Admittance Y is reciprocal of impedance. Its variation with frequency is illustrated in Figure
Y = 1 /Z
Figure: Admittance Curve
Figure illustrates the variation of current with frequency. Current is maximum at resonance frequency ω. At rms value of current I0 /√2 , power becomes half of its maximum value. Thus the points, corresponding to I0 /√2 on the resonance curve, are known as the half power points. Frequency ω1 and ω2 corresponding to these two points are known as the lower and upper half power frequency.
Figure: Resonance Curve