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dB
Calculating the drop in dB will result in the following:
A�dB � 20 log10 � VL
� � 20 log10 �
Vs
0.890Vs
�
Vs
� 20 log10 0.890 � �1.01 dB
substantiating the fact that there is a 1-dB attenuation.
As mentioned earlier, there are other methods for attenuation that are
more sophisticated in design and beyond the scope of the coverage of
this text. However, the above designs are quite effective and relatively
inexpensive, and they perform the task at hand quite well.
Noise Filters
Noise is a problem that can occur in any electronic system. In general,
it is the presence of any unwanted signal that can affect the overall
operation of a system. It can come from a power source (60-Hz hum),
from feedback networks, from mechanical systems connected to electrical
systems, from stray capacitive and inductive effects, or possibly
from a local signal source that is not properly shielded—the list is endless.
The manner in which the noise is eliminated or handled is normally
analyzed by someone with a broad practical background and with
a sense for the origin for the unwanted noise and how to remove it in
the simplest and most direct way. In most cases the problem will not be
part of the original design but a second effort in the testing phase to
remove unexpected problems. Although sophisticated methods can be
applied when the problem can be serious in nature, most situations are
handled simply by the proper placement of an element or two of a value
sensitive to the problem.
In Fig. 23.89 two capacitors have been strategically placed in the
tape recording and playback sections of a tape recorder to remove the
Recording
head
C n
100 pF
C1 R 1
R 3
R
P
P
R
Short circuit to
high-frequency
noise
Compensation control of high frequencies
R 2 C C
Playback
network
Record phase
(a)
Applied
signal
Coupling
capacitor
Playback
head
R 3
FIG. 23.89
Noise reduction in a tape recorder.
P
R
P
R
(b)
APPLICATIONS ⏐⏐⏐ 1077
Filter to reduce
stray pickup
R s
C C
Coupling
capacitor
Playback phase
C s
100 pF
Amplifier
and notch
filter