Branch line coupler with sharp roll-off slope and good out-of-band rejection

A new 3 dB branch line coupler with a sharp roll-off slope characteristic is introduced. The bandpass characteristic with a good out-of-band rejection property reduces noise, intermodulation, and interference problems. The filter characteristics have been obtained with four asymmetrical λ/4 open circuited coupled lines above four DGSs (Defected Ground Structure) in microstrip configuration. Also the sharp selectivity has been obtained by using four λ/2 open circuited shunt stubs. Theoretical model for the circuit is derived based on a TEM approximation. The coupler has been simulated and fabricated. Measurement results show excellent roll-off slopes in lower transition band for |S₂₁| and |S₃₁| such as 60dB/700MHz and 40dB/550MHz, respectively, at around 3.5 GHz. And insertion losses for |S₂₁| and |S₃₁|, both better than 4.1 dB have been obtained. Power and phase imbalances are 0.4 dB and ± 6⁰, respectively. Return loss and isolation characteristics are both better than 20 dB over 42% fractional bandwidth.