Upload
stella-blair
View
218
Download
2
Embed Size (px)
Citation preview
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 fc
Modulating baseband signal m(t) can be analog or digital signal
tfj cetgts 2)(Re)(
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
)]([)( where
)]()([)]([)( 21
tgfG
ffGffGtsfS cc
)]()([)( 41
cgcgvffPffPfP
ECE 4710: Lecture #19 3
Amplitude Modulation
Complex AM envelope given by Ac 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(2fc 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)
)](1[)( tmAtg c
tftmAtftgts ccc 2cos)](1[2cos)()(
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:
Amin 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
ECE 4710: Lecture #19 5
AM Signal Spectrum
AM Bandpass Spectrum )]([)( tsfS
)]()()()([)( 21
ccccc ffMffffMffAfS
LSB + USB = DSB
LC
RF BW = 2 baseband
signal BW = 2 B
RF BW
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 Ac [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 recoveryVery simple and inexpensive Rx circuitry
1002
)](min[)](max[100
2 % Modulation minmax tmtm
A
AA
c
ECE 4710: Lecture #19 7
AM Modulation %
If Mod % > 100% then m(t) < -1 and Ac[1 + m(t)] < 0 Overmodulation The carrier will have an instantaneous phase change of 180° :
Ac cos(2fc t) Ac cos(2fc 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
1)( if
1)( if
,0
,)2cos()](1[)(
tm
tmtftmAts cc
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
])()(21[)()(21|)(| 222122
212 tmtmAtmtmAtsP ccAM
)(22212
21 tmAAP ccAM
CarrierPower
SidebandPower
%100)(1
)(2
2
tm
tmE
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
Vin > Vout, Diode ON, C charges
Vin < Vout, Diode OFF, C
discharges slowly thru R
Vin > Vout, Diode turns back ON,
C charges again until Vin < Vout
Produces “noisy” representation of g(t) envelope carrier + m(t) !!
ECE 4710: Lecture #19 10
Simple AM Rx
)(ts )(ts )(tm
Antenna
EnvelopeDetector LPF
AnalogOutput )(tmHPF
)(ts )(ts )(ts
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 (Ac)
)(ts
ECE 4710: Lecture #19 11
Simple AM Rx Circuit
Antenna
EnvelopeDetector LPF
AnalogOutput )(tmHPF
)(ts )(ts )(ts
EnvelopeDetector
LPF HPF
)(ts
)(ts )(ts
)(tm
1C 2C3C
1R2R
3R
ECE 4710: Lecture #19 12
AM Signal Spectrum
f +fc
fc0
AM Spectrum@ Rx Input
f +fc fc 0
AM Spectrumafter
Envelope Detector
High Frequency
“Noise”
f0
AM Spectrumafter LPF
f
AM Spectrumafter HPF
0
Carrier DC removed but
low frequency distortion
introduced!!B
B
BB
B
B
cfRCB
21
LPF RC :
ECE 4710: Lecture #19 13
AM Signal Spectrum
AM Spectrum AFTER HPF Must remove DC component
caused by carrier (Ac) 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
f0 BB
1 1 0 1 0 0 1PSD for
Polar NRZ
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
tftmAts cc 2cos)](1[)(
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
ECE 4710: Lecture #19 16
AM Tx from PWM
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
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