Lecture 4 1
The AM Radio
Lecture 4 2
The AM Radio
• Understanding the AM radio requires knowledge of several EE subdisciplines:– Communications/signal processing (frequency
domain analysis)– Electromagnetics (antennas, high-frequency
circuits)– Power (batteries, power supplies)– Solid state (miniaturization, low-power
electronics)
Lecture 4 3
The AM Radio “System”
Transmitter Receiver
Lecture 4 4
Signal
• The radio system can be understood in terms of its effect on signals.
• A signal is a quantity that may vary with time.– Voltage or current in a circuit– Sound (pressure wave traveling through air)– Light or radio waves (electromagnetic energy
traveling through free space)
Lecture 4 5
Frequency
• The analysis and design of AM radios (and communication systems in general) is usually conducted in the frequency domain using Fourier analysis.
• Fourier analysis allows us to represent signals as combinations of sinusoids (sines and cosines).
Lecture 4 6
Frequency
Frequency is the rate at which a signal oscillates.
High Frequency Low Frequency
Lecture 4 7
Electromagnetic Waves
• Visible light is electromagnetic energy with frequency between 380THz (Terahertz) and 860THz.– Our visual system perceives the frequency of the
electromagnetic energy as color.– Red is 460THz, green is 570THz, and blue is
630THz.• An AM radio signal has a frequency of between
500kHz and 1.8MHz.• FM radio and TV uses different frequencies.
Lecture 4 8
Sound Waves
• Sound is a pressure wave in a transmission medium such as air or water.
• We perceive the frequency of the wave as the “pitch” of the sound.
• A single frequency sound sounds like a clear whistle.
• Noise (static) is sound with many frequencies.
Lecture 4 9
Fourier Analysis
• Mathematical analysis of signals in terms of frequency
• Most commonly encountered signals can be represented as a Fourier series or a Fourier transform.
• A Fourier series is a weighted sum of cosines and sines.
Lecture 4 10
Example-Fourier Series
Square wave
Fourier Series representation of the square wave
tkkk
]24[cos)12(
41
Lecture 4 11
Fourier Series Example (Cont.)One term
Five terms
Lecture 4 12
Frequency-Summary
• Signals can be represented in terms of their frequency components.
• The AM transmitter and receiver are analyzed in terms of their effects on the frequency components signals.
Lecture 4 13
AM Transmitter
• Each AM station is allocated a frequency band of 10kHz in which to transmit its signal.
• This frequency band is centered around the carrier frequency of the station– A station at 610 on your dial transmits at a
carrier frequency of 610kHz– The signal that is broadcast occupies the
frequency range from 605kHz to 615kHz
Lecture 4 14
AM Transmitter
• Transmitter input (signal source) is an audio signal.– Speech, music, advertisements
• The input is modulated to the proper carrier frequency.
• Modulated signal is amplified and broadcast
Lecture 4 15
Transmitter Block Diagram
Signal
SourceModulator
Power
Amplifier
Antenna
Lecture 4 16
Modulator
The modulator converts the frequency of the input signal from the audio range (0-5kHz) to the carrier frequency of the station (i.e.. 605kHz-615kHz)
frequency5kHz
Frequency domain representation of input
Frequency domain representation of output
frequency610kHz
Lecture 4 17
Modulator-Time DomainInput Signal
Output Signal
Lecture 4 18
Power Amplifier
• A typical AM station broadcasts several kW– Up to 50kW-Class I or class II stations– Up to 5kW-Class III station– Up to 1kW-Class IV station
• Typical modulator circuit can provide at most a few mW
• Power amplifier takes modulator output and increases its magnitude
Lecture 4 19
Antenna
The antenna converts a current or a voltage signal to an electromagnetic signal which is
radiated throughout space.
Lecture 4 20
AM Receiver
• The AM receiver receives the signal from the desired AM station as well a signals from other AM stations, FM and TV stations, cellular phones, and any other source of electromagnetic radiation.
• The signal at the receiver antenna is the sum of all of these signals (superposition).
• The AM receiver separates the desired signal from all other received signals using its frequency characteristics.
Lecture 4 21
AM Receiver
• We present a superhetrodyne receiver-this is the type used in most modern radio and TV receivers.
• The desired signal is first translated to an Intermediate Frequency (IF).
• The desired signal is then recovered by a demodulator.
Lecture 4 22
Receiver Block Diagram
RF
Amplifier
IF
Mixer
IF
Amplifier
Envelope
Detector
Audio
Amplifier
Antenna
Speaker
Lecture 4 23
Antenna
• The antenna captures electromagnetic energy-its output is a small voltage or current.
• In the frequency domain, the antenna output is
0 frequency
Undesired SignalsDesired Signal
Carrier Frequencyof desired station
Lecture 4 24
RF Amplifier
• RF stands for radio frequency.• RF Amplifier amplifies small signals from the
antenna to voltage levels appropriate for transistor circuits.
• RF Amplifier also performs a bandpass filter operation on the signal– Bandpass filter attenuates the frequency
components outside the frequency band containing the desired station
Lecture 4 25
RF Amplifier-Frequency Domain
• Frequencies outside the desired frequency band are attenuated
• Frequency domain representation of the output:
0 frequency
Undesired SignalsDesired Signal
Carrier Frequencyof desired station
Lecture 4 26
IF Mixer
• The IF Mixer shifts its input in the frequency domain from the carrier frequency to an intermediate frequency of 455kHz:
IF Mixer
0 frequency
Undesired Signals
Desired Signal
455 kHz
Lecture 4 27
• The IF amplifier bandpass filters the output of the IF Mixer, eliminating essentially all of the undesired signals.
IF Amplifier
0 frequency
Desired Signal
455 kHz
Lecture 4 28
Envelope Detector
• Computes the envelope of its input signal
Lecture 4 29
Audio Amplifier
• Amplifies signal from envelope detector
• Provides power to drive the speaker
Lecture 4 30
Hierarchical System Models
• Hierarchical modeling is modeling at different levels of abstraction
• We can “divide and conquer”• Higher levels of the model describe overall
function of the system• Lower levels of the model describe detail
necessary to implement the system
Lecture 4 31
Systems in EE
• In EE, a system is an electrical and/or mechanical device, a process, or a mathematical model that relates one or more inputs to one or more outputs.
• In the AM receiver, the input is the antenna voltage and the output is the sound energy produced by the speaker.
SystemInputs Outputs
Lecture 4 32
Top Level Model
AM ReceiverInput Signal Sound
Lecture 4 33
Second Level Model
RF
Amplifier
IF
Mixer
IF
Amplifier
Envelope
Detector
Audio
Amplifier
Antenna
Speaker
Power Supply
Lecture 4 34
Low Level ModelEnvelope Detector.
Half-wave
Rectifier
Low-pass
Filter
Lecture 4 35
Circuit Level ModelEnvelope Detector
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