1-13 In summary, in a series-LC circuit with a source voltage that is below the resonant frequency (100 kHz in the example), the resultant reactance (X), and therefore impedance, is higher than at resonance. In addition current is lower, and the voltage drops across the reactances are lower. All of the above follow in sequence due to the fact that X_{C} is greater than X_{L} at any frequency lower than the resonant frequency. How the Ideal Series-LC Circuit Responds to a Frequency Above Resonance (300 kHz) Given: Again, X_{L} and X_{C} are not equal. This time, X_{L} is larger than X_{C}. (If you don't know why, apply the formulas and review the past several pages.) The resultant reactance is 2000 ohms (X_{L}-XC= 3770 -1770 = 2000 ohms.) Therefore, the resultant reactance (X), or the impedance of our perfect circuit at 300 kHz, is 2000 ohms. By applying Ohm's law as before: In summary, in a series-LC circuit with a source voltage that is above the resonant frequency (300 kHz in this example), impedance is higher than at resonance, current is lower, and the voltage drops across the reactances are lower. All of the above follow in sequence from the fact that XLis greater than X_{C} at any frequency higher than the resonant frequency. Summary of the Response of the Ideal Series-LC Circuit to Frequencies Above, Below, and at Resonance The ideal series-resonant circuit has zero impedance. The impedance increases for frequencies higher and lower than the resonant frequency. The impedance characteristic of the ideal series-resonant circuitresults because resultant reactance is zero ohms at resonance and ONLY at resonance. All other frequencies provide a resultant reactance greater than zero. Zero impedance at resonance allows maximum current. All other frequencies have a reduced current because of the increased impedance. The voltage across the reactance is greatest at resonance because voltage drop is directly proportional to current. All discrimination between frequencies results from the fact that X_{L} and X_{C} completely counteract ONLY at the resonant frequency.

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