Lesson 1/Learning Event 3
(2) Impedance mismatch also causes reflection loss, which is a
technical term to describe "bouncing back" of the unused part of the
energy in the signal wave.
Assume that a voice-frequency AC signal is
traveling down a line in which an impedance mismatch exists.
When the
wave reaches the point where the mismatched impedances are joined
together, part of the wave is reflected back to the source.
This
reflection loss causes both attenuation and distortion.
(3) You may have little control over the impedance of circuits in
communication equipment.
The important rule to observe is to avoid
upsetting any of the matched impedance conditions that have been
engineered into the equipment.
b. Attenuators and Pads.
In various elements of a communication
system it is necessary to control the gain of amplifiers and the losses of
other circuits without disturbing the matched impedance conditions.
Attenuators and pads are employed for this purpose. Attenuators and pads
always introduce I2R losses because they are made of resistors.
(1) A pad is a fixed attenuator which inserts a desired loss between
two impedances that may or may not be matched, depending on circuit
conditions.
Pads may be designed to suit either condition, but usually
impedances will be matched in a well-engineered system.
For example,
practically all fixed-plant communication equipment and auxiliary items
are designed for input and output impedances of 600 ohms.
(2) The "T" and "H" pads shown in figure 1-3 are the most common types
used in military communications. One side of the unbalanced or "T" pad is
grounded, while the balanced or "H" type pads usually do not have grounded
connections. The "T" pads or attenuators are used on unbalanced circuits,
while the "H" pads or attenuators are used on balanced circuits.
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