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(2) dBm
dB described above is based upon the power ratio between the input termi-
nal and the output terminal. In effect, this is a relative indication.
Absolute indication can be achieved provided that the reference for com-
parison is fixed to a certain value. Generally, the following is supposed:
As a result
Therefore
If we assume that 1 mW is taken to be the standard power, then 0.775 V
becomes the standard voltage for voltage conversion. The absolute indica-
tion is as follows and is expressed in units of dBm.
A–36
• A very wide range of numerical values can be expressed as small
numerical values by using dB. Small ratios can also be expressed con-
veniently.
Example:
When the power ratio is 1, 0 dB is used; when 10, 10 dB is used, and
when 100, 20 dB is used. Even at 1,000,000, only 60 dB is used. So, a
smaller numerical value range is sufficient.
• When expressed as a ratio, the total gain and total attenuation must be
calculated by the product of each gain and each attenuation, respec-
tively. Whereas, the calculation is simpler if total gain and total attenu-
ation area expressed as dB. The total gain and total attenuation can be
calculated by simply adding each gain and each attenuation, respec-
tively.
Current I
Voltage E
Fig. A-37 Assumption in 600 ohm impedance series
2
E
P
=
R
2
E
=
P×R
E
=
=
P×R
1×10
E
I
=
=
1.29[mA]
R
P
Power 10 log
[dBm]
10
1mW
Standard power
600
1mW
Load resistance
=
0.775[V]
-3
×600
:1mW
:600
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