Heathkit IG-18 User Manual

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VARIABLE

RESISTOR

Ivl

a a Q

0°

h

r

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IC

I G -18

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RI

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a a Q

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-18

AMPLIFIER

UNDER

TEST

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1

c!!:>

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AMPLIFIER

UNDER

TEST

Page

53

~

A

.:~

9'9

I I

AC

VOLTMETER

~

B

:a

9'9

I

AC

VOLTMETER

Figure 9

Set

the

Generator

to a low frequency,

such

as

20 Hz,

short

out the

variable

resistor,

and

set

the

signal

level

of

the

Generator

to give a con-

venient

reading

on the

AC

voltmeter.

Remove

the

short

from

the

variable

resistor.

The

value of

the

variable

resistor

is

then

adjusted

until the

reading

on

the

meter

drops

to

1/2

of

the

former

value. The

resistance

of

the

variable

resistor

is

then equal to

the

unknown input

im-

pedance of

the

amplifier.

The

resistance

of

the

variable

resistor

can

be

measured

with an

ohm-

meter.

If

this

same

check

were

made

at

higher

frequencies,

the

input impedance would

appear

to

be

smaller

because

some

of the

signal

would

be

shorted

out by

the

input

capacitance

of

the

am-

plifier.

.

For

low input

impedances,

of a few hundred

ohms

or

less,

connect the

circuit

as

shown

in

Fig-

ure

9B.

Fixed

series

resistor

Rl

should

be

at

least

ten

times

the

input

resistance

to be

meas-

ured.

The

value of

this

resistor

is

not

impor-

tant

if

it

is

made

large

enough. The output

voltage

is

set

to

some

convenient value,

as

be-

fore,

with

variable

resistor

R2

disconnected.

When

R2

is

connected again,

it

is

adjusted

until

the

meter

reading

drops

to half of

the

former

value. The

resistance

of

variable

resistor

R2

is

then equal

to

the input

resistance

of

the

am-

plifier.

FREQUENCY

MEASUREMENTS

An

oscilloscope

and

the

Generator

can

be

used

to

measure

the

frequency

of an unknown

signal.

Place

the

oscilloscope

horizontal

frequency

selector

in

the

horizontal

input pOSition. Con-

nect

a

sine

wave

from

the

Generator

to the

horizontal

input of the

oscilloscope.

Connect

the

unknown

frequency

to

the

vertical

input of

the

oscilloscope.

Do

not

use

square

waves to

make

these

tests.

When

the

size

of

the

generator

signal

is

ad-

justed

to

approximately

the

same

size

as

that

of the unknown signal, .and

the

oscilloscope

is

properly

adjusted,

waveforms

called

Lissajous

figures

will be

seen

on

the

oscilloscope.

In-

terpreting

these

waveforms

correctly

will show

the

frequency

of the unknown

signal.

Questions and Answers:

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Heathkit IG-18 Specifications

General

Brand	Heathkit
Model	IG-18
Category	Test Equipment
Language	English

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