Figure 122. How dbm is used in a telephone system.
c. Then the power represented by -30 dbm
little hard to understand, think back to the money
(actually .001 mw) enters the repeater and becomes
example. Remember how at the end of the year you
amplified. The repeater provides a gain of 36 db.
had saved above the reference level of 0.
This brings the power at the output of the repeater up
You called this + a NET GAIN for the whole
to a new level of +6 dbm (which represents 4 mw).
d. Next, as the power moves toward the
THE STANDARD FREQUENCY FOR
end of the line, it suffers another loss of 12 db. This
gives a final OUTPUT LEVEL of -6 dbm (actual
power is .25 mw).
a. You know now that 0 dbm represents 1
mw of power. And this 1 mw, in turn, represents the
voice power. Now, since voice power is made up of
ac voltage and current you must also be concerned
a. You can see that db losses and gains are
with frequency. This means that when you state that
added algebraically. That is, a +36 db and a -30 db
the input level to a circuit is 0 dbm you must also
gives a +6 db. And a +6 db and a -12 db gives a -6
specify the frequency.
b. The standard frequency used for testing
b. Notice that db and dbm are not used in
in telephone work is 1,000 Hertz (1 kHz). When
the same way. The db is used to express the amount
you make db loss and gain measurements you'll use
of loss or-gain. Then, after the loss or gain has taken
a device that feeds a frequency of 1 kHz at a level a1
place, dbm is used to express the new power level
0 dbm to the line input. Then, at various points
arrived at because of the loss or gain.
along the line, another device will be used to
measure the db loss or gain. This device will not
c. By using 0 dbm as the reference level
only tell how much loss or gain there is but it will
you can easily tell the net loss in the circuit. Even
also indicate the level in dbm.
though the power level changed several times along
the circuit the received power is at a level of -6 dbm.
c. These devices are used as shown in
The letter m after db tells us that this is 6 db below
figure 123. The one that supplies the power is called
the input level of 0 dbm. So we can say that,
an oscillator. And the one that measures the power
regardless of how much loss or gain has taken place,
is called a decibel meter.
the circuit NET LOSS is only 6 db. If this seems a