SEA 222 - Page 41

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which

protects

the

following

circuitry

from

intermodulation

problems.

The

filtered

45

MHz

I.F.

Signal

is

then

applied

to

the

first

bilateral

I.F.

Amplifier,

Ul.

After

amplification

in

Ul,

the

45

MHz

I.F.

Signal

is

.combined

in

A2

with

a

38.6

MHz

signal

from

the

VCXO,

down

converted

tp

the

6.4

MHz

second

I.F.

Frequency and

passed

through

F2,

a narrow band

signal

sideband

filter.

The

filtered

s i gna 1

is

then

amp

1

ifi

ed

in

bi

1atera1

I.F.

Amplifier,

U2,

and combined

with

the

6.4

MHz

BFO

frequency

in

mixer A3. The

output

from

A3

is

a low

level

audio

signal

which

is

then

amplified

by

the

low

noise

audio

preamplifier,

U3.

Note

that

the

6.4

MHz

filter

selects

the

LOWER

sideband.

This

is

actually

the

UPPER

sideband

because

of

the

frequency

inversion

which

occurred

at

the

first

mixer.

After

being

amplified

to

a

suitable

level

by

the

low

noise

pre-

amplifier

stage,

the

audio

signal

is

then

sent

to

three

different

circuits:

The

squelch

limiter/amplifier,

U7,

provides

a hard

limited

signal

to

the

CPU.

This

signal

is

examined

by

the

squelch

software

routine

which

determines

the

presence

of

voice

signals.

The 1

ow

1eve1

audio

is

a 1

so

sent

to

the

audio

derived

AGC

circuitry

which

consists

of

USR

and

USC.

These

two

operational

amplifiers

provide

amplification

and

filtering

to

the

audio

signal

which

is

then

rectified

by

CR15.

The

diode

output

is

a

fast

attack,

slow

release

AGC

voltage

which

is

then

applied

to

the

AGC

control

buss

of

the

I.F.

Amplifier

stages

through

USO.

The

third

low

level

audio

signal

is

sent

to

the

signal

audio

pre-

amplifier/filter,

USA,

where

it

is

approximately

doubled

in

amplitude

and band

limited

to

a

cutoff

frequency

of

about

3000 Hz.

After

being

amplified

and

filtered,

the

receiver

audio

is

then

passed

on

to

the

front

panel volume

control

where

the

operator

can

control

the

audio

level

applied

to

the

input

of

the

power

amplifier

stage,

U6.

The

output

from

U6

provides

the

loudspeaker

signal.

5.3

RECEIVE

RF

CIRCUITRY

AND

FIRST

MIXER

As

previously

discussed,

an incoming

signal

is

first

passed

through

a

system

of

cascaded

low and

high

pass

filters,

a

T/R

relay

and a

P.I.N.

Diode

device

which

is

designed

to

protect

the

sensitive

input

mixer from damage due

to

high

RF

overloads.

On

the

RF

board,

switching

diodes

CRS

and

CR6

are

foward

biased

by

the

+lOV

Rx

buss,

thus

passing

the

received

signal

to

the

double

balanced

mixer

Al.

The

use

of

a

hot

?S

I