диафрагмированные волноводные фильтры / e9c5201d-2e7c-4e5e-b0b5-295e0b737479

Short Abstract— A novel diplexer design is introduced that uses filter characteristics with transmission zeros (TZs). These TZs are realized by dedicated cavity resonances at the single filter input ports. This approach allows an overall compact design since there are no structural constraints as in conventional configurations with cross coupled cavities. All cavities are arranged in an H-plane structure. The interfacing of the respective cavities at all ports by E-plane T-junctions facilitate the compactness. They are realized at one side (below the cavity structure) for easy integration and mounting. The frequency allocations of the different channels can be easily exchanged by mounting the diplexer in a 180-degree position. This is possible due to a point symmetry for the single channel ports with respect to the common port. The approach has been verified by several diplexer designs for Ka-band applications. As an example, a 28GHz diplexer with 6th order filters is presented – good coincidence of theoretical and measured results demonstrate the validity of the approach.

Keywords-component; waveguide diplexer; asymmetric filter characteristics; extracted pole filters; symmetrical interface ports

I.INTRODUCTION

Transceiver equipment for radio or satellite transmission systems often use waveguide diplexers for the separation of the receive and transmit signals served by the antenna. These diplexers are commonly of the branching type, i.e., the two filters are directly connected to a suited 3-port waveguide junction providing the common port (e.g., [1]). In the eighties 3D electromagnetic CAD tools based on methods such as mode matching or finite element methods became available. Since then there has been a continuous effort focused on suitable 3- port junction structures for the interconnection of the filters (e.g., [2-6]). In most of these designs standard inline cavity filters are used providing a standard Chebyshev response with theoretically all transmission zeros at infinity. In recent designs [5,6] filter responses with transmission zeros at finite frequencies were used. Thus, tailored filter functions can be realized to achieve the required isolation between the frequency bands with a lower filter order compared to the standard Chebyshev designs. The main advantages are lower insertion loss and smaller size due to the lower number of

cavities. However, the introduced designs [5,6] use cross couplings in triplet or quadruplet cavity structures for the realization of the transmission zeros. Therefore, there are constraints and limitations for the overall diplexer structure including a suitable alignment/positioning of the three interface ports.

In this contribution a diplexer design is presented that uses filter characteristics with transmission zeros at finite frequencies. The transmission zeros are brought about by assigned cavity resonances directly coupled to the single filter input ports. The remaining filter resonances are sequentially coupled from the respective single port interface to the common port. Hence, this concept allows a flexible arrangement of the cavities without the restrictions of grouped cross coupled cavity configurations. This flexibility can be used for overall compact diplexer arrangements considering the single port locations symmetrical to the common port position. The latter aspect provides easy change of the lower and upper frequency band allocations at the respective interfaces by mounting the unit in a 180-degree position, since all interfaces are situated on one side. The advantages of this novel concept are demonstrated by a 28GHz diplexer with 6th order filters. Good coincidence of computed and measured results prove the overall design approach.

II.FILTER DESIGN

A diplexer design task generally starts with the evaluation of appropriate filter functions for a given specification. The main requirements of the 28GHz diplexer design are summarized in table 1. The most relevant rejection requirement for the filter characteristics results from the isolation between the served frequency band which is the closest to the passband. In addition the environmental and technological aspects have to be considered. For example, a low cost realization from aluminum at 28GHz, has to take into account the frequency change of ± 35MHz over the operating temperature range due to the material expansion. This is, the filter passband has to be enlarged and the stopband requirements have to be considered closer to the passband.

TABLE I. SUMMARY OF SPECIFICATIONS

(between common and single ports in the respective useful band)

The investigations of all relevant aspects for a compact and low cost design yield 6th order filter functions with a 600MHz passband (return loss 23dB) and one transmission zero 800MHz above or below the passband, respectively, i.e., at the passband of the other diplexer filter. Fig. 1 shows, for example, the synthesized filter characteristic for the lower band filter of the diplexer. Each transmission zero is implemented by a cavity resonance coupled to one interface port of the filter (extracted pole, cf. [7]), while the other resonances are sequentially coupled from the input to the output port. Thus, all couplings of the filters can have the same nature since there are no particular sign conventions for the generation of transmission zeros as in cross coupled filter structures. That is, both filters have the same configuration which facilitates a symmetrical arrangement for the final diplexer interfacing.

The normalized coupling values of the single filters are obtained by following e.g. the synthesis in [7], which also provides the frequency offsets of the resonances and the required phases between the extracted pole cavity and the first

GS=1

0

Figure 1. Synthesized characteristic of the filter for the lower passband

sequentially coupled resonator. Both filters exhibit very similar coupling values (cf. Fig. 2). The main difference are the frequencies of the extracted pole cavities and the phases between these cavities and the respective first sequentially coupled filter cavities.

An H-plane cavity structure has been chosen for the realization of these filter characteristics. The required phase between the input and the first sequential coupled cavity is realized by the length of a waveguide section. The cavities at the interfaces are directly coupled by E-plane junctions, that is, the interfaces are perpendicular to the cavity structure. The inline structure of the filters is folded to accommodate with an overall compact design with symmetrical interface locations (cf. next section).

Extracted pole cavity

Extracted pole

cavity

Lower band interface port

Figure 3. Outline of diplexer structure

U-shapes determined for the diplexer configuration. That is, the design considers the intercavity irises at the respective side-walls to accommodate with the desired U-shape structure. In a second step, these structures are used for a first diplexer design. The little differences of the cavity sizes and the phase waveguide sections of the two filters will cause a deviation from the desired symmetry of the interface locations. Hence, a final design step is necessary considering suitable adjustments of the cavity alignments to obtain exactly symmetrical locations for the interface ports.

The design of the single filter as well as the diplexer structures has been supported by the CAD tool ‘Microwave wizard’ from MICIAN GbR. These computations also considered finite radii in the structures which are evident for a realization with state-of-the-art CNC milling techniques. The computed results of the final diplexer structure with radii – providing also the symmetry of the interface ports – are shown in Fig. 4.

III.DIPLEXER CONFIGURATION

Fig. 3 depicts an outline of the diplexer configuration with the H-plane cavity filter structures. All interfaces are realized by E-plane T-junctions. The five sequentially coupled cavities of each filter are folded to compose a compact U-shape. One end of each U structure is directly connected via an iris to the common port T-junction located in the center of the unit. The cavity at the other end of each U structure is coupled via an iris to a short waveguide section. The length of this section is determined according the respective phase condition obtained from the synthesis. These waveguide sections are also connected to E-plane T-junctions. The opposite port of each of these T-junctions, couples via an iris the respective 6th cavity of the filters – which implements the transmission zero. The single channel interfaces are provided by the perpendicular ports of these T-junctions. Consequently, all interfaces are situated at one side of the diplexer perpendicular to the filter cavity structures (cf. Fig. 3). This overall configuration facilitates compactness and the final integration in an equipment.

The initial designs of the filter structures start according the

IV. REALISATION AND RESULTS

The diplexer is realized in a flat sheet from aluminum containing the structural part with the cavities, irises and T- junctions (see Fig. 6). The waveguide interfaces with WR34 size are situated on the backside of the structural part. This part is completed by a simple lid containing some screws for post manufacturing fine tuning. Both, structural part and lid are silverplated and finally joined by soldering. The measured results shown in Fig. 5 exhibit good agreement with the computed responses. The insertion loss at the respective center frequencies is less than 0.6 dB which corresponds to a Q efficiency of 65% (the theoretical unloaded Q factor is app. 5000). Regarding the useful bands including the margin for the frequency drift over temperature (this is a considered bandwidth of 470MHz) the insertion loss is still below 0.9 dB. Wide spurious free bands are achieved due to the application of the fundamental mode resonances for the cavity designs. This compact and easy acting design facilitates large scale and low cost series production.

Rejection / Return loss (dB)

0

-50

Rejection / Return loss (dB)

0

-50

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