C.2
dBm AND dBW
619
networks into an equivalent single network regarding gain or loss. We are often faced
with such a problem in the real world. Remember, we add the dB values in each network
algebraically.
Look what happens when we combine these four networks into one equivalent network.
We just sum:
+12 - 28 + 7 - 11 = -20, and -20 dB is a number we can readily handle.
Thus the equivalent network looks like the following:
To see really how well you can handle dBs, the instructor might pose a difficult problem
with several networks in series. The output power of the last network will be given and
the instructor will ask the input power to the first network. Let us try one like that so the
instructor will not stump us.
First sum the values to have an equivalent single network:
+23 + 15 - 12 = +26 dB.
Thus,
We first must learn to ask ourselves: Is the input greater or smaller than the output?
This network has gain, thus we know that the input must be smaller than the output. By
how much? It is smaller by 26 dB. What is the numeric value of 26 dB? Remember,
20 dB is 100; 23 dB is 200, and 26 dB is 400. So the input is 1/400 of the output or
40
/400 (mW) = 0.1 mW.
C.2
dBm AND dBW
These are the first derived decibel units that we will learn. They are probably the most
important. The dBm is also a ratio. It is a decibel value related to one milliwatt (1 mW).
The dBW is a decibel value related to one watt (1 W). Remember the little m in dBm
refers to milliwatt and the big W in dBW refers to watt.
The values dBm and dBW are measures of real levels. But first we should write the
familiar dB formulas for dBm and dBW:
Value (dBm)
= 10 log P
1
/(1 mW),
Value (dBW)
= 10 log P
1
/(1 W).
Summary :
C.2 dBm AND dBW 619 networks into an equivalent single network regarding gain or loss. But first we should write the familiar dB formulas for dBm and dBW: Value (dBm) = 10 log P 1 /(1 mW), Value (dBW) = 10 log P 1 /(1 W).
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