диафрагмированные волноводные фильтры / 1b01ac8b-5b54-4e9c-98a9-699b7f15fb4b

SUMMARY

For the realization of the smart community, the Internet of Things (IoT) and sensor technology in the millimeterwave band is necessary. The development of these technologies requires accurate spectrum analysis in the frequency domain over 100 GHz. A tunable preselection filter is a key device in building such a spectrum analyzer. In this paper, we propose use of a ridge waveguide to achieve frequency tuning over a wider bandwidth. In addition, we demonstrate an example design and prototype for a filter with a relative bandwidth of 38% (75 GHz to 110 GHz) and 32% (78.1 GHz to 107.5 GHz), respectively. C 2016 Wiley Periodicals, Inc. Electron Comm Jpn, 99(12): 20–27, 2016; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11909

Keywords: millimeter-wave; tunable filter; FabryPerot resonator.

1.Introduction

To meet the increasing energy demand and reduce environmental load in recent years, it is required to realize the smart community, where home, buildings, and transportation systems are connected by networks and energy is effectively utilized. In Japan, under the leadership of Ministry of Economy, Trade and Industry, the forum of systems related to smart community was established, and Japan Smart Community Alliance (JSCA) based on this forum has started [1]. In this proposal [2], for the area of telecommunications, not only the conventional telecommunication devices, but also Internet of Things (IoT) where each home electronics, energy devices, automobiles, and so far are connected to networks are mentioned as key technologies. As transportation systems, a system in which automobiles are networked as sensors through the development of sensor and control technologies is listed. One of the

problems of the former technology is that the communication traffic drastically increases [3], and a technology to use millimeter-wave and terahertz bands is important from the viewpoint of effective usage of limited frequency source. For the latter technology, the millimeter-wave radar of 77 GHz/79 GHz band [4] is the key technology; it is urgent task to develop telecommunication and sensor technology in the millimeter-wave and terahertz bands.

We have developed spectrum analyzer in the frequency domain over 100 GHz, which is necessary to develop the aforementioned technologies. As its preselector, we have proposed a novel tunable filter which constitutes a Fabry–Perot resonator inside waveguide (FPW filter) [5– 12]. As the center frequency of the transmissive range for this filter is determined only by the distance between two half-mirrors mounted inside waveguide, wide frequency tuning range can be realized in principle. In the previous prototyping, a tunable filter with a relative bandwidth of 24 % (100 HGz to 140 HGz) was achieved. This also has a feature not to consume a large amount of power as YIG tunable filter (YTF) [13] due to small linear actuator-based operation, which allows downsizing.

In this paper, to realize wider tuning range, we propose a novel configuration using ridge waveguide. As an example, we also report a design and a prototype evaluation of the filter with a relative bandwidth of 38% (frequency tuning range: 75 GHz to 110 GHz). The electromagnetic field analysis in this paper was carried out using CST Microwave Studio.

2. Operation Principle of FPW filter and Challenge for Wide Frequency Tuning Range

FPW filter is a kind of filter which is Fabry–Perot resonator [14] inside waveguide. Figure 1 shows the basic constitution of the FPW filter. Two half-mirrors are provided inside rectangle waveguide (height: a, width: b) which transmits only basic mode (TE10 mode), and the distance

C 2016 Wiley Periodicals, Inc.

Fig. 1. Basic constitution of FPW filter. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Fig. 2. E-plane cross section of conventional FPW filter.

between two half-mirrors is L = λg/2. The half-mirror has a structure with arbitrary reflection or transmission coefficient and its material as well as shape is selected according to its characteristics. In the example shown in Fig. 1 [9], the mirror has a structure with slit of 0.07 mm width in the direction of H-plane on Si substrate coated with thin Au film. Here, it is defined that center frequency of desired transmission band is fc and wavelength inside waveguide is λg. Inside the waveguide, only TE10 mode can be transmitted, and hence the line length of transmission wave is uniquely determined, the waveguide surrounded by two half-mirrors works as Fabry–Perot resonator, which serves as a bandpass filter with fc in this configuration. Accordingly, if the line length (L) can be sufficiently changed, wideband tunable filter can be achieved. Preselector of spectrum analyzer is required to have wide frequency tuning range with a few hundred MHz bandwidth at 3 dB as well as insertion loss of less than 10 dB. It is considered that the filter with this configuration is suitable.

Figure 2 shows the cross section of the filter designed as a tunable filter. To change L, the inner waveguide has a movable structure by inserting the inner waveguide with the half-mirrors into the outer waveguide. Furthermore, choke structure is constructed to prevent the leak of transmission wave from the gap between the outer waveguide and the movable waveguide, which is needed to move the inner waveguide. We reported FPW filter produced with this structure previously, and we have already achieved the

frequency tuning range with the relative bandwidth of 24% (100 GHz to 140 GHz) [9].

Through the investigation afterward, we figured out that the following two problems should be solved for the frequency tuning range expansion [8].

The first problem is the increase of insertion loss by higher-order mode. If diameter of movable inner waveguide is equal to the size of standard waveguide, the inner diameter of outer waveguide (equal to size of resonator) is bigger than size of standard waveguide by the thickness of movable inner waveguide. As a result, the frequency forming higher-order mode shifts to lower one, causing a problem that loss appears due to mode conversion within operating frequency range. Thus, movable inner waveguide with a thickness as thin as possible is desired. However, due to the mechanism requirement, the thickness of at least about 0.1 mm to 0.2 mm is needed, and hence the generation of higher-order mode is inevitable. To solve this problem, it is necessary to get its thickness thicker and to lower the frequency lower at which higher-order mode is formed, but this method also makes the frequency for the next higherorder mode shift to lower, which makes further frequency tuning range expansion difficult to realize.

The second problem is disable of choke due to unnecessary resonance. As shown in Fig. 2, this filter needs choke structure to prevent the leak of electromagnetic wave from the resonator to the gap between outer waveguide and movable waveguide. However, if the distance between the choke and the head of movable waveguide approach the half of the wavelength inside waveguide λg, which is determined by the size of outer waveguide, the resonance arises and the prevention capability of the choke is reduced. Thus, the leak from the gap between movable waveguide and outer waveguide increases, and it is possible the insertion loss also increases. Conventional technology solves this problem by minimizing the distance between choke and head of movable waveguide at the lower limit of operation frequency bandwidth (equal to the longest L), which shifts resonant frequency to a value higher than the operation bandwidth. However, if frequency tuning range is further expanded, the change amount of L becomes larger, which results in a problem that resonant frequency appears within operation bandwidth. In this paper, we propose a novel method to solve these problems, and aim to realize even wider frequency tuning range than the previously reported FPW filter [9].

3. Proposal of Frequency Tuning Range Expansion Technology with Ridge Waveguide

To solve the aforementioned problems, we propose a filter with two technologies shown in Fig. 3 for the frequency tuning range expansion.

Fig. 3. E-plane cross section of propositional filter.

The first technology is to introduce ridge waveguide [15] to increase the region of higher-order mode. The ridge waveguide has an effect to shift cut-off frequency to lower band. Using this feature, waveguide can be downsized with operation bandwidth kept. Thus, by reducing the outer size of movable inner waveguide to the extent of inner diameter of standard waveguide, inner diameter of outer waveguide (resonator size) becomes equal to the size of standard waveguide, which is expected that frequency for higher-order mode can be shifted to higher bandwidth than operating one.

The second technology is to suppress unnecessary resonance by mounting choke on movable inner waveguide. As mentioned above, application of a ridge waveguide can thicken movable waveguide. Conventional choke is arranged with the length of λg/4 perpendicular to gap, but if choke is constructed on movable waveguide by arranging in parallel to gap, it is expected that the distance between choke and movable waveguide becomes constant and unnecessary resonance can be expelled from the operating bandwidth. By introducing these technologies, further frequency tuning range expansion can be expected, which is difficult to be realized with conventional FPW filter.

4.FPW Filter Design with Wider Tuning Range

Technologies

To verify the wide frequency tuning range technologies mentioned in the previous section, we designed FPW filter in real and tested its effect. In this design, the operation frequency bandwidth is 75 GHz to 110 GHz and the input/output plane is the waveguide of WR-10, which are different from the previously reported FPW filter [9].

First, a ridge waveguide used as movable waveguide was designed in detail. There are many kinds of ridge waveguides according to their arrangement. In this design, a double-ridge waveguide with symmetrical structure as shown in Fig. 4 is adopted to facilitate its design by simulation software. Also, considering to arrange a choke structure on it, the thickness of the ridge waveguide is set as 0.3 mm. The inner size of WR-10 waveguide is 2.54 mm ×

Fig. 4. Schematic of double-ridge waveguide. Table 1. Design parameters of double-ridge waveguide

Fig. 5. Comparison of cut-off frequencies between normal waveguide and double-ridge waveguide. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

1.27 mm, and hence the inner size of the ridge waveguide to be designed is calculated as 1.94 mm × 0.67 mm by subtracting the thickness of its own thickness. Using this dimensional condition, the ridge part was designed to control the cut-off frequency below that of WR-10 waveguide (59.1 GHz).

There are multiple combinations of e and h to satisfy the condition described above. Considering the processability during its production, the design parameters used this time are shown as Table 1. The simulation result is further shown in Fig. 5. The solid line is S21 frequency characteristics of the designed ridged waveguide, and the broken line is that without ridge. According to Fig. 5, the ridge structure enables cut-off frequency to reduce from 77.3 GHz to 55.6 GHz, and the cut-off frequency of WR10 can be achieved under 59.1 GHz even if waveguide is downsized. Subsequently, the choke mechanism was designed. Figure 6 is the enlarged view of the choke mechanism shown in Fig. 3. As the thickness of the movable

Fig. 7. Comparison of simulation results between proposal choke and conventional choke. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

inner waveguide is 0.3 mm in the previous discussion, the depth and the width Cw of the choke were, respectively, arranged to 0.2 mm and about 1/4 of the wavelength inside waveguide of the outer waveguide (λg), while the gap between the movable and the outer waveguides was 0.03 mm. To verify the effect of the choke, the models with the structure of the proposed (horizontal) choke in Fig. 6 and the conventional (vertical) one in Fig. 2 (width: 0.2 mm, depth: 1.1 mm), respectively, mounted in the 2.54 mm × 0.03 mm waveguide simulating a gap between movable and outer waveguides were simulated, respectively. Figure 7 shows the result of the simulation, where the broken line indicates the frequency characteristics of the conventional (vertical) choke and the solid line indicates that of the proposed (horizontal) one (Cw = 1.1 mm). As shown in Fig. 7, the stop amount (decreasing amount of S21) of the proposal choke is smaller than that of the conventional one, but it is found that an amount of over 10 dB is ensured throughout the entire operation bandwidth. Accordingly, it is estimated that the proposed choke can prevent electromagnetic wave leaked from the gap sufficiently. It is further possible to increase the stop amount by arranging multiple chokes on movable waveguide in the direction away from resonator. The optimal number of chokes should be selected for the whole design of FPW filter.

Finally, the whole design of FPW filter was performed with ridge waveguide and choke mechanism. Halfmirror has a structure consisted of Au-plated metal plate

Fig. 9. S21 frequency characteristics of half-mirror.

with slit in the direction of H-plane in the center part and ridges on upper and lower sides of inner side of the slit (Fig. 8(b)). The design value was arranged such that S21 is around −20 dB to obtain the stop amount necessary for preselector. The simulation model and the frequency characteristics of the designed half-mirror are shown in Figs. 8 and 9, respectively. Figure 10 shows the designed FPW filter.

Figure 11 shows the simulation results of the designed FPW filter when its resonator length L is changed from 3.1 mm to 1.5 mm every 0.04 mm. Figure 11(a) indicates that the increase of the insertion loss of about 1 dB occurs, where the minimum value appears at around 92 GHz. This is because the stop amount of the choke decreases as it gets away from around 92 GHz. Aside from this, the insertion loss increases sharply in the direction from around 110 GHz to 108.3 GHz. The frequency of the higher-order mode (LSE11) calculated from the size of the resonator (2.65 mm × 1.47 mm) is 116.7 GHz. The tendency that the insertion loss increases from the frequency a few GHz higher than that of higher-order mode agrees with the previous simulation result [9]. The results described above suggest that the proposed ridge waveguide can shift the frequency of higher-order mode to higher than the operating one. As shown in Fig 11(a), it is verified that the increase of the insertion loss does not exist in the operation bandwidth, and therefore the effect to avoid the performance decrease of the choke due to unnecessary resonance in the operation bandwidth using the proposed

Fig. 10. Schematic of design filter. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

choke structure can be verified. According to these results, the effect of the proposed technologies for wider tuning range is confirmed, and it is demonstrated that the relative bandwidth of the frequency tuning range is expanded from 24% to 38%. No load (Q) calculated by the simulation is approximately 2400.

5.Prototype and Evaluation Result Using Prototype

Figure 12 shows the prototype of FPW filter. Also, Fig. 13 shows the E-plane cross section presenting the structure of the prototype. The double-ridge structure of the movable waveguide is connected to the outer waveguide through conversion by the taper structure constructed on a common rectangular waveguide at the head of the movable waveguide opposite to the half-mirror. The choke structure is also constructed on the taper side, so as to suppress the leak of electromagnetic wave to the gap. Choke mechanism is also prepared on the taper side to suppress leakage to the gap. The movable waveguide is controlled by outer linear actuator, thereby it can move back and forth within the filter and the total length of the filter does not change. The dimension of the filter without the actuator and its attachment port is 76.5 mm × 25 mm × 25 mm.

Figure 14 shows the result measured by this prototype. For the measurements, frequency extender for E band (WR-12, 56 GHz to 94 GHz) and F band (WR-8, 90 GHz to

Fig. 11. Simulation results of design filter (L = 3.1 mm to 1.5 mm, 0.04-mm step). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Fig. 12. Overview of prototype. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Fig. 14. Measured frequency characteristics of prototype. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

140 GHz) was used, and vector network analyzer was used for the measurements. The prototyped FPW filter (WR10) and the frequency extender were connected by taper waveguide, and the loss of taper waveguide is separately measured and was excluded from the result in Fig. 14.

When the measurement result in Fig. 14(a) is observed, the insertion loss increases and S21 decreases at around 78, 100, and 110 GHz. From the experience to assemble device, it is estimated that the deviation of the movable waveguide due to assemble error affects the insertion loss, and thus it was checked by the simulation. Figures 15 and 16 show the simulation results in the cases that the centers of the outer and the movable waveguide are shifted to E-plane direction (parallel to the short side of the outer waveguide) and to H-plane direction (parallel to the

Fig. 16. Simulation results with H-plane shift. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

long side of the outer waveguide) for 20 μm, respectively. As shown in Figs. 15 and 16, at frequencies around 100, 110, and 85 GHz, the insertion loss increases due to the deviation of movable waveguide. These results indicate that the reason why the insertion loss in Fig. 14(a) increases is the deviation of the movable waveguide from the center by

assembling. It should be noted that the frequencies at which the insertion loss increases do not completely meet with the result by the simulation. It is estimated that the actual device involves even more complicated factors such as deviation and inclination in the rotating direction.

As those described above, the insertion loss at certain frequencies increases. If the operation range of this filter is that S21 ≥ −10 dB in the transmission bandwidth, the relative bandwidth of the frequency tunable range is 32% (78.1 GHz to 107.5 GHz), which has wider frequency tunable range than the conventional filter does, and its effect was verified. If the frequencies at which the insertion loss increases can be suppressed by improving the assembling method, it is expected to achieve the frequency tunable range as calculated by simulation. The measurement reproducibility is mainly dependent on that of actuator position. The position reproducibility of the actuator used this time in the prototype is ±0.2 μm, and the deviation of approximately ±50 MHz was observed at the center frequency.

6.Conclusion

We proposed the wider frequency tunable bandwidth of FPW filter for spectrum analyzer over 100 GHz, which is necessary to develop telecommunication and sensor technology in millimeter-wave band. In addition, the design and the evaluation of the prototype were performed, and its effect was verified. As a result, it was confirmed by the simulation that the relative bandwidth of the frequency tunable range can be improved from 24% to 38% using this bandwidth expansion method. The relative bandwidth of 32% could be verified by the experiment, although there is a problem that the insertion loss increases due to the assembling. This frequency tuning range expansion method can cover the whole operation bandwidth of waveguide only with one filter, which can reduce the parts number including junctions and lower insertion loss at the same time. Therefore, the realization of broadband spectrum analyzer with low cost and high dynamic range is expected. In the future, by improving the assembling procedure and reducing the influence of the assembling error with optimal design, we aim to realize the relative bandwidth of 38% as verified by the simulation.

Acknowledgments

Part of this study was carried out with the support of “Research and Development for Expansion of Electric Wave Source” by Ministry of Internal Affairs and Communications. We are very thankful for each member of the management committee for the important advice and discussion.

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