Double tuned band-pass filter with inductive coupling - has capacitors determining attenuation poles provided as T networks

摘要

The filter has a respective series capacitor and filter circuit coupled to its input (1) and output (2) with the coupling inductance bridged by a parallel capacitor. A second parallel capacitor is connected across the input (1) and output (2), the two capacitors determining the filter attenuation poles. Either or both capacitors is provided by a T network having two fixed capacitors (C4,C5, C4', C5') in the horizontal arm and a variable capacitor (CT, CT') in the transverse arm, with each variable capacitor (CT, CT') connected to earth.