Stubbed Quarter-Wave Coupled Line BPF

A microstrip quarter-wave bandpass filter is designed on a 20 mil thickness RO4003C substrate in order to meet the following design goals:

In this design, stubs are added so as to have a lower coupling in the input and output coupled lines. This leads to a significantly wider gap in these lines with respect to the classical QW coupled lines filter. The design goals are essentially the same as in the classical quarter-wavelength coupled line example.

Once this was done, the circuit was laid-out and simulated in Sonnet Lite. Care must be taken in the box size definition to avoid unintended resonant modes caused by the bounding conditions. Also, some adjustments were made to fit the Sonnet results to the filter prototype response.

Finally, the results obtained from Sonnet are compared to those obtained with the Qucs-S simulation with MS transmission line models. As expected, the passband insertion loss is higher than that predicted in the electrical-level simulation.

Also, the results of the EM simulation of the classical and the stubbed QW coupled line filters are compared. As shown below, the stubbed version has higher insertion loss than its classical counterpart, but it also has higher out-of-the-band rejection due to the presence of the open stubs.