Microstrip Stepped-Z LPF
A microstrip stepped impedance lowpass filter is designed on a 20 mil thickness RO4003C substrate in order to meet the following design goals:
| Parameter | Value | Band |
| Insertion loss | < 1.5 dB | [0, 2] GHz |
| Stopband rejection | > 20 dB | > 4 GHz |
| Return loss | < -15 dB | [0, 2] GHz |
The first step is to determine the order of the filter that satifies the design goals. For doing this, the Qucs-S filter design tool is used as it automatically generates the Qucs-S schematic. For this first approach, ideal transmission lines are used as shown below:
Ideal Stepped-Z LPF Circuit
Then, the filter is synthesized using microstrip lines according to the RO4003C properties.
Notice that the high impedance lines must be wide enough to be fabricated into a standard PCB manufacturing facility, so let's keep them wider than 100 μm.
Once the filter is simulated with microstrip transmission line models, the circuit is laid out and assembled in Sonnet Lite for EM simulation. Care must be taken in the box size definition to avoid unintended resonant modes caused by the bounding conditions.
Layout
Insertion Loss
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 cutoff frequency in the EM simulation is lower with respect to the electrical simulation
