A bandpass filter can be designed as two coupled L and C tuned
circuits.

The two circuits are coupled together via the mutual inductance
between the two spaced-apart coils. As in a 455 KHz interstage
transformer in a screening can.

The degree of coupling is approximately 1/Sqrt(Q1*Q2), where Q1 and Q2
are the tuned-circuit Q's which are determined by the filter's
terminating resistances.

Closer coupling increases the bandwidth.

For details see Terman.
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