Radio Communication

SL – Option F.1

• Radio communication includes any form of communication that uses radio (EM) waves to transfer information– TV, mobile phones, wireless internet, satelite

radio

Radio Waves

• Electromagnetic wave that is created by an alternating (AC) current– Made up of

perpendicular electric and magnetic fields

Radio communication

• Principle– A changing electric field must be created in

order to produce a radio wave• Simplest oscillating circuit is made up of a coil of

wire and a capacitor• The circuit will oscillate at a given frequency based

on the turns in the coil and the size of the capacitor• When the electrons oscillate, the waves are sent

out in all directions– This is called a transmitter

Radio Communication

• A second oscillator circuit, placed away from the transmitter will detect the signal– Electrons in the circuit will respond to the EM

waves and an alternating current will be produced

• The amplitude of this oscillation is generally very small compared to the sent signal

Modulation

• In order to send information we need more than just a signal– A single signal frequency would just be a tone

• The frequency needs to be changed or modulated to represent the change in data to carry an actual signal– Morse code was simply turning the signal on

and off

Modulation

• Amplitude and frequency modulation are accomplished by superimposing a second wave upon the first– Carrier wave

• The means by which the information is transmitted

– Signal wave • Contains information or

message to be transmitted

Practice Problem

• A carrier wave is modulated by a single signal wave. As a result of the modulation, a maximum amplitude of the carrier wave occurs every 2.3ms. Between each maximum there are 2.1 x 10^5 complete oscillations of the carrier wave. Determine the frequency of the signal wave and of the carrier wave.

Practice Problem Solution

• For the signal wave

• 1/fs = 2.3x10^-3 s

• 435Hz

• For the carrier wave

• Fc = (2.1x10^5)/(2.3x10^-3)

• 91MHz

Simple sinusoidal wave

• y= A sin(2pi f t) – A = amplitude– f = frequency

Power Spectrum

• This is the plot of A^2 vs f

• Ex y = 10sin(200pi t) + 5sin(400pi t)

Power

Frequency (Hz)

100 200

What is the frequency of each?

Amplitude Modulation• The frequency of the carrier wave is

constant

• The signal wave is used to vary the amplitude of the carrier wave

Side frequency

• When a carrier wave is modulated by a signal wave, 2 additional waves are actually produced– This can be shown mathematically but it is not

necessary at this point– The 2 additional frequencies are then:

c s

c s

f f

f f

Side frequency

• This produces what is known as a power spectrum– A chart showing the different frequencies that

make up a signal

Bandwidth• It is defined as the range of frequencies in the

modulated signal• Signals, especially more complex ones (radios)

often need more than one frequency to transmit information– With these and their sidebands, a band of frequencies

gets taken up by each signal, this is known as bandwidth

• Radio stations are allocated a 9 kHz bandwidth• Phones are give 3 kHz• Analog TV signals get 6 MHz since they need to transmit

video as well

Side bands• If the signal wave has multiple frequencies, you

have a range of frequencies to deal with

• Sidebands– Lower sideband

• (fc-fH) to (fc-fL)

– Upper sideband• (fc+fH) to (fc+fL)

• Bandwidth for multiple frequencies• Δf = (fc+fH) – (fc-fH)• =2fH

Practice Problem

• A carrier wave of frequency 535 kHz is modulated by frequencies in the range 50Hz to 5.0kHz. Determine the frequencies transmitted and the bandwidth.

Max and Min Amplitude

• (Ac + As) – (Ac – As) = 2As

• Modulated wave has a total A of 2As

Simple AM transmitter

Frequency Modulation

• The amplitude of the carrier wave is kept constant

• The signal wave is used to vary the carrier waves frequency

Frequency Modulation

• In an FM signal, the carrier wave deviates from its original frequency f by an amount f– This is known as the peak frequency deviation– Δf = f – fc

• f = maximum frequency of modulated carrier wave• fc = maximum frequency of carrier wave

Frequency Modulation

• An FM signal also has a modulation index– If the carrier signal f is modulated by Δf:

• B = Δf/fI– Where fI is the highest frequency in the carrier signal

• The larger the modulation index of an FM wave, the more sidebands are produced and the wider the bandwidth– In FM signals the bandwidth is equal to:

2 ( + mod )BW x peak deviation highest ulating frequency

FM Transmissions

• In typical FM transmissions Δf = 75kHz and B is high, usually 5.

• A high quality FM radio station has a frequency deviation of 75kHz and contains audio signals varying from 50 Hz to 15 kHz. What is the modulation index and the bandwidth of the FM transmissions?

Comparing AM and FM

• Bandwidth– AM uses 9kHz per channel while FM uses

200 kHz• This means that FM stations are farther apart and

also why they use higher frequencies

• Range– AM signals have a much larger range than FM

due to the lower frequencies• AM waves are reflected off of the ionosphere while

FM signals shoot off into space

Comparing AM and FM

• Quality– Other sources of EM waves, which are all

around us, affect AM waves much more significantly

• This is why AM stations are more susceptible to noise

• Cost– A simple AM receiver can be built very

cheaply, while FM receivers are more complex and costly

AM Receiver• Aerial – the antennae, a long conducting rod that EM

waves can oscillate electrons in• Tuning Circuit – A circuit that can be ‘tuned’ to resonate

at a particular frequency• r.f. (radio frequency) amplifier – amplifies the tuned

signal• Demodulator – removes the carrier wave from the signal• a.f. (audio frequency)

amplifier – amplifies the audio signal

• Speaker