Allied Radio Knight KG-640 Manuel de l'opérateur - Page 11

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HOW

TO MEASURE

DB (Decibles)

AC output voltages are often measured in units called Decibels,

which are used to indicate power levels in amplifiers or general

telephone work. The DB

scale

(bottom

meter

scale)

is based

on the voltage developed across a 600 ohm line when .001 watt

is dissipated.

This

voltage

is assigned

the

reference

0 db.

Such a voltage deflects the pointer to .775 volts on the 0-2 volt

AC range. Therefore, a direct meter reading in terms of deci-

bels can be made

only when

the meter

is connected

across

600-ohm

resistive

load. Otherwise

only

relative

db measure-

ments

can be obtained. However,

in a large number

of cases,

relative

measurements

are

appropriate,

since

reference

con-

ditions are defined by other factors and only relative variations

are important.

To measure

DB, proceed as follows:

[] Plug the black test lead

into the jack marked

—COM.

Plug

the

red

test

lead

into

the

jack

marked

OUTPUT.

NOTE:

When

no DC

voltage

is present,

the VOA

jack can

be used since the DC blocking capacitor is not required.

(] Rotate the selector switch to the highest AC range

and

work

down

to one

which

will provide

a reading

at the

right hand side of the scale.

[J The meter pointer will indicate a reading in DB.

AC VOLTAGE

RANGE