Classical LC filter design is becoming increasingly rare in much of the RF design community. This is in part due to the domination of direct-conversion in up- and down-converters along with the widespread availability of excellent off-the-shelf filter components, such as surface-acoustic-wave (SAW) filters. However, tunable bandpass filters are still required for many applications, whether for discrete designs or on-chip integrated-circuit (IC) designs. If insertion loss must be as low as possible, a peaked-response lowpass filter may be used in order to side-step the higher insertion loss of a true bandpass filter. If attenuation is required in both stopband regions, however, a bandpass filter is generally required.
Bandpass filters (BPFs) have an inherently higher insertion loss than lowpass filters (LPFs) given similar design requirements. A first-order BPF can be designed for low passband insertion loss, but it will suffer from poor stopband attenuation roll-off and rounded passband shape. A second-order BPF can be designed to have appreciably faster stopband transition regions and a reasonably flat passband shape, but normally exhibits higher passband insertion loss than the first-order filter. Only the second-order filter design case will be considered here, whereas extensive details for the first-order filter design can be found in ref. 8.