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

+

-

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

+

-

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

S. Mandayam/ ECOMMS/ECE Dept./Rowan University

SummarySummary