Lesson 1/Learning Event 3
Learning Event 3: ATTENUATION AND ATTENUATORS.
Attenuation can be defined as loss of signal power, and may be
classed as either intentional or unintentional. Loss of signal power can
be attributed, in most cases, to impedance mismatch (reflection loss) and
to heat loss. Reflection loss is unintentional as is also heat loss due
to resistance in lines. However, heat loss is intentional when values of
resistance are willfully inserted in the circuit.
Heat loss is usually
called
I2R loss.
a. Importance of Impedance Matching. A communication system consists
of many parts which must be connected together by means of telephone lines
or other facilities.
Each element of the system has a given input and
output impedance which must be matched to the line facilities.
The
purpose of impedance matching is twofold: To transfer maximum power with
minimum loss, and to prevent reflection loss with consequent phase
distortion of the signal elements.
(1) Maximum transfer of power is demonstrated in figure 1-2 by three
simple DC circuits consisting of a battery and two resistors.
Each
circuit represents a transmission line, with R1 being the source impedance
and R2 the load impedance. The battery represents the source of energy.
Calculations shown on the figure prove that power (0.24 watt) delivered to
the load is greatest when impedances are matched (both 600 ohms).
When
load impedance is greater or lower than source impedance, power delivered
to the load is reduced (0.18) watt.
Figure 1-2.
Maximum Power Transfer from Source to Load.
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