Lesson 1/Learning Event 3
(5) The usual practice in communication systems is to employ a fixed-
gain amplifier and to vary the input signal level by means of an
attenuator. Assume, for example, that we will use an amplifier which has
a fixed gain of 52 dB, a 60-dB input attenuator variable in 1 dB steps,
maximum undistorted power output of 34 dBm, and input signal level of -10
dBm.
If the signal with no attenuation is placed directly into the
amplifier, 52 dB gain will increase the signal level to 42 dBm output;
distortion is the result.
The attenuator must, therefore, be placed in
the input circuit of the amplifier, not the output; it does no good to
reduce the level of a signal after it has been distorted.
(6) Assume that this amplifier with -10 dBm input signal level is to
drive the next stage with signal power of 20 dBm.
The input attenuator
must be adjusted to provide a 22-dB loss, thus reducing the signal level
to -32 dBm.
In this way the 52 dB gain of the amplifier can raise the
signal level from -32 dBm to the required level of 20 dBm.
c. Energy
Level
Diagram.
Transmission
characteristics
of
a
communication system can be illustrated by an energy level diagram. Such
a diagram shows levels, gains, and losses.
Analysis of an energy level
diagram shows that the gains or losses of circuits, stated in decibels,
can be added algebraically; however, levels in the dBm cannot be added.
On the other hand, dB figures can be added algebraically to dBm figures
and the result is the change from the original power level to a new level
in dBm.
Example 1:
If the input to an amplifier is 5 dBm and the output is
25 dBm, the gain of the amplifier is the difference
between these two levels, or 20 dBm.
Example 2:
If the input to an amplifier is 5 dBm and the gain of the
amplifier is 20 dB, the output level is the sum of the
two, or 25 dBm.
Example 3:
If the input to an amplifier is 25 dBm and the amplifier
gain is 20 dB, the output level is the difference between
the two, or 5 dBm.
d. Energy Level Diagram of Equal-Level Patching. A slightly different
type of level diagram is shown in figure 1-6. This figure illustrates the
energy level diagram of an equal-level patching system.
The 2W receive
line illustrated terminates in an item of equipment.
Attenuators and
fixed-gain amplifiers are used to adjust the levels to required values.
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