POWERFUL MULTIPLE HARMONICS REJECT FILTER DESIGN, TESTING AND ACCEPTANCE CONSIDERING ULTRA OVERMODED WAVEGUIDE CONDITION Rousslan Goulouev, Daniel Garcia, Qiang Shi, Colin McLaren Honeywell Aerospace Unit 10, Triangle Business Park, Stoke Mandeville, Aylesbury HP22 5SX Email:
[email protected] INTRODUCTION Waveguide low-pass or harmonic reject filters are commonly used in RF/microwave radio and communication systems and play important role rejecting spurious harmonics from transmitter. Most publications refer to up to third or fourth harmonic rejection requirements for a harmonic filter used in satellite communication systems. Highly atypical rejection requirements (see Table 1) extending up to 17th harmonic arose due to specifics of application of MetOp-SG SCA Wind Scatterometer [1]. The condition of possible existence of hundreds of propagational waveguide modes (484 counted in WR-159 at 92 GHz) creates more challenges and risk factors than dealing with conventional waveguide harmonic filters due to additional problems, which are associated with general lack of public knowledge on suitable and applicable design concepts, design procedures, computational methods, analysis tools, testing procedures and acceptance rules. The current paper presents MetOp-SG harmonic filter (HFIL) and a summary of concomitant study on the problems listed above.
Fig. 1. MetOp SCA harmonic filter 3D EM model drawn in HFSS (a) and operational prototype (b) DESIGN CONCEPT Among the existing information on waveguide harmonic filters the four major classes of low-pass filters can be considered based on E-plane corrugated waveguide, “Waffle-iron” structure, ridged waveguide and absorptive concept [2-5]. Although all those types of filters are continuously being modified, there still can be certain characteristic features associated with them in respect of performance, cost and risk factors, if used in space applications. Two conceptual solutions based on the trapped mode and E-plane corrugated waveguide have been found matching the specification by analysis. Although the trapped mode filter is commonly recommended due to “optical beaming” [3] of the upper remote frequencies to RF absorbing terminations, the practical realization is considered to face a number of risk factors associated with uncertainties in simulation accuracy and materials properties over such high frequencies. Despite reflective character of rejection and presence of high-order modes propagation bands, the E-plane corrugated waveguide filter concept is selected because of technological simplicity (made from two metal parts) and higher accuracy of overall design process (due to 2D nature of E-plane structures). DESIGN METHOD The common design procedure for a regular corrugated filter is introduced in [6] and established in three steps: 1. Selecting the height of waveguide channel stopping the propagation of cross-polarized waveguide modes (TE01 and higher) 2. Selecting right width of corrugated structure defining the layout of TEN0-modes pass-bands on the frequency axis (those pass-bands must not fall into specified stop-band frequencies)
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Designing corrugated structure having continuously wide stop-band (from the roll-off to the last point of the rejection spec) for TE10-mode. According to the researchers [6,7] following the steps 1 to 3 “automatically” leads to required attenuation of propagation of the stop-band frequencies carried by any other waveguide mode. The greatest challenge, however, is 3rd step, i.e. finding the “TE10 compliant” 2D design solution providing ultra-wide spurious free stop-band. The traditional design methods based on distributed networks [6] are found not efficient, because they usually lead to designs, which are technologically infeasible or not compliant to some of key requirements by further simulations. On the contrary, a “non-prototyped” design method based on distribution of “cavity transmission zeros” [7] (some modifications are applied) is found to be more fit to the requirements, as it based on preselected gap size and iris thickness of corrugations minimizing conductivity losses and maximizing multipaction fringing effects [8]. THEORY The design and analysis methods are based on the single cavity EM model corresponding to the problem of scattering on a cavity formed by a uniaxial connection of three rectangular waveguides, which is stated and solved in [7] in terms of variational mode-matching method [9]. The solution is expressed in closed non-matrix terms for the Y-matrix 11
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divided into four sub-matrices Ykl , Ykl , Ykl and Ykl associated with reflection and transmission paths of scattering. The Y-matrix in turn can be normalized into GAM and converted into GSM [10]. Since the corrugated structure (Fig. 1 (a)) is inline built from the single cavity elements, the GSM of whole filter is computed using common GSM cascading methods [10]. The analytical study of the cavity model results several important conclusions used further for significant simplification of the modal analysis and the design process in general. First of all, the transmission response of a single 12
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corrugation cavity might have a transmission zero point. The first order zero corresponds to Y0,0 = Y0,0 = 0 (in a single mode approximation) with the position and the bandwidth (loaded Q-factor) can be designed [7]. Secondly, the analysis 12
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of convergence of the sums in the expressions for Ykl and Ykl submatrices show very fast convergence due to exponential grow of terms sin ( β n L ) since the localized modes become evanescent [7]. This means that the accuracy of the position of the transmission zero and its Q-factor are accurately computed even for very high frequencies with a reasonable number of localized modes taken into account. And thirdly, the analysis shows the dominance of the diagonal elements in all GAM submatrices for the basic E-plane cavities forming the corrugated structure. That practically validates reviewing only diagonal members of GSM in the modal analysis. The last conclusion is intuitionally confirmed by the researches of corrugated filters in publications [6,7]. The “tapering function” used in [7] is found, however, inefficient if applied to the current stop-band requirements as it leads to a corrugated structure mostly rejecting a bandwidth from near-band to about the 3rd harmonic only. The optimal corrugations profile is found if using a tapering function (1) with γ ≈ 2.5 and ε ≈ 8.5 giving a greater plurality of cavities better distributing the transmission zeros over high frequencies. γ π ⋅ i hi = hmin ⋅ 1+ ε ⋅ cos N
(1)
The rest of dimensions are also tapered in the same way using the propositions, selection criteria and limitations defined in [7]. In contrary to [7], the HFIL corrugations are tapered from the centre to the ends, which is preferable for matching with the interface waveguide in terms of image parameters [11] using no stepped transformers. SIMULATION The variational mode-matching modal analysis applied to HFIL final design model with a large number of accessible TE nm and TE nm -modes with the n ≤ 28 and m ≤ 5 showed full compliance with the specified rejection requirements. Some selected near-band and out-of-band characteristics are shown on Fig.2 and Fig.3. In addition to domestic mode-matching software, HFSS and SPARK 3D used to additional verification and power handling analysis. MEASUREMENT The fundamental problems of measuring a waveguide component in overmoded propagation and scattering conditions are commonly known [13]. Currently there is no a known technologically feasible way of measuring multi-moded GSM values if the number of waveguide modes is more than two. Nevertheless, the waveguide harmonic reject or low-pass
filters are commonly measured with taper transitions [6,7]. The accuracy of such a measurement is not studied nor practically evaluated.
Fig. 2. Simulated near-band frequency response (a) and multi-mode transmission versus frequency plot for symmetric vertically polarized modes (b)
Fig. 3. Simulated multi-mode transmission (dB) versus frequency (GHz) plots for asymmetric vertically polarized modes (a) and cross polarized modes (b) Due to crucial importance of the HFIL testing, appropriate test philosophy and measures are developed, which are formulated in several items: 1. Compliance to rejection over all specified bands if measured in traditional way with the tapers. This measurement commonly called “TE10-mode measurement”. 2. Demonstration of a good correlation between the measurement and simulation for TE10-mode. 3. Supplementary TE20-mode and TE50-mode measurements are also to show compliance with specs. 4. Demonstration of a good correlation between the measurement and simulation for TE20 and TE50-modes. 5. Evaluation of the measurement errors due to the overmoded interaction occurring between the tapers and DUT. A separate study is performed on measurement errors based on mode-matching models of the tapers/transducers connected to each other (during calibration) and with DUT (during measurement). The study shows that usually the original transmission response (when HFIL is loaded with semi-infinitive waveguides) is significantly distorted with numerous spurious spikes. It is, however, shown that the resulting measurement error is less if measuring higher attenuation values, which is explained by narrower bandwidth of the spurious spikes and their greater absorption due to conductivity losses.
Fig. 4. A 3D model of taper-DUT-taper drawn by HFSS (left) and transmission response measurement simulation over the vicinity of 5th harmonic (right) It is also shown a general invalidity of such measurement over the TE n0 propagational zones (see Fig. 2 (b) and Fig. 3(a)) due to dominant existence of another mode. Within the restrictions, the overall measurement error is estimated as not greater than ±15 dB if measuring 60 dB and more. It has to be noted the estimate is applicable only to regular corrugated filters and cannot be applied to filters of other types because of specifics of scattering discussed above. According to the philosophy of testing, since the HFIL measurement results are compliant with the specs when measured with appropriate tapers/transducers showing adequate margins greater than corresponding measurement error values, and the measurement results well correlate with the simulation results for the selected waveguide modes over the selected frequency spots, the rest of simulation results on the other GSM paths and over frequency bands are also validated.
Fig. 5. Selected measured rejection plots for 4th (a) and 17th (b) harmonics bands The TE10-mode transmission response testing is performed over the vicinities of each harmonic from the second to 17th with long transitions tapered from appropriate single moded waveguide to WR-159 and the HFIL put between the tapers. The measured values of rejection are found not to be detectable in most cases due to the noise limit (see Fig. 5.), which confirms the modal analysis results giving high values of attenuation for the worst case of modal scattering. The near-band measured data is also found in a good correlation to simulation results. Due to a large number of plots all measured and simulation results are not presented here except the selected ones, but they are summarized in the performance and compliance table (see Table 1).
TE20 AND TE50 MODE TRANSDUCERS Special TE10 to TE20 and TE10 to TE50 mode transducers are designed, fabricated and used for the supplementary test measures mentioned above. The TE20-mode transducer is designed based on common concept of gradual transforming TE10-mode into TE20-mode through T-shaped cross sections [14].
Fig. 6. 3D view of TE10-TE50 mode transducer drawn in HFSS (left) and output excitation of high order waveguide modes near 5th harmonic frequency The TE50-mode transducer concept described in [14] is found technologically too complicated and large by size. A new type of TE50-mode transducer is developed based on an H-plane stepped and E-plane corrugated structure. The H-plane steps are formed in such a way giving a significant excitation of TE50-mode and The E-plane corrugated structure is designed to filter TE50-mode from the rest of the other waveguide modes excited by the H-plane steps.
Fig. 7. HFIL simulation versus measurement correlation plots for transmission magnitudes of TE20 (a) and TE50 (b) modes RESULTS AND COMPLIANCE A novel design approach has been developed for a high power multi-harmonic reject waveguide corrugated filter. The filter has been fabricated and tested. The simulation and measurement results are in a good agreement with each other and compliant to the requirements. The worst case simulation and measured results versus corresponding requirements of specification are summarized below in Table 1 showing compliance for all listed parameters.
Table 1. Summary of key HFIL RF requirements, simulated and measured results Parameter Spec Simulated Measured Pass-Band, GHz 5.355 ± 0.002 5.355 ± 0.002 5.355 ± 0.002 Return Loss, dB >28 >40 >38 Insertion Loss, dB 120(1) 105(2) 3rd harmonic [16.059, 16.071] GHz TBD >120(1) 103(2) 4th harmonic [21.416, 21.424] GHz >120(1) 103(2) > 50 5th harmonic [26.775, 26.785] GHz > 120(1) 91(2) > 55 th (1) 6 harmonic [32.124, 32.136] GHz > 120 87(2) > 75 th (1) 7 harmonic [37.483, 37.497] GHz > 120 85(2) > 65 8th harmonic [42.832, 42.848] GHz > 120(1) 78(2) > 60 th (1) 9 harmonic [48.191, 48.209] GHz > 120 78(2) > 60 10th harmonic [53.540, 53.560] GHz > 120(1) 82(2) > 65 (1) Harmonics 11,12,13,14,15 and 16 > 60 >73(2) TBD 17th harmonic [91.023, 91.057] GHz > 60 80(1) 85(2) Peak power (multipaction), W >2800 57000(4) N/A L x W x H dimensions, mm 265 x 85 x 70 265 x 85 x 70 265 x 85 x 70 Interface WR-159 WR-159 WR-159 Notes: 1) worst case of waveguide mode transmission, 2) limited by VNA noise floor
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