Boss Audio Systems BV8.5GA User Manual download pdf (Page 46)

Good AM-

SSB

-CW

performance

at a fraction

of

a superhet's

cost

BY

JIM WHITE,

W5LET

DIRECT-

CONVERSION

COMMUNICATION

RECEIVER

AN

essential ingredient

for the

neophyte

in

amateur radio

is

a

good receiver

-one that

is

sensitive

enough to pick

up

signals that

are

down near

the noise

level

and selec-

tive

enough

to

separate

adjacent

signals

and provide

clear

copy. With-

out

such

a

receiver,

one can

only look

forward

to

"unanswered"

CQ's

and

lots

of

frustration.

However,

a good

communications

receiver

can cost

anywhere from $250

to $500,

and

most

beginners don't

have that kind

of

money. Even

a good used

receiver

can

cost

$150.

As

an alternative, a direct -

conversion receiver

should

be tried. It

performs well

over a

range

of 3.5 to

4.3

MHz

on

AM,

SSB, and CW

and

is easily

constructed

at a cost near $30.

Direct

conversion

is a much -

neglected

type

of design that

can

best

be described

by

comparing

it to the

more

common

system, superhetero-

dyning.

In the

superhet

system

(Fig. 1),

the first

stage

is

an r -f

amplifier. This

is

followed

by

a

mixer

where

the

signal is

48

combined

with the

output

of a local

oscillator.

The

frequency

of

the

latter

is

a certain

amount

above

or below

that

of the

r -f

and the difference

is

called the

intermediate

frequency.

The

output

of

the mixer

contains

a

high- frequency

component

and a

low- frequency

component.

These

two

signals are produced

by

superheterodyning; that

is, combin-

ing two

signals

to

produce one

at a

frequency

equal to the

sum of the fre-

quencies of the

original

signals, and

one at a frequency

equal to their

dif-

ference. At

this

point we filter

out the

high

-frequency

component

and am-

plify the lower

in

a

stage

that

has high

gain and a narrow

passband,

which

affords

selectivity.

The

output of the i -f

amplifier is

sent

to

a

detector,

which

may

be of two types:

for AM

reception,

it

is an envelope

detector (a

diode fol-

lowed

by a low -pass filter);

for

SSB

and

CW,

a product detector,

which is

really a second mixer, fed

by a

beat

frequency

oscillator

(BFO),

is

used. The difference

component of

this

heterodyning process

is an au-

dio signal,

which

is then ampli-

fied through

one or

more stages

and

passed

on

to

phones

or a speaker.

As

you

can see,

there are usually

four or

more

stages

that

must

be

properly tuned

in conjunction

with

each other

for proper signal

recovery

in

a superhet

receiver.

Most quality

communications

receivers

have two

or three i -f

stages,

with

separate

mix-

ers, local oscillators, and tuned amp-

lifiers for

each stage.

These

complica-

tions drive the cost

of

receivers

out

of

the reach

of a

large

portion of

new-

comers to the hobby.

The

direct -conversion

technique

is

a

much

simpler process. The

block

diagram

of

this

system is

shown

in Fig.

1.

The r -f

amp

supplies

the mixer

with

an amplified

version

of the

signal

re-

ceived from the

antenna.

The mixer

is

also

fed

an

r -f

signal of

the

same fre-

quency as the incoming

carrier from a

local

oscillator

whose frequency

is

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