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nlf

OPERATING

AND

SERVICE

MANUAL

(HP

PART

NO.

400D/H/L-903}

MODEL

400D

SERIALS

PREFIXED:

310-

MODEL

400H

SERIALS

PREFIXED:

313-

MODEL

400L

SERIALS

PREFIXED:

313-

AND

SPECIF.

HO2-400D

SERIALS

PREFIXED:

310-

VACUUM

TUBE

VOLTMETER

Appendix

C,

Manual

Backdating

Changes,

adapts

this

manual

to:

Models

400D/H02-400D,

Serial

Nos.

313-28977

and

below

Models

400H/L,

Serial

Nos.

310-52371

and

below

Models

400DR/HR/LR,

_All

Serial

Nos.

Copyright

Hewlett-Packard

Company

1961

P.O.

Box

301,

Loveland, Colorado,

80537

U.S.A.

00102-5

Printed:

APRIL

1966

T.

O.

33A1-12-349-1

Figure

1-1.

Vacuum

Tube

Voltmeters

Models

400D, 400H,

400L

1-0

00102-2

T.O.

33A1-12-349-1

Section

I

Paragraphs

1-1

to

1-5

SECTION

|

GENERAL

DESCRIPTION

1-1.

INTRODUCTION.

(Sce

figure

1-1.)

1-2.

This

manual

contains

operating

and

servicing

instructions,

anda

parts

breakdown,

for

the

Models

400D, 400H,

and

400L

Vacuum

Tube

Voltmeters

manu-

factured

by

the

Hewlett-PackardCompany.

The

Model

400D

Voltmeter

is

similar

toa

military

counterpart,

Electronic

Voltmeter

ME-30A/U,

in

appearance

and

operation,

but

contains

modified

electrical

circuits

to

obtain

improved

performance.

Applicable

Federal

Stock

Numbers

for

the

voltmeters

are

as

follows:

Model

400D:

6625-643-1670

Model

400H:

6625-557-8261

Model

400L:

6625-729-8360

1-3.

The

Models

400D,

400H,

and

400L

Voltmeters

are

the

same

except

forthe

differences

listed

in

Fig-

ure

1-2.

a.

Voltage

Range:

400D/H

-

0.1

millivolt

to

300

volts;

400L

-

0.3

millivolt

to

300

volts,

in

12

ranges

providing

full-scale

readings

of

the

following

voltages:

0.001 0.100

10.00

0.003

0.300 30.00

0.010

1.000

100.00

0.030 3.000

300.00

b.

Decibel

Range:

-72

to

+52

db,

in

12

ranges.

c,

Frequency

Range:

10

cps

to

4

mc.

d.

Input

Impedance:

10

megohms

shunted

by

15

pf

(15

uf)

on

ranges

1.0

volt

to

300

volts;

25

pf

on

ranges

0.001

volt

to

0.3

volt.

e.

Stability:

Line

voltage

variations

of

+10%

do

not

reduce

the

specified

accuracy,

and

line

voltage

transients

are

not

reflected

in

the

meter

reading.

Electron

tube

deterioration

to

75%

of

normal

transconductance

affects

accuracy

less

than

0.5%

from

20

cps

to

1

mc,

{.

Amplifier:

OUTPUT

terminals

are

provided

so

that

the

voltmeter

can

be

used

to

amplify

small

signals

or

to

enable

monitoring

of

waveforms

under

test

with

an

oscilloscope.

Output

voltage

is

approximately

0.15

volt

rms

on

all

ranges

with

full-scale

meter

deflection.

Amplifier

frequency

response

is

same

as

the

voltmeter.

Internal

impedance

is

approximately

50

ohms

over

entire

frequency

range,

a.

The

front

panel

meters

are

different

in

each

model,

as

described

in

paragraph

1-6.

b.

The

accuracy

specifications

are

different

for

each

model,

as

described

in

figure

1-2.

1-4.

DESCRIPTION.

1-5.

The

Hewlett-Packard

Models

400D,

400H,

and

400L

Vacuum

Tube

Voltmeters

are

general

purpose,

portable

electronic

a-c

voltmeters

of

high

sensitivity

and

stability.

They

are

suited

to

both

laboratory

and

field

use.

Models

400D/H

measure

a-c

voltages

from

0.

001

to

300

volts

and

Model

400L from

.003

to

300

volts

rms

fullscale,

witha

frequency

bandwidth

cover-

ing

10cps

to4

megacycles.

The

voltmeters

are

com-

pact,

accurate,

and

rugged

and

have

fast

meter

re-

sponse,

high

input

impedance,

stable

calibration

ac-

curacy,

and

freedom

from

the

effects

of

normal

line

voltage

variations.

The

voltmeters

are

designed

for

long

instrument

life

with

a

minimum

of

servicing.

g.

Accuracy:

Model

400D

-

+2%

of

full

scale,

20

cps

to

1

me;

+3%

of

full

scale,

20

cps

to

2

mc;

+5%

of

full

scale,

10

cps

to

4

mc.

Model

400H

-

+1%

of

full

scale,

50

cps

to

500

ke;

+2%

of

full

scale,

20

cpsto

1

me;

+3%

of

full

scale,

20

cpsto

2

me;

45%

of

full

scale,

10

cpsto

4me.

Model

400L

-

+2%

of

reading

or

+1%

of

full

scale,

whichever

is

more

accurate,

50

cps

to

500

ke.

+3%

of

reading

or

+2%

of

full

scale,

whichever

is

more

accurate,

20

cps

to

1

mec.

+4%

of

reading

or

+3%

of

full

scale,

whichever

is

more

accurate,

20

cps

to

2

mc,

+5%

of

reading

10

cps

to

4

mc.

h.

Power

Requirement:

115/230

volts

410%,

50

to

1000

cps,

approximately

100

watts.

i,

Size:

11-3/4

in.

high,

7-1/2

in,

wide,

12

in,

deep,

j.

Weight:

23

Ibs.

18

lbs;

shipping

weight

approximately

Figure

1-2.

Table

of

Specifications

00102-3

1-1

Section

I

Paragraphs

1-6

to

1-12

1-6,

Each

model

voltmeter

has

three

calibrated

scales

on

the

panel

meter.

The

Models

400D

and

400H

have

two

linear

VOLTS

scales,

0

to

1

and

0

to

3,

and

one

DECIBELS

scale,

-12

to

+2

db.

The

meters

used

in

the

Models

400H

and

400L

are

larger

and

include

a

mirror

to

eliminate

parallax

in

viewing

and

to

facilitate

use

of

the

higher

scale

calibration

accuracy

of

these

models.

The

Model

400L

VOLTS

scales

are

logarithmic

in

calibration,

from

0.3

to

1

and

0.8

to

3;

and

the

DECIBELS

scale

is

linear.

In

all

models,

the

VOLTS

scales

are

calibrated

to

indicate

the

root-mean-square

(rms)

value

of

an

applied

sine

wave.

Actual

meter

deflection

is

proportional

to

the

average

value

of

the

applied

signal,

thereby

minimizing

additional

meter

deflection

due

to

noise

and

harmonic

distortion.

1-7,

A

voltmeter

output signal

is

provided

at

the

front

panel

OUTPUT

terminals.

This

output

is

proportional

to

the

meter

reading

and has

a

waveshape

similar

to

the

applied

signal.

This

signal

level

is

about

0,15

volts

rms

for

a

full-scale

meter

reading,

regardless

of

the

input

signal

level.

The

internal

impedance

at

the

OUTPUT

terminal

is

50

ohms

over

the

full

frequency

range.

High-impedance

loads

(above

100K)

will

not

adversely

affect

the

accuracy

of

the

voltmeter,

This

output

is

valuable

for

increasing

the

sensitivity

of

bridges,

etc.,

where

distortion

added

to

the

waveform

is

not

a

factor,

1-8,

The

voltmeter

chassis

is

constructed

of

aluminum

alloy

throughout.

The

panel

is

finished

in

non-reflecting,

light-grey

baked

enamel;

the

cabinet

is

finished

in

dark-blue,

baked

wrinkle

paint,

The

cabinet

is

equipped

with

rubber

feet

and

a

leather

carrying

handle.

Control

markings

on

the

front

panel

are

engraved

and

black

filled.

INPUT

and

OUTPUT

terminals

are

special

binding

posts

which

accept

either

bare

wire

or

banana

plugs;

the

3/4-inch

spacing

between

binding

posts

accepts

standard

dual-banana

plugs.

The

"ground"

side

of

the

INPUT

and

OUTPUT

terminals

is

connected

to

the

instrument

chassis

which

is

in

turn

connected

to

the

power

line

ground

through

the

third

(round)

prong

of

the

plug

on

the

power

cable.

1-2

T.O,

33A1-12-349-1

1-9.

The

voltmeter

is

equipped

with

a

non-detachable

power

cord.

Test

leads,

which

may

be

plain

wire

leads

or

coaxial

cable,

and

test

probes

must

be

supplied

by

the

user.

1-10.

Instruments

designated

Models

400DR,

400HR,

and

400LR

are

rack

mount

configurations

of

the

400D,

400H,

and

400L,

respectively.

They

are

identical

to

their

cabinet

model

counterparts

in

every

other

re-

spect.

They

are

designed

to

be

mounted

ina

stan-

dard

19inchwide

x

7inch

high

relay

rack

space.

Re-

fer

to

Appendix

C

for

Replacement

Parts

information.

1-11.

ACCESSORIES.

1-12.

Accessory

instruments

for

the

voltmeter

are

available

(not

supplied)

to

increase

its

range

of

opera-

tion

and

application,

such

as

increasing

voltage

mea-

surement

range

and

input

impedance,

converting

to

current

measurement,

providing

line

matching,

ete.,

as

follows:

a.

H-P

11004A

Line

Matching

Transformer.

Pro-

vides

balanced

135-ohm

or

600-ohm

input,

5

ke

to

600

ke.

b.

H-P11005A

Bridging

Transformer.

Allows

volt-

age

measurement

on

balanced

lines.

20

cps

to

45

kc.

c,

H-P

11039A

Capacitive

Voltage

Divider.

Safely

measures

power-frequency

voltages

to

25

kilovolts.

Division

ratio,

1000:1.

Input

capacity,

15

pf

+1

pf.

d.

H-P

11041A

Capacitive

Voltage

Divider.

Accu-

racy

+3%.

Division

ratio,

100:1.

Input

impedance,

50

megohms,

resistive,

shunted

with

2.75

pf

capa-

city.

Maximum

voltage,

1500

volts.

e.

H-P

456A

AC

Current

Probe.

Allows

current

measurements

without

breaking

the

circuit,

Sensitivity

1

mv/ma

+2%

at

1

kc.

Maximum

input

1

amp

rms;

2

amp

peak,

Output

noise

less

than

50

uv

rms,

f.

H-P

11029A-11034A

Shunt

Resistors,

For

mca-

suring

currents

as

small

as

1

microamp

full

scale,

Accuracy

+1%

to

100

ke,

+5%

to

4

me

(470A,

45%

to

1

mc),

Maximum

power

dissipation,

1

watt.

00102-3

T.O.

33A1-12-349-1

Section

IL

Paragraphs

2-1

to

2-11

SECTION

II

INSTALLATION

2-1.

UNPACKING

AND

INSPECTION.

2-2.

There

are

no

special

precautions

for

unpacking

the

voltmeter.

Save

the

shipping

carton

and

packing

materials

for

possible

storage

or

reshipment.

When

unpacking,

inspect

instrument

and

packing

materials

for

signs

of

damage

in

shipment.

Make

an

operation

check

as

directed

in

paragraph

2-10

to

determine

if

performance

is

satisfactory.

If

there

is

any

indication

of

damage,

immediately

file

a

claim

with

the

transport

service

used

or

other

cognizant

authority.

2-3.

LINE

VOLTAGE

REQUIREMENT.

2-4.

The

voltmeter

is

wired

at

the

factory

for

use

on

115-volt

a-c

power.

This

voltage

may

vary

+10%

without

adverse

effect

upon

voltmeter

performance.

The

volt-

meter

can

be

wired

for

use

on

230-volt

a-c

power

by

reconnecting

the

dual

primary

windings

on

the

power

transformer

as

shown

in

the

schematic

diagram

in

Section

V.

When

using

230-volt

power,

change

from

a

l-amp

to

a

1/2-amp

slow-blow

fuse.

If

necessary,

provide

an

adapter

for

attaching

the

standard

115-volt

plug

on

the

voltmeter

to

the

230-volt

outlet.

2-5.

POWER

LINE

CONNECTION.

2-6.

The

three-conductor

power

cable

on

the

voltmeter

is

terminated

in

a

polarized

three-prong

male

connector.

The

third

contact

is

an

offset

round

pin

added

to

a

stand-

ard

two-blade

connector,

which

grounds

the

instrument

chassis

when

used

with

the

appropriate

receptacle.

To

connect

this

plug

in

a

standard

two-contact

receptacle,

use

an

adapter.

The

chassis

ground

connection

is

brought

out

of

the

adapter

in

a

green

pigtail

lead

for

connection

to

a

suitable

ground.

2-7.

The

power

plug

normally

supplied

with

the

volt-

meter

is

made

of

molded

rubber

and

is

an

integral

part

of

the

power

cable.

On

certain

military

contracts,

a

modification

of

the

Model

400D,

termed

the

H02-400D,

is

equiped

with

a

removable

plug

having

the

same

pin

configuration

but

constructed

of

corrosion-resistant

material.

In

all

other

respects

the

H02-400D

is

the

same

as

the

Model

400D

and

carries

the

same

Federal

Stock

Number.

00102-2

The

lower

INPUT

and

OUTPUT

signal

terminals

on

the

panel

of

the

voltmeter

are

connected

directly

to

the

chassis

of

the

voltmeter.

Any

voltage

applied

to

the

lower

terminal

will

be

shorted

directly

to

ground.

If

the

ground

con-

nection

in

the

power

cord

is

disconnected

by

use

of

an

adapter,

the

entire

voltmeter

cabinet

will

carry

whatever

potential

is

applied

to

the

lower

terminal

and

may

be

a

hazard

to

the

operator.

2-8.

INSTALLATION.

2-9.

The

voltmeter

is

a

portable

instrument

requiring

no

permanent

installation.

The

voltmeter

is

for

bench-

top

operation,

standing

on

its

rubber

feet

with

its

front

panel

near

the

vertical

plane.

A

bail

is

provided

for

raising

the

front

of

the

cabinet

to

obtain

a

better

viewing

angle.

2-10.

OPERATION

CHECK.

2-11.

The

voltmeter

is

ready

for

use

as

received

from

the

factory.

The

simple

check

described

below

can

be

made

by

incoming

inspectors

to

determine

if

electrical

damage

was

incurred

in

shipment.

If

more

complete

proof

of

instrument

performance

is

required,

the

over-all

performance

check

described

in

paragraph

5-22

must

be

used.

Make

a

simple

performance

check

as

follows:

a.

Connect

voltmeter

to

the

power

line

through

a

variable

transformer.

Set

transformer

for

115

volts,

turn

on

and

allow

a

five-minute

warmup.

b.

Measure

any

sine

wave

voltage,

excepting

the

power

line,

from

0.01

to

300

volts

whose

exact

voltage

is

known.

Note

that

the

lower

INPUT

terminal

is

connected

to

the

power

line

ground.

c.

While

making

the

above

measurement,

adjust

the

line

voltage

from

103

to

127

volts,

The

reading

on

the

meter

must

not

change

by

more

than

the

width

of

the

pointer.

2-1

Section

HI

T.O.

33A1-12-349-1

REFERENCE

NUMBER

DESIGNATION

FUNCTION

Panel

meter

Indicates

rms

volts

and

decibels

of

sine

wave

signals.

Indicator

light

Indicates

that

voltmeter

is

turned

on.

ON

Power

switch

Applies

line

power

to

voltmeter.

INPUT

terminals

Receive

voltage

to

be

measured

or

signal

to

be

amplified.

RANGE

(DB-VOLTS)

switch

|

Selects

full-scale deflection

sensitivity.

OUTPUT

terminais

Supply

signal

level

proportional

to

meter

reading,with

same

waveform

as

applied

to

INPUT

terminals.

Zero

adjust

screw

Meter

zero

adjust

screw

(for

400D

and

400H

only).

Figure

3-1.

Voltmeter

Front

Panel,

Showing

Controls

and

Connectors

3-0

00102-2

T.O.

33A1-12-349-1

Section

III

Paragraphs

3-1

to

3-9

SECTION

Ill

OPERATING

INSTRUCTIONS

3-1.

INSTRUMENT

TURN-ON.

3-2.

The

voltmeter

is

ready

for

use

as

received

from

the

factory

and

will

give

specified

performance

after

a

few

minutes

warmup.

See

Section

II

for

information

regarding

connection

to

the

power

source

and

to

the

voltage

to

be

measured.

Controls

are

shown

in

figure

3-1.

3-3.

GENERAL

OPERATING

INFORMATION.

3-4.

METER

ZERO

CHARACTERISTIC.

When

the

Model

400D

and

400H

Voltmeters

are

turned

off,

the

meter

pointer

should

rest

exactly

on

the

zero

calibration

mark

on

the

meter

scale.

If it

does

not,

zero-set

the

meter

as

instructed

in

paragraph

5-7.

The

meter

supplied

in

the

Model

400L

Voltmeter

is

not

provided

with

a

mechanical

meter

zero

adjustment.

When

the

voltmeter

is

turned

on

with

the

INPUT

terminals

shorted,

the

meter

pointer

may

deflect

upscale

slightly;

this

deflection

does

not

affect

the

accuracy

of

a

reading.

NOTE

When

the

voltmeter

RANGE

switch

is

set

to

the

lowest

ranges

and

the

INPUT

terminals

are

not

terminated

or

shielded,

noise

pickup

can

be

enough

to

produce

up

to

full-scale

meter

deflec-

tion.

This

condition

is

normal

and

is

caused

by

stray

voltages

in

the

vicinity

of

the

instru-

ment.

For

maximumaccuracy

on

the

-001-volt

range,

the

voltage

under

measurement

should

be

applied

to

the

voltmeter

through

a

shielded

test

lead.

3-5.

METER

SCALES.

The

two

voltage

scales

on

each

of

the

voltmeter

models

are

related

to

each

other

by

a

factor

of

1710

(10

db).

In

conjunction

with

the

calib-

rated

RANGE

switch

steps,

this

provides

an

intermediate

range

step

spaced

10

db

between

"power

of

ten"

ranges,

which

are

20

db

apart.

The

relationship

of

the

DECIBELS

scale

to

the

0

to

1

VOLT

scale

is

determined

by

making

0

db

on

the

DECIBELS

scale

equal

to

the

voltage

required

to

produce

1

milliwatt

in

600

ohms

(0.775

volts).

Thus,

the

DECIBELS

scale

reads

directly

in

dbm

(decibels

referred

to

one

milliwatt)

across

a

600-ohm

circuit,

and

can

be

used

to

measure

absolute

level

of

sine

wave

signals.

It

can

also

be

used

to

measure

relative

levels

of

any

group

of

signals

which

have

the

same

waveform,

across

any

constant

circuit

impedance.

The

RANGE

switch

changes

voltmeter

sensitivity

in

10-db

steps

accurate

to

within

+1/8

db.

The

RANGE

switch

position

indicates

the

value

of

a

full-scale

meter

reading.

3-6.

CONNECTIONS.

Voltmeter

test

leads

must

be

provided

by

the

user.

The

type

of

leads

and

probes

used

will

depend

upon

the

application,

as

listed

below:

a

For

connection

to

low-impedance

signal

sources,

plain

wire

leads

often

are

sufficient.

00102-2

b.

For

high-impedance

sources,

or

where

noise

pickup

is

a

problem,

low-capacity

shielded

wire

must

be

used

with

a

shielded,

dual

banana

plug

for

connection

to

the

voltmeter

terminals.

c.

If

a

probe

is

used,

it

should

also

be

shielded

to

prevent

pickup

from

the

hand.

d.

For

signals

above

a

few

hundred

kilocycles,

the

capacity

of

the

test

leads

must

be

kept

to

a

minimum

by

using

very

short

leads,

preferably

unshielded.

An

alligator

clip

should

be

used

at

the

test

end

so

that

connection

can

be

made

without

adding

the

capacity

of

the

user's

hands.

3-7.

MAXIMUM

INPUT

VOLTAGE,

Do

not

apply

more

than

600

volts

dc

to

the

INPUT

terminals.

To

do

so

ex~-

ceeds

the

voltage

rating

of

the

input

capacitor.

3-8.

If

an

applied

voltage

momentarily

exceeds

the

selected

full-scale

voltmeter

sensitivity,

a

few

seconds

may

be

required

for

circuit

recovery,

but

no

damage

will

result.

3-9.

INPUT

VOLTAGE

WAVEFORM.

The

voltmeter

is

calibrated

to

indicate

the

root-mean-square

value

of

a

sine

wave;

however,

meter

pointer

deflection

is

proportional

to

the

average

value

of

whatever

waveform

is

applied

to

the

input.

If

the

input

signal

waveform

is

not

a

sine

wave,

the

reading

will

be

in

error

by

an

amount

dependent

upon

the

amount

and

phase

of

the

harmonics

present,

as

shown

in

figure

3-2

below.

When

harmonic

distortion

is

less

than

about

10%,

the

error

which

results

is

negligible.

INPUT

VOLTAGE

CHARACTERISTICS

METER

INDICATION

Fundamental

=

100

100

Fundamental

+10%

100

2nd

harmonic

Fundamental

+20%

2nd

harmonic

100-102

Fundamental

+50%

100-110

2nd

harmonic

Fundamental

+10%

3rd

harmonic

Fundamental

+20%

3rd

harmonic

Fundamental

+50%

3rd

harmonic

Note:

This

chart

is

universal

in

application

since

these

errors

are

inherent

in

all

average-respond-

ing

type

voltage-measuring

instruments.

96-104

94-108

90-116

Figure

3-2.

Effect

of

Harmonics

on

Voltage

Measurements

3-1

Section

TI

Paragraphs

3-10

to

3-16

=—

POSSIBLE

GROUND

LOOP

+

T.O.

33A1-12-349-1

INSERT

UNGROUNDED

ADAPTER

TO

BREAK

o

T

USE

GROUND

LOOP

eae

ONE

SIDE

OF

POWER

LINE

IS

ALSO

GROUNDED

Figure

3-3.

Test

Setup

for

Avoiding

Ground

Loop

3-10. Since

the

voltmeter

meter

deflection

is

propor-

tional

to

the

average

value

of

the

input

waveform,

it

is

not

adversely

affected

by

moderate

levels

of

random

noise.

The

effect

that

noise

has

on

the

accuracy

of

the

meter

reading

depends

upon

the

waveform

of

the

noise

and

upon

the

signal-to-noise

ratio.

A

square

wave

has

the

greatest

effect,

a

sine

wave

intermediate

effect,

and

"white"

noise

has

the

least

effect

on

the

meter

reading.

3-11.

If

the

noise

signal

is

a

50%

duty

cycle

square

wave

and

the

signal-to-noise

ratio

is

10:1

(between

peak

voltages),

the

error

will

be

about

1%

of

the

meter

reading.

If

the

noise

signal

is

"white"

noise

and

the

signal-to-noise

ratio

10:1,

the

error

is

negligible.

3-12.

LOW-LEVEL

MEASUREMENTS

AND

GROUND

CURRENTS.

3-13.

When

the

voltmeter

is

used

to

measure

signal

levels

below

a

few

millivolts,

ground

currents

in

the

meter

test

leads

can

cause

an

error

in

meter

reading.

Such

currents

are

created

when

two

or

more

ground

connections

are

made

between

the

instruments

of

a

test

setup

and/or

between

the

instruments

and

the

power

line

ground.

Two

ground

connections

complete

an

electrical

circuit

(ground

loop)

for the

voltages

which

are

generated

across

all

instrument

chassis

by

stray

fields,

particularly

the

fields

of

transformers.

These

ground

currents

can

be

minimized

by

disconnecting

the

ground

lead

in

the

power

cord

from

either

the

voltmeter

or

the

signal

source

being

measured,

at

the

power

outlet

as

shown

in

figure

3-3,

and

by

making

sure

that

in

the

test

setup

no

other

ground

loop

is

formed

that

can

cause

a

ground

current

to

flow

in

the

voltmeter

test

leads.

Although

the

resultant

voltage

developed

across

a

test

lead

is

in

the

order

of

micro-

volis,

it

is

enough

to

cause

noticeable

errors

in

measurements

of

a

few

millivolts.

The

presence

of

ground

currents

can

sometimes

be

determined

by

simply

changing

the

grounds

for

the

instruments

in

the

3-2

setup

and

watching

for

a

change

in

meter

reading.

If

changing

the

ground

system

causes

a

change

in

meter

reading,

ground

currents

are

present.

3-14.

MEASUREMENT

OF

VOLTAGE.

3-15.

The

meter

has

two

VOLTS

scales,

0

to

1

and

0

to

3.

When

the

RANGE

switch

is

set

to

.001,

.01,

-1,

1,

10,

or

100

VOLTS,

read

the

0

to

1

scale.

When

the

RANGE

switch

is

set

to

.003,

.03,

.3,

3,

30,

or

300

VOLTS,

read

the

0

to

3

scale.

The

lower

(black)

signal

INPUT

and

OUT-

PUT

terminals

and

the

instrument

case

are

connected

tothe

power

system

ground

when

the

instrument

is

used

witha

standard

three-

terminal

(grounding)

receptacle.

Connect

only

ground-potential

circuits

to

the

black

INPUT

and

OUTPUT

terminals.

3-16.

Operate

the

instrument

as

follows:

a.

Connect

the

voltmeter

to

the

a-c

power

source.

b.

Turn

the

Power

switch

ON

and

allow

a

warmup

period

of

approximately

five

minutes.

c.

Disconnect

any

external

equipment

from

the

OUT-

PUT

terminals.

d.

Set

the

RANGE

switch

to

the

VOLTS

range

which

will

read

the

voltage

to

be

measured

at

mid-scale

or

above.

If

in

doubt,

select

a

higher

VOLTS

range.

e.

Connect

the

voltage

to

be

measured

to

the

INPUT

terminals.

00102-2

T.O,

33A1-12-349-1

AVOID

A

SHORT

CIRCUIT

ACROSS

THE

POW-

ER

LINE!

To

measure

power

line

voltage,

first

connect

only

the

upper

(red)

INPUT

terminal

to

each

side

of

the

power

line,

in

turn,

leaving

it

connected

to

the

side

that

causes

meter

indi-

cation.

Then

connect

the

lower

(black)

INPUT

terminal

(grounded

internally)

to

the

other

side

of

the

line.

If

this

procedure

is

not

followed,

the

power

line

may

be

short-circuited

through

the

grounded

INPUT

terminal

of

the

voltmeter.

f.

Read

the

meter

indication

on

the

appropriate

VOLTS

scale,

in

accordance

with

the

full-scale

value

indicated

on

the

RANGE

switch.

Evaluate

the

reading

in

terms

of

the

full-scale

value

indicated

on

the

RANGE

switch.

Study

the

following

examples:

Example

1

When

the

RANGE

switch

is

in

the

.1

VOLTS

range,

read

the

0

to

1

VOLTS

scale.

If

the

meter

indicates

.64

on

that

scale,

the

voltage

being

measured

is:

.64

(meter

indication)

x

[eritetiseslerted|

1

[voltage

range

T

(full-scale

value)

~

*°64

volt

Example

2

When

the

RANGE

switch

is in

the

30

VOLTS

range,

read

the

0

to

3

VOLTS

scale.

If

the

meter

indicates

1.6

on

that

scale,

the

voltage

being

measured

is:

1.6

(meter

indication)

x

30

Fascseaueces|

voltage

range

3

(full-scale

value)

—

ne

volts

3-17.

MEASUREMENT

OF

DECIBELS.

3-18.

The

DECIBELS

meter

scale

is

provided

for

measuring

dbm

directly

across

600

ohms

and

for

measuring

db

ratio

for

comparison

purposes

when

each

measurement

is

made

across

the

same

circuit

impedance.

To

measure

signal

level

directly

in

dbm

(0

dbm

equals

1

milliwatt

into

600

ohms)

proceed

as

follows:

a.

Connect

the

voltmeter

to

the

a~c

power

source,

b.

Turn

the

Power

switch

ON

and

allow

a

warmup

period

of

approximately

five

minutes.

c.

Disconnect

any

external

equipment

from

the

OUT-

PUT

terminals.

d.

Set

the

RANGE

switch

to

the

DB

range

which

will

give

an

upscale reading

of

the

signal

to

be

measured.

If

in

doubt,

select

a

higher-level

scale.

e.

Connect

the

voltage

to

be

measured

to

the

INPUT

terminals,

00102-2

Section

TI

Paragraphs

3-17

to

3-21

f.

Note

the

meter

indication

on

the

DECIBELS

scale

{-12

to

+2

db).

The

signal

level

is

the

algebraic

sum

of

the

meter

indication

and

the

db

value

indicated

by

the

RANGE

selector.

Study

the

following

examples:

Example

1

If

the

indication

on

the

DECIBELS

scale

is

+2

and

the

RANGE

switch

is

in

the

+20

DB

position,

the

level

is

+22

dbm.

Example

2

If

the

indication

on

the

DECIBELS

scale

is

+1.5

and

the

RANGE

switch

is

in

the

-40

DB

position,

the

level

is

-38.5

dbm.

3-19.

To

measure

db

across

impedances

other

than

600

ohms,

follow

the

above

procedure

and

evaluate

the

results

as

follows:

NOTE

Since

the

measurement

is

made

across

other

than

600

ohms,

the

level

obtained

in

step

f

is

in

db,

but

not

in

dbm.

a.

To

obtain

the

difference

in

db

between

measure-

ments

made

across

equal

impedances,

algebraically

subtract

the

levels

being

compared.

b.

To

obtain

the

reading

of

a

single

measurement

in

dbm,

note

the

impedance

across

which

the

measure-

ment

is

made

and

refer

to

the

Impedance

Correction

Graph,

described

in

paragraph

3-20.

c.

To

obtain

the

difference

in

dbm

between

measure-

ments

made

across

different

impedances,

convert

each

measurement

to

dbm

using

the

Impedance

Correction

Graph

described

in

paragraph

3-20.

Then

algebraically

subtract

the

dbm

leveis

being

compared,

3-20.

IMPEDANCE

CORRECTION

GRAPH.

3-21.

As

the

voltmeter

DECIBELS

scale

is

calibrated

to

indicate

dbm

for

measurements

made

across

600-ohm

circuits,

a

correction

factor

must

be

used

when

meas-

urements

are

made

across

circuit

impedances

other

than

600

ohms,

if

absolute

dbm

levels

are

desired.

The

correction

factor

is

not

necessary

in

measuring

relative

db

levels

(not

dbm)

across

the

same

impedance,

but

it

is

required

for

comparison

of

db

levels

measured

across

different

impedances.

The

Impedance

Correction

Graph

in

figure

3-4

gives

the

correction

factor

for

conversion

of

the

meter

reading

to

dbm

when

the

impedance

of

the

circuit

under

test

is

known.

To

use

the

graph,

read

the

conversion

factor

corresponding

to

the

test

circuit

impedance

and add

it

to

the

meter

reading

determined

by

the

method

of

paragraph

3-17.

Observe

the

algebraic

sign

of

the

correction

factor

in

making

the

algebraic

addition.

Use

the

following

examples:

Example

1

If

the

measurement

is

made

across

90

ohms,

the

indication

on

the

DECIBELS

scale

is

+2,

and

the

RANGE

switch

is

atthe

+30

DB

position,

the

level

in

dbm

is

obtained

as

follows:

3-3

Section

II

Paragraphs

3-22

to

3-25

+

2

(meter

indication)

+30

(RANGE

switch

position)

+32

(sum)

+

8

(correction

factor

from

the

Impedance

+40

dbm

Correction

Graph)

Example

2

For

the

same

conditions

as

given

above,

except

that

the

measurement

is

made

across

an

impedance

of

60,000

ohms,

the

level

in

dbm

is

obtained

as

follows:

+

2

(meter

indication)

+30

(RANGE

switch

position)

+32

(sum)

~-20

(Correction

factor

from

the

Impedance

+12

dbm

Correction

Graph)

3-22.

USE

OF

VOLTMETER

AMPLIFIER.

3-23.

The

amplifier

in

the

voltmeter

may

be

used

for

amplifying

weak

signals.

With

full~scale

meter

deflec-

tion,

the

open-circuit

output

of

the

amplifier

is

approxi-

mately

0.15

volt

rms

regardless

of

the

RANGE

switch

position.

The

impedance

looking

into

the

OUTPUT

terminals

is

approximately

50

ohms.

The

frequency

3-4

T.O.

33A1-12-349-1

response

and

calibration

of

the

voltmeter

may

be

affected

by

the

impedance

of

a

load

applied

to

the

OUTPUT

terminals.

To

check

the

effect

of

the

applied

load:

observe

the

meter

reading

obtained

with

no

load

connected

to

the

OUTPUT

terminals

and

then

note

any

shift

of

reading

when

the

external

circuit

is

connected

to

the

OUTPUT

terminals.

If

the

shift

is

negligible,

the

measurement

is

not

being

affected

appreciably

by

the

load.

Whenever

the

input

signal

is

changed,

i.e.,

a

different

frequency

or

band

of

frequencies

is

applied,

repeat

the

quick

check

described

above.

3-24.

Maximum

gain

from

the

amplifier

is

obtainable

only

on

the

lowest

(.001

volts)

range,

since

output

level

is

the

same

for

all

bands.

This

is

due

to

the

10-db

amplification

loss

per

step

inserted

by

the

RANGE

switch

as

it

is

turned

clockwise.

Amplification

may

also

be

obtained

on

the

.003,

.01,

.03,

and

1

volt

ranges.

3-25.

When

the

voltmeter

is

used

as

an

amplifier,

select

a

range

which

gives

a

meter

deflection

near

full

scale.

Off-scale

signals

more

than

twice

the

value

of

the

position

of

the

RANGE

switch

will

cause

severe

distortion.

00102

-2