METHOD AND APPARATUS FOR ELIMINATING IN-BAND RIPPLE FROM BAND-PASS FILTER RESPONSES

摘要

A circuit produces a band-pass filter amplitude response with substantially reduced ripple in the pass-band. The circuit of the invention employs a band-pass filter circuit that can be optimized to meet the out-of-band rejection specification of particular application, and a complementary filter that has an amplitude response that when additively combined with the amplitude response of the band-pass filter produces an overall band-pass response that is substantially flat over the pass-band. The two filters are isolated from one another such that the two filter responses combine additively without interaction between them. An optional amplifier can also be coupled to the input of the complementary filter to offset insertion loss resulting from the combination of the two responses. For band-pass filters that produce a rounded in-band amplitude response, the complementary filter is preferably a shallow notch filter. One preferred embodiment of the shallow notch is a series RLC circuit. One preferred embodiment of the invention can be employed to the process of frequency up-conversion. The complementary filter is a series RLC circuit, the band-pass filter is one optimized to reject out-of-channel interference signal components, and a mixer for performing the up-conversion provides the isolation between the two filters.