AN

ALO

G &

DIG

ITA

L C

OM

MU

NIC

AT

ION

SY

ST

EM

A

MPLIT

UD

E M

OD

ULA

TIO

NPart IV

Part IV

LO

NG

C

Su

perh

etro

dyn

eR

eceiv

er

Superh

etro

dyn re

ceiv

ers c

onvert a

ll incom

ing sig

nals to

a lo

wer fre

quency, k

now

n a

s the

inte

rmedia

tefre

quency (IF

), at w

hic

h a

single

set o

f am

plifie

rs is

use

d to

pro

vid

e a

fixed le

vel o

f sele

ctiv

ity a

nd

use

d to

pro

vid

e a

fixed le

vel o

f sele

ctiv

ity a

nd

sensitiv

ity.

The k

ey c

ircuit is th

e m

ixer

–act a

s a sim

ple

am

plitu

de

modula

tor to

pro

duce su

m a

nd d

iffere

nce fre

quencie

s.

Hete

rodyne m

eans to

mix

two fre

quencie

s togeth

er in

a

nonlin

ear d

evic

e o

r to tra

nsla

teone fre

quency to

anoth

er u

sing n

onlin

ear m

ixin

g.

2

Su

pe

rhe

trod

yn

eR

ec

eiv

er

3A

M N

on-C

ohere

nt S

uperh

etro

dyne R

eceiv

er B

lock D

iagra

m

Su

perh

etro

dyn

eR

eceiv

er

4

fLO

should

alw

ays b

e a

t a fre

q 4

55kH

z

above th

e in

com

ing c

arrie

r freq.,fc

Su

pe

rhe

trod

yn

eR

ec

eiv

er

RF S

ectio

n

Consist o

f pre

-sele

cto

r and R

F a

mplifie

r

Pre

-sele

cto

r is a b

road-tu

ned B

PF w

ith a

n a

dju

stable

centre

frequency th

at is tu

ned to

the d

esire

d c

arrie

r frequency. M

ain

purp

ose

of th

e p

re-se

lecto

r is by se

lectiv

ity, it a

ccepts o

nly

the 5

purp

ose

of th

e p

re-se

lecto

r is by se

lectiv

ity, it a

ccepts o

nly

the

desire

d fre

quency o

r pre

vent u

nw

ante

d ra

dio

frequency (Im

age

Fre

quency) fro

m e

nte

ring th

e re

ceiv

er a

nd a

ll oth

ers a

re

reje

cte

d.

RF A

mplifie

r –to

am

plify

the sig

nal b

efo

re e

nte

ring th

e m

ixer

Su

pe

rhe

trod

yn

eR

ec

eiv

er

Mix

er/

Converte

r Sectio

n

This se

ctio

n in

clu

des a

radio

frequency o

scilla

tor sta

ge (L

O) a

nd

a m

ixer/

converte

r stage (th

e first d

ete

cto

r) whic

h is a

nonlin

ear

devic

e u

sed to

convert R

F’s to

IF’s (R

F to

IF fre

quency

transla

tion). IF

signal in

com

merc

ial A

M b

roadcast a

re b

etw

een

450kH

z a

nd 4

60kH

z. T

he m

ost c

om

mon IF

use

d is 4

55kH

z.

Reaso

n to

transla

te R

F ->

IF is to

obta

in a

dequate

sele

ctiv

ity.

6

Reaso

n to

transla

te R

F ->

IF is to

obta

in a

dequate

sele

ctiv

ity.

-RF sig

nal a

re d

ow

n c

onverte

d to

IF.

-Shape o

f the e

nvelo

pe re

main

s the

sam

e, a

lthough th

e c

arrie

r and S

Bs

freq a

re tra

nsla

ted fro

m R

F->

IF, the

orig

inal in

fo c

onta

ined in

the

envelo

pe re

main

unchanged.

Su

pe

rhe

trod

yn

eR

ec

eiv

er

Mix

er/

Converte

r Sectio

n

LO

is alw

ays m

ain

tain

ed a

t a fre

quency w

hic

h is h

igher, b

y a

fixed a

mount, th

an th

e in

com

ing R

F sig

nals.

The m

ixer in

the re

ceiv

er c

om

bin

es th

e sig

nal fro

m th

e R

F

am

plifie

r and th

e fre

quency in

put fro

m th

e lo

cal o

scilla

tor to

pro

duce th

ree fre

quencie

s:

A ‘d

iffere

nce’ fre

quency o

f local o

scilla

tor fre

quency -

RF

7

A ‘d

iffere

nce’ fre

quency o

f local o

scilla

tor fre

quency -

RF

signal fre

quency.

A ‘su

m’ fre

quency e

qual to

local o

scilla

tor fre

quency +

RF

signal fre

quency.

A c

om

ponent a

t the lo

cal o

scilla

tor fre

quency.

Mix

ing tw

o sig

nals to

pro

duce su

ch c

om

ponents is c

alle

d a

‘hete

rodyne’ p

rocess.

FrequencyConversion

Concept o

f a m

ixer.

Fre

qu

en

cy

Co

nv

ers

ion

Pro

ce

ss

Diffe

rence c

om

ponent:

1455-(9

99,1

000,1

001)k

Hz

9

Consid

er a

situatio

n sh

ow

n a

bove.

The A

M sig

nal in

to th

e m

ixer is a

1000kH

z c

arrie

r that h

as b

een m

odula

ted b

y a

1kH

z

sinew

ave, th

us p

roducin

g sid

e fre

qs a

t 999 k

Hz a

nd 1

001 k

Hz.

Sum

com

ponent:

1455+(9

99,1

000,1

001)k

Hz

All o

rigin

al in

puts

Fre

qu

en

cy

Co

nvers

ion

Pro

cess

The

IFam

plifie

rhas

atu

ned

circ

uit

that

accepts

com

ponents

only

near

455kH

z,

inth

iscase

454kH

z,

455kH

zand

456kH

z.

Sin

ce

the

mix

er

main

tain

sth

esa

me

am

plitu

de

pro

portio

nth

at

existe

dw

ithth

eorig

inal

AM

signal

input

at

999kH

z,

1000kH

zand

1001kH

z,

the

signal

now

passin

gat

999kH

z,

1000kH

zand

1001kH

z,

the

signal

now

passin

g

thro

ugh

the

IFam

plifie

rsis

are

plic

aof

the

orig

inal

AM

signal.

The

only

diffe

rence

isth

at

now

itscarrie

rfre

qis

455kH

z.

Afre

quency

conversio

nhas

occurre

dth

at

has

transla

ted

the

carrie

rfro

m1000kH

zto

455kH

z–

afre

quency

inte

rmedia

teto

the

orig

inal

carrie

rand

inte

lligence

frequencie

s.10

Su

pe

rhe

trod

yn

eR

ec

eiv

er

How

LO

tracks th

e R

F a

mplifie

r so th

at th

e d

iffere

nce b

etw

een th

e

two fre

quencie

s is main

tain

ed a

t a c

onsta

nt v

alu

e?

Exam

ple

–th

e ra

dio

is tuned to

receiv

ed a

bro

adcast sta

tion w

hic

h

transm

it at 8

00kH

z.

The L

O w

ill be ru

nnin

g a

t 1255khz. T

he d

iffere

nce fre

quency is

1255kH

z –

800kH

z =

455kH

z.

11

A S

uperh

etro

dyne R

eceiv

er T

uned to

800kH

z

Su

pe

rhe

trod

yn

e R

ec

eiv

er

Sim

ilarly

if the ra

dio

is tuned to

receiv

ed a

statio

n tra

nsm

itted a

t

700kH

z

The L

O w

ill be ru

nnin

g a

t 1155kH

z a

nd th

e d

iffere

nce is still

main

tain

ed a

t the re

quire

d 4

55kH

z.

This fre

quency d

iffere

nce th

ere

fore

rem

ain

consta

nt re

gard

less o

f the

frequency to

whic

h th

e ra

dio

is actu

ally

tuned a

nd is c

alle

d th

e

inte

rmedia

te fre

quency (IF

).

12

When th

e fre

q o

f LO

is tuned a

bove R

F : H

igh sid

e in

jectio

n

fLO

= f

RF

+ f

IF

When th

e fre

q o

f LO

is tuned b

elo

w R

F : L

ow

side in

jectio

n

fLO

= f

RF

-fIF

13

Ima

ge

Fre

qu

en

cy

Pre

vio

usly

, for a

700kH

z b

roadcast sta

tion, th

e L

O o

f 1155kH

z

giv

ing th

e d

iffere

nce (IF

) frequency o

f the re

quire

d 4

55kH

z.

What h

appen if w

e w

ere

to re

ceiv

e a

noth

er sta

tion b

roadcastin

g

on a

frequency o

f 1610kH

z?

This w

ould

also

mix

the L

O fre

q o

f 1155kH

z to

pro

duce th

e IF

freq o

f

14

This w

ould

also

mix

the L

O fre

q o

f 1155kH

z to

pro

duce th

e IF

freq o

f

455kH

z. i.e

this sta

tion a

lso re

ceiv

ed th

e sa

me tim

e a

s one a

t 700kH

z.

Sta

tion 1

: Fre

quency 7

00kH

z, L

O 1

155kH

z, IF

= 4

55kH

z

Sta

tion 2

: Fre

quency 1

610kH

z, L

O 1

155kH

z, IF

= 4

55kH

z

An

Image F

requency

is an u

nw

ante

d fre

quency th

at c

an a

lso

com

bin

e w

ith th

e L

O o

utp

ut to

cre

ate

the IF

frequency.

Imag

eF

req

uen

cy

(fim

)

An im

age fre

q is a

ny fre

q o

ther th

an th

e se

lecte

d ra

dio

freq c

arrie

r th

at if a

llow

ed to

ente

r a re

ceiv

er a

nd m

ix w

ith th

e L

O, w

ill pro

duce a

cro

ss-pro

duct fre

q th

at is e

qual to

the IF

.

An im

age fre

q is e

quiv

ale

nt to

a se

cond R

F th

at w

ill pro

duce a

n IF

th

at w

ill inte

rfere

with

the IF

from

the d

esire

d R

F.

Once a

n im

age fre

q h

as b

een m

ixed d

ow

n to

IF, it c

annot b

e

filtere

d o

ut o

r suppre

ssed. F

or R

F to

pro

duce a

cro

ss pro

duct e

qual

15

filtere

d o

ut o

r suppre

ssed. F

or R

F to

pro

duce a

cro

ss pro

duct e

qual

to th

e IF

, it must b

e d

ispla

ced fro

m th

e L

O fre

q b

y a

valu

e e

qual to

th

e IF

:

For h

igh-sid

e in

jectio

n:

Giv

ing:

IFLO

imf

ff

LO

IFRF

ff

f

IFRF

imf

ff

2

Imag

eF

req

uen

cy

(fim

)

Fig

ure

: Rela

tive F

requency S

pectru

m fo

r the R

F, IF, LO

and Im

age

Fre

quencie

s for a

superh

etro

dyne re

ceiv

er u

sing H

igh-sid

e In

jectio

n

16

Exam

ple

2-

Dete

rmin

e th

e im

age fre

quency fo

r a

standard

bro

adcast A

M re

ceiv

er tu

ned to

a sta

tion a

t

1320kH

z.

Su

pe

rhe

trod

yn

eR

ec

eiv

er

IF S

ectio

n

Consists o

f a se

ries o

f IF a

mplifie

rs and B

PFs

These

stages c

onta

in m

ost o

f the a

mplific

atio

n in

the re

ceiv

er a

s

well a

s the filte

ring th

at e

nable

s signals o

n o

ne fre

quency to

be

separa

ted fro

m th

ose

on th

e n

ext.

Most o

f the re

ceiv

er g

ain

and se

lectiv

ity is a

chie

ved in

the IF

sectio

n.

17

sectio

n.

Dete

cto

r Sectio

n

Convert th

e IF

signals b

ack to

the o

rigin

al so

urc

e in

form

atio

n.

Audio

Am

plifie

r Sectio

n

Com

prise

s severa

l casc

aded a

udio

am

plifie

rs and o

ne o

f more

speakers.

The n

um

ber o

f am

plifie

rs use

d d

epends o

n th

e a

udio

signal p

ow

er

desire

d.

Au

tom

atic

Ga

inC

on

trol

Circ

uit

(AG

C)

The A

GC c

ircuit is u

sed to

pre

vent v

ery

strong sig

nals fro

m

overlo

adin

g th

e re

ceiv

er.

It can a

lso re

duce th

e e

ffect o

f

fluctu

atio

ns in

the re

ceiv

ed sig

nal stre

ngth

.

The A

GC c

ircuit m

akes u

se o

f the m

ean D

C v

olta

ge le

vel p

rese

nt

at th

e o

utp

ut o

f the d

iode d

ete

cto

r.

If the sig

nal stre

ngth

incre

ase

, the m

ean D

C v

olta

ge le

vel a

lso

incre

ase

.

18

incre

ase

.

If the m

ean D

C v

olta

ge le

vel e

xceeds a

pre

dete

rmin

ed th

resh

old

valu

e, a

volta

ge is a

pplie

d to

the R

F a

nd IF

am

plifie

rs in su

ch a

way a

s to d

ecre

ase

their g

ain

to p

revent o

verlo

ad.

As so

on a

s the in

com

ing sig

nal stre

ngth

decre

ase

s, the m

ean D

C

volta

ge le

vel is re

duced b

elo

w th

e th

resh

old

, the R

F a

nd IF

am

plifie

rs retu

rn to

their n

orm

al o

pera

tion.

Au

tom

atic

Ga

inC

on

trol

Circ

uit

(AG

C)

19