Flat group delay low pass filters using two CPW topologies

Improved and small flat group delay low pass filters are presented. The design of these filters is based on two CPW stub topologies. The improvement of the flatness group delay is achieved by loading the stub at the appropriate positions. The straight stub filter is realised and measured, and a good agreement is observed. The second topology is based on a reshaped open circuit stub. The optimized structures are well matched beyond the cutoff frequency with a flat group delay.