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