THE DECIBEL SYSTEM  14188_51
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THE DECIBEL SYSTEM  14188_50
MEASUREMENT METHODS
Neets Module 16Introduction to Test Equipment
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You do not need to use the formula in most applications. The following shows conversions of dBm
to mW:
+20dBm
=
100mW
+10dBm
=
10mW
+7dBm
=
5mW
+6dBm
=
4mW
+4dBm
=
2.5mW
+3dBm
=
2mW
0dBm
=
1mW
3dBm
=
.5mW
10dBm
=
.1mW
For a +10 dBm level, start with the 1 milliwatt reference and move the decimal point one place to the
right (+10 dBm = 10 mW). Another 10 dB increment brings the power level to +20 dBm, thereby moving
the decimal point another place to the right (+20 dBm = 100 mW). For a

10 dBm level, again start with
1 milliwatt, but this time move the decimal point one place to the left (

10 dBm = .1 mW). An additional
10 dB decrease results in another decimal point shift to the left (

20 dBm = .01 mW).
For a 3 dB increase, you double the power. For a 3 dB decrease, you reduce the power by onehalf
(+3 dBm = 2 mW and

3 dBm = .5 mW). A +6 dBm level is an additional 3 dB change from +3 dBm. In
this case, you just double the power level of the +3 dBm (+6 dBm = 4 mW).
Q3.
What milliwatt value is equal to +6 dBm?
The dB change can be made in either direction. For example, +7 dBm is a decrease from +10 dBm.
Reducing the +10 dBm power by onehalf, we have +7 dBm, or 5 mW. A +4 dBm power level is a 3 dB
decrease from +7 dBm (+4 dBm

2.5 mW). By using this simple method, you can quickly find any
power level that corresponds to a given dBm.
Some test instruments you will be using are calibrated in decibels and have a 1 milliwatt zero
reference level. Figure 21 illustrates such an instrument. Notice that this is an ac
voltmeter
in which the
upper scale of the meter indicates ac voltage and the lower scale indicates decibels. The zero powerlevel
indicator on the decibel scale is located at, or near, center scale. If the power in the line being measured is
more than the reference value, the meter will indicate a value to the right of the zero mark (+dB). If the
power is less than the reference value, the meter will indicate a value to the left of the zero mark (

dB).
Such meters are useful when recording measurements where a direct indication in decibels is desired.
However, you must remember that this meter is still a
voltmeter
and that power measurements are not
meaningful unless the circuit
impedance
is known. If you feel the need to review how to calculate power
in ac circuits, refer to NEETS, Module 2.
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