диафрагмированные волноводные фильтры / 69671c08-caaf-4388-a18d-0d7353a820c2

Abstract— This letter presents a novel configuration of a high- Q bandwidth (BW) reconfigurable waveguide (WG) filter tuned with only two tuning elements regardless of the filter order. The filter is realized in rectangular WG technology and is capable to tune the BW without deviating the center frequency. Furthermore, the configuration is scalable to higher order filters without the need for additional tuning mechanisms. For the proof of concept, a four-pole prototype filter is designed, fabricated, and tested at the Ku-band. The measured BW tunability of the

filter is nearly 35% from 225 to 320 MHz. The center frequency remains unaltered at 13.375 GHz over the BW range.

Index Terms— Reconfigurable filter, tunable bandwidth (BW), waveguide (WG) filter.

I. INTRODUCTION

THERE have been significant developments in recent years to enhance the lifespan of a communication satellite by refueling the satellite on-orbit using robotic mechanisms [1]. Hence, a communication satellite with a longer lifespan inevitably demands a flexible payload to meet the requirements

of the ever-increasing need for a high data rate [2]–[4]. High-quality factor (Q) tunable filters constitute an essential

part of a flexible payload. However, one of the key requirements for such reconfigurable filters in such applications is to minimize the number of tuning elements. In this regard, frequency reconfigurable high-Q filters have been proposed in both coaxial and waveguide (WG) technologies that utilize only one tuning element and yet maintain constant absolute bandwidth (BW) over the tuning range [5]–[8].

With regards to BW reconfigurable high-Q filters, the challenge is to maintain the same center frequency with a minimum number of tuning elements as BW is varied. Reference [9] proposes a four-pole circular WG cavity filter at the K -band, which utilizes nonresonating cavities as coupling structures for seamless tuning of interresonator and input– output couplings. The filter in [9] is designed at 20 GHz and achieves a BW variation from 54 to 72 MHz. However, the filter utilizes N + 1 (where N is the filter order) tuning elements. To reduce the number of tuning elements, [10] proposes a two-pole dual-mode circular WG cavity filter at the X-band. The filter is designed at 11.2 GHz and achieves a BW variation from 26 to 52 MHz. Though the filter reduces

Manuscript received October 24, 2019; revised January 21, 2020 and March 23, 2020; accepted April 15, 2020. (Corresponding author: Gowrish B.)

The authors are with the Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada (e-mail: gowrish.biit@gmail.com; rrmansour@uwaterloo.ca).

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

Digital Object Identifier 10.1109/LMWC.2020.2989283

the number of tuning elements required for resonators by 50%, it requires independent tuning elements for each of the couplings.

Especially, for higher order filters, it is highly desirable to minimize the number of tuning elements. This not only enhances the reliability and reduces the cost but also significantly eases the complexity of the control system in a closed-loop reconfigurable radio architecture. The proposed high-Q WG filter configuration in this letter aims to drastically reduce the number of tuning elements. The proposed BW reconfigurable filter utilizes only two tuning elements irrespective of the filter order. For the proof of concept, a four-pole prototype filter is designed, fabricated, and tested at the Ku-band. The measured BW tunability of the filter is more than 35% from 225 to 320 MHz. The center frequency remains unaltered at 13.375 GHz over the BW range. Section II describes the methodology adopted in designing the proposed filter. Measured results and observations of the prototype filter are expounded in Section III, followed by concluding remarks in Section IV.

II. BANDWIDTH RECONFIGURABLE WG FILTER

Fig. 1 depicts the 3-D model of the proposed BW reconfigurable high-Q WG filter. Interresonator and input/output couplings are realized using metal septa located in the E-plane of the WG. BW reconfigurability is achieved by linearly moving the two metal inserts (modified sidewall) referred to as “tuning elements.” The linear movements of the two tuning elements provide the flexibility to tune the absolute BW without altering the center frequency of the filter.

In Fig. 2, we consider an ideal WG cavity to illustrate the variation of the physical coupling coefficient of an E-plane metal septum as the sidewalls of the WG cavity are linearly displaced. The linear displacements of the sidewalls effectively change the location of the metal septum within the WG and, hence, change the coupling values that lead to BW variations. As expected, the coupling is minimum when the septum is at the center of the WG, and coupling increases as the septum approaches either of the sidewalls of the cavity. Thus, two tuning elements with linear displacement mechanisms are sufficient to change all the coupling values and hence the BW in the proposed WG filter.

With regards to the impact of resonator loading (i.e., the shift in the resonant frequency due to changes in coupling values), Fig. 3 depicts the loaded resonant frequency of metal septum versus coupling. It can be seen that the loaded resonance frequency remains uniform over a wide range of

Fig. 2. Impact of septum position on the coupling value (k: physical coupling coefficient, fr: resonant frequency, W_s1 < W_s2 < W_s3).

Fig. 3. Resonator loading: impact of coupling structure on the resonant frequency of a loaded resonator (septum width W-s varied from 25 to 10 mm, and iris width W-i varied from 0.5 to 4.25 mm).

coupling values. To highlight this unique feature of septum filters, we also show in Fig. 3 the variation of the loaded frequency in the case of inductive iris filters. It is noted that the loaded resonance frequency varies significantly in this case with the coupling values.

Hence, an inherently E-plane metal septum filter is robust to resonator loading impact caused by coupling variations. As a result, no additional tuning elements to correct each of the loaded resonators are required. Thus, the proposed BW reconfigurable filter requires only two tuning elements to tune the BW while maintaining the same center frequency irrespective of the filter order.

For practical implementation, the tuning elements have an air gap of 0.5 mm between the top and bottom metal walls. The tuning elements are shaped to push the spurious resonant modes (resulting from the air gap) out of the required band [8]. Fig. 4 depicts the simulated reflection coefficient (S11) and the transmission coefficient (S21) of the proposed filter. The insertion loss of the filter varies from 0.25 dB (at 315 MHz) to 0.37 dB (at 210 MHz). The filter is designed using the coupling matrix approach [11] and simulated using a 3-D Electromagnetic ANSYS HFSS [12].

III. FABRICATION AND MEASUREMENT

Fig. 5 depicts the photograph of the fabricated prototype filter (disassembled and assembled). The filter is made from aluminum. M2.5 dowel pins and screws are used for alignment and assembly of the filter. M2 threaded Nylon screws are used

Fig. 6. Measured results: reflection coefficient (S11) and transmission coefficient (S21).

as support rods (see Fig. 1). The Kapton tape is used at the WG ends to ensure the presence of the air gap between tuning elements and WG sides. Compression springs are used over the support rods to precisely adjust the linear displacements of the tuning elements. The measured S11 and S21 results of the BW reconfigurable prototype filter are depicted in Fig. 6. The insertion loss of the filter varies from 0.56 dB (at 320 MHz) to 0.83 dB (at 225 MHz). The measured group

delay and the spurious response of the filter are shown in Figs. 7 and 8, respectively. Table I compares the proposed BW reconfigurable high-Q filter with other approaches.

IV. CONCLUSION

This letter has presented a novel configuration of a BW reconfigurable WG filter that uses only two tuning elements irrespective of the filter order. The proposed filter configuration demonstrates that it can achieve relatively wide BW variations without deviating the center frequency. A four-pole prototype filter is designed, fabricated, and tested at the Ku-band. The measured BW tunability of the filter is nearly 35% from 225 to 320 MHz at 13.375 GHz. To the best of our knowledge, this is the only BW reconfigurable filter that can be tuned with only two tuning elements regardless of the filter order.

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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS

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