Lei Chen and Feng Wei

Bandpass filters are key components for ultrawideband (UWB) communications systems operating at frequencies from 3.1 to 10.6 GHz. A variety of different design approaches have been implemented to achieve different notch depths and frequencies with these filters, but the authors have proposed a bandpass filter comprised of a multiple-mode resonator (MMR) to achieve dual notched bands for excellent suppression of unwanted spurious signals in the filter stopband. The MMR is formed with two semicircular defected ground structures (S-DGSs) and simplified composite right/left-handed (SCRLH) resonator. Both computer simulations and measurements of a prototype filter indicate high levels of out-of-band rejection with acceptable in-band insertion loss and return loss.

A number of different filter structures have been studied for UWB applications, including filters based on cascaded broadside-coupling1 and MMRs.2,3 Most of these bandpass filters exhibited good frequency performance and were suitable for practical implementation. With its wide frequency range, however, UWB devices and systems are susceptible to interference from existing wireless applications, including 5.8-GHz wireless local-area networks (WLANs) and 8-GHz satellite-communications (satcom) systems. As a result, a bandpass filter designed for UWB applications should also incorporate one or more notches to suppress interference from these other wireless systems.

Various filter design techniques were studied in refs. 4 and 5, but only one of these included a bandpass filter with a notched band. Bandpass filters with multiple notched bands were reported in refs. 6 and 7, but they were based on multilayer structures requiring complex fabrication techniques and not readily compatible with existing microwave-integrated-circuit (MIC) fabrication processes.

The proposed UWB bandpass filter is based on previous work by the authors. It features dual notches with high rejection and a wide stopband, implemented within a relatively small circuit. The notches can be tuned as desired by adjusting the dimensions of the SCRLH resonators. The S-DGSs help improve out-of-band performance. The bandpass filter features a 3-dB passband from 3.0 to 10.7 GHz, with the two notch bands extending from 5.8 to 5.9 GHz and from 8.0 to 8.1 GHz. The upper stopband provides better than 20-dB rejection through 20 GHz.

The use of CRLH transmission lines with series inductive-capacitive (LC) series tank and shunt LC tank circuits has been studied extensively for a variety of microwave circuits. A SCRLH resonator is comprised of a high/low-impedance short line and grounded stub with metalized via hole (Fig. 1). Compared with a conventional CRLH transmission-line unit, the SCRLH resonator does without the series capacitances. A SCRLH resonator can help achieve dual stop-band performance when placed next to a microstrip line and it can be equivalent to a two shunt-connected series LC resonant circuit (Fig. 2). According to refs. 8 and 9, an SCRLH resonator exhibits an inherent dual-mode property, with the two resonant frequencies given by Eq. 1:

ω1 = 1/(LLCR)0.5
ω2 = (1 + 4LL/LR)0.5/(LLCR)0.5 (1)

The characteristics of coupled SCRLH resonators of various dimensions were studied by means of simulation (Fig. 3), using Version 11.0 of the High-Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from Ansys. As can be seen, the dual notched bands both decrease as parameter LR1 increases. But only the upper notched band increases as LR2 decreases, and only the lower notched band increases as LL1 decreases. Therefore, by adjusting the resonator dimensions, dual notched bands can be achieved at desired frequencies.

A microstrip DGS has a periodic etched defect on the ground plane and can deliver good stop-band characteristics in high-frequency microstrip circuits. An S-DGS is comprised of two semicircular defected areas and one narrow connecting slot on the ground plane (Fig. 4).10 It exhibits a higher quality factor (Q) and better stop-band characteristics than a conventional DGS. The characteristics of a S-DGS unit as a function of frequency can be modeled by means of a series-connected parallel LC resonant circuit.

The MMR was originally used to design the UWB filter, to obtain a narrow stopband. Because of the addition of the S-DGS units to this design, the effects of an S-DGS structure on the frequency characteristics of the MMR were studied, with Fig. 5 simulated S-parameters for the MMR with and without the S-DGS. The simulations show that the S-DGS unit can improve the out-of-band performance of a conventional MMR while also achieving a wide and deep upper stopband.

The UWB bandpass filter with dual notches was designed with one MMR, two S-DGSs, and one SCRLH resonator cascaded along with the microstrip line (Fig. 6). The SCRLH resonator was used to achieve the dual notched bands. This structure is simple but can effectively block interference that may appear within the UWB communications frequency range. The design was simulated with Version V11.0 of HFSS and then fabricated on RT/duroid 5880 microwave laminate from Rogers Corporation with thickness of 1 mm and dielectric constant (er) of 2.2. The dimensions for the various filter parameters were as follows: W0 = 3.0 mm, W1 = 0.5 mm, W2 = 0.2 mm, W3 = 0.3 mm, L1 = 8.5 mm, L2 = 13.5 mm, Ldgs = 3.0 mm, Wdgs = 0.3 mm, Rdgs = 0.8 mm, WR1 = 5.5 mm, WR2 = 0.5 mm, WL1 = 0.6 mm, LR1 = 1.5 mm, LR2 = 3.0 mm, LL1 = 1.0 mm, and Wgap = 0.15 mm. The radius of the via hole is 0.2 mm.

The fabricated prototype bandpass filter was characterized with a model N5230A microwave vector network analyzer (VNA) from Agilent Technologies. Figure 7 offers a comparison between simulated and measured results. It is apparent that the fabricated filter has a low-loss passband from 3.0 to 10.7 GHz and dual notched bands from 5.8 GHz to 5.9 GHz and from 8.0 GHz to 8.1 GHz, respectively. The return loss is better than -10 dB over the passband. Both the transition bands at upper and lower cutoff frequencies are steep. Measured rejection is more than 10 dB at the midband of the notched band and the upper-stopband with 20 dB rejection to 20 GHz. Measurements of group delay show flat response across the operating band. Deviations between simulations and measurements are likely due to reflections from the connectors and substrate losses. Figure 8 shows the prototype.

In summary, the UWB bandpass filter was implemented with a combination of MMR and SCRLH structures and achieved the desired passband with low loss while also adding two notches with high rejection, using a commercially available printed-circuit-board (PCB) material. The prototype filter is small in size and compatible with standard MIC fabrication processes for integration into more complex UWB designs.

References
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