диафрагмированные волноводные фильтры / 29dccb58-5cd6-40a9-97b2-3cc76d975b92

Abstract—A higher order rectangular waveguide metal insert noncontiguous diplexer at Ku band is investigated in this paper. During the design process for a diplexer a E-plane metal insert filter at Ku band was also investigated. The result demonstrates a Chebyshev response with 0.05 dB ripple in the two pass-bands and very high rejection in the stop-band giving higher isolation between the frequency bands.

KeywordsFilter synthesis; metal-insert filter; Waveguide diplexers.

I.INTRODUCTION

Passive waveguide devices, such as E-plane filters are the key systems in the application of the microwave and millimeter waves applications. The E-plane metal insert filter gains its importance due to its easy design, low cost, high-Q and very low loss due to absence of the dielectric materials. Besides these, it has an advantage of high power carrying capacity well suited for satellite and radar communication purposes. The theoretical analysis using equivalent circuits and design procedures of metal insert filters are well-known [1]-[3].

The design of a E-plane filter comprises of E field parallel alignment of resonators and gaps between each resonator filled by thin metallic plate. Here in this manuscript, the design of a higher order E-plane filter is reported. A MATLAB program on general synthesis of E-plane filters is developed. Design of the 9 pole filter based on full wave electromagnetic simulations is carried out using a Moment of Method solver (CST MICROWAVE STUDIO).

Subsequently the design of a Ku band E-plane diplexer is reported in this manuscript. Two higher order filters at different central frequencies are combined using a H-plane power splitter to realize the overall design.

Paper accepted for presentation at the International Conference on Devices and Communications (ICDeCom-11), Ranchi, India, http://www.icdecom.bitmesra.ac.in

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Figure 1.a) E-plane metal insert filter. b) Metal septum with periodic hollows c) Equivalent circuit with impedance inverters

II.THEORY

Section of transmission line of finite thickness metallic plate with rectangular cross-sections is used as a unit element for the design of the waveguide filter (TE10 mode). The equivalent circuit of this unit filter is an inductor in shunt with two series capacitors along the direction of propagation as shown in figure 2.

Figure 2. Unit filter section with equivalent circuit

The equivalent circuit parameters for E-plane discontinuity can be calculated by set of equations [4]. These metal septums are the impedance inverters used to invert and scale the impedance ZL connected to its output port with the input impedance Zin. It operates like a quarter-wavelength line of characteristic impedance K at all frequencies [5, 6].

Zin = K 2

Zl

As shown in Figure 1, the gaps between the metallic septum li are half-wave resonators varying with characteristic impedance. Numerical solutions of the series reactance Xs, and parallel reactance Xp as shown in figure.2 can be calculated from normalized geometric parameters t' = t/λo ,a' = a/λo and d' = d/λo, where, λo is the wavelength in free space[2]. The geometric parameters are kept between the following limits for most of the applications.

0.001 ≤ t'≤ 0.05

0.55 ≤ a'≤ 0.95

0.025 ≤ d '≤ 0.80

A. Design of the filter

Provided the values of the pass-band ripple (La) the stopband attenuation (Ls), the two pass-band edge frequencies(f1 and f2) the waveguide size (a) and the metal septum thickness (t), the following steps are used for synthesis of filters[2].

•Determine the number of resonators required to achieve desired result.

•Prototype realizations from given filter parameters. There are several prototype functions out of which Chebychev function was selected for its considerable sharp cut-off.

•Find out the individual septum length and spacing between them.

•Full wave simulation and optimization if required.

A MATLAB code was used for the filter synthesis. The program takes less than 10 seconds to converge with error of less than 0.0001 mm for the physical dimensions of the filter.

Figure.3. Solution of the septum length from the calculated reactance

Figure.3 shows a convergence graph of normalized reactance parameters plotted against the normalized length of the septum.

B. Design example

A WR 62 waveguide dimension was selected for the Ku band filter design. A filter was designed for the following specifications.

Filter specifications:

Passband ripple (la) = 0.01 dB; Stopband attenuation (ls) = 40 dB;

Lower cutoff frequency (f1) = 15.75 GHz; Upper cutoff frequency (f2) = 16 GHz;

Upper stopband edge frequency (fs) = 16.1 GHz;

In order to achieve good selectivity a higher order filter with 9 resonators is selected for the design. The thickness of the metal septum was taken as 0.6 mm. As the thickness reduces, the manufacturing process becomes more rigorous. With the help of the reactance parameters the physical dimensions such as resonators lengths and the impedance inverter lengths were calculated using the program. The septums are considered to be made of electrical conductors with very low loss. The complete proposed design of such filter is shown in figure.4.

Figure.4. Complete design of the filter.

The frequency response of the designed filter is shown in figure.5. The response shows a pass-band with center frequency at fo=15.88 GHz and bandwidth of 200 MHz The pass-band has a maximum insertion loss of about 0.05 dB. The return loss (S11) in the pass band is greater than 21dB. The response has a quick transition from pass band (S21=0.01dB) at 16 GHz to stop band (S21=-60 dB) at 16.1 GHz.

Figure.5. Frequency response of the filter

The out of band rejection is also greater than 100 dB in the frequency range between 13 GHz to 20 GHz (figure.6). The broadband characteristics show a reentrant mode appearing at 20.8 GHz.

Figure.6.Broad band frequency response of the filter

The variation in thickness of the metal insert results in shifting of the passband which is to be taken care of for very thin metal insert at higher frequencies.

III.DIPLEXER DESIGN AND RESULTS

The design of a diplexer with the help of a H-plane tee and two bandpass filter is developed. The block diagram of the diplexer with two channels is shown in figure.7. The technique is to synthesize two band-pass filters with different frequency band characteristics in the Ku band and combine them as shown in figure.8.

A H-plane waveguide Tee is used for its easy design and fabrications. A proper matching of the filters with the H-plane Tee is necessary.

Figure.8. Diplexer with two filters and a H-plane Tee

To avoid higher order modes due to filter junction the matching length for the filters are chosen sufficiently long (≈ λ g / 2 ) . Smaller the difference between the band

frequencies of the two filters better and easier is the matching of the diplexer.

Figure.9. Frequency response of the diplexer

The frequency response of the diplexer is as shown in figure.9. Two bands of 200 MHz each are seen at central frequencies 15.5 GHz and 15.95 GHz respectively. The steep response of the filter yields the isolation greater than 60dB at 15.75 GHz between both the filters bands. It is observed that the diplexer response is highly dependent on the power divider being used. The diplexer is optimized using the variables ml1, ml2, nl and nw as shown in figure.10.