Boss Audio Systems BV8.5GA User Manual download pdf (Page 30)

Popular Electronics

NOVEMBER

1974

-VER

since

we published

coostruc

riot

plans for

the

world's

first

hob-

byist

experimenter's

laser

in De-

cember

of

1969,

POPULAF

ELEC-

TRONICS

has

kept readers

abreasi

of

.laserdevelopments.Forexample.

in January

1970,

we

gave details

on

now to

use

the laser for making-

three

-dimensional

holocrams.

Then,

in May

1970.

we publishes

plans for

assembling

a rase-

voice

communicator

(which,

incidentally

.

was featured

for

several weeks

in

the

Smithsonian

Institution)

Now.

we

have another

break

through

-plans for

building

the

world's

first

experimenter's

laser

video

(TV)

system

for a moderate

S150.

(The

TV

camera

and receiver

are

extra

items.)

THE

POPULAR

ELECTRONICS

laser TV

system gives

you an

advance

look

at a communication

system

of the future.

There

are two

key

devices

in

the

system.

One is

the

composite

laser

tube

video

modulator

"transmitter"

thatworks

in

conjunction

with

a

low -cost

TV

camera. The

other is

the

detector,'r

-f modulator

"receiver"

that feeds

an ordinary TV receiver.

(See box

on Class -1 requirements.)

32

The

helium -neon laser

tube used

in

the System

employs the latest

aluminum

cathode design.

The

tube

is used

in conjunction

with

solid -state rrodulation

and detec-

tion

circuits.

With

the Laser

TV

system,

you

can expect

a

range

up

to

50 ft

with-

out

special optics.

For

extended

range,

ycu

can use

a telescope

and

/or

a

converging

lens.

More

about

range

later in

the

article.

Overall

System. The

block

dia-

gram

of

the

overall

laser TV

system

is

shown in Fig.

1. The

videc

(or

audio, not

both

simultaneously)

output signalfrom

the TV

camera

is

typically

on the

order

of

1

volt

peak -to-

peak. This

signal

is fed to

the laser

modulator,

which

is de-

signed

to provide

a gain

of 2

mA /volt

Hen,e,

the 1 -volt

p -p

v deo

output

signal from

the

camera,

after passing

through

the

mod-

ulator, is

converted to

a 2 -mA p -p

signal teat

current-

drives

the laser

tube.

The

gain

of the laser

tube

is

about

0-1 mW /mA,

while

the laser

detector has

a gain

of

1.25

mA

/mW.

The

r -f oscillator

in the

detector is

tunable

over a 60- to 72 -MHz

range

to permit

the system

to operate

on

TV

channel

3 or channel 4,

whichever is not in

use in your

area.

The

output

of the r -f modulator

is

an amplitude

-modulated

(AM)

signal

that is

adjustable

from 0 to

5

mV

rms.

This

signal

can

be fed

into

an

ordinary TV

receiver

through

its

vhf

antenna

terminals

by means

of

300 -ohm

twin -lead

cable.

Laser

/Modulator

Circuit.

In

the

laser /modulator

power

supply,

shown

schematically

in =ig.

2,

T1.

C5 -C8,

and

D9

-D14

are

ar-anged in

a

voltage

doubler

conf guration

that

serves as the

main

high

-

voltage

supply. This

supply

deliv-

ers about

1700

volts

to the laser

tube.

Diodes

Dl

-D8

and

capacitors

C1

-C4

form

two more

voltage

doub-

lers

that

are

"stacked"

on

top of

the main

high

-voltage

supply for

ionizing

the

gas in

the

laser

tube.

As

soon as ionization

is

complete,

current

starts to flow

through

the

laser tube.

However,

the

values

of

C1

-C4

are

too low to

su3port the

5 -mA tube

current.

So, the

starting

voltage

collapses

and

only the

main

sustaining

voltage remains.

Transformer

T2

and its

as-

sociated rectifiers

(D15

-018)

and

filter

capacitors (C9 -C14) make

up

POPULAF ELECTRONICS

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