S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Electrical Electrical Communications SystemsCommunications Systems
0909.331.010909.331.01 Spring 2005Spring 2005
Shreekanth MandayamECE Department
Rowan University
http://engineering.rowan.edu/~shreek/spring05/ecomms/
Lecture 12aLecture 12aApril 26, 2005April 26, 2005
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
PlanPlan• Broadcast Transmitters and Receivers
• AM Broadcast Transmitter• Class-C Amp Collector Modulator• TRF Receiver• Superheterodyne Receiver
• Performance of Comm. Systems corrupted by Noise• Performance Measures: Digital and Analog• Thermal (Johnson) Noise
• Amplitude: Recall Random Variables: lab1.ppt• Power Spectral Density• Autocorrelation function• Wiener-Khintchine Theorem
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
ECOMMS: TopicsECOMMS: Topics
Probability
Inform ation
Entropy
Channel Capacity
Discrete
Pow er & Energy Signals
Continuous Fourier Transform
Discrete Fourier Transform
Baseband and Bandpass Signals
Com plex Envelope
Gaussian Noise & SNR
Random VariablesNoise Calculations
Continuous
Signals
AMSw itching M odulator
Envelop Detector
DSB-S CProduct M odulatorCoherent Detector
Costas Loop
SSBW eaver's MethodPhasing M ethod
Frequency M ethod
Frequency & Phase M odulationNarrowband/WidebandVCO & Slope Detector
PLL
Analog
Source EncodingHuffm an codes
Error-control EncodingHam m ing Codes
Sam plingPAM
QuantizationPCM
Line Encoding
Tim e Division M uxT1 (DS1) Standards
Packet Sw itchingEthernet
ISO 7-Layer Protocol
BasebandCODEC
ASKPSKFSK
BPSK
QPSK
M -ary PSK
QAM
BandpassM ODEM
DigitalDigital Com m Transceiver
Systems
Electrical Comm unication Systems
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
AM Broadcast TransmitterAM Broadcast Transmitter
Temp.StabilizedCrystal Oscillator
BufferAmplifier
Class-CModulatedPower Amp.Stage
Class-CModulatedPower Amp.Stage
ImpedanceMatchingNetwork
AudioAmplifier
ModulatorDriverAmplifier
Demodulator
AudioInput
+
-
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Amplifier ClassificationAmplifier Classification
ic
t
ic
t
ic
t
ic
t
Class A Class B
Class AB Class C
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
BJT Collector ModulatorBJT Collector Modulator
Circuit
Output Characteristics
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Circuit
OperationBJT Collector ModulatorBJT Collector Modulator
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
AM ReceiverAM Receiver
• Purpose• Demodulate received signal
• Requirements• Carrier frequency tuning • Filtering• Amplification
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Tuning CircuitsTuning CircuitsSeries Tuned Circuit
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Vr
C
L
I
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Series Tuned CircuitSeries Tuned Circuitfunction [f,A]=resonance(f0,Q)%ECOMMS Spring 00 Class Demo%S. Mandayam, ECE Dept., Rowan University%To illustrate series resonance and Q-factorclose all;%defining frequency axisf=f0-f0/2:0.1:f0+f0/2;%calculating relative responsey=(f/f0)-(f0./f);A=20*log10(1./(1+(y*Q).^2).^0.5);plot(f-f0,A);grid on;xlabel('Shift from resonant frequency in Hz');ylabel('Relative Response in dB');title('Series Tuning Circuit');
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Tuning CircuitsTuning CircuitsParallel Tuned Circuit
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-
Vr
C
L
I
Active Tuned Circuits?
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Tuned Radio Frequency (TRF) Tuned Radio Frequency (TRF) ReceiverReceiver
ActiveTuningCircuit
DetectorCircuit
LocalOscillator
BandpassFilter
BasebandAudio Amp
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
HeterodyningHeterodyning
Heterodyning(Upconversion/Downconversion)
SubsequentProcessing(common)
AllIncomingFrequencies
FixedIntermediateFrequency
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Superheterodyne ReceiverSuperheterodyne Receiver
RFAmplifier
H1(f)Mixer
LocalOscillator
IFAmplifier
H2(f)Detector Audio
Amp
Commontuning
fRF
or fc
fLO
fIF fIFfm fm
Downconversion
fIF = |fRF - fLO|
Upconversion
fIF = fRF + fLO
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Performance of Communications Performance of Communications Systems Corrupted by NoiseSystems Corrupted by Noise
DigitalBit Error Rate (BER)
AnalogOutput SNR
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Noise Noise
• A random, unwanted fluctuation in signal amplitude
• Thermal (Johnson) Noise• Amplitude vs. time: Gaussian PDF Model
• See Lab1 Pre-lab Lecture: lab1.ppt
• We also want to know how much noise power there is per Hz – why?
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Why?Why?
f
|W(f)|
Baseband0
f
|W(f)|
Bandpass
0 fc-fc
f
|W(f)|
Bandpass
0 fIF-fIF
1. Modulation 2. Demodulation
3. Demodulation
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Power Spectral Density Power Spectral Density (PSD)(PSD)
• Normalized power of a waveform in the frequency domain
• Used for measuring signal/noise power loss/transfer in communications system blocks
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Autocorrelation FunctionAutocorrelation Function• Measure of
• similarity of a waveform observed at times seconds apart• how rapidly a random waveform fluctuates with time
0
Rx()
(time delay)
Slowly fluctuating signal
rapidly fluctuating signal
Rw() Pw(f) FWiener-Khintchine Theorem
Matlab Demo:autocorr.m
S. Mandayam/ ECOMMS/ECE Dept./Rowan University
Thermal (Johnson) NoiseThermal (Johnson) Noise
0 0.5 1 1.5 2 2.5 3
x 1012
0
0.5
1
1.5
2
x 10-21 PSD of thermal noise
frequency, Hz
PS
D,
W/H
z
)1(2)(
/
kThfne
hffP
Matlab script:psd_noise.m