диафрагмированные волноводные фильтры / c792d9d5-b8ee-4f11-b74f-534a41110189

V Senthil Kumar

Communication Systems Group

UR Rao Satellite Centre, HAL Airport Road

Bengaluru, India. senthil@isac.gov.in

Abstract— A high performance, waveguide E-Plane filter has been designed for Ka-band Ground Station Applications to restrict the band of frequencies. E-plane filter with all metal insert configuration has been selected for the application to obtain low insertion loss. Inductive irises are fabricated on thin metallic structure with high degree of fabrication accuracy and the strip is sandwiched between two waveguide halves form the E-plane filter assembly. Fabrication accuracy of 10 microns is demanded by the high frequency filter to meet the stringent electrical requirements of the Ground Station system. Silver plating has been carried out interior surfaces of the waveguide filter to reduce the insertion loss. Low loss of 0.25 dB is aimed at 26.25 GHz center frequency with 1.5 GHz pass band. Stopband rejection of 80 dB is obtained at 30 GHz frequency, at which high power uplink signals are transmitted from the Ground station for communicating with GEO Stationary Satellites.

Keywords— waveguide filter; millimeter wave; metal strip;

I. INTRODUCTION

Microwave filters used in Ground Stations shall meet stringent requirements like, low insertion loss, good return loss, and high stopband rejection. Presently, Millimeter wave frequency band has acquired significant importance in Satellite communications due to its large available bandwidth to support few Giga bits per second data rate. Microwave filters need high Q resonators for achieving narrow pass band, in general waveguide medium is selected for such designs [1]. A circuit consisting of a metal sheet with appropriate patterns that is inserted in the middle of the waveguide parallel to E-plane is called E-plane circuits. These E-plane filters are high Q, low cost, and suitable for mass production. In 1970s, a maximally flat filter was designed and analyzed by Y. Konishi [2] and subsequently many researchers explored many designs in various frequency bands with improvements in electrical parameters. Thickness of the conductive strip was assumed to be infinitesimally small by Y. Konishi for the analysis. Finite thickness of the conductive strip was considered and analyzed by Y. Tajima [3], where good agreement between analysis and measurement was reported. Y Shih analyzed the effect of the metallization thickness of the septum with an efficient computer aided design (CAD) program for E-plane filters [4]. In this paper, design theory for a band pass filter with inductive strips is presented with electromagnetic simulation results. Low insertion loss performances of the E-plane filter will result in higher G/T (Gain to Noise Temperature) for the Ground station at Ka-band data receive applications.

Dhanesh G Kurup

Department of Electronics and Communication Engineering

Amrita School of Engineering, Amrita Vishwa

Vidyapeetham, Bengaluru, India

dg_kurup@blr.amrita.edu

II. MILLIMETER WAVE FILTERS

Waveguide medium is conventionally selected for filters realization in millimeter wave frequencies as waveguide dimensions at this frequency band is compact. Inductive Iris, Inductive post, Transverse strips or transverse diaphragms based waveguide filters are attractive to provide high Q for achieving narrow pass band. In such filters, the inductive, capacitive structures are part of the waveguide and these structures are machined directly in the waveguide. In upper frequency band of millimeter waves, band pass filters are realized in waveguide with Fin-line configuration for achieving high degree of fabrication accuracy. Fin-line structures are printed on the microwave substrate using photolithographic method where few microns fabrication accuracy is possible. Microwave subsystems are being developed in Ka-Band, V- band and Q band for satellites to utilize the available bandwidths. To support the above millimeter wave based satellite communication, ground stations are being developed. The driving parameters of the design of satellite receive terminals are G/T ratio, and size of the antenna system. G/T value of the ground station can be met by increasing the gain of the antenna and or by reducing the noise temperature of the antenna feed systems. Antenna feed system loss increases the noise temperature and also reduces the overall gain. Insertion loss of the band pass filter which is part of the antenna feed system plays important role in the Ground station G/T as this band pass filter is placed prior to LNA (Low Noise Amplifier) in the ground station antenna system.

III. E-PLANE FILTERS

E-Plane filter consisting of inductive strips on thin metal sheet, which is sandwiched between two waveguide halves. E –plane waveguide filters have high Q, low insertion loss, high power handling capability and their input /output interfaces enable these filters to find many applications in Ground stations. This E-Plane filter is realized by cascading half-wave rectangular resonators. If metal septum is placed at center of broad wall of the waveguide, the structure will act as inductive and the magnitude will depends on the length of the inductive septum. Slot height is equal to the waveguide height to obtain higher Q value in filters. In all cases, the thickness of the septum is very thin and less than 1% of the wavelength [6]. E- Plane filters are capable of providing 0.7% to 7% bandwidth and best suited for millimeter wave frequencies.

IV. ELECTRICAL SPECIFICATIONS

Following specifications were derived for the filter from the Ground station system, which will be used for receiving data information from all Ka-band satellites. Main purpose of this filter is to improve the noise performance of the receiving system and also to provide Electro-magnetic compatibility with the other Ka-band ground antennas, which are used for up linking very high RF power at 30 GHz to communicate with GEO stationary communication satellites. High rejection of 80 dB for uplink frequency signals prevents the LNA from damage and de-sensitization. This band pass filter restricts any other nearby signals entering in to the receive band of 25.5 to 27 GHz thus improving the noise temperature performance of the receive system.

TABLE I: ELECTRICAL SPECIFICATIONS OF THE FILTER

V. DESIGN OF E-PLANE FILTERS

WR34 standard waveguide is chosen for the design of E- plane filter to have compatibility with other subsystems of the antenna feed system. Cut-off frequency of the WR34 waveguide is 17.4 GHz which is far below the operating frequency of 25.5 to 27 GHz. Hence insertion loss of filter will be better in the WR34 waveguide (recommended frequency range is 20 to 33 GHz). As shown in Fig.1, E-plane filter will have a circuit consisting of a metal sheet with appropriate patterns that is inserted in the middle of a waveguide parallel to the E-plane. To meet above center frequency (26.25 GHz) with 1.5 GHz pass band and rejection requirements of 80 dB at 30 GHz frequency, 7 pole filter of Chebyshev type is designed. Low pass coefficients (g) corresponding to 20 dB return loss value and K impedance inverter parameters are calculated from standard design equations given in [1]. Thickness of the septum is selected from the thumb rule i.e., 0.1 to 1% of the wavelength [6] and this design use 0.25 mm. Adding metallic inserts centered in the E-plane of split block waveguide housing is a well-established technique for realizing low-cost and mass producible microwave configurations [9]. Unlike Finline filters, dielectric losses are absent in metal insert E-Plane filters, hence the E-plane structure has a higher transmission Q factor. High power handling capability has made E-plane filters as better choice for many applications in transmitter systems [10]. But the present ground station application need for receive applications where high power handling capability is not demanded.

Fig. 1 Waveguide E-plane Metal-insert Filter.

Fig.2. Equivalent circuit for Band pass filters with Impedance inverters

The equivalent circuit shown in Fig.2 is used for any band pass filter such as waveguides, coaxial circuits and microstrip circuits. The order or degree of the filter (n), the values X1, X2, X3, ….Xn and impedance inverters coefficients K01, K12, K23,….Knn+1 are calculated. The design of the bandpass filter with equivalent circuit along with analytical derivation was described in [2]. The design procedure is as follows: First, equivalent T network of an inductive strip inserted in the middle of the waveguide is estimated. The design method of required filter is derived by applying the equivalent network of the inductive strip to the usual method of filter design.

The design involves in finding the septum length for the i’th

septum using Kii+1 and low pas coefficients (g0, g1, gn). The i’th resonator length is calculated from second equation and λ0 (free

space wavelength of the centre frequency). Computer based program will be very useful in calculating the resonator lengths and inductive strip widths to minimize the calculation time. Many CAD tools are commercially available for the above calculation especially for the E-Plane filters as this type of filter has many practical applications [17]. After calculation of the strip dimensions, it is essential to optimize the return loss and insertion loss parameters by varying the inductive strip width and resonator length dimensions. In this paper, WASP-NET software [18] is used for the optimization.

VI. ELECTRO MAGNETIC SIMULATION

Electromagnetic simulation is essential for this complex filter to avoid fabrication iterations. The simulation of the E- Plane filter, with the help of WASP-NET software package [18], which uses Mode-Matching Technique has been carried out. Optimisation also carried out for the insertion loss and return loss parameters of the filter. Table2 shows the optimized values of the strip widths and resonator lengths. The simulated response of the filter is shown in Fig.3 for the finalized /optimized design. From the simulated results, it is found that the centre frequency is 26.25 GHz and its stop band rejection is better than 80dB at 30 GHz. Simulated return loss value is better than 20 dB over the pass band. Fig.4 shows the group delay variation of the filter and the group delay is less than 3.6 n Sec over the pass band, which meets the specification of 6 n Sec. When we use bandwidth efficient modulation techniques in the data reception, the group delay of the filter should be minimum for minimizing bit errors. Simulated in band ripples in the insertion loss is also shown in Fig.5 and the value is better than 0.05dB. Perfect conductor is assumed for the waveguide and the inductive strips in the simulation and hence the simulated insertion loss value is very less. But in practical conditions, the insertion of this type is filter will be of the order of 0.2 to 0.5 dB depending on the surface finish and surface plating used. Optimized parameters were modelled in another Electromagnetic simulator (High Frequency Structure Simulator software), which gives more accurate simulation. Based on the yield analysis the accuracy of the patterns for the filter is maintained within 10 microns.

Guide dimensions (WR34) : a × b = 8.64 mm × 4.32 mm

Fig.3 Simulated S-Parameter Response of E-Plane Filter

Fig.4 Simulated Group Delay Response of E-Plane Filter

Fig.5 Simulated In-band Ripple Response of E-Plane Filter

VII. FABRICATION, ASSEMBLY & TESTING

WR34 Waveguide was fabricated into two symmetrical structures with a center cut along the length. Inductive irises are fabricated on thin (0.25 mm) Be-Cu sheet using Wire Cut EDM method with high accuracy. Photochemical etching method can also be followed for fabricating the septum. Fabrication cost of the metal inserts is very less (10%) compared to waveguide fabrication while mass producing these septum. To minimize the insertion loss, plating inside the waveguide filter with 4 microns thickness of Silver. At this KaBand frequency, the skin depth is about 0.6 microns and hence above plating thickness is adequate. Photograph of the fabricated unit is shown in Fig.6. Fabrication is carried out in ISRO fabrication facility, Bengaluru. Two waveguide structures along with central septum is assembled using SS screws. Number of screws were decided based on the operating wavelength and RF leakage levels. Six mounting screws are placed in the waveguide assembly to maximize the filter electrical performances. Standard rectangular waveguide WR34 flanges are provided for interfacing with the rest of the waveguide based antenna feed systems.

Waveguide to Coaxial Adapters were attached to the waveguide filter for S-Parameter measurements using Vector Network Analyzer. Developed filter in assembled condition is shown in Fig.6. Waveguide calibration was not carried out but insertion loss of the waveguide to coaxial adapters was normalized. Hence insertion loss measurements are accurate compared to return loss measurements.

Fig.6. Photoigraph of the Ka-Band E-Plane filter

CONCLUSION

High Performance E-Plane waveguide filter has been developed for Ka-band Ground Station applications. Bandpass filter with low insertion loss is achieved at 26 GHz center frequency by selecting metal insert E –Plane filter type. Order of the filter is calculated based on the stopband rejection requirements. After calculating the dimensions of inductive strips and resonator cavities using standard design procedures listed in literature, EM simulation is necessary to avoid the fabrication iterations. Extensive EM simulation is carried out for this complex electro-magnetic structure using commercially available software [18]. Optimization also carried out to obtain good return loss performances in the required pass band. Rectangular waveguide is realized in two part construction and the inductive irises are realized in thin Beryllium-Copper metal sheet. Waveguide parts are fabricated on Aluminum Alloy using CNC machines. Assembly of waveguide pieces sandwiching the metal insert structures is carried out for electrical measurements. Designed filter meets all the requirements of the ground Station Antenna system. High performances like, low insertion loss, good impedance matching, and high stopband rejection of the E-Plane filter will result in higher G/T for the ground station. Designed filter allow Electro Magnetic Compatibility with other Ground Station Antenna systems which are operating at different frequency bands and power levels in Ground Station Complex for communicating with other Satellites. Band pass filter allow only the required band of frequency (25.5 to 27 GHz) will reach the receive system of the Ground station.

ACKNOWLEDGMENT

Authors extend their thanks to Dr. A.V.G Subramanyam for his support during modeling and simulation. The authors would like to also thank Sri. K.M. Subrahmanyam of Communication Systems Group, U R Rao Satellite Centre for his efforts in mechanical fabrication and assembly of the waveguide E-plane filter.

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