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Electrical Communications Systems 0909.331.01 Spring 2005

Electrical Communications Systems 0909.331.01 Spring 2005. Lecture 12a April 26, 2005. Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring05/ecomms/. Plan. Broadcast Transmitters and Receivers AM Broadcast Transmitter

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Electrical Communications Systems 0909.331.01 Spring 2005

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  1. Electrical Communications Systems0909.331.01Spring 2005 Lecture 12aApril 26, 2005 Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring05/ecomms/

  2. 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

  3. ECOMMS: Topics

  4. Buffer Amplifier Impedance Matching Network Temp. Stabilized Crystal Oscillator Class-C Modulated Power Amp. Stage Class-C Modulated Power Amp. Stage Demodulator + Modulator Driver Amplifier Audio Amplifier Audio Input S - AM Broadcast Transmitter

  5. ic p 2p wt ic ic ic p 2p wt p 2p p 2p wt wt Amplifier Classification Class B Class A Class C Class AB

  6. BJT Collector Modulator Output Characteristics Circuit

  7. BJT Collector Modulator Operation Circuit

  8. AM Receiver • Purpose • Demodulate received signal • Requirements • Carrier frequency tuning • Filtering • Amplification

  9. I + C r V L - Tuning Circuits Series Tuned Circuit

  10. Series Tuned Circuit function [f,A]=resonance(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 f=f0-f0/2:0.1:f0+f0/2; %calculating relative response y=(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');

  11. Tuning Circuits Parallel Tuned Circuit I + r V C L - Active Tuned Circuits?

  12. Tuned Radio Frequency (TRF) Receiver Active Tuning Circuit Detector Circuit Bandpass Filter Baseband Audio Amp Local Oscillator

  13. All Incoming Frequencies Fixed Intermediate Frequency Heterodyning (Upconversion/ Downconversion) Subsequent Processing (common) Heterodyning

  14. 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

  15. Performance of Communications Systems Corrupted by Noise Analog Output SNR Digital Bit Error Rate (BER)

  16. 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?

  17. |W(f)| |W(f)| 0 fc -fc f Bandpass 0 2. Demodulation 1. Modulation f Baseband |W(f)| 3. Demodulation -fIF fIF 0 f Bandpass Why?

  18. 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

  19. F Rw(t) Pw(f) Rx(t) Slowly fluctuating signal rapidly fluctuating signal t (time delay) 0 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

  20. -21 PSD of thermal noise x 10 2 1.5 PSD, W/Hz 1 0.5 0 0 0.5 1 1.5 2 2.5 3 frequency, Hz 12 x 10 Thermal (Johnson) Noise Matlab script: psd_noise.m

  21. Summary

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