1. Amplitude Modulated Radio Frequency Transmission System Instructor: Dr. Fu By: Megan Myles, David Jackson, and Edwin Wambwa

2. Introduction An amplitude modulated radio frequency transmission system consists of a modulator in which a sinusoidal high frequency carrier waveform cos(2πf c t) is amplitude modulated (AM) by a lower frequency signal v m (t), containing the information to be transmitted. This is also known as the intelligence signal.

3. Modulation There is only one sine wave at a given frequency, so how does the information get carried at a particular frequency? Modulation = Encoding information on a signal Analog radio modulation technologies: –FM = Frequency Modulation –AM = Amplitude Modulation –PM = Phase Modulation

4. AM Modulation

5. AM/FM Difference But AM can transmit over longer distances because AM frequencies bounce off ionosphere and diffract around hills and buildings but FM frequencies are absorbed, causing “shadows” PM is analogous to FM.

6. Basic Principles of AM Radio Transmissions Am Modulator/transmitter Tuned Circuit Receiver RF Amplifier Peak or envelope detector Audio Frequency Transistor Amplifier Frequency Spectrum of a AM signal

7. AM Block Diagram Project Plan

8. LC Tuned Circuit Inductor = 8.2 µH Capacitor =.33 µF Resonant Freq. = 96.75 kHz Resistor = 1 kOhm

9. AM Transmitter Block.

10. Project Schematics LC resonant frequency = 1Vp-p 96.75117220964837 kHz Intelligence signal = 5 kHz, -1Vdc offset, 400mVp-p

11. AM Waveform Key Yellow- Carrier Signal Blue- Intelligence Signal Purple- Transmitter output modulated waveform

12. AM Frequency Spectrum Key Cursor 1- Center Frequency 96.75kHz Cursor 2- Upper Sideband 101.7kHz

13. Transmitted AM signal 3dB cutoff difference between center and sidebands

14. AM Receiver Circuit

15. Peak (Envelope) Detector. Key 2πf c > 1/R 3 C 2 > 2πf i 607905.5438 > 40000 > 31415.92654 R3 = 2.5 kOhms C2 =.01 µF

16. Trouble Shooting. AM envelope disappeared when transmitter was connected to receiver Used resistors as a simulated test resistance for antenna Receiver output signal obtained Used potentiometer to fine tune antenna resistance Receiver output matched original intelligence input

17. Complete System/Recovered Intelligence. Key Yellow=carrier Input (1Vp-p, 96.75117220964837kHz) Blue=intelligence input (400mVp-p, 5kHz, -1Vdc offset) Purple= amplitude modulated transmitter output (20Vp-p, 96.75117220964837kHz) Green=receiver output (400mVp-p, 5kHz, 90 degrees out of phase with original intelligence)

18. Project cost Components and Parts: $55 Some components were acquired from Yomi’s stock pile. Man Hours:Approx 60Hrs.

19. Challenges. Sound output after designing a higher gain amplifier circuit (we designed for a single toned sound output). Signal transmission using aerials instead of hard wiring the circuit. Implementation of a tuning circuit (AFC) in order to receive an audible signal

20. Conclusions. The experiment was well understood and gave us a better insight into our understanding of AM and FM. We implemented similar labs throughout the course, but were able to combine methods of radio transmission into a complete transmission system.

21. Fin.