the following methods: short circuiting, dissipation,
open circuiting, or a combination of all three.
Discrete components are normally used to
achieve interference reduction at the source.
Capacitors, resistors, and inductors are used to short
circuit, dissipate, and open circuit the interference,
Short circuiting of interference is done by using
capacitors connected across the source. The perfect
capacitor looks like an open circuit to dc or the power
frequency, and progressively as a shoft circuit to ac as
the frequency is increased.
The function of a capacitor in connection with
radio interference filtering is to provide a
low-impedance, radio-frequency path across the
source. When the reactance of the capacitor is lower
than the impedance of the power lines to the source,
high-frequency voltages see the capacitor as a shorter
path to ground.
The capacitor charges to the line
voltage. It then tends to absorb transient rises in the
line voltage and to provide energy for canceling
transient drops in the line voltage.
The efficiency of a perfect capacitor in bypassing
radio interference increases indirect proportion to the
frequency of the interfering voltage, and in direct
proportion to the capacitance of the capacitor. All
capacitors have both inductance and resistance. Any
lead for connecting the capacitor has inductance and
resistance as a direct function of lead length and
inverse function of lead diameter. Some resistance is
inherent in the capacitor itself in the form of dielectric
leakage. Some inductance is inherent in the capacitor,
which is usually proportional to the capacitance.
The effect of the inherent resistance in a
high-grade capacitor is negligible as far as its filtering
action is concerned. The inherent inductance, plus
the lead inductance, seriously affects the frequency
range over which the capacitor is useful. The bypass
value of a capacitor with inductance in series varies
At frequencies where inductive reactance is much
less than capacitive reactance, the capacitor looks
very much like a pure capacitance. As the frequency
approaches a frequency at which the inductive
reactance is equal to the capacitive reactance, the net
series reactance becomes smaller until the resonant
frequency, a point of zero impedance, is reached. At
this point, maximum bypass action occurs. At
frequencies above the resonant frequency, the
inductive reactance becomes greater than the
capacitive reactance. The capacitor then exhibits a
net inductive reactance, whose value increases with
frequency. At frequencies much higher than the
resonant frequency, the value of the capacitor as a
bypass becomes lost.
The frequency at which the reversal of reactance
occurs is controlled by the size of the capacitor and
the length of the leads. For instance, the installation
of a very large capacitor frequently requires the use of
long leads. As an example of the influence of lead
length upon the bypass value of a capacitor, the
following data is presented to a typical 4-microfarad
capacitor whose inherent inductance is 0.0129 henrys:
Note that for the 4-µF capacitor, each additional
inch of lead causes the capacitance-inductance
crossover point to be reduced.
In figure 10-2, notice the capacitance-to-
inductance crossover frequencies for various lead
lengths of a 0.05 microfarad capacitor. Also, notice
the difference in the crossover frequencies for the
3-inch lead for the 4-µF capacitor, discussed above,
and for the 3-inch lead for the 0.05-capacitor
referenced in figure 10-2.
Coaxial Feedthrough Capacitors
Coaxial feedthrough capacitors are available with
capacitances from 0.00005 to about 2 µF. These
capacitors work well up to frequencies several times
those at which capacitors with leads become useless.