Racal Instruments 9916 User Manual

Racal Instruments 9916

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4.7

The

potential

at

C53

is

appl

ied

via

R63

and

I

C25b

to

the

base

of

Q1

O.

An

increase

of

input

signal

amplitude

causes

an

increase

in

potential

at

Q10

emitter.

4.8

The

P1N

diodes

of

the

attenuator

exhibit

an

impedance

which

is

inversely

proportional

to

the

current

carried.

As

the

AGC

potential

increases

D14

current

increases,

and

the

potential

at

the

D12/D13

junction

increases.

This

reduces

the

current

carried

by D12

and

D13,

which,

in

turn,

increases

the

potential

at

the

D1l/D12

and

D13/D15

junctions

and

increases

the

current

in Dl1

and

D15.

The

overall

effect

is

an

.

increase

in

the

impedance

of

the

series

attenuator

arm

and

a

decrease

in

impedance

of

the

shunt

arms.

.

4.9

The

AGC

voltage

is

available

at

a two

pin

outlet

on

the

rear

panel

to

provide

an

indication

of

relative

amplitude

for

'A'

Channel

signals.

-Low

Level

Inhibit

.10

The

output

of

the

AGC

detector

is

applied

to

IC25q!1.

This is

an

operational

ampl

ifier

connected

as a

Schmitt

trigger.

The

triggering

level

is

set

by

adjusting

R64.

If

the

input

signal

level

is

too

low

to

give

stable

counting

lC25q112 is

at

logic

10',

and

IC9b,

part

of

the

signal

selection

system

described

in

paragraph

4.17,

is

inhibited.

Overload

Protection

and

Indication

4.11

Signals

applied

to

the

'A'

Channel

input

are

passed

to

the

peak

to

peak

detector

C35,

D9, D10

and

C37.

Increasing

signal amp!

itude

resul

ts

in

the

detector

-output

making

IC29/2

more

negative

via

the

potential

divider

R39/R40.

4.12

IC29

is

a

Schmitt

trigger

which

triggers

when

pin

2

goes

more

negative

than

pin

3.

This

occurs

when

the

input

voltage

is

approximately

5

volts

r.m.s.,

and

de-energises

RLA

to

isolate

the

ampl

ifier

from

the

input.

At

the

same

time

08

is

switched

on

and

lights

LPl

to

give

warning

that

overload

has

occurred.

;'

Channel

Amplifier

'4.13

The

signal

to be

measured

is

applied

to

the

front

panel

BNC

socket

SK50,

and

passes,

via

Cl00,

R200

and

the

Cl/R3

combination,

to

the

high

impedanc~

FET

first

stage

01.

The

gate

of

01

is

protected

by R3,

D1

and

the

gate!

channel

'diode

.

4.14

The

output

at

the

source

of

01

is

passed

via

the

~mitter

follower

02

to

the

AGC

attenuator

D2/D3.

The

attenuator

output

is

amplified

in

1C30,

buffered

by

Q4,

-and

applied

to

the

Schmitt

trigger

IC21

c.

The

signal,from

IC21

c

passes

via

a

line

receiver

[C21b,

which

is

used as a

gate,

to

05

and

IC8b.

The

output

of

IC21b

is

also

taken

to

the

Low

Frequency

Multiplier

(if

fitted)

and

then

'to

IC9a.

The

LF

Multiplier

is

described

In

-paragraph

4.71.

IC8b

and

IC9a

are

part

of

the

signal

selection

system

described

in

paragraph

4.

17.

4-2

9916

Vol.

2

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Racal Instruments 9916 Specifications

General

Brand	Racal Instruments
Model	9916
Category	Measuring Instruments
Language	English