Sharp FDD-412A - Page 32

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FDD412A/B

:

Correc-—

Input

Differences

between

ae

ee

tion

data

input

data

intervals!

difference

values

O@E886

©

(c)

Operating principle

of

VFO

The

VFO

circuit

is

intended

to

generate

the

window

signal.

In

practice,

the

intervals

between

read

data

vary

depending

on

several

factors

including

medium

revolutions.

This

keeps

the

window

signal

from

correct

separation

as

a

fixed

generator

output.

It

becomes

necessary

for

the

window

signal

to

synchronize

with

data

interval

variations.

This

is

achieved

by

a

circuit

as

shown

in

Fig.

2.25.

correcting

differ-

and

window

signal

ences

are

created.

are

checked

Window

signal

Fig.

2.25

Operating

Principle

of

VFO

(d)

Correspondence

to

block

diagram

(Fig.

2.22)

(i)

Data

SS:

Converts

input

data

waveshapes

into

pulse

waveshapes

for

easy

comparison

in

order

to

verify

differ-

ences

(in

FM

mode:

1.0

Us-wide

pulse;

in

MFM

mode:

0.5

s-wide

pulse).

(ii)

Data

separator:

Compares

the

generator

output

and

data

pulses

in

phase.

For

a

phase

difference,

a

signal

with

the

pulse

width

proportionate

to

the

amount

of

difference

is

output.

The DEC

(decrease)

or

INC

(increase)

signal

is

output

depending

on

the

phase

shift

direction

(leading

or

lagging).

(iti)

Charge

pump/L.P.F.:

Produces

the

control

voltage

(VE)

for

control

on

the

VCO

generator

output.

The

voltage

is

output

in

proportion

to

the

pulse

width

(i.e.,

phase

difference)

of

the

DEC

or

INC

signal.

(iv)

vco:

This

voltage

controlled

oscillator

provides

oscil-

lation

at

frequencies

proportionate

to

the

input

control

voltage

(center

frequency:

2MHz).

(e)

Other

operations

(i)

HIGH

GAIN:

Promotes

VFO

follow-up

characteristics.

When

the

VFO

SYNC

signal

is

given,

it

is

preferable

to

complete

the

follow-up

as

soon

as

possible.

Once

the

follow-up

is

completed,

it

then

is

necessary

to

keep

off

effects

of

noise

and bit

dislocation.

This

signal

turns

on

the

follow-up

characteristics.

,

(ii)

2ND

FILE

DATA:

The

FDD

equipped

with

VFO

can

separate

data

read

by

an

FDD

without

VFO

by

connecting

a

model

B

FDD.

This

signal

allows

data

from

a

model

B

FDD

to

enter

the

VFO

circuit

of

this

FDD.

2us

Input

data

A

DATAYSSoutpatel

(LSP

te

ee

:

;

yoo

ctock

UCP

LUE

ep

i

:

if

\,

;

DEC

L

Lf

]

|

|

INC

aa

aaa

a

EE

Fig.

2.26

Typical

VFO

Follow-up

Bereewron

state