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Voltage 20 log
10
0.775V
Let's look at how dB and dBm are actually used. As shown in the Fig. A-
38, if we assume the presence of attenuation on the transmission line and
the exchange, then the signal that is transmitted from the transmitter at a
level of -13 dBm becomes -27.5 dBm at the receiver.
-13dBm
Transmitter
Subscriber line
Attenuation: -7 dB
For reference, Table A-2 shows the conversion table for ratio and dB.
Table A-3 shows the conversion table for dBm and power/voltage on a 600
ohm impedance system.
Table A-2 Conversion Table
for Ratio and dB
dB
Power
+20
100.0
+10
10.0
0
1.0
-10
0.1
-20
0.01
(3) dBV
dBV is the absolute indication of the voltage when the standard voltage is
taken to 1 V. If the voltage of the measurement point is taken to be E V,
then dBV is defined in the following equation:
E
20 log
[dBV]
10
1
The dBm indication of the voltage on a 600 ohm impedance system is
described as:
E
20 log
[dBm]
10
0.775
where impedance of the measure-
E
[dBm]
ment point is 600 ohm
-20dBm
-20.5dBm
Exchange
Internal
attenuation: -0.5 dB
Fig. A-38 How dB and dBm are used
Table A-3 Conversion Table for dBm
and Power/Voltage on a 600 ohm
Voltage
dBm
10.0
3.2
1.0
0.3
0.1
-27.5dBm
Receiver
Subscriber line
Attenuation: -7 dB
Impedance System
Power
Voltage
0
1.0mW
0.775V
-10
0.1
0.245
-20
0.01
0.0775
-30
0.001
0.0245
-40
0.0001
0.00775
1
2
3
4
5
6
A
çi
A–37
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