Telecommunications: Radio Transmission and Reception - Amplitude and Frequency Modulation , Study notes of Basic Electronics

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Phys 351, Fall 2008

Lecture 15 1

Telecommunications: Radio (and TV) transmission and reception

Frequency mixing, heterodyne-ing

Modulation and demodulation

Phys 351, Fall 2008

Lecture 15 2

Basic motivation and ideas

Audio frequencies are 20Hz < Δω < 20kHz, but difficult to transmit and

to receive directly because of interference from ambient EM.

Trick: use circuits tuned to specific RF bands (500kHz - 100MHz) which are waves

that are easy to transmit and mix in audio frequency waves

as modulation of the radio frequency signal.

Two main methods:

Amplitude Modulation (AM):

small changes in radio frequency electric field amplitude

Frequency Modulation (FM):

small changes in radio wave frequency

Radio tuned to (“listens exclusively” to)

a carrier frequency ω 0

(e.g. 91.5MHz)

with enough bandwidth Δω

to span all encoded audio frequencies.

ω 0

Δω

Amplitude

ω

Phys 351, Fall 2008 Lecture 15 3

Modulation methods

Frequency mixing:

trig identity: 2(cosω 0 t)(cosω A t) =

cos[(ω 0 +ω A )t] + cos[(ω 0 −ω A )t]

where audio frequency is ω A

and carrier is ω 0

.

AM: add amplitude of audio

directly to carrier amplitude.

FM: vary the carrier frequency

by amount proportional to

amplitude of audio signal at

rate of change set by

frequency of audio signal.

Audio amplitude

quiet

loud

2 π/ω A

2 π/ω A

Phys 351, Fall 2008 Lecture 15 4

For some value k < 1

(so that carrier always has positive amplitude)

with k proportional to loudness, construct

will give a pure tone at frequency ω A of amplitude k/2.

So for an audio signal g(t) encode the AM radio signal as

signal = [1+g(t)] (cosω 0 t)

to get:

where g(ω) is the Fourier transform of g(t)

AM signal

ω 0

g()

2

!

cos{( )t}

2

k

cos{( )t }

2

k

cos t

signal[ 1 kcos t](cos t )

0 A 0 A 0

A 0

= !+ !+! + !"!

= +!!

ω

signal(ω)

ω 0

ω

signal(ω)

ω A ω A

Phys 351, Fall 2008 Lecture 15 5

Super-heterodyne

“Super-het” receiver: Mixes

carrier with local oscillator ω 1

to make an intermediate

frequency signal (455 kHz)

to which internal circuits are

tuned so that gains, etc can be

controlled easily and noise

excluded.

Tuning dial sweeps

both ω 0 and ω 1

to keep ω int

= constant.

ω int

ω 0 ω 1

Phys 351, Fall 2008 Lecture 15 6

Super-het receiver

×

ω 1

oscillator

Tuning

dial

Wide-band

RF amp

antenna

Tuned

455kHz

amp mixer

Quartz

capacitor

Rectifier/

audio

filter

audio

amplifier

Once signal is mixed down to ω int

, it is filtered against a stable (quartz) oscillator

and amplified

The signal is rectified and filtered to remove ω int

leaving audio frequency signal

that is driven through speaker.

Phys 351, Fall 2008

Lecture 15 7

The complete circuit

Phys 351, Fall 2008

Lecture 15 8

FM signals

Generically much greater SNR than AM to encode sound into the

carrier frequency rather than the amplitude, which is directly corrupted by

any interfering waves.

For audio f(t) encode:

Notice that now the “phase” of the RF must be detected and used to recreate the

audio f(t).

Usually requires more sophisticated and delicate circuitry (phase-locked loop

which tunes a tank circuit to a resonant frequency ω int ) but pays off

in much improved SNR

signal cos{[ kf(t)]t}

0

= !+

Phys 351, Fall 2008 Lecture 15 9

FM receiver = “phase detector”

!

signal = cos{[ "

0

+ kf ( t )] t }

Signal:

Reference:

Sig ⊕ Ref:

XOR

Phase det. Low pass

VCO

V

sig

Voltage controlled

oscillator

!

V

ref

= cos{ "

0

t + #}

!

V

sig

= cos{ "

1

t }

!

V

sig

" V

ref

= cos{(

0

+

1

) t + $}

+ cos{(

0

%

1

) t % $}

Bandpass

!

V

det

= cos{( "

0

# "

1

) t # $( t )}

!

V

audio

= cos{ "( t )}

and again

Phys 351, Fall 2008 Lecture 15 10

TV signals and reception

Same game as FM except must encode three colors and intensity of video raster,

sync signal, and audio in different sidebands

Of course analog communication will soon be like the T. rex

Digital radio and digital TV have all of the advantages of D/A accuracy and are just

as impervious to noise and interference.

Choice between the two (analog and digital) is dependent on bandwidth (digital

needs much more) and complexity of circuitry – AM radio is VERY simple:

you can even build one yourself….