Microwave Journal
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Non-Uniform Defected Microstrip Structure Lowpass Filter

May 14, 2012

A defected microstrip structure (DMS) has properties similar to a defected ground structure (DGS), but without any leakage through the ground plane. In this article, new lowpass filters with ultra-wide stopband, using non-uniform DMS that is beneficial to higher order harmonic suppression, are presented. A comparison is made to lowpass filters using standard 50 Ω microstrip lines and stepped-impedance microstrip lines. The proposed lowpass filters have the advantages of good frequency selectivity, ultra-wide stopband and simple circuit topology and it shows that the proposed filter, with stepped impedance microstrip line, has better out of band suppression and enhanced return losses than that with a uniform microstrip line and, simultaneously, the DMS units are smaller, which decrease the circuit size. The validity of the design is demonstrated by experiment.

Currently, the defected ground structure (DGS)1-4 has been widely employed for RF circuit design and improving RF components performance. DGS increases the effective capacitance and inductance of a microstrip line. It is more efficient for harmonic suppression, especially when using periodical DGSs,4 which results in a greatly improved stopband, because the EM field is highly concentrated under the microstrip line (ML). However, DGS introduces wave leakage through the ground plane, which brings difficulties with encapsulation.

Compared with DGS circuit, the defected microstrip structure (DMS)5-7 has no enclosure problems, because there is no leakage through the ground plane. DMS is easier to integrate with other microwave circuits and has an effectively reduced circuit size compared with DGS. Simultaneously, DMS exhibits the properties of slow-wave, rejecting microwaves at certain frequencies and has an increasing electrical length for certain circuits, which are similar to DGS, but without any manipulation of the ground plane. The defection in the microstrip line creates resonance characteristics in the frequency response.

Lowpass filters reject the higher harmonics and spurious responses of circuits, which plays a very important role in RF circuits and systems. The conventional implementation of a lowpass filter (LPF) involves the use of open stubs or stepped impedance microstrip lines. However, these structures have a gradual cut-off response. The filter rejection characteristic can only be improved by increasing the number of sections, and this method increases the passband insertion loss and the filter’s physical size. In the past few years, some new methods, such as complementary split ring resonators (CSRR),8 and DGS, especially the periodical DGSs,4 have been applied to LPF design.

In this article, a new technique for developing DMSassisted lowpass filters with ultra-wideband is presented. The conductor plane of a standard 50 Ω transmission line and a stepped-impedance transmission line are perturbed by non-uniform DMS to generate an ultra-wide rejection band that is beneficial to higher order harmonics suppression. New lowpass filters, with good performances of transmission zero and ultra-wide stopband, are designed and akind of lowpass filter with DMS cells reduction is fabricated and measured. The experimental results demonstrate the validity of the new design.

 

Figure 1

Fig. 1 Defected microstrip structures.

Figure 2

Fig. 2 Simulated S-parameter of a 50 Ω microstrip line with a T shaped DMS unit.

Defected Microstrip Structure

A conventional DMS consists of a horizontal rectangular slot and a vertical rectangular slot in the middle of a conductor strip, as shown in Figure 1. The figure also shows an L-shaped DMS, which is also called a spurline. Similar to the DGS, DMS increases the electrical length of the microstrip line and disturbs its current distribution. As a result, the effective capacitance and inductance of the microstrip line increase. Accordingly, the DMS has slow-wave and stopband characteristics, as the simulation shown in Figure 2 demonstrates, and the dimensions as well as the performance of the DMS are shown in Table 1. It shows that the cut-off frequency of the DMS increases, as the dimension b decreases, so new lowpass filters with ultra-wide stopband can be designed by using these characteristics, because non-uniform DMSs extend the stopband greatly. Here, the simulated results are obtained by using a 50 Ω microstrip line with a substrate relative permittivity of 2.2 and a thickness of 0.8 mm. The DMS may have more obvious stopband performance by using a substrate with higher permittivity. The electrical performances of stopband for DMS can be simulated by a parallel LC resonant circuit,9 and the equivalent circuit of a DMS matches the response of the 1st Butterworth lowpass prototype, as Figure 3 shows. Where:

Table 1

Figure 3

Fig. 3 Equivalent circuit of the DMS and Butterworth lowpass prototype.

Math Equation 1

Here, w0 and wc denote the resonant frequency and cut-off frequency of the parallel LC resonator, respectively, Z0 is the characteristic impedance, and g1 is the normalized parameter of the first Butterworth lowpass prototype.

Figure 4

Fig. 4 Qe vs. parameter for a T-shaped and L-shaped DMS.

Figure 5

Fig. 5 Qe vs. parameter c for T-shaped and L-shaped DMS.

Figure 6

Fig. 6 Qe vs. parameter f for T-shaped and L-shaped DMS.

Calculated variation curves of the external quality factor Qe, versus DMS parameters a, c and f are shown in Figures 4 to 6, respectively. It shows that for a T-shaped DMS, Qe has no obvious variation with parameters a increasing, while Qe increases greatly with parameter a increasing. For L-shaped DMS, Qe increases with parameter a increasing, while it decreases with parameter a increasing. Both have adequate Q values.

Non-uniform DMS Lowpass Filters

Lowpass filter with L-shaped DMS

Figure 7

Figure 7 L-shaped DMS lowpass filters.

Figure 8

Fig. 8 Stepped impedance ML and equivalent circuit.

In order to demonstrate a lowpass filter with ultra-wide stopband implementation, L-shaped and T-shaped DMS lowpass filters with non-uniform cells are proposed and are designed on Duroid substrates with a relative permittivity of 2.2 and a thickness of 0.8 mm. All the filters are symmetrical structures. The L-shaped DMS lowpass filters with 50 Ω microstrip line (ML) and stepped impedance microstrip line are shown in Figure 7(a) and (b), respectively, and the equivalent circuit model of the stepped impedance ML is shown in Figure 8, where Z01 and Z02 can be calculated as 50 Ω and 43.73 Ω, respectively. Figure 9 shows the simulated frequency responses of the lowpass filter. It can be seen that with a 50 W uniform ML, the filter has a cut-off frequency of 7.3 GHz, while the filter has a lowered cut-off frequency of 6.12 GHz with the stepped impedance ML. Both have good frequency selectivity and ultra-wide stopband of more than 12.7 GHz, which is 1.6 times the cut-off frequency. Calculated variation curves of the cut-off frequency versus the filter parameter d are shown in Figure 10. It shows that the cut-off frequency increases with d increasing and the cut-off frequency of the filter with a 50 Ω ML always has a larger value than that with a stepped impedance ML, when they have the same parameter d.

Figure 9

Fig. 9 Qe Simulated frequency responses of the lowpass filter with eight L-shaped DMS cells.

Figure 10

Fig. 10 Variation of the cut-off frequency vs. parameter d.

Figure 11

Figure 11 T-shaped DMS lowpass filters.

Figure 12

Figure 12 Simulated frequency responses of the lowpass filter with 50 Ω microstrip line and nine T-shaped DMS cells.

Lowpass filter with T-shaped DMS

Proposed lowpass filters with ultra-wide stopband by using non-uniform T-shaped DMS are shown in Figure 11. Their operation principle is the same as that of the L-shaped DMS filter. The simulated frequency responses of the lowpass filter with 50 Ω microstrip line by using nine T-shaped DMS non-uniform cells are shown in Figure 12. It can be seen that the lowpass filter has transmission zeros, good out of band suppression and an ultra-wide stopband of more than 10 GHz. The filter cut-off frequency decreases with the parameter d decreasing. Figure 13 shows the simulated frequency responses of the lowpass filter with different DMS cells. It can be seen that the lowpass filter with nine DMS cells and seven DMS cells nearly have the same cut-off frequency of approximately 9.45 GHz. However, the former has a wider stopband than the later because more non-uniform DMS cells bring a larger reject band range. Figure 14 shows the frequency responses of the lowpass filter with nine DMS cells, using uniform microstrip line and stepped impedance microstrip line. Figure 15 shows the variation curves of cut-off frequency versus the filter physical parameter b. It can be seen that the lowpass filter with stepped impedance ML shows a lower cut-off frequency, which correspondingly introduces a wider stopband than that with a 50 Ω uniform ML. It also can be seen that the filter cut-off frequency decreases with the parameter b increasing.


Figure 13

Fig. 13 Frequency responses of the lowpass filter with seven and nine DMS cells, using a 50 Ω microstrip line.

Figure 14

Fig. 14 Frequency responses of the lowpass filter with nine DMS cells, using 50 Ω microstrip line and stepped impedance microstrip line.

It is known that the stopband is related to the number of DMS cells, and a wider stopband always leads to a larger circuit size. Here, lowpass filters with five T-shaped DMS cells are also designed, in order to reduce the circuit size, as Figure 16 shows. The simulated frequency responses are shown in Figure 17. It shows that the filter has an ultra-wide stopband of approximately 2.45 times the cut-off frequency of 5.8 GHz and the stepped impedance ML introduces a better out of band suppression and a better passband return loss than the uniform ML. Variation curves of cut-off frequency versus parameters b and d for the lowpass filter with reduced DMS cells are shown in Figures 18 and 19, respectively. They show that the cut-off frequency decreases with b increasing, while it increases with d increasing, and a larger d introduces a larger difference on filter cut-off frequency between uniform and stepped impedance MLs.

Figure 15

Figure 15 Cut-off frequency vs. parameter b for the lowpass filter with nine DMS cells.

Figure 16

Figure 16 T-shaped DMS lowpass filter with reduced cells.

Figure 17

Fig. 17 Frequency responses of the lowpass filter with reduced DMS cells, using 50 Ω and step impedance microstrip lines.

Figure 18

Fig. 18 Cut-off frequency vs. parameter b for the lowpass filter with reduced DMS cells.

In order to verify the design, a lowpass filter, as shown in Figure 16(b), was fabricated and measured and the hardware, which hasa circuit size of no more than 40×3.2 mm is shown in Figure 20. The figure also shows the measured performances, taken with an Agilent E5071C vector network analyzer. The measured results are similar to the simulation. The measured discrepancies are mainly due to the simulation precision and fabrication uncertainty.

Figure 19

Fig. 19 Cut-off frequency vs. parameter d for the lowpass filter with reduced DMS cells.

Figure 20

Fig. 20 The fabricated filter (a) and its performance (b).

Conclusion

New microstrip lowpass filters, with ultra-wide stopband by using periodical non-uniform DMS, are designed and a comparison between lowpass filters with uniform 50 Ω and stepped-impedance microstrip lines is made. It shows that, with stepped-impedance microstrip line, the filter has enhanced return losses and better out of band suppression. The new design is demonstrated by measurement. Compared with DGS assisted lowpass filter, the DMS lowpass filter has a simpler topology and is easier to fabricate. Most importantly, it has no enclosure problem, because there is no leakage from the ground plane.

Acknowledgment

This work was supported in part by the Open Research Fund of China State Key Laboratory of Millimeter Waves (K201107).

References

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