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:
| Parameter | Value | Band |
| Insertion loss | < 1 dB | [4.8, 5.2] GHz |
| Stopband rejection | > 25 dB |
< 4 GHz > 6 GHz |
| Return loss | < -15 dB | [4.8, 5.2] GHz |
The design goals were chosen so that the design can be simulated in Sonnet Lite. This software has a memory limitation of 64 MB which means that large designs, like an o5 or a o7 filter cannot be simulated. Taking the classical QW coupled lines filter as the baseline, open stubs are added at the input and output. Having parametrized all the circuit features in Qucs-S, the variables are tuned manually so as to have the desired frequency response.
The Qucs-S filter design tool is specially convenient for designing the filter as it automatically generates an schematic which is used for electrical-level 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.
The design must be compatible with the capabilities of a standard PCB manufacturing facility, so it is important to keep the separation between the first two lines wide enough. This minumum gap could be something close to 100 μm. Notice that wider passbands would require tighter coupling between the first (last) pair of coupled lines and the gap would be etched correctly in a real PCB manufacturing process.
Now the transmission lines are synthesized using the Qucs-S filter tool by taking into account the substrate properties. The circuit variables are parametrized so as to easily tune them.
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
.png)
.png)
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.
