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, respectively. CAPACITORS 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. Function 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. Limitations 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 with frequency. 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: LEAD LENGTH CROSSOVER FREQUENCY 1 inch 0.47 MHz 2 inches 0.41 MHz 3 inches 0.34 MHz 4 inches 0.30 MHz 6 inches 0.25 MHz 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. 10-8