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.