диафрагмированные волноводные фильтры / 596757af-8988-4d26-bdd8-e372861492c3

Abstract—In this paper, a dual pole frequency reconfigurable waveguide bandpass filter with multiple transmission zeros (TZs) has been demonstrated. Two identical planar inserts have been placed onto the transverse plane of a standard X-band rectangular waveguide at an optimized distance of 8.41 mm to get the response. The inserts consists of varactor diode and chip capacitor loaded split ring resonators (SRR) and asymmetrical rectangular slots. The diodes and chip capacitors have been used for frequency reconfiguration whereas asymmetrical rectangular slots have been used to independently control the transmission zeros (TZs) location. Numerical simulations are performed using CST microwave studio (version 14).

Keywords— rectangular waveguide, bandpass, transmission zeros (TZs), varactor diode.

I. INTRODUCTION

In modern microwave and millimeter wave communication systems, electronically reconfigurable/ tunable based waveguide bandpass filters are becoming more and more popular. The popularity has been achieved because the center frequency can be tuned within a given bandwidth by using a single component like, micro-electro mechanical system (MEMS) [1], varactor diode [2–3], optical components [4], memristor [5], etc.

In present day communication systems, electromagnetic interference is a big concern. To reduce / overcome the effect of interference, the filters should have sharp skirt selectivity and good stop band performance. These can be achieved by introducing TZs in the stopband. There are different techniques to introduced TZs in the filter response, like, use of cross coupling between the resonators [6], and frequency selective surfaces [7].

The objective of this paper is to design a compact, lightweight, reconfigurable, dual pole waveguide bandpass filter with multiple TZs.

II. 1. RESONATOR ANALYSIS AND FILTER DESIGN

The schematic diagram of the reconfigurable resonator and its placement inside a standard WR-90 are shown in Fig. 1. The resonator consists of a split ring resonator,

loaded with a chip capacitor (120 pF, operating frequency 10 MHz - 40 GHz) and a varactor diode (Skyworks SMV1231), for frequency reconfiguration. The resonator has been designed on Roger RO4350 dielectric substrate of relative dielectric constant 3.66, loss tangent 0.004, slab thickness 0.762 mm, and copper thickness 0.035 mm.

Fig. 1. Reconfigurable resonator and its placement into the standard WR-90 rectangular waveguide.

When the bias voltages of the varactor diodes are changed, their capacitances are also changed. This, in turn, changes the net capacitance of the resonator and hence the resonance frequency. The variation of the resonance frequency with bias voltage is shown in Fig. 2. It reveals that as the bias voltages increases, resonance frequency shifts towards higher frequency. The optimized dimensions of the SRRs are as r0= 3.1 mm, r1 = 2.8 mm, and r2 = 4 mm. The chip capacitor was used to enhance the net capacitance of the resonator, so that the resonance can be achieved at the desired frequency point.

Fig. 2. Unit cell responses for different bias voltages.

Once the unit cell has been analyzed, the next step is to develop a dual pole, reconfigurable waveguide bandpass filter. In the proposed work two unit cells (Fig. 1) have been placed on the transverse plane of a WR–90 waveguide at an optimized distance of 8.41 mm. At 10 GHz, separation between the resonators is 92.15°, which approximately quarter wavelength.

The simulated frequency responses of the filter at two extreme bias voltages are shown in Fig. 3. The 3D model of the filter has been shown as inset in the same Fig.3. The Fig.3 shows poor passband to stopband transition. Therefore to improve it two asymmetrical slots have been introduced in the unit cell of Fig. 1, as shown in Fig. 4. Each slot introduces one TZ and helps to control it. Since there are two unit cells in the structure, there will four slots and four TZs. The Fig. 4, represents combination three different parallel LC (two asymmetrical slot and one SRR) circuit connected in series. At resonant frequency SRR provides infinite shunt impedance, therefore all the power transferred to the load and provides a pole. At different frequency, SRR represents inductor / capacitor. The asymmetrical slot resonator resonates at two different frequencies, provides short circuit shunt path and results two TZs.

Fig. 3. S-parameter responses with 0V and 5V bias voltages.

Fig. 4. Asymmetrical slot resonator and SRR.

To demonstrate the independent control of the TZs, the parametric analysis of the unit cell (Fig. 4) has been carried out for different asymmetrical slot dimensions (L1 and L2). The results are plotted in Fig. 5. It reveals that with increase in L1, the lower TZs down shifts in frequency while the upper TZ and resonance frequency remain constant. Similarly with increase in L2 upper TZs down shifts in frequency while the lower TZ and resonance frequency remains constant. The variation of slot width W1 and W2 has

been shown at Fig. 6. The Fig. reveals that, by varying slot width the location of upper and lower TZs remains constant.

Fig. 5. Variations of lower transmission zero and upper transmission zero with variation in the slot length L1 and L2.

Fig. 6. Variations of lower transmission zero and upper transmission zero with variation in the slot width W1 and W2.

Based on the above analysis the slot dimensions have been optimized for TZs at 8.40, 9.03, 10.67 and 11.56 GHz. The optimized dimensions are found as L1 =13 mm, L2 = 8.3 mm, W1 = W2 = 0.5 mm for unit cell 1 and L1 =14.9 mm, L2 = 9.3 mm, W1 = W2 = 0.5 mm for unit cell 2. The location of asymmetrical slot from centre is 4.5 mm. The asymmetric slots in unit cell 1 provides TZs at 8.40 and 10.67 GHz whereas the asymmetric slots in unit cell 2 provides TZs at 9.03 and 11.56 GHz.

III. RESULT AND DISCUSSION

To analyze the tolerance between the two identical inserts and its effect on the frequency response of the filter has been shown in Fig. 7. It reveals that an acceptable frequency response can be achieved for l = 8.41 mm. The simulated frequency response of the proposed filter is shown in Fig. 8. The 3D model of the filter has been shown as inset in the same Fig. 8. The Fig. reveals that the resonance frequency varies between 9.77 GHz to 10.07 GHz when bias voltage varies between its extreme ranges (0 V to 15 V). The corresponding 3–dB bandwidth varies between 582 MHz to 661 MHz with passband insertion loss 1.25 dB and passband return loss better than 20 dB.

Fig. 7. Parametric analysis of the structure for different lengths ( ).

Fig. 8. Comparison of the simulated S-parameters (magnitude) of the filter for 0 V and 15 V biasing voltages.

IV. CONCLUSION

This paper presents a frequency reconfigurable waveguide bandpass filter with multiple TZs. The reconfigurabilty has been achieved by using varactor diode and location of TZs can be independently tuned by using asymmetrical slot. The total length of the proposed filter is 10 mm, which is compact and makes the filter light weight.

REFERENCES

[1]L. Pelliccia, S. Bastioli, F. Casini and R. Sorrentino, “High Q tunable waveguide filters using ohmic RF MEMS switches,” IEEE Trans. Microw. Theory Techniques, vol. 63, pp. 3381–3390, 2015.

[2]N. Mohottige, U. Jankovic, D. Budimir, and U. Jankovic, “Compact E–plane varactor–tuned bandpass filters,” Antennas and Propag. Society Int. Symp. (APSURSI), pp. 790–79, 2013.

[3]A. Bage and S. Das, “A Frequency Reconfigurable Dual Pole Dual Band Bandpass Filter for X–Band Applications,” Prog. In Electromag.Res. Lett., vol. 66, pp. 53–58, 2017.

[4]N. Mohottige, D. Budimir and C. J. Panagamuwa, “Optically reconfigurable E–plane waveguide resonators and filters,” 43rd European Microw. Conf., pp.798–801, 2013.

[5]M. Potrebić, D. Tošić and D. Biolek, “Reconfigurable microwave filters using memristors,” Int. J. Circ. Theor. Appl., vol. 46, pp. 113– 121, 2018.

[6]J.S. Hong and M. J. Lancaster, “Couplings of microstrip square open– loop resonators for cross–coupled planar microwave filters,” IEEE Trans. Microw. Theory Tech., vol. 24, pp. 2099–2109, 1976.

[7]M. Ohira, H. Deguchi, M. Tsuji, and H. Shigesawa, “Novel waveguide filters with multiple attenuation poles using dual–behavior resonance of frequency–selective surfaces,” IEEE Trans. Microw. Theory Tech., vol. 53, pp. 3320–3326, 2005.