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CST – Computer Simulation Technology

Microstrip Bandstop and Lowpass Filters

A 3-section folded line bandstop filter was designed for a maximally flat response at a center frequency of 1.5 GHz and with a fractional bandwidth, D= 0.3. The filter was implemented in a microstrip platform with a permittivity of the substrate εr=2.2 and a substrate height h=31 mil. The thickness of the metallization layer is 0.31 mil.

The physical parameters of the folded line filter sections were optimized using the Agilent ADS circuit simulation software to provide the closest values of the lowpass filter prototype values g's and electrical lengths for a given set of filter specifications. By cascading the folded line filter sections, the overall filter response was obtained. The layout generated by Agilent momentum was exported to CST MICROWAVE STUDIO® (CST MWS).

The artwork, substrate layers and metallization layers can be exported from Agilent momentum to CST MWS through a single menu....



Figure 1: Bandstop filter model imported into CST MWS from Agilent momentum

The overall footprint of the folded line bandstop filter only measured 1015.1 sq.mm. The smallest normalized width defined as the ratio of the smallest width in the design to the substrate height is conveniently larger (0.5). This facilitated practical realization of the folded line bandstop filter with less stringent dimensional tolerances.



Figure 2: Response comparison for the folded line bandstop filter

Figure 2 shows a comparison of the circuit simulation results using Agilent ADS with the full wave EM simulation results using CST MWS and Agilent momentum. The time domain solver was used in the case of CST MWS. Further validation is provided with the help of the measured results. The S-parameters are shown between 0.5 to 2.5 GHz. The bandstop behavior can be clearly seen between 1.4 to 1.6 GHz.



Figure 3: Surface current in the stopband at 1.5 GHz


Figure 4: Surface current in the lower passband at 1.0 GHz

Similarly, a 3-section folded line lowpass filter was designed for a maximally flat response with a cut-off frequency of 1.5 GHz. The folded line lowpass filter was also implemented in a microstrip platform with a permittivity of the substrate εr=2.2 and substrate height h=31 mil. The thickness of the metallization layer is 0.31 mil. The layout of the folded line lowpass filter is shown in figure 5. The overall footprint of the folded line lowpass filter only measured 534.8 sq.mm. The largest normalized width defined as the ratio of the largest width in the design to the substrate height is conveniently smaller (3.63). This facilitated practical realization of the folded line lowpass filter.



Figure 5: 3D view of the folded line lowpass filter

Figure 6 shows a comparison of the circuit simulation results for the folded line lowpass filter using Agilent ADS with the full wave EM simulation results using CST MWS and Agilent momentum. The time domain solver was used in the case of CST MWS. Further validation is provided with the help of the measured results. The S-parameters are shown between 0.5 to 2.5 GHz.



Figure 6: Response comparison for the folded line lowpass filter


Figure 7: Surface current in the passband at 1.0 GHz


Figure 8: Surface current in the stopband at 2.0 GHz

References

[1] H. Peddibhotla and R.K. Settaluri, “Compact Folded-line Bandstop and Lowpass Filters,” Micro. Optical Tech. Letters, vol. 42, issue 1, pp.44-46,May 2004.

[2] H. Peddibhotla and R.K. Settaluri, “Miniaturized High Performance Lowpass and Bandstop Filters for Wireless Applications,” Proc. IMAPS Conf. on Ceramic Interconnect Tech., Apr. 2003.

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