TRI-BAND BPF DESIGN

Based on the theory of MSIRs, the tri-band BPF centered at 2.4, 4 and 5.6 GHz is designed (see Figure 1). It comprises two open-end feed lines, a pair of MSIRs and two pairs of OLRRs. The MSIRs determine the first passband center frequency, fractional bandwidth and selectivity. The chosen parameters are: k1= 0.75, k2 = 0.3, l1 = 7.6 mm, l2 = 3.93 mm, l3 = 3.6 mm, l4 = 3.24 mm, l5 = 0.1 mm, w1 = 0.195 mm, w2 = 0.4 mm and w3 = 1.8 mm.

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The OLRRs are traditional half wavelength resonators determining the second and third passband responses. The difference is that the OLRRs used in the second passband are meandered. Two extra transmission zeros are produced between the first and second and the second and third passbands. This improves passband isolation and selectivity.

The bandwidths depend on the coupling coefficients and external quality factors. That is, s1, s2, d1 and d2 affect the bandwidths; s1 = 0.4, s2 = 0.6, d1 = 0.1 and d2 = 0.2 mm. The length L of the open-end feed line is about a half wavelength at the first operating frequency of 2.4 GHz and the impedance is 50 Ω. Here L = 30 and w = 0.5 mm.

Figure 5 shows the current distribution of the feed line at 2.4, 4 and 5.6 GHz. The resonators are placed at positions of maximal current distribution for strong magnetic coupling.

Figure 5

Figure 5 Current distribution of open-ended feed line.

The filter is fabricated on a substrate with a relative permittivity of 9.6 with a dielectric height of 0.5 mm. The overall size of the filter is 30 × 9 mm.

RESULTS AND DISCUSSION

The tri-band BPF (see Figure 6) is simulated using ADS Momentum and measured using a Keysight 8719ES network analyzer (see Figure 7). Measured results show good agreement with the simulation. The passbands are centered at 2.4, 4 and 5.6 GHz with 3 dB bandwidths of 5.9, 44 and 22.4 percent, respectively, with insertion loss less than 1.1, 0.8 and 0.1 dB. The locations of transmission zeros are at 2.72, 3.2, 3.68, 4.35, 4.8 and 6.6 GHz, providing rejection greater than 35 dB.

Figure 6

Figure 6 Tri-band BPF photograph.

Figure 7

Figure 7 Tri-band BPF simulation and measurements.

Table 1 compares this performance with other work, showing that this filter contains the most transmission zeros and the best isolation between passbands.

CONCLUSION

A novel tri-band BPF combines MSIRs and OLRRs. The proper impedance ratios and electrical lengths of the MSIRs realize a wide upper stopband. The meander OLRRs produce two extra transmission zeros at the upper and lower sides of the second passband, which improves band-to-band rejection. Consequently, the low-cost tri-band BPF is only 30 × 9 mm2 with operating frequencies at 2.4, 4 and 5.6 GHz. The insertion loss of each band is less than 1.1 dB and band-to-band rejection is better than 35 dB. High selectivity, compact size and low insertion loss makes it attractive for use in 5G applications.

References

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