Figure 121. The use of a reference level.
c. Using this method, at the end of a year-
being equal to 0 db. Then, to make sure no one
you'd be able to tell exactly what your net savings
forgets that 1 mw is the reference level, a small letter
were for that year. All you'd need to do is check the
m is tacked on after 0 db like this: 0 dbm. The letter
last month. In the graph you can see that the last
m, of course, stands for 1 mw. Summing up, then:
month shows a level of + (above 0). This
The standard reference level used in telephone work
means that, although your account has changed
is 1 mw which is expressed as 0 dbm. Remember,
many times during the year above and below 0,
whenever you see 0 dbm it means 1 milliwatt of
you still come out with a NET GAIN of (above
power.
0). The actual amount of money that this
represents is 0. Now just as we use an amount of
4-26.
HOW YOU USE THIS REFERENCE
money here for a reference level, in telephone work
LEVEL
we use an amount of power as a reference level.
a. The easiest way to find out is to look at
4-25.
THE REFERENCE LEVEL FOR DBM IS 1
what happens in an average telephone system like
MILLIWATT OF POWER
that shown in figure 122.
a. The reference level used in telephone
b. In the figure you see a telephone system
work is .001 watt (1 mw) of power. This level was
with a graph (energy level diagram) below it. The
chosen because it represents the average amount of
graph shows what's happening to the voice power as
power generated by the voice in a telephone
it travels along the system. You can see that the
transmitter. And by using 1 mw you can compare
INPUT LEVEL is 0 dbm (meaning 1 mw). The first
all db losses and gains in a circuit to this reference
section of line extending from the line input to the
level.
repeater gives a loss of 30 db. This brings the power
level at the repeater input down to -30 dbm (30 db
b. For convenience, 1 mw is designated as
below the reference level of 0 dbm).
344 L4
154
Previous Page
