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<p>In general, neither impedance nor admittance can vary with time, since they are defined for complex exponentials in which . If the complex exponential voltage to current ratio changes over time or amplitude, the circuit element cannot be described using the frequency domain. However, many components and systems (e.g., <a href="page.php?w=varicap">varicap</a>s that are used in <a href="page.php?w=Tuner_%28radio%29">radio tuners</a>) may exhibit non-linear or time-varying voltage to current ratios that seem to be <a href="page.php?w=LTI_system_theory">linear time-invariant (LTI)</a></p><p>
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