1. ECE 4710: Lecture #19 1 Bandpass Review  Modulated bandpass signal  where g (t) is complex envelope of baseband signal  Desired modulated signal, s (t), obtained by: 1)Selection of appropriate modulation mapping function g [m(t)] and 2) Multiplying g (t) by sinusoidal carrier with frequency f c  Modulating baseband signal m(t) can be analog or digital signal

2. ECE 4710: Lecture #19 2 Bandpass Review  Spectrum of bandpass signal is  Frequency translated version of baseband spectrum  PSD of bandpass signal is  Chapter 5 goals:  Study g(t) and s(t) for various types of analog & digital modulation methods »AM, DSB-SC, FM, OOK, BPSK, FSK, QPSK, MPSK, QAM  Evaluate spectrums and examine basic Tx and Rx structures

3. ECE 4710: Lecture #19 3 Amplitude Modulation  Complex AM envelope given by  A c is for the carrier power level  m(t) is modulating source signal  analog or digital »For AM  m(t) is normally considered to be analog audio signal  Bandpass AM signal is  If m(t)  -1 then amplitude of cos(2  f c t ) is always > 0  No negative values means complex envelope is purely real  g(t) = x(t) + j y(t) = x(t)  in-phase component only!!  Envelope of g(t) = | g(t) | = x(t)  baseband signal information completely represented by AM signal envelope »No need for **carrier phase information** in s(t) to correctly recover m(t) »Primary advantage for AM  m(t)  -1 is the normal case for AM (e.g., broadcast AM)

4. ECE 4710: Lecture #19 4 AM Signal Waveform m(t) assumed to be sinusoidal just for demonstration purposes Envelope of s(t) precisely represents m(t) if: A min  0 or m(t)  -1 Carrier signal phase in s(t) is the same throughout all amplitude variations of m(t) Only in phase component of s(t) needed to recover and in- phase signal = | g(t) | = baseband signal envelope

5. ECE 4710: Lecture #19 5 AM Signal Spectrum  AM Bandpass Spectrum  LSB + USB = DSB LC RF BW = 2  baseband signal BW = 2 B RF BW

6. ECE 4710: Lecture #19 6 AM Modulation %  AM modulation percentage  100% modulation if: max[ m(t) ] = +1 & min[ m(t) ] = -1  If Mod %  100% then m(t)  -1 and A c [1 + m(t) ] > 0 »Required by FCC for AM broadcast radio  Overmodulation of AM signal (% Mod > 100%) causes AM signal BW to be much larger than baseband signal BW if standard 2 quadrant mixer is used in Tx »Enables envelope detection in Rx for baseband signal recovery  Very simple and inexpensive Rx circuitry

7. ECE 4710: Lecture #19 7 AM Modulation %  If Mod % > 100% then m(t) < -1 and A c [1 + m(t) ] < 0  Overmodulation  The carrier will have an instantaneous phase change of 180° : A c cos(2  f c t )   A c cos(2  f c t )  g(t) is now complex and envelope of s(t) is no longer  m(t)  If a two-quadrant multiplier is used for generating s(t) in Tx then »Low values of m(t) are “clipped” »Distortion is introduced and signal BW is increased significantly »Not allowed by FCC for AM Broadcast  Four-quadrant multiplier could be used to successfully generate overmodulated AM signal »Must use product detector (mixer) NOT envelope detection in Rx to properly recover m(t)  Tx and Rx much more expensive

8. ECE 4710: Lecture #19 8 AM Signal Power  If modulating signal m(t) has no DC then  Modulation Efficiency  Highest AM efficiency is only 50%  best case!!  Other power “wasted” on carrier »Needed for envelope detection but it does NOT improve S/N @ Rx Carrier Power Sideband Power

9. ECE 4710: Lecture #19 9 Envelope Detection Simple diode circuit that takes RF input s(t) and produces output  envelope  | g(t) | 1 2 3 1 2 3 V in > V out, Diode ON, C charges V in < V out, Diode OFF, C discharges slowly thru R V in > V out, Diode turns back ON, C charges again until V in < V out Produces “noisy” representation of g(t) envelope  carrier + m(t) !!

10. ECE 4710: Lecture #19 10 Simple AM Rx Antenna Envelope Detector LPF Analog Output HPF Modulated Signal at Rx input Envelope Detector output has high frequency noise due to imperfect diode-RC response LPF produces smooth output by attenuating high frequency noise HPF output eliminates DC component due to carrier envelope ( A c )

11. ECE 4710: Lecture #19 11 Simple AM Rx Circuit Antenna Envelope Detector LPF Analog Output HPF Envelope Detector LPFHPF  

12. ECE 4710: Lecture #19 12 AM Signal Spectrum f +f c  f c 0 AM Spectrum @ Rx Input f +f c  f c 0 AM Spectrum after Envelope Detector High Frequency “Noise ” f 0 AM Spectrum after LPF f AM Spectrum after HPF 0 Carrier DC removed but low frequency distortion introduced!! B B B BB BB BB LPF RC :

13. ECE 4710: Lecture #19 13 AM Signal Spectrum  AM Spectrum AFTER HPF   Must remove DC component caused by carrier ( A c ) to recover m(t)  Distortion of low frequency portion of m(t) permanently introduced  Consequence of AM and simple Rx circuitry which is main advantage of AM  Distortion is OK for audio signals (human speech has no energy < 300 Hz)  Distortion is NOT OK for most data signals  causes ISI f 0 B BB 1 1 0 1 0 0 1 PSD for Polar NRZ

14. ECE 4710: Lecture #19 14 AM Transmitter  AM is a linear modulation method  Carrier envelope is linear representation of m(t)  AM Tx must preserve linearity of s(t) or signal will be distorted  One approach:  Generate low power s(t) and then use Class A or B linear amplifier for Tx power amplifier (PA) »Not efficient in converting DC power supply to RF energy  Typically 40-60% efficient »Energy lost to heat  Expensive for high power broadcast AM (5-20 kW stations)  Cooling of PA and other components also expensive

15. ECE 4710: Lecture #19 15 AM Transmitter  Better approach:  Amplify carrier signal only to very high level using efficient Class C or D power amplifiers »Typically 80-95% efficient  Amplitude modulate the high-power carrier by using m(t) to bias the DC supply of the PA »PA output signal will vary  to “DC” supply signal »“DC” signal is not really constant DC  variable  m(t)  One technique generates Pulse Width Modulating signal from m(t) »Pulse width is  to signal amplitude »Pass PWM through high power switch to generate high voltage signal »Use LPF to generate “DC” signal for PA

16. ECE 4710: Lecture #19 16 AM Tx from PWM

17. ECE 4710: Lecture #19 17 Amplitude Modulation  Primary Advantages: 1)Linear so that carrier envelope  m(t) for mod % < 100 2)Envelope detection enables simple Rx circuit for baseband signal recovery 3)Low cost Rx’s built 4)Efficient use of bandwidth  RF BW  2 B  Primary Disadvantages: 1)Signal power “wasted” on carrier 2)Poor Rx performance for low S/N ratios 3)High power, expensive Tx’s required for good S/N at Rx 4)Simple Rx with envelope detector cannot be used for data signals where line code PSD’s have signal power @ low frequencies

18. ECE 4710: Lecture #19 18 AM Big Picture Amplitude Modulation is: 1) Bandwidth Efficient 2) Power ( S/N ) Inefficient 3) Cannot be used for most digital/data signals