Section V: Improved Circuit Model
The equivalent inductance along with the coupling gap capacitance present in the microstrip feed line introduces a series resonance and hence an additional frequency response to the circuit. This additional frequency is the passband. The introduction of a series capacitor to the original circuit, shown in Fig. 13, includes the modules responsible for the series and parallel resonances. Furthermore, the parallel RC combination used to model the fringing effects remain as part of the circuit and no further changes are made in the original circuit. The circuit parameters obtained using (5) and (6) are provided in Table V.
\(L_{\text{bandpass}}=\frac{L_{\text{bandstop}}}{1-(\frac{f_{\text{bandpass}}}{f_{\text{bandstop}}})^{2}}\)(5)\(C_{\text{bandpass}}=\frac{25.33}{L_{\text{bandpass}}*{f^{2}}_{\text{bandpass}}}\)(6)
Moreover, the choice of the stop band and passband frequencies has to be in a manner such that \(f_{\text{bandpass}}<f_{\text{bandstop}}\). An example simulation was carried out by considering 0.70 GHz as the passband frequency and 0.77 as the stop-band frequency and the corresponding EM simulation and circuit simulation results are depicted in Fig. 14. The results clearly demonstrate the effectiveness of the proposed equivalent circuit that models both the bandpass and bandstop behavior.
Figure 13- \(\pi\) type circuit model for bandpass/bandstop filter
Figure 14-EM and circuit simulation of passband and stopband frequencies
Table V: Parameters for New Circuit Model