Figure 123. Use of oscillator and transmission measuring set.
4-29.
SUMMARY
g. To accomplish this, 1 mw is designated
Here are the most important points covered
as being equal to 0 dbm.
in this information sheet:
h. This 0 dbm is then used as a reference
a. The db is a transmission measuring unit
level and all losses and gains are compared to 0
used to express power loss and gain in a telephone
dbm. This means that a level of -3 dbm is 3 db
system.
below the reference level. And, since a 3 db loss
represents a 50 percent power loss, a level of -3 dbm
b. A minus sign placed before db (-3 db)
is equal to .5 mw. Similarly, a level of +3 dbm is
indicates a power loss.
equal to 2 mw since a gain of 3 db doubles the
power.
c. A plus sign placed before db (+3 db)
indicates a power gain.
i. The frequency used with the reference
level of dbm is 1,000 Hertz (1 kHz).
d. Db losses and gains in a circuit are
added algebraically.
j. This, then, is the standard frequency and
testing power used in telephone work: 1 kHz at 0
e. The db by itself indicates an amount of
dbm.
power loss or gain but it does not tell exactly how
much power is involved.
k. In transmission measurement 1 kHz at 0
dbm is supplied by an oscillator. And a decibel
f. A reference level of 1 mw of power is
meter is used to measure the amount of db loss or
used with the db so that the db can express a definite
gain at many points along the circuit.
amount of power.
344 L4
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