Metra Electronics DAT Combo Manual do Utilizador descarregar pdf (Página 53)

RADIO

-TV

EXPERIMENTER

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CHECK

INTERNATIONAL

CRYSTAL MODEL

C -12B

CB Frequency Meter

It should

be

evident

to

every

CBer that

the

FCC is bent

on

a

real

crackdown, for the

monthly

list of

fines and

forfeitures now

runs

several pages

rather

than several lines.

And

a quick perusal

of

the list shows

that

next to

transmitted obscenities,

off -frequency

operation

ranks near

the top of the

pink

-

ticket

list.

But

there is really

no reason why

any

CBer should

risk losing his license because

of

off -frequency

operation.

For the truth of

the

matter is that

any

communications serv-

ice

shop or CB

club shop

should

be

equipped

with

a

frequency meter.

A

frequency

check is difficult?

Nonsense.

With

a frequency

meter

specifically

designed

for CB,

such as International

Crystal's model

C -12B,

it

takes but ten

seconds

to

check

each channel.

Equally

important, operation

is

so simple

the check

could be performed

by

a child.

Twenty -three

Plus. The

C -12B

is a hy-

brid (tube

and transistor),

battery

-powered

frequency

meter specifically

designed for the

Citizen's

Band. It

has 23 switch

-selected

frequencies

plus

a spare

(the

24th

position).

In

addition

to

checking frequency with

a

claimed

accuracy of .0015

%, the

meter

will

also measure

percent

modulation

and

the transceiver's

RF power output.

The meter

is supplied with

a separate pick-

up box (called

the PK)

that provides

a dum-

my load

for

the transmitter

and acts as an

attenuator when

the frequency

meter

is

used

as

a precision

signal generator.

The

meter's

direct output

provides

an unmodulated

signal

(for

alignment,

say) of 100 microvolts;

with

the PK

box in the

circuit, the output

at the

end

of the

PK's

test

cable is one

microvolt.

CB

TRANSMITTER

RF

PK

RF

BOX

RF

CRYSTAL

REFERENCE

OSCILLATOR

MIXER

50A

LOAD

AUDIO

AMPLIFIER

AND

COUNTER

On The

Beat. Block diagram shows how

the

frequency

tests

are

performed.

The

out-

put of

a precise crystal -controlled oscillator

is

mixed (beat) with

the transceiver's output

signal.

The difference

signal below is

first

amplified, then

rectified,

and the resultant

DC

passed

to a calibrated

meter. The

greater

the difference

frequency,

the

higher

the

meter reading.

This,

in turn,

is

interpolated

into

deviation from

center -frequency.

Let's

look

at a practical

example.

Suppose

you wanted

to

check out a

channel -9

crystal.

Setting

the frequency

meter

to

channel 9 pro-

duces

an internal

27.065 MHz

signal.

If

the

signal

from

the transceiver were

off-

frequen-

cy

by

100 Hz,

its output would

be 27,065,100

Hz.

And when

27,065,000

Hz is beat

against

27,065,100

Hz,

the output

from

the

mixer

will

be

the

difference

between

the two

fre-

quencies-a

100 -Hz

beat

note.

This is then

amplified,

rectified

and the

resultant

DC

displayed

on

the

meter

as 100 -

Hz

deviation.

A

special switch

is provided

that

tells you

whether

the deviation

is above

or below

center

channel.

If the

signal

from

the

transmitter

were

exactly 27.065

MHz,

there would

be no

(zero)

beat

note

and the

meter would

therefore

indicate

"0

" -no

de-

viation

from

center

channel.

The

meter is

calibrated

from

0

to 3000

Hz

deviation, with

METER

Block diagram

shows

frequency

measurement

system.

PK

box

samples

RF

signal fed

into the

dummy load -feeds

it to

mixer.

FEBRUARY -MARCH,

1967

57

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