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Understanding AM Broadcast Transmitters and Superheterodyne Receivers

This lecture covers essential concepts in electrical communications, focusing on AM broadcast transmitters and receivers. Key topics include the Class-C amplifier, collector modulator, TRF (Tuned Radio Frequency) receiver, and the superheterodyne receiver. The discussion highlights the function and requirements of demodulation, carrier frequency tuning, and amplification. Additionally, tuning circuits and their roles in frequency response and Q-factor are demonstrated. This lecture provides a comprehensive overview for students in Electrical and Communications Engineering.

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Understanding AM Broadcast Transmitters and Superheterodyne Receivers

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  1. Electrical Communications SystemsECE.09.331Spring 2008 Lecture 14aApril 29, 2008 Shreekanth Mandayam ECE Department Rowan University http://engineering.rowan.edu/~shreek/spring08/ecomms/

  2. Plan • Broadcast Transmitters and Receivers • AM Broadcast Transmitter • Class-C Amp Collector Modulator • TRF Receiver • Superheterodyne Receiver

  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 08 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. Summary

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