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The decibel (dB) is a measurement unit
important in describing networking signals. The decibel is related to
the exponents and logarithms described in prior sections. There are
two formulas for calculating decibels:
- dB = 10 log10 (Pfinal / Pref)
- dB = 20 log10 (Vfinal / Vreference)
The variables represent the following
values:
- dB measures the loss or gain of the
power of a wave. Decibels are usually negative numbers representing
a loss in power as the wave travels, but can also be positive values
representing a gain in power if the signal is amplified
- log10 implies that the
number in parenthesis will be transformed using the base 10
logarithm rule
- Pfinal is the delivered
power measured in Watts
- Pref is the original
power measured in Watts
- Vfinal is the delivered
voltage measured in Volts
- Vreference is the
original voltage measured in Volts
The first formula describes decibels in
terms of power (P), and the second in terms of voltage (V). Typically,
light waves on optical fiber and radio waves in the air are measured
using the power formula. Electromagnetic waves on copper cables are
measured using the voltage formula. These formulas have several things
in common.
Enter values for dB and Pref to discover the correct power.
This formula could be used to see how much power is left in a radio
wave after it has traveled over a distance through different
materials, and through various stages of electronic systems such as a
radio. To explore decibels further, try the following examples using
the flash activities:
- If Pfinal is one microWatt (1 x 10-6
Watts) and Pref is one milliWatt (1 x 10-3
Watts), what is the gain or loss in decibels? Is this value positive
or negative? Does the value represent a gain or a loss in power?
- If the total loss of a fiber link is
-84 dB, and the source power of the original laser (Pref)
is one milliWatt (1 x 10-3 Watts), how much power is
delivered?
- If two microVolts (2 x 10-6
Volts) are measured at the end of a cable and the source voltage was
one volt, what is the gain or loss in decibels? Is this value
positive or negative? Does the value represent a gain or a loss in
voltage?
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Interactive Media Activity
Interactivity: Calculating Gain
This activity allows the user to enter the final
voltage and the reference voltage to get gain in decibels.
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Interactive Media Activity
Interactivity: Using Decibels
This activity allows the user to enter a value for the
decibels and a value for the reference power resulting in the
final power.
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