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906 ⏐⏐⏐ RESONANCE<br />

Setting the input impedance for the network of Fig. 20.25 equal to<br />

this value will result in the following relationship:<br />

Z � �0.707R<br />

which can be written as<br />

Z � � R 1<br />

———<br />

1 1<br />

�� � j<br />

R �qC � �� qL�<br />

1<br />

——— �<br />

1 1<br />

���1 � jR<br />

�2�<br />

R �qC � �� qL��<br />

or �<br />

and finally �<br />

The only way the equality can be satisfied is if the magnitude of the<br />

imaginary term on the bottom left is equal to 1 because the magnitude<br />

of 1 � j 1 must be equal to �2�.<br />

The following relationship, therefore, defines the cutoff frequencies<br />

for the system:<br />

R�qC � � � 1<br />

Substituting q � 2pf and rearranging will result in the following<br />

quadratic equation:<br />

f 2 � � �0<br />

having the form af 2 R<br />

———<br />

R<br />

�<br />

1<br />

1 � jR<br />

�2�<br />

�qC � �� qL�<br />

1<br />

———<br />

1<br />

�<br />

1<br />

1 � jR<br />

�2�<br />

�qC � �� qL�<br />

1<br />

�<br />

qL<br />

f 1<br />

� �2 2pRC 4p LC<br />

� bf � c � 0<br />

1<br />

1<br />

with a � 1 b ��� and c ���2<br />

2pRC<br />

4p LC<br />

Substituting into the equation:<br />

f �<br />

�b � �b� 2 ��� 4�a�c�<br />

��<br />

2a<br />

will result in the following after a series of careful mathematical manipulations:<br />

1 1 1<br />

f1 � �� 4pC�<br />

�� �� �<br />

R R<br />

1 1 1<br />

f2 � �� �� � �<br />

4pC�<br />

R �R � 2� � � � �4<br />

� 2� � � � � � 4<br />

C ��� �L<br />

L� C � ��<br />

ƒ r<br />

(20.39a)<br />

(20.39b)<br />

Since the term in the brackets of Eq. (20.39a) will always be negative,<br />

simply associate f 1 with the magnitude of the result.<br />

The effect of R l, L, and C on the shape of the parallel resonance<br />

curve, as shown in Fig. 20.28 for the input impedance, is quite similar

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