Simple Coil & Capacitor Bandpass (Band-Pass) Filters

Author: R.J.Edwards G4FGQ © 6th January 2003

The majority of L & C filters consist of one or more cascaded basic L-C sections used in applications where a small loss in the pass-band is of no consequence and frequencies of 'infinite' attenuation in the stop-band are not required. E.g., simple, switchable, bandpass filters at the input of radio receivers or at the output of radio transmitters where power-handling components may be needed.

This program assists with design of such simple filters without the operating inconvenience encountered when using programs intended for designing more complicated filters. The two possible basic filter sections, T & Pi, are computed. Note: T and Pi sections should not be cascaded with each other.

Input data are the pair of filter cut-off frequencies, terminating resistances, and the variable frequency at which overall filter insertion loss between its terminations is computed. For simplicity, internal filter loss is neglected. Internal loss depends on coil Q. It results in an increase in attenuation in the pass-band and a decrease in attenuation in the stop-bands. To approximate the computed filter response coil Q must exceed the computed minimum Q value although a useful band-pass response can be obtained at lower values.

Values of L uH and C pF components are output. A tolerance of +/- 5 or 10 percent will be adequate in many cases where the bandwidth/mean-frequency ratio exceeds 0.25 When the ratio is less than 0.25, bandwidth may be widened to ensure low attenuation over the whole required pass-band. Or variable preset L and C components can be used. Insertion loss at band-edges is always 3.01 dB.

When coil Q is low and approaches 1/ratio, greater loss must be accepted in the pass-band and filter design must be changed to the case of two over-coupled tuned circuits as for a double-tuned IF transformer.

Insertion loss is defined as that when the filter is inserted between generator and a termination both of the same resistance as specified by the input data.

In the case of a Pi network, to obtain more feasible L and C component values, the filter section can be based on a high value of terminating resistance and input and output connections can be tapped down the input/output coils. By the same means the filter can be designed to operate between different impedances.

The impedance transforming ratio is proportional to the square of tapping-turns turns ratio on the end coils of the Pi-section.

NOTE: For frequencies out-of-program-range, multiply/divide L & C by the frequency ratio.

 

 

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