year went on, if you deposited in February, you'd plot + (above 0). Then, if in March you had to
withdraw , you'd plot this to show that you had - (below 0). And as the year progressed, you'd plot
the other months as shown.
c. Using this method, at the end of a year you'd be able to tell exactly what your net savings were for that
year. All you'd need to do is check the last month. In the graph you can see that the last month shows a level of
+ (above 0). This means that, although your account has changed many times during the year above and
below 0, you still come out with a net gain of (above 0). The actual amount of money that this
represents is 0. Now just as we use an amount of money here for a reference level, in telephone work we
use an amount of power as a reference level.
8. The reference level for dBm is 1 mW of power.
a. The reference level used in telephone work is .001 watt (1 mW) of power. This level was chosen
because it represents the average amount of power generated by the voice in a telephone transmitter. And by
using 1 mW, you can compare all dB losses and gains in a circuit to this reference level.
Figure 3-12. The use of a reference level.
b. For convenience, 1 mW is designated as being equal to 0 dB. Then, to make sure no one forgets that 1
mW is the reference level, a small letter m is tacked on after 0 dB like this: 0 dBm. The letter m, of course,
stands for 1 mW. Summing up, then: The
SS0031
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