amplitude modulated radio frequency transmission system instructor: dr. fu by: megan myles, david...
TRANSCRIPT
Amplitude Modulated Radio Frequency Transmission System
Instructor: Dr. FuBy: Megan Myles, David Jackson,
and Edwin Wambwa
Introduction
An amplitude modulated radio frequency transmission system consists of a modulator in which a sinusoidal high frequency carrier waveform cos(2πfct) is amplitude modulated (AM) by a lower frequency signal vm(t), containing the information to be transmitted. This is also known as the intelligence signal.
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
AM Modulation
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.
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
AM Block DiagramProject Plan
LC Tuned Circuit
• Inductor = 8.2 µH• Capacitor = .33 µF• Resonant Freq. =
96.75 kHz• Resistor = 1 kOhm
mA
mA
mA
LR
R
CR
1I
2I
3I
AM Transmitter Block
.
Project Schematics
• LC resonant frequency = 1Vp-p 96.75117220964837 kHz
• Intelligence signal = 5 kHz,
-1Vdc offset, 400mVp-p
AM Waveform
Key
• Yellow- Carrier Signal
• Blue- Intelligence Signal
• Purple- Transmitter output modulated waveform
AM Frequency Spectrum
Key
• Cursor 1- Center Frequency 96.75kHz
• Cursor 2- Upper Sideband 101.7kHz
Transmitted AM signal 3dB cutoff difference between center and
sidebands
AM Receiver Circuit
Peak (Envelope) Detector
. Key
•2πfc > 1/R3C2 > 2πfi
•607905.5438 > 40000 > 31415.92654
•R3 = 2.5 kOhms
•C2 = .01 µF
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
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)
Project cost
Components and Parts: $55Some components were
acquired from Yomi’s stock pile.
Man Hours:Approx 60Hrs
.
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
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.
Fin.