Intermodulation distortion in active microwave filters

Active microwave filters have potential application in high-volume consumer products such as cellular-radio handsets. Active negative-resistance circuits may be coupled into physically small low-Q-factor filter structures to cancel resonator losses and produce apparently ideal filter characteristics. However, at large input-signal levels, the nonlinear active devices cause distortion effects, particularly with multiple signal inputs. Results from an experimental and computer-simulation study of saturation and intermodulation distortion in active negative-resistance microwave bandpass and bandstop filters are presented. These results have been generalised into simple formulas to enable the designer to determine the level of distortion in any specific filter of this type.