Elenco Electronics MO-1251 Manual de usuario descargar pdf (Pagina 15)

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10.7

MHz

.01

INPUT

MPX

AUDIO

INPUT

-L C4

470pF

.01

.05

120i2

IC1

MC1358

1µF

50V

SOURCE

10K

INPUT

LEVEL

1µFÄ

50V

33µH

+ 15V

5-60pF

10pF

2.2K

+ 15V

C10

1µF

50V

470pF

Rö

100K

+15V

C13

.01

C11

tµF

50V

DECODER

STAGE

1012

{ If

C14

16'j

PILOT

LED1

R9 O R10

;'µ

.L.

14115 C16

147

C12

.0022

C24

0039

R20

R21

4.3K

4.7K

+15V

IC2-b

LM358

C25

C26

015

R22

5:1K;1

+ 15V

0.1

2N3904

4.0

R23

10K

MATRIX

INPUT

LEVEL

C19

470pF

R19

25K

PLL

ADJUST

+15V

-C17

10T.47

R11'

IC3

100K

LM1800

-41

.1+

C18

.0047

.-'1AAr

R12

100K

L+ R

OUTPUT

+It-L.2

MATRIX

C27

DECODER

10,F

(FIG.71

50V

FIG. 3 -THE

DECODER STAGE

converts

the multiplexed

audio signal

into

L +R and L-

signals.

standard 4 .5 -MHz audio demodulator

that is used in many television receivers.

The circuit is more or less

what you find in

the databook. The major exception is that

the

standard de- emphasis

capacitor has

been eliminated

order

ensure

that the

L -R signal is presented to the decoder. If

present, the capacitor would roll off the

high- frequency L -R

signal.

The output of ICI

is routed to

1, which

allows you to choose between

the inter-

nally demodulated signal and an exter-

nally demodulated one. Buffer amplifier

IC2 -a then provides a low- impedance

source for driving IC3, an LM1800 stereo

demodulator. As with IC1, IC3 is used in a

conventional manner.

Our

circuit differs

from

the cookbook circuit, however, in

that the component values associated with

the phase -locked loop have been altered

so that the loop will lock on the 15.734 -

kHz MTS pilot rather than on the 19 -kHz

FM -radio pilot.

When IC3 is locked on a stereo signal,

the outputs presented at

pins 4 and 5 are

the discrete left- and

right- channel sig-

nals, respectively. In order

to provide

noise reduction to the

L -R signal,

must re- combine

the discrete outputs

into

sum and difference

signals. Op -amp

IC4-

a is used to regenerate

the L -R signal.

is wired as a difference

amplifier,

wherein

the inputs are summed

together

(+ L - R). Capacitor C18

bridges the

left -

and right -channel

outputs of the

demod-

ulator.

Although it decreases

high -fre-

quency separation

slightly,

it also reduces

high- frequency

distortion.

After building

the circuit, you

may want to compare

sound output

with and without C18.

The L

R signal

is taken

from the

LM1800 at pin

2, where it appears

conve-

niently at the output

of an

internal buffer

amplifier.

The raw L -R

signal is applied

to IC4-

b, a 12 -kHz lowpass

filter.

The L + R sig-

nal

also fed through a

12 -kHz lowpass

filter in order to keep

the phase shift

un-

L -R

NOISE

REDUCTION

STAGE

(FIG.E)

dergone

by both

signals equal.

If only

one

were filtered, there

would

be a loss

high- frequency

separation

when the

left

and

right channel

signals

were recovered.

Next, as shown

Fig. 4, the

L -R

signal

is fed to Q2.

That transistor

has

three functions.

It allows

to add a

level

control

to the

L -R signal

path;

provides a

low source

impedance

for driv-

ing the

following

circuits;

and it inverts

the signal

180 °. (Think

the signal

at the

collector of

Q2 as - (L

- R)).

Inversion

necessary

compensate

for

the 180°

in-

version

in the

compander.

comes the

expander

stage;

this

where

we would use

a dbx

decoder

if we

could get

one (see

sidebar).

the collec-

tor of Q2

is a 75 -µs

de- emphasis

network

(R27 and C29)

that functions

just

the

network associated

with Ql

(in Fig.

3).

Note that Q2

feeds both

Q3 and

IC5 -a,

- 12 db per octave

highpass

filter. The

output

of that

filter

drives the

rectifier

in-

put of

IC6, an

NE570.

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 85 86

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Elenco Electronics MO-1251 Manual de usuario Pagina 15

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