Filters with an almost constant stopband

We use a resistive-reactive crosscoupling within elliptic high- and low-pass filters to provide sharp-cutoff highpass or lowpass responses, which then reach an almost constant stopband attenuation value a short distance from the passband. This almost-constant stopband is maintained down to DC for highpass types and to at least twice the cutoff frequency for lowpass configurations. Similar resistive-reactive crosscoupling configurations can be used to provide constant stopband levels for bandpass and bandstop networks.