W. Cheng, X.H. Wang, Y. Tuo, Y.F. Bai, and X.W. Shi
Bandpass filters with large fractional frequency bandwidths are needed for applications in the ultrawideband (UWB) frequency range from 3.1 to 10.6 GHz. A fractional bandwidth of about 110 percent at a center frequency of 6.85 GHz serves UWB applications cleared by the Us Federal Communications Commission (FCC), opening 3.1 to 10.6 GHz for low-power, commercial UWB use.
By implementing a microstrip bandpass filter based on a multiple- mode resonator (MMR) with rectangular groove for fine tuning the resonator frequencies, passband performance with return loss of better than 10 dB and insertion loss of less than 1.5 dB was achieved, with group-delay variation of less than 0.3 ns. The use of a MMR with a stepped-impedance configuration was originally reported to make use of its first three resonant modes as part of a bandpass filter design covering the full UWB frequency range. In ref. 1, an UWB filter with aperture-backed microstrip line MMR was reported to show its low insertion loss within the passband with 8 dB return loss. The proposed filter design exhibits improved return loss of 10 dB within the passband, with low passband insertion loss of 2 dB, using MMR techniques.2 A diminutive filter with two short open-circuited stubs was designed to achieve wide upper-stopband performance; it measured 3-dB cutoff frequencies of 3.4 GHz at the low end and 10.3 GHz at the high end, as reported in ref. 3. Even though all of the above-described UWB filters have exhibited satisfactory performance levels across the wide UWB passband, there is always a need for improved return-loss and insertion-loss performance levels in the UWB frequency range, and improvements may be possible by applying modified MMR techniques.4
In the improved MMR UWB bandpass filter design, the first three resonant modes were constructed to realize five transmission poles with low return loss across the full passband. By building upon earlier MMR filter designs, the current design was modified to reallocate the first three resonant modes close to the lower-end, center, and upper-end of the target UWB passband.
Meanwhile, the degree of coupling of the input/output parallel-coupled lone sections5,6 has largely been increased in the current MMR design, resulting in improved passband performance as shown by both computer-aided-engineering (CAE) simulations and measurements of a prototype filter. All parameters predicted by CAE simulation, including insertion/return loss and group delay, were experimentally confirmed in a wide frequency range including the UWB passband.
The proposed UWB filter consists of a nonuniform MMR in the center section and two identical coupled lines located at the left and right sections. The backside aperture on the ground plane is used not only to tighten the degree of coupling of the coupled lines but also to realize the specified impedance ratio for the side-to-central sections in the MMR.1 As is well known, the impedance ratio of the three sections or the length of the MMR can be modified to adjust frequency dispersion within the passband of the UWB; the approach for designing specific parameters for this is described in ref. 4.
Figure 1 shows the topology of the new MMR microstrip bandpass filter and its key parameters, with the units shown in millimeters. In contrast to the MMR designs in refs. 1 and 2, a parallel-coupled dual line structure3-6 has been used in this UWB bandpass filter. This type of coupling structure is expected to strengthen the degree of coupling between the input/ output port and the MMR resonator compared to a conventional parallel coupled line, which can increase the S21 magnitude of the UWB filter and widen the passband of the filter. In addition, slotting a groove in the MMR as shown in Fig. 1 creates a new structure and way to finely adjust the three resonators within the UWB filter. Combining these structures, a UWB filter with favorable passband performance was designed and characterized.
The slot in the MMR is used to slightly regulate the frequency dispersion and improve the bandpass performance. By changing the length, width, and location of the groove, the first three resonant frequencies within the UWB passband (3.1 to 10.6 GHz) will be reassigned in order to achieve good filter performance. Figure 2(c) shows variations of frequency dispersion with and without the groove, respectively, of Figs. 2(a) and 2(b). It is clear that three resonant frequencies within 3.1 to 10.6 GHz are slightly regulated by slotting a groove in the MMR. Variations of the frequency dispersion with parameters L, W, and d, respectively, are shown in Figs. 3(a), 3(b), and 3(c), where L is the length of the groove, W is the width of the groove, and d is the distance between the groove and the center part of the MMR, as illustrated in Fig. 1. The plots in Fig. 3 show simulated S21 magnitude under weak coupling conditions with fixed d = 0.3 mm and W = 1.2 mm and varying L in Fig. 3(a), simulated S21 magnitude under weak coupling conditions with fixed d = 0.13 mm and L = 13.6 mm and varying W in Fig. 3(b), and simulated S21 magnitude under weak coupling conditions with fixed L = 13.6 mm and W = 1.2 mm and varying d in Fig. 3(c).
Comparing the curves in Fig. 3, it is obvious that parameter W plays an important role in regulating the three resonant frequencies compared to L and d. The three plots in Fig. 3 show that the slot in the MMR can slightly adjust frequency dispersion and improve UWB passband filter performance, although the variation in frequency dispersion is not as pronounced as in ref. 4. It is still worthwhile and significant to note that applying a slot in an MMR in this manner can be useful for other filter designs.
After slight adjustments of certain dimensions were determined, the UWB MMR bandpass filter was designed, simulated, and measured. Figure 4 provides views of the top and bottom of the proposed UWB filter. In the design, the simulation and optimization are carried out by using commercial HFSS 11.0 software. The UWB filter is fabricated on a substrate with dielectric constant of 2.2 and a thickness of 0.787 mm, and its filtering performances are measured by a model N5230A vector network analyzer (VNA) from Agilent Technologies.
Figure 5 shows the comparison between the predicted and measured frequency responses of S21 (insertion loss) and S11 (return loss) magnitudes as well as group delay. The predicted S-parameters prove that the newly designed UWB filter has high return loss (better than 11 dB) and low insertion loss (less than 0.8 dB) across a wide range of frequencies (3.9 to 10.7 GHz) including the UWB passband. The measured results show good return loss (better than 10 dB) with low insertion loss (better than 1.5 dB), including the loss of the SMA connectors used in the filter. The measured group delay varied between 0.15 and 0.45 ns with a maximum variation of 0.3 ns, thus indicating the good linearity of the proposed UWB filter. The attenuation is better than 20 dB in the upper stopband through 14 GHz.
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In summary, an UWB microstrip bandpass filter based on a uniform MMR with dual-parallel coupling lines and a rectangular groove was successfully implemented. The three resonant frequencies were reassigned by properly regulating the length (L), width (W), and location (d), of the rectangular groove. The UWB filter achieved good passband performance, including insertion loss of less than 1.5 dB and return loss of better than 10 dB, as well as group delay variation less than 0.3 ns. Measured performance was in close agreement with simulated results.
This work is supported in part by the National Science Foundation of China under Grant 60801039 and the Guangdong Province Major Science and Technology Project 2009A080207006.
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