диафрагмированные волноводные фильтры / 71eafcf0-2f6a-48ee-81b0-72d77b9fb1e5

Abstract— This letter presents a simple waveguide filter structure having planar structure that can control transmission zeros (TZs) and center frequency independently. The planar structure is composed of the conventional complementary splitring resonator (CSRR) and asymmetric slot structures. The planar structures resonate at particular frequencies and introduce total 2N TZs, above and below the passband of an Nth-order filter, and thus also improve the stopband performance of a planar CSRR-based waveguide bandpass filter. To demonstrate it, a third-order CSRR-loaded bandpass filter has been considered and analyzed. It has been shown that by varying the length of the asymmetric slot structures the TZs can be independently controlled.

Index Terms— Bandpass filter (BPF), complementary split-ring resonators (CSRR), transmission zeros (TZs).

I. INTRODUCTION

MODERN radar and radio navigation systems require low loss, low cost, low weight, high power, compact microwave filters for their operation, and complementary splitring resonator (CSRR)-loaded waveguide bandpass filter is an excellent option for this [1]. However, such filters have poor stopband performance and are affected by out-of-band interfering signals easily. In order to reduce the interference level, these filters should have very good stopband characteristics, which can be achieved by introducing a number of transmission zeros (TZs). To achieve design flexibility, these TZs should be independently controllable. In practice there exist different techniques to independently control the TZs. A very common approach is implementation of cross coupling between the non adjacent resonators or between the dual mode and multimode cavities [2]. Many other techniques like extracted pole cavity [3], frequency dependent techniques [4] also exist. Cascading of a group of resonators [5], [6] is also a useful solution for controlling TZs. Glubokov and Budimir [7] presented an inline extracted pole filters using non resonating nodes that can generate TZs equal to the order of the filter. In [8] and [9], E-plane waveguide bandpass filters have been implemented to achieve two TZs at both sides of the passband. Ohira et al. [10]–[13], proposed few waveguide

Manuscript received October 19, 2016; revised January 10, 2017 and March 14, 2017; accepted May 11, 2017. (Corresponding author: Amit Bage.)

The authors are with the Department of Electronics Engineering, IIT (ISM), Dhanbad 826004, India (e-mail: bageism@gmail.com; sushrut_das@ yahoo.com).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LMWC.2017.2723983

filters with multiple attenuation poles using dual-behavior resonance of FSS / resonators. The filter, proposed in [10], uses three FSSs and each of them has completely different shape. The authors proposed the application of the genetic algorithm to find the shape of the FSSs, which made the filter design complex and time consuming. The FSSs in [11] and [13] are also complex and require a time consuming optimization to set the center frequency and TZs at the desired locations. The filter, proposed in [12], uses two different types of resonators and a waveguide step. The results reveal that the resonance antiresonance frequencies are not independent. Rosenberg et al. [14] proposed a double-slot iris structure to form waveguide bandpass / bandstop filter. Each of these two slots has different dimensions and provides a path for signal propagation. The slot dimensions were adjusted to form a destructive interference between the signals of these two paths at the TZ frequency. Irrespective of the shape of the irises or technology, none of these filter structures allow independent control of the center frequency and TZs. Kyrylenko and Mospan [15] presented doubleand triple-slot iris-based singleand dual-band bandstop filters with controllable quality factor.

This letter presents a CSRR and asymmetric slots-based planar insert geometry as an alternative of those in [11]–[13] for designing waveguide bandpass filters. The location of TZs can be independently controlled by varying the respective slot length whereas location of resonant frequency can be independently tuned by varying the dimensions of the CSRRs. Therefore the planar insert is capable of controlling TZs and center frequency independently, which was not possible in [11]–[13]. The proposed insert geometry is also simpler than those in [11]–[13]. It has been shown that for the Nth-order filter, 2N TZs are introduced in stopbands, which improves the stopband characteristic of a planar CSRR-loaded bandpass filter. To demonstrate it, a third-order CSRR-loaded equiripple bandpass filter has been designed using a standard design procedure [1]. To validate the analysis, simulated results have been compared with measured results which show a good agreement.

II. ASYMMETRIC SLOT-LOADED CSRR STRUCTURE

The geometries of a conventional CSRR structure and of the proposed asymmetric slot-loaded CSRR structure are shown in Fig. 1. They have been placed on the transverse plane of a standard WR–90 waveguide and simulated using Ansoft HFSS (V. 14). The simulated frequency responses of the

2

Fig. 1. Resonator structures. (a) Conventional CSRR. (b) Proposed structure.

Fig. 2. Comparison of the frequency responses of the structures of Fig. 1.

circuits are compared and shown in Fig. 2. Fig. 2 reveals that center frequencies of both structures are equal, which means that asymmetric slot structures have no effect on center frequency. However, they can introduce two TZs in the insertion loss characteristic, on both sides of the passband. In fact the equivalent circuit of Fig. 1(b) can be represented as a series combination of three different parallel resonators (one corresponding to the CSRR and the others corresponding to the two asymmetric slots) that is placed in shunt in a transmission line. At 9.9 GHz the resonator, corresponding to the CSRR, resonates and provides infinite shunt impedance. Therefore, all the power is transferred to the load and a transmission pole results. At other frequencies the CSRR behaves either as inductor / capacitor. If the asymmetric slot dimensions are chosen such that their resonance frequencies lie outside the band of interest, then they will also behave as inductor and the other as capacitor. A careful choice of the slot dimension can make one slot to behave as inductor and the rest as capacitor, within the band of interest. With these conditions fulfilled, the structure will behave as a series resonator. The resonator resonates at two different frequencies to provide short circuit shunt path, and results in two TZs (one below the transmission pole where the CSRR behaves as inductor, and the other above the transmission pole where the CSRR behaves as capacitor).

Since the center frequency is primarily determined by the CSRR, above analysis implies that the position of the lower and upper TZs can be set by varying asymmetric slot dimensions (alternatively, varying the inductance and capacitance provided by them). To establish this, the proposed structure has been simulated for different slot lengths (either varying L1 or L2). The results are shown in Fig. 3. It reveals that the variations of a particular slot length (L1 or L2) primarily affects either upper or lower TZ. The corresponding variations of other TZ and center frequency are relatively small. This provides the flexibility of independent control of lower and upper TZs.

III. FILTER DESIGN

Based on the analysis, presented in [1], initially a thirdorder equiripple bandpass filter has been designed for center

IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS

Fig. 3. Variations of F0 (resonance frequency), lower TZ, and upper TZ with (a) L1 and (b) L2.

TABLE I

LUMPED ELEMENT VALUES OF EQUIVALENT CIRCUIT MODEL

frequency 9.92 GHz, lower cutoff frequency 9.57 GHz, higher cutoff frequency 10.28 GHz, and passband ripple 0.26 dB. Next, asymmetric slot structures have been incorporated in the design to insert lower TZs at 8.2, 8.88, 9.27, GHz and upper TZs at 10.54, 11.14, and 12.11 GHz. The schematic of the filter and its equivalent circuit is shown in Fig. 4. In Fig. 4(b) E and K represents the electrical length and impedance of inverter whereas F represent the center frequency. The CSRRs with asymmetric slot structures have been fabricated on Roger RO4350 substrate with relative permittivity (εr) 3.66, dielectric loss tangent [tan(δ)] 0.004, substrate height 0.762 mm, copper thickness 0.035 mm, and placed on the transverse plane of a standard WR–90 waveguide at 9.02-mm distance. The ‘h’ indicates the copper and substrate thickness. The values of lumped elements are tabulated in Table I.

Once the values of the lumped elements have been found, the dimensions of the CSRR and the slots can be adjusted using the method described in [1]. The obtained dimensions are tabulated in Table II.

IV. RESULTS AND DISCUSSION

The proposed filter has been simulated using Ansoft HFSS (Version 14), fabricated and measured using Keysight PNA network analyzer (Model: N5221A). The simulated (with 3-D EM model and with the equivalent circuit in Fig. 4) and measured frequency responses are compared in Fig. 5, which

Fig. 4. Proposed third-order bandpass filter. (a) 3-D view. (b) Lumped element equivalent circuit model.

TABLE II

DIMENSIONS OF THE RESONATOR

Fig. 5. Comparison of the simulated and measured S-parameters of the filter.

TABLE III

COMPARISON OF FILTER CHARACTERISTICS

shows a good agreement between them. The mismatches between the circuit simulated response and measured/HFSS response, especially at the upper stopband is due to the parasitic capacitances between the CSRR and asymmetric slots that have been neglected in the equivalent circuit of Fig. 4.

Table III shows a comparison of the characteristics of the proposed filter with few other similar reported filters [10]–[13].

Other characteristics are more or less same. Therefore, the proposed work can be considered as an alternative of the filters of [10]–[13]. In Table III f0, RL, BW, and IL represent the center frequency, return loss, bandwidth, and insertion loss, respectively.

V. CONCLUSION

This letter presents a simple way to insert and control the TZs in the stopband of a CSRR-loaded waveguide bandpass filter using asymmetric slot structures. Necessary explanations have been given for the TZs along with their control mechanism. The method provides a flexible way to insert TZs at some specific target frequencies, which helps the filter to reject those frequencies with higher levels as compared to other stopband frequencies. The introduction of six TZs also improves the stopband performance compared to a simple planar CSRR-based waveguide bandpass filter.

REFERENCES

[1]A. Bage and S. Das, “A compact, wideband waveguide bandpass filter using complementary loaded split ring resonators,” Prog. Electromagn. Res. C, vol. 64, pp. 51–59, May 2016.

[2]A. E. Atia and A. E. Williams, “Non-minimum phase, optimum amplitude, bandpass waveguide filters,” IEEE Trans. Microw. Theory Techn., vol. MTT-22, no. 4, pp. 425–431, Apr. 1974.

[3]J. D. Rhodes and R. J. Cameron, “General extracted pole synthesis

technique with applications to low-loss TE011 mode filters,” IEEE Trans. Microw. Theory Techn., vol. 28, no. 9, pp. 1018–1028, Sep. 1980.

[4]S. Amari and J. Bornemann, “Using frequency-dependent coupling to generate finite attenuation poles in direct-coupled resonator bandpass filters,” IEEE Microw. Guided Wave Lett., vol. 9, no. 10, pp. 404–406, Oct. 1999.

[5]S. Amari, U. Rosenberg, and J. Bornemann, “Singlets, cascaded singlets, and the nonresonating node model for advanced modular design of elliptic filters,” IEEE Microw. Wireless Compon. Lett., vol. 14, no. 5,

pp.237–239, May 2004.

[6]R. Levy and P. Petre, “Design of CT and CQ filters using approximation and optimization,” IEEE Trans. Microw. Theory Techn., vol. 49, no. 12,

pp.2350–2356, Dec. 2001.

[7]O. Glubokov and D. Budimir, “Extraction of generalized coupling coefficients for inline extracted pole filters with nonresonating nodes,”

IEEE Trans. Microw. Theory Techn., vol. 59, no. 12, pp. 3023–3029, Dec. 2011.

[8]J. Y. Jin, X. Q. Lin, Y. Jiang, L. Wang, and Y. Fan, “A novel E-plane substrate inserted bandpass filter with high selectivity and compact size,” Int. J. RF Microw. Comput.-Aided Eng., vol. 24, pp. 451– 456, Jul. 2014.

[11]M. Tsuji, H. Deguchi, and M. Ohira, “A new frequency selective window for constructing waveguide bandpass filters with multiple attenuation poles,” Prog. Electromagn. Res. C, vol. 20, pp. 139–153, Mar. 2011.

[12]M. Ohira, T. Matsumoto, Z. Ma, H. Deguchi, and M. Tsuji, “A new type of compact evanescent-mode waveguide bandpass filter using planar dual-behavior resonators,” in Proc. Asia–Pacific Microw. Conf., 2011, pp. 1023–1026.

[13]M. Ohira, H. Deguchi, and M. Tsuji, “A novel resonant window having dual-behavior resonance for pseudo-elliptic waveguide filter,” in Proc. 36th Eur. Microw. Conf., 2006, pp. 1083–1086.

[14]U. Rosenberg, S. Amari, and F. Seyfert, “Pseudo-elliptic direct-coupled resonator filters based on transmission-zero-generating irises,” in Proc. 40th Eur. Microw. Conf., 2010, pp. 962–965.

[15]A. Kyrylenko and L. Mospan, “Twoand three-slot irises as bandstop filter sections,” Microw. Opt. Technol. Lett., vol. 28, no. 4, pp. 282–284, 2001.