диафрагмированные волноводные фильтры / 3f54dd28-740e-4d63-b466-001fa84bbb5d

Abstract— This paper presents the design of a millimeter-wave waveguide bandpass filter with wide spurious free stopband, and absorptive properties for the second and third harmonics. The filter consists of the combination of a reflective evanescent mode filter and a absorptive leaky-wave filter, the latter being employed as an harmonic pad. Specifically, while the reflective filter provides the required input-to-output rejection of the harmonics, the absorptive filter provides the required matching, thus preventing the harmonics from being reflected at the input port. The absorptive filter is properly designed and analysed in HFSS so as to absorb all of the TE and TM waveguide modes which are in propagation up to 140 GHz. The experimental results of this filter will be presented at the Conference.

I. INTRODUCTION

The evanescent mode approach is one of the most effective solutions for the design of very compact filters having relatively low insertion loss and high power handling capability [1], [2]. Thanks to their wide upper stopband, these filters are often employed within waveguide assemblies as quasi-lowpass filters. As is for any reflection-type filter, the power at the stopband frequencies is reflected at input port, and although this is perfectly fine for most of the applications there are instances in which such a reflected power is undesired as it significantly impacts the performance of the system. As an example, the absorption of the reflected power is commonly crucial when the filter is fed by a high power transmitting tube, which is the case for high power radar systems. In order to solve the various problems associated with the presence of multiple reflections between the rejection filter and the microwave source, such as frequency pulling, hot-spotting, and potential oscillations of the amplifiers, extensive research was carried out starting in the sixties on reflection-less waveguide filters [3]-[5]. All these filters were intrinsically absorptive filters, where the absorption was responsible for both suppression of the reflected and transmitted signal.

An alternative solution to realize reflection-less filters consists of preceding a reflective filter with an absorptive filter, the latter being constructed as a frequency selective pad which minimally affects the operating frequency (passband of the reflective filter) while attenuating the undesired frequency range (stopband of the reflective filter). The frequency selective pad could be designed in principle as a

highpass/lowpass diplexer having the highpass port terminated,

but unfortunately this implementation is unpractical when the function of the diplexer must be guaranteed beyond the unimodal bandwidth (and for all the various higher order modes) of a standard sized rectangular waveguide. A leaky-wall filter appears to be the most suitable solution for a rectangular waveguide realization, and although the concept of cascading a leaky-wall filter with a reflective filter was mentioned in [4], virtually no practical implementation or further analyses have been published until very recent times, when an interesting implementation has been proposed in [6]. The solution proposed in [6] employs a slotted waveguide structure as leaky-wall filter cascaded with a reflective waffle-iron filter. The various slots couple to a common auxiliary waveguide which is then terminated on a resistive load. In this structure, the resonant frequency and the external q-factor (coupling) of the slots determine the location and bandwidth of the absorbed frequency range. This solution seems to give the best results when a specific and limited range of frequencies must be absorbed. Moreover, the accuracy pertaining to the manufacturing of the resonating slots becomes more important as the frequency increases (the length of the slot is especially important in setting its resonant frequency).

In this paper we propose a different solution which is suitable for a broadband continuous absorption of the stopband and that is suitable for the implementation of very high frequency structures. Our solution employs a different type of leaky-wall filter similar to those proposed in [3] and [5], where the coupling elements to the absorptive loads are non-resonating dielectric-loaded waveguides, commonly referred to as branches. The non-resonating properties of these coupling elements makes the filter naturally broadband (whatever is above the cut-off frequency of the branches will be coupled and absorbed to some extent) and eases the manufacturing process (the size of the waveguides only determines the branches cut-off frequency, which does not need to be precise).

II. V-BAND FILTER

The V-band filter design described herein has a 43-46 GHz passband and -40 dBc stopband ranging from 60 to 138 GHz. Passband insertion loss has to be below 1 dB, while the

(a)

(b)

Fig. 1 Cascaded leaky-wall and evanescent mode filter: (a) block diagram;

(b) equivalent diagram for the absorbed stopband frequencies.

Fig. 2 Basic structure of the periodic leaky-wall filter.

passband return loss should be no less than 15 dB. Moreover, a maximum of 2.5:1 VSWR has to be guaranteed at the 2nd and 3rd harmonics frequency ranges, namely 86-92 GHz and 129-138 GHz. Input and output interfaces of the filter are standard WR-22 waveguide flanges.

The block diagram of the proposed filter is shown in Fig. 1. The leaky-wall filter behaves as a frequency selective pad which offers minimum insertion loss at the passband frequencies of the evanescent mode filter, while behaving as an attenuator at those stopband frequencies that needs to be absorbed (Fig. 1b). In such a configuration, thanks to the equivalent short circuit provided by the evanescent mode filter at its stopband frequencies, the power at the reflected frequencies will pass twice (back and forth) throughout the leaky-wall filter. As a result, the leaky-wall filter performance in terms of absorption has to be only half of the total required input return loss. In this particular case, 7.36 dB being the return loss required at the harmonic frequencies (2.5:1 VSWR), the leaky-wall filter has to provide something like 4 dB of absorption in order to meet the requirements. Besides taking care of the rejection of the transmitted signal on the harmonic frequencies, the evanescent mode filter significantly reduces the length of the leaky-wall filter, as compared to a

design that would use only the leaky-wave design for all of the harmonic attenuation..

A. Leaky-wall filter

The designed leaky-wall waveguide filter is a periodic structure consisting of a straight section of WR-22 waveguide where several branches made out of dielectric-loaded circular waveguides are attached to both broad and narrow walls of the rectangular waveguide. The basic structure which is then periodically repeated along the length of the waveguide section is depicted in Fig. 2. Each waveguide broad wall has 5 branches while each narrow wall has 3 branches. The dielectric loading of the circular waveguide branches is Quartz. This material has a dielectric permittivity of 4 (which is high enough to ensure good performance in terms of peak power), and it is a well-known material for having excellent loss tangent values even at very high frequency (observe that low loss branches are especially recommended when high average power is required [3]). At the operating frequency, which is well below the cutoff frequency of the dielectricloaded circular waveguides, the branches are completely reflective thus not allowing any energy to be coupled into them. The passband signal is therefore virtually unaffected by the presence of the branches. On the other hand, for those frequencies above the cut-off frequency of the dielectricloaded circular waveguides, a significant portion of the signal energy is coupled into those branches. The branches are then terminated on a resistive load so that all the energy that was coupled into them is not reflected back to the main rectangular waveguide.

The diameter of the branches (1.22 mm) is designed so that the cutoff frequency of the branches is somewhere around 70 GHz, which is between (but far enough removed) with respect to) the passband and the second harmonic. The locations of the branches are selected so as to couple (and therefore absorb) the energy from several higher order modes. To this purpose, although the inherent structure symmetry would suggest that in theory we could only consider those modes which do possess at the same time even-symmetry with respect to the E- plane and odd symmetry with respect to the H-plane (such as TE30, TM12, TE12), in practice all waveguide modes which are in propagation below 138 GHz needs to be considered. As a matter of fact a simple waveguide bend or twist preceding the leaky-wall filter within the whole waveguide assembly of the system would be more than enough to excite other modes

(such as the TE10, TE20, TM11, and TE11) which would still need to be absorbed by the filter.

In order to optimize the efficiency of the full-wave simulator (we used HFSS) when a large number modes are involved, the basic structure in Fig. 2 has been analysed considering all the four possible combination of symmetries (four separate simulations have been carried out). Radiation boundary conditions are employed at the ends of the branches to simulate the presence of a non-reflective absorptive load, actually used in the final design. Note that waveguide ports could have been used to but the computation time and would drastically increase. The reflection and transmission

Fig. 3 HFSS simulations: reflection and transmission of the modes with evenand odd-symmetry with respect to the E- and H-plane, respectively (such as TE10, TE30, TE12, TM12). The fundamental TE10 mode is indicated in red.

Fig. 4 HFSS simulations: reflection (a) and transmission (b) of the modes with oddand even-symmetry with respect to the E- and H-plane, respectively (such as TE01, TE03, TE21, TM21).

Fig. 5 HFSS simulations: reflection and transmission of the modes with even-symmetry with respect to both E- and H-plane (such as TE11, TM11, TE31, TM31).

Fig. 6 HFSS simulations: reflection (a) and transmission (b) of the modes with odd-ymmetry with respect to both E- and H-plane (such as TE20, TE40, TE22, TM22).

coefficients (S11 and S21) of these modes in for the various symmetries are shown in Fig. 3, 4, 5, and 6. Observe that above their cutoff frequency, all the considered modes shows a good matching from the input port and a decent level of absorption. As an example, the dominant TE10 mode (red curve in Fig. 3) loses about 1 dB when passing through the basic element for frequencies above 70 GHz. Moreover, the simulated insertion loss of the TE10 mode (which is the only mode in propagation at the passband frequencies) at 46 GHz is about 0.1 dB.

Based on the HFSS simulations, four basic unit cells should be cascaded to form the periodic structure of the leaky-wall filter. This would provide the equivalent of a 4 dB pad, and

therefore a input matching of 8 dB. In order ot have some margin, five sections have been actually employed.

B. Evanescent Mode Filter

The evanescent mode filter is a 7-pole reflective filter which provides the required rejection of the transmitted signal up to 138 GHz. To this purpose, the cross-section of the filter is extremely compact (approx. 1x1 mm). This compact section includes five resonators which slightly penetrates into holes in the upper wall so as to realize the proper capacitive loading of the evanescent mode section. In order to obtain a decent width for the passband (thus minimizing the losses), the coupling between the WR-22 and such a little waveguide section must be maximized somehow. In this design this was accomplished

by using two resonating quarter-wave ridged waveguide sections which are embedded into the filter as the first and last resonators. The evanescent mode filter structure is shown in Fig. 7.

III. RESULTS

The complete structure of the filter comprising the leakywall and the evanescent mode filter is depicted in Fig. 8. The distance between the two filters has been dimensioned to optimize the filter return loss at the harmonics frequency ranges (it is about quarter-wave length in this design).

The HFSS simulation of the structure is depicted in Fig. 9. More than 10 dB of return loss will be provided at the harmonic frequencies, while more than 8 dB of return loss will be provided in all the stopband from 75 to 138 GHz. Observe that thanks to the reflective evanescent mode structure the filter response is very selective in the vicinity of the passband (this would not have been possible by only using an absorptive filter).

The experimental results of a manufactured prototype will be presented at the Conference.

VI. CONCLUSION

A millimeter-wave waveguide bandpass filter with harmonics absorption has been proposed. The filter employs the cascade of a reflective type evanescent mode filter and an absorptive type leaky-wall filters, the latter being used essentially as a frequency selective pad. HFSS analysis for all the involved waveguide modes has been provided. The experimental results of the filter will be presented at the Conference.

REFERENCES

[1]R. V. Snyder, “New application of evanescent mode wave-guide to filter design,” in IEEE Trans. Microwave Theory and Tech., vol. 25, no. 12, pp. 1013-1021, Dec. 1977.

[2]G. Craven and R. Skedd, Evanescent mode microwave components, Artech House, 1989.

[3]E. Wantuch, R. M. Maines, “Novel high power harmonic suppressor,“ PGMTT National Symposium Digest , Vol 62 , No 1, 1517 May 1961, pp 70 – 71.

[4]L. Young, B. M. Schiffman, and E. G. Cristal, \High-power filters for the suppression of spurious frequencies," in PTGMTT International Symposium Digest, vol. 64, no. 1, May 1964, pp. 122 { 126.

[5]R. V. Snyder, “An improved low-pass harmonic absorber," in IEEE MTT-S International Microwave Symposium Digest, vol. 76, no. 1, June 1976, pp. 122.

[6]T. Stander, P. Meyer, P. W. Van der Walt, “Compact high power broadband absorbitive filters using slotted waveguide harmonic pads,” in IET Microwave, Antennas & Propagation, Vol. 8, No 9, pp, 673678, June 2014.

Fig. 7 Evanescent mode filter.

Fig. 8 Complete filter structure.

Fig. 9 HFSS simulations of the complete filter.