Reconstruction lumped-element bandpass filter suitable for lowpass delta-sigma RF transmitters

This paper proposes a filter realised using only lumped-element components, implemented as a highly selective bandpass filter suitable for lowpass delta-sigma (LPΔΣ) RF transmitters. The proposed filter is characterised by low insertion loss, high selectivity and a transfer function tailored for filtering the close-up out-of-band noise of LPΔΣ RF transmitters. The circuit design is based on a modified loaded-stub ring-resonator structure, however, implemented using 4 π-shape lumped-element resonators with LC tanks. The measurements show good agreement with simulation and the proposed filter provides a fractional 3-dB bandwidth of 14.3 %, insertion loss of less than 1.6 dB, suppression of more than 18 dB on both sides of the desired band, and a sharp cut-off frequency response. This filter is combined with the delta-sigma transmitter to show the effective reduction of the out-of-band quantisation noise signals.