampling – Pulse Amplitude Modulation (PAM)

22. Quantized PAM Signal

23. Figure 3.11Illustration of the quantization process. (Adapted from Bennett, 1948, with permission of AT&T.)

24. Figure 5-20-continued From Analog to PCM The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

25. Figure 5-20-continued From Analog to PCM The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

26. Figure 5-20-continued From Analog to PCM The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

27. Figure 5-19 Pulse Coded Modulation The McGraw-Hill Companies, Inc., 1998 WCB/McGraw-Hill

28. Nyquist’s Sampling Theorem A band-limited signal of finite energy, which has no frequency components higher than W Hertz, may be completely described by specifying the values of the signal at instants of time separated by (1/2W) seconds or can be recovered from a knowledge of its samples taken at a rate of 2W samples per second. fs = 2 × W Sampling frequency Bandwidth of signal

29. Sampling frequency = message bandwidth Message signal cannot be recovered from the sampled signal !! Impact of Sampling on the Frequency Domain

30. Sampling frequency Message bandwidth Impact of Sampling on the Frequency Domain Message signal Frequency Content Frequency Content of the sampled message signal fs = 2 × W

31. Revisit Analog Modulation Schemes • Amplitude Modulation (AM) • Frequency Modulation (FM) • How to produce AM Signal?

32. Amplitude Modulation

33. Amplitude Modulation Modulating signal VAM(t) vS(t) cos C t vC Carrier Amplitude Carrier Frequency Carrier Signal: Message Signal or modulating signal: Modulated Signal: Modulation Index

34. Amplitude Modulation • Modulation Index M is determined by the peak amplitudes of the carrier and the modulating signal. • In practice, carrier signal amplitude vC is usually fixed and the M ratio is changed by varying the amplitude of the modulating signal vS. • Hence, higher vS produce higher M but M < 1. • M is kept as high as possible to ensure good SNR of the received AM signal for recovery. • When M > 1, over-modulated carrier signal distorts the information – Clipping or saturation.

35. Amplitude Modulation Envelope of the modulated signal has the same shape with the message signal. Envelope is distorted Illustrating the amplitude modulation process. (a) Baseband signal vs(t). (b) AM wave for M < 1 for all t. (c) AM wave for M > 1 for some t.

36. AM Signal: where modulating signal: Thus, Double-side band components Carrier Component (DSB) Wasted energy in carrier component because it contains no information (LSB) (USB) Lower side band Upper side band AM : Double-sided band (DSB) Action: To suppress the carrier

37. Amplitude Modulation • AM is the earliest type of modulation in history. • Its main advantage is its simplicity. – linear modulation technique • AM is wasteful in power consumption. Although the carrier signal does not carry any information, it is still transmitted. • AM is wasteful in bandwidth usage. The upper sideband is reflection of the lower sideband. One sideband is sufficient to express the frequency content of the message signal. Yet, AM still transmits one unnecessary sideband.

38. Spectrum of AM wave (a) Spectrum of AM Signal: both carrier and double-sided bands |v| f fS fc-fS fc fc+fS (b) Spectrum of Double-Sided Band - Carrier Suppression (DSB-SC) |v| f fc-fS fc fc+fS

39. Double Sideband-Suppressed Carrier Modulation (DSB-SC) Modulating signal DSB-SC signal vS(t) cos C t Carrier Frequency • Balance Modulator vS(t) DSB-SC Carrier Oscillator -90o vC(t) -90o

40. Double Sideband-Suppressed Carrier Modulation (DSB-SC) modulating signal: DSB-SC signal:

41. Single Sideband-Suppressed Carrier Modulation (SSB-SC) Sideband filter (crystal filter) Modulating signal SSB-SC signal DSB-SC signal vS(t) cos C t Carrier Frequency Bandpass filter applied at the DSB-SC signal to generate SSB-SC signal. Problem: It is very difficult and costly to design a bandpass filter that is sharp enough to select only one sideband !

42. Demodulation of AM signal Modulating signal DSB-SC signal cos C t Carrier Frequency • Balance Modulator vS(t) DSB-SC Carrier Oscillator -90o vC(t) -90o

43. Demodulation of AM signal • Most basic: Envelope detector (for AM signal only) Charging/Discharging voltage AM signal Cc – vAM(t) + vs(t) diode D To remove DC component & smoothen vs(t) C R • As VAM(t) increases in amplitude, the diode conducts (forward bias) and capacitor C start to charge-up very quickly to 1st peak vp1 with a time constant  = Cr, where r is the diode’s forward resistance (usually very small when diode is conducting). • As VAM(t) decreases in amplitudes, the diode switch-off (reverse bias) and capacitor C start to discharge slowly with a time constant  = CR, where R must be greater than r. • When VAM(t) increases again, D conducts and C charges up rapidly to 2nd peak vp2 and when VAM(t) decreases again D is off and C discharges slowly and this is repeated according to the amplitude of VAM(t) signal. • If CR is too small, C discharge too rapidly; results in ripple amplitude in demodulated output. • If CR is too large, C discharge too slowly; vs(t) fails to follow the envelope results in distortion (or diagonal clippling) in demodulated output. • Hence, time constant  must be optimum.

44. Optimum AM Demodulation Ripple amplitude in AM Demodulation – RC too small Diagonal Clipping/distortion in AM Demodulation – RC too large

45. |v| f fc-fS fc fc+fS fS Demodulation of (DSB-SC) signal (1) • Synchronous detection Low Pass Filter Recovered modulating signal DSB-SC signal cos C t local oscillator • Local oscillator produce the exactly coherent oscillation output that is synchronized with the original carrier in both frequency and phase. • The output is then filter by low-pass filter that only allowed the desired signal to pass through. Desired signal Unwanted signal

46. Demodulation of (DSB-SC) signal (2) • Costal Loop / Phase Lock Loop (PLL) Output: LPF DSB-SC Loop filter VCO -90o -90o LPF • The frequency fc is know a priori to the demodulator and generated by the voltage control oscillator, VCO. • PLL circuit (VCO + Loop filter) try to lock the phase so that local oscillation is synchronized with original fc.of the DSBSC signal. • Once synchronization is achieved, the difference in phase will be eliminated, thereby, recover the modulating signal.

47. Frequency Modulation (FM) vs fC fi In FM, the information is conveyed by varying the frequency of the carrier signal fC in step with the instantaneous amplitude of the modulating signal vs.

48. Frequency Modulation (FM) • FM signal is produced by a frequency modulator which converts the voltage variation in the modulating signal vs to a frequency variation of the carrier signal • The “instantaneous” frequency fi is the sum of carrier frequency fC and the “frequency deviation” as the result of the ‘amplitude-frequency’ conversion. fi vs(t) Frequency Modulator • When no modulating signal is applied, the output frequency is the same as the carrier frequency since ; no deviation is observed. • When a modulating signal is applied, the instantaneous output frequency fi will start to vary/deviate from fc with the amount of . • The conversion can be seen from the graph fi kf = fc Conversion gain vs 0

49. Constant speed Carrier Signal: 2 1 FM Modulated Signal: Angular displacement : Fact in FM : (Distance = speed × time) Instantaneous frequency fi of the cosine wave is: Instantaneous angular displacement varying speed Frequency Modulation (FM)

50. General FM signal can be expressed as: where i is the instantaneous angular displacement: • Recall that instantaneous frequency iof FM: • hence i can be re-written as: FM modulation index: Frequency Modulation (FM)