Quarter-Wave Notch Filter
A microstrip quarter-wave notch filter is designed on a 20 mil thickness RO4003C substrate in order to meet the following design goals:
| Parameter | Value | Band |
| Insertion loss | < 1 dB | < 4 GHz |
| Stopband rejection | > 20 dB |
< 4.5 GHz > 5.3 GHz |
| Return loss | < -15 dB | < 4 GHz |
The filter must have a deep notch in 5 GHz. For designing such a filter, tge Qucs-S filter design tool is specially convenient since it automatically generates an schematic which can be used for simulation and tuning. As a first approach, ideal transmission lines are used as shown below:
Ideal QW Coupled Lines Circuit
Then, the filter is synthesized using microstrip lines according to the RO4003C properties. Notice that such high impedance lines are difficult to implement in a real PCB manufacturing process since they require line widths below 100 μm. In order to handle this problem, the minimum width is fixed to 100 μm and the wider line is tuned in Qucs-S to that to get a suitable frequency response, giving priority to the lower frequencies.
Once the filter response is tuned, the circuit is 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.
Layout
Insertion Loss
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Group Delay
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 in the EM simulation is slightly shifted towards lower frecuencies with respect to the circuit simulation with microstrip transmission line models.
