The Decibel Measurement System  Continued
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The Decibel Measurement System
The TravelingWave Tube
Neets Module 11Microwave Principles
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Table 21.—Decibel Power Ratios
Source Level (dB)
Power Ratio
1
=
1.3
3
=
2.0
5
=
3.2
6
=
4.0
7
=
5.0
10
=
10 = 10
_{1}
20
=
100 = 10
_{2}
30
=
1000 = 10
_{3}
40
=
10,000 = 10
_{4}
50
=
100,000 = 10
_{5}
60
=
1,000,000 = 10
_{6}
70
=
10,000,000 = 10
_{7}
100
=
10
_{10}
110
=
10
_{11}
140
=
10
_{14}
Examine table 21 again, and take particular note of the power ratios for source levels of 3 dB and 6
dB. As the table illustrates, an increase of 3 dB represents a doubling of power. The reverse is also true. If
a signal decreases by 3 dB, half the power is lost. For example, a 1,000 watt signal decreased by 3 dB will
equal 500 watts while a 1,000 watt signal increased by 3 dB equals 2,000 watts.
The attenuator is a widely used piece of test equipment that can be used to demonstrate the
importance of the decibel as a unit of measurement. Attenuators are used to reduce a signal to a smaller
level for use or measurement. Most attenuators are rated by the number of decibels the signal is reduced.
The technician's job is to know the relationship between the dB rating and the power reduction it
represents. This is so important, in fact, that every student of electronics should memorize the
relationships in table 21 through the 60 dB range. The technician will have to apply this knowledge to
prevent damage to valuable equipment. A helpful hint is to note that the first digit of the source level (on
the chart) is the same number as the corresponding power of 10 exponent; i.e., 40 dB = 1
10
^{4}
or 10,000.
A 20 dB attenuator, for example, will reduce an input signal by a factor of 100. In other words, a 100
milliwatt signal will be reduced to 1 milliwatt. A 30 dB attenuator will reduce the same 100milliwatt
signal by a factor of 1,000 and produce an output of 0.1 milliwatt. When an attenuator of the required size
is not available, attenuators of several smaller sizes may be added directly together to reach the desired
amount of
attenuation
. A 10 dB attenuator and a 20 dB attenuator add directly to equal 30 dB of
attenuation. The same relationship exists with amplifier stages as well. If an
amplifier
has two stages rated
at 10 dB each, the total
amplifier gain
will be 20 dB.
When you speak of the dB level of a signal, you are really speaking of a logarithmic comparison
between the input and
output signals
. The input signal is normally used as the reference level. However,
the application sometimes requires the use of a standard reference signal. The most widely used reference
level is a 1milliwatt signal. The standard decibel abbreviation of dB is changed to dBm to indicate the
use of the 1milliwatt standard reference. Thus, a signal level of +3 dBm is 3 dB above 1 milliwatt, and a
signal level of

3 dBm is 3 dB below 1 milliwatt. Whether using dB or dBm, a plus (+) sign (or no sign at
all) indicates the output signal is larger than the reference; a minus (

) sign indicates the output signal is
less than the reference.
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