Admittance Curve:

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