SEA 222 - Page 45

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-

'

.:i

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~

)

S.3.9

S.4

S.4.1

output

of

USC

is

then applied

to

the

AGC

detector

diode,

CR

16.

The

output

of

CR16

consists

of

positive

voltage peaks

which

are

propor-

tional

to

the

output level

from

the

product

detector

and

are

referenc7d

to

the

offset

voltage applied

to

the

non-inverting input

of

USC.

This

voltage charges

AGC

time constant

capacitor,

C81,

through

resistor

R70

which

provides

an

AGC

attack

time of approximately

2S

MSec,

while

the

AGC

discharge

resistor,

R71,

provides a

release

time constant of about

2 seconds. This

fast

attack,

slow

release

AGC

voltage

is

buffered

by

non-inverting follower,

USO

and

applied

to

the

I.F.

Amplifier

AGC

port

through

steering

diode

CR17.

THE

AUDIO

T/R

GATES

Integrated

circuit

U4

and

MOSFET

Ql

provide T/R gating functions in

the

audio

amplifier

circuitry

as follows:

U4B

is

turned

ON

in

the

receive

mode

and

connects

preamplifier

audio

to

the

AGC

amplifier/detector

circuit.

Turning

this

device

OFF

in

the

transmit

mode

prevents

the

AGC

storage

capacitor

from

"pumping

up"

on

transmitter

microphone audio

from

the

preamplifier

circuit.

Similarly,

U4A

serves

to

disconnect

the

microphone

audio

circuitry

from

the

product

detector

in

the

receive

mode.

This avoids

any

noise

contribu-

tion

from

the

microphone

amplifier/clipper

circuitry.

Ql

and

U4C

are operated

by

the

squelch signal from

the

CPU.

A

positive

output

from

the

CPU

turns

Ql

ON,

thus

shorting

the

audio signal across

the

volume

control,

while

the

CPU

signal

is

inverted

by

U4D

and

is

then used

to

turn

U4C

OFF,

greatly

attenuating

the

audio signal

applied

to

USA

from

the

audio

preamplifier.

Note

that

the

squelch

signal

is

controlled

by

the

CPU

and

is

positive

during transmission

periods.

The

overall function of

the

gate

circuitry

is

to

provide a

thumpless squelch gate

and

a rapid,

clickless

transition

between

the

transmit

and

receive

modes.

THE

TRANSMITTER

BLOCK

DIAGRAM

Figure S.4.1

shows

the

block diagram of

the

SEA

222

in

the

transmit

mode.

The

microphone signal

is

fed

to

a

amplifier/clipper/filter

circuit

(U8)

for

speech wave-shaping and

is

then

applied

through

emitter

follower

Q2

to

the

balanced modulator,

A3.

The

6.4

MHz

carrier

oscillator

is

also

applied

to

A3.

The

resulting

douhle

sideband

suppressed

carrier

signal

is

amplified

in

the

bilateral

6.4

MHz

I.F.

Amplifier,

U2,

passed through the 6.4

MHz

lower sideband

filter

F2

where

it

is

converted

to

a

single

sideband suppressed

carrier

signal

and

then

mixed

with a 38.6

MHz

signal in mixer

A2.

The

output

from

A2

at

45

MHz

is

amplified

by

Ull

and

then

passed

through

the

45

MHz

I