the following methods: short circuiting, dissipation,open circuiting, or a combination of all three.Discrete components are normally used toachieve interference reduction at the source.Capacitors, resistors, and inductors are used to shortcircuit, dissipate, and open circuit the interference,respectively.CAPACITORSShort circuiting of interference is done by usingcapacitors connected across the source. The perfectcapacitor looks like an open circuit to dc or the powerfrequency, and progressively as a shoft circuit to ac asthe frequency is increased.FunctionThe function of a capacitor in connection withradio interference filtering is to provide alow-impedance, radio-frequency path across thesource. When the reactance of the capacitor is lowerthan the impedance of the power lines to the source,high-frequency voltages see the capacitor as a shorterpath to ground.The capacitor charges to the linevoltage. It then tends to absorb transient rises in theline voltage and to provide energy for cancelingtransient drops in the line voltage.LimitationsThe efficiency of a perfect capacitor in bypassingradio interference increases indirect proportion to thefrequency of the interfering voltage, and in directproportion to the capacitance of the capacitor. Allcapacitors have both inductance and resistance. Anylead for connecting the capacitor has inductance andresistance as a direct function of lead length andinverse function of lead diameter. Some resistance isinherent in the capacitor itself in the form of dielectricleakage. Some inductance is inherent in the capacitor,which is usually proportional to the capacitance.The effect of the inherent resistance in ahigh-grade capacitor is negligible as far as its filteringaction is concerned. The inherent inductance, plusthe lead inductance, seriously affects the frequencyrange over which the capacitor is useful. The bypassvalue of a capacitor with inductance in series varieswith frequency.At frequencies where inductive reactance is muchless than capacitive reactance, the capacitor looksvery much like a pure capacitance. As the frequencyapproaches a frequency at which the inductivereactance is equal to the capacitive reactance, the netseries reactance becomes smaller until the resonantfrequency, a point of zero impedance, is reached. Atthis point, maximum bypass action occurs. Atfrequencies above the resonant frequency, theinductive reactance becomes greater than thecapacitive reactance. The capacitor then exhibits anet inductive reactance, whose value increases withfrequency. At frequencies much higher than theresonant frequency, the value of the capacitor as abypass becomes lost.The frequency at which the reversal of reactanceoccurs is controlled by the size of the capacitor andthe length of the leads. For instance, the installationof a very large capacitor frequently requires the use oflong leads. As an example of the influence of leadlength upon the bypass value of a capacitor, thefollowing data is presented to a typical 4-microfaradcapacitor whose inherent inductance is 0.0129 henrys:LEAD LENGTHCROSSOVER FREQUENCY1 inch0.47 MHz2 inches0.41 MHz3 inches0.34 MHz4 inches0.30 MHz6 inches0.25 MHzNote that for the 4-µF capacitor, each additionalinch of lead causes the capacitance-inductancecrossover point to be reduced.In figure 10-2, notice the capacitance-to-inductance crossover frequencies for various leadlengths of a 0.05 microfarad capacitor. Also, noticethe difference in the crossover frequencies for the3-inch lead for the 4-µF capacitor, discussed above,and for the 3-inch lead for the 0.05-capacitorreferenced in figure 10-2.Coaxial Feedthrough CapacitorsCoaxial feedthrough capacitors are available withcapacitances from 0.00005 to about 2 µF. Thesecapacitors work well up to frequencies several timesthose at which capacitors with leads become useless.10-8

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