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Description
o Background: The practical inverter model has frequency-dependent parameters, which mean that the designed filter may produce results that differ from the targeted filter specifications in terms of FBW changed and center frequency shifts. To address this issue, a frequency conversion function is proposed that takes into account the frequency dependency of the inverter characteristic impedance or admittance. While a conventional circuit has been proposed to improve the frequency dependence parameters, the frequency dependency for the impedance of the inverter has not yet been resolved.
o Objectives: We present a proposed method for designing a wideband bandpass filter (BPF) that compensates for frequency dependency based on the image admittance and image phase.
o Methods: We have added a new admittance compensation method to the conventional method that compensates for phase frequency variations. While an idealized impedance or admittance inverter has characteristic immittances K or J and a phase shift of 90° at all frequencies, practical inverter models used in implementing the BPF structure exhibit frequency dependency based on image admittance and phase terms. After compensating for admittance and phase, the proposed circuit is obtained. We have carried out simulations of the 3rd and 5th order based on both the conventional and proposed BPF.
o Results: Our proposed method efficiently improves both the frequency shift and unwanted bandwidth when designing filters with more than 20% of bandwidth. In contrast, the conventional method exhibits frequency degradations at a 10% Fractional Bandwidth (FBW).
o Discussion or Conclusion: This paper presents a BPF design method that takes into account the frequency variations of the inverter. The proposed filter design method improves upon existing approaches by compensating for frequency variations in the image admittance.
We verify the proposed design theory through simulation and measurement results. Furthermore, we demonstrate the effectiveness of the proposed design method by using a capacitive coupled BPF with a SAW resonator. Our results confirm that the designed BPF not only satisfies 10% FBW, but also exhibits no frequency degradation.
References
[1] D. Ahn et al., “Accurate Recursive Inverter Formula for the Correction of Phase Variation Effect on Bandpass Filters,” in 29th European Microwave Conference, pp. 203-206, 1999
[2] Hong, J-S., "Micros-trip filters for RF/microwave applications", Hoboken, John Wiley & Sons, (2011).
| Keywords | Bandpass Filter (BPF), Frequency dependency, Broad bandwidth |
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