Marconi Instruments TF 2502 - Measurement Precautions; Making Power Measurements

Marconi Instruments TF 2502

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(b)

The

catch

on

the

terminating

load.

(c)

The

plug

and

socket

of

the

cable

joining

thedetector

unit

and

the

monitor.

2.4

MEASUREMENT

PRECAUTIONS

When

malting

a

measurement,

the

following

points

should

be

borne

in

mind:

1)

Ideally,

the

impedance

presented

by

the

power

meter

to

a

transmitter

under

test

should

be

purely

resistive.

In

practice,

the

power

meter

impedance

will,

inevitably,

include

a

small

reactive

component

which

will

be

reflected

back

into

the

transmitter

tuned

output

circuit,

partially

detuning

it.

In

order

to

obtain

a

true

measure

of

the

power

output

capabilities

of

a

transmitter,

its

output

stage

should

always

be

tuned

to

produce

maximum

indication

on

the

power

meter.

2)

In

circumstances

where

the

power

meter

is

required

for

use

merely

as

a

terminating

device,

it

is

better

to

make

connection

directly

to

the

terminating

load

as

this

presents

a

particularly

high

quality

resistive

termination

having

a

v.

s.

w.

r.

of

about

1.

04.

To

remove

the

load

for

this

purpose

see

Sect.

2.

6.

3)

Because

of

the

time

lag

in

thermocouple

heating,

and

the

low

resistance

of

the

meter

circuit,

wait

a few

seconds

before

reading

the

power.

4)

The

peak

voltage

that

can

be

applied

to

the

power

meter

is

limited

by

the

in-

line

detector

insulation;

this

is

rated

to

withstand

60 V

peak,

which

exceeds

the

peak

voltage

of

a 100%

sine

wave

amplitude

modulated

signal

of

10 W

mean

power.

5)

In

the

event

of

long-term

storage

in

conditions

of

extreme

humidity,

check

50

Hz

calibration

before

use

(refer

to

Sect.

5. 4).

If

necessary,

dry

out

by

placing

the

power

meter

in

an

oven (+50

°C

to +70 °C)

for

at

least

one

hour.

2.5

MAKING

POWER

MEASUREMENTS

To

make

a

power

measurement

proceed

as

follows:

1)

Set

the

Range

switch

to

suit

the

expected

power

level

of

the

source.

If

this

is

unknown,

use

the 10 W

position

as

a

precaution.

2)

Turn

the

INSERT

PROBE

and

Sensitivity

controls

fully

counter-clockwise.

3)

Connect

the

source

under

test

to

the

Input

socket

and

note

the

reading

on

the

DETECTOR

meter.

Bring

this

reading

to

about

half

scale

by

turning

up

the

Sensi-

tivity

control.

If

this

is

insufficient,

turn

the

INSERT

PROBE

control

clockwise.

NOTE:

If

the

probe

is

inserted

too

far,

the

v.

s.

w.

r.

of

the

instrument

may

increase.

Do

not

insert

it

until

the

Sensitivity

control

is

at

maximum.

8 2502 (1a)