Electrical Communications Systems 0909.331.01 Spring 2005

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Electrical Communications Systems0909.331.01
Spring 2005
Lecture 12aApril 26, 2005

• Shreekanth Mandayam
• ECE Department
• Rowan University
• http//engineering.rowan.edu/shreek/spring05/ecom
  ms/

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Plan

• 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

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ECOMMS Topics
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AM Broadcast Transmitter
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Amplifier Classification
Class B
Class A
Class C
Class AB
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BJT Collector Modulator
Output Characteristics
Circuit
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BJT Collector Modulator
Operation
Circuit
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AM Receiver

• Purpose
• Demodulate received signal
• Requirements
• Carrier frequency tuning
• Filtering
• Amplification

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Tuning Circuits
Series Tuned Circuit
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Series Tuned Circuit
function f,Aresonance(f0,Q) ECOMMS Spring 00
Class Demo S. Mandayam, ECE Dept., Rowan
University To illustrate series resonance and
Q-factor close all defining frequency
axis ff0-f0/20.1f0f0/2 calculating relative
response y(f/f0)-(f0./f) A20log10(1./(1(yQ).
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')
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Tuning Circuits
Parallel Tuned Circuit
I

r
V
C
L
-
Active Tuned Circuits?
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Tuned Radio Frequency (TRF) Receiver
Active Tuning Circuit
Detector Circuit
Bandpass Filter
Baseband Audio Amp
Local Oscillator
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Heterodyning
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Superheterodyne Receiver
fm
fm
fIF
fIF
RF Amplifier H1(f)
IF Amplifier H2(f)
Audio Amp
Mixer
Detector
fRF or fc
Local Oscillator
fLO
Common tuning
Downconversion fIF fRF - fLO
Upconversion fIF fRF fLO
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Performance of Communications Systems Corrupted
by Noise
Analog Output SNR
Digital Bit Error Rate (BER)
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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?

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Why?
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Power Spectral Density (PSD)

• Normalized power of a waveform in the frequency
  domain
• Used for measuring signal/noise power
  loss/transfer in communications system blocks

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Autocorrelation Function

• Measure of
• similarity of a waveform observed at times t
  seconds apart
• how rapidly a random waveform fluctuates with time

Matlab Demo autocorr.m
Wiener-Khintchine Theorem
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Thermal (Johnson) Noise
Matlab script psd_noise.m
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Summary