Original sentence: However, elements like varicaps have non-linear or time-varying voltage–current ratios that appear to be linear for small signals; hence, they can be roughly described as having a time-varying impedance.In general, neither impedance nor admittance can be time varying as they are defined for infinite-time-horizon signals.That is, this description is an approximation; over large signal swings, the voltage–current relationship is non-linear and cannot be described by impedance
Edited sentence: However, many systems may exhibit non-linear or time-varying voltage–current ratios that appear to be LTI for small signals over small observation windows; hence, they can be roughly described as having a time-varying impedance.In general, neither impedance nor admittance can be time varying as they are defined for complex exponentials for –∞ < t < +∞.That is, this description is an approximation; over large signal swings or observation windows, the voltage–current relationship is non-LTI and cannot be described by impedance