Metra Electronics DAT Combo Instrukcja Użytkownika Pobierz pdf (Strona 56)

You

can add a

Beat -Frequency

Os-

cillator to just

about

any all -band

receiver

without

digging into

the

chassis. All

connections are

external

but you

can

get

better

shortwave

reception

with

a direct

connection.

INSTANT

Many

receivers

for the

general

public

have

a short

wave

band

but

no BFO (beat

-

frequency oscillator).

Most hams today

use

either

Morse

code (CW) or

single -sideband

(SSB)

voice

(both of

which

require

a

BFO),

and

cannot be tuned

in on receivers

without

a

BFO circuit.

But there

is a

solution.

Go

to a war- surplus

dealer, purchase

the lowest

priced longwave

receiver he has

at hand,

and you're in business.

Here's How.

Just

about all modern

re-

ceivers,

especially those which

are intended

for the general

public,

are put together

using

a

superheterodyne

circuit

(not that there

is

anything necessarily "super"

about the home

-

entertainment version

of it).

The signal

from

the

antenna

is

picked up by the RF

stage

of

the receiver

-at the station's

actual

frequency. In

the

type

of set we're dealing

with

here, little

amplification takes place in

the RF (radio frequency)

stage.

Instead

it

is immediately

converted to a fixed inter-

mediate frequency (IF),

and

usually

centers

on 455 kHz (kc).

Because this narrow

band

of frequencies is fixed,

tuned -amplifier

cir-

cuits can be built much

more economically.

Now

if

you had a receiver

intended for

amateur

or

communications

listening, it

would

have

a

beat -frequency oscillator oper-

62

by

C. M. Sranbury II

ating very

near

the

intermediate

frequency.

The

BFO is

actually

a

miniature

transmitter

(oscillator)

built into the receiver

and put-

ting out

a microvolt

signal. For

example,

in Lafayette's

brand new

HA -700

the

BFO

operates

at either 452.5 or

457.5 kHz

(and

if necessary

can be

adjusted for any value in

between).

When

a

CW carrier is tuned dead

on, it

appears

in

the IF stage

at

exactly

455

kHz, beats with

that BFO

just 2.5 kHz

away

and in turn produces

an audio

note of 2500

Hz

(cps).

(1 kHz

equals 1000

Hz,

of

course.)

The dots

and dashes

are then easily

readable (heard

as dots

and dashes).

What To Do?

But

we're

forgetting -

you've inherited

a SW receiver

without

a

beat -frequency oscillator.

So

obviously what

you

must do is

add a

BFO

to

your

present

receiver. And because

you are

just a

be-

ginner,

this

must be

accomplished in the

simplest

way

possible.

Which brings us back

to that

war

-surplus longwave receiver.

Most

of

these are blessed

with

a BFO which oper-

ates at

the LW

sets'

own

IF (somewhere

below

200 kHz)

and all

will

tune the

SW

rigs' IF. (Be

careful

-some were

regener-

ative circuits not superhets.)

By now

I'm

sure the idea

is beginning

to

dawn on

you.

If

not then consider this

little experiment.

RADIO

-TV

EXPERIMENTER

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