диафрагмированные волноводные фильтры / b4bbe185-8b11-420f-975e-95d31cbf5eae

Abstract—This paper describes the design of a folded rectangular waveguide cavity band pass filter, incorporating virtual negative coupling (VNC) by means of circular iris, at X-band. Thus, the filter is robust from mechanical aspects, and hence apt for space application. The filter realized at 8310 MHz with 1.2 percent fractional bandwidth, has a sharp roll-off due to VNC. As a result, rejection better than 40 dBc is achieved at just 100 MHz offset from band centre. Compared to traditional all pole filter, introduction of VNC has resulted in around 20% size reduction.

Keywords— band pass filter, cavity resonator, rectangular waveguide , virtual negative coupling

I. INTRODUCTION

Band pass filters implemented using rectangular waveguide (WG) cavity have been profoundly used in space applications, especially beyond X band [1, 2]. Traditionally, WG cavity resonators are coupled through iris window (inductive or capacitive), to realize all pole filters. However, surge in the demand for high data rate, has opened up new avenues in filter design. Coupling matrix based optimal filter design has been matured and well proven across technology [3, 4]. By introducing appropriate transmission zeros (TZ), an optimal filter (with minimal order) can be designed for application at hand. This approach, not only results in compactness but also minimizes the insertion loss (IL), compared to traditional all pole filter design [4].

Introduction of TZ in a filter, demands for two types of inter resonator coupling, say positive and negative. One of the challenging aspects in such a filter design, specifically for space application, is ensuring the robustness of coupling mechanism for mechanical rigidity. This issue is well addressed by the concept of 'Virtual Negative Coupling' (VNC) [4-8]. In VNC, the coupling mechanism (i.e. iris opening) remains same for both positive and negative coupling. However, negative coupling is achieved by forcing phase reversal between the resonators.

In this paper, a folded rectangular WG cavity filter employing VNC is designed, simulated and realized at 8310 MHz with a stringent specification for space application. As a result, a pair of symmetric TZ's is realized in the filter's response, thus leading to sharp rejection at the adjacent channel. Section II describes the design of the WG filter incorporating VNC. Measured results are depicted in Section III, followed by conclusion in section IV.

Fig. 1. Perspective view of the WG cavity filter with VNC

Fig. 2. Top view and side view of the filter, depicting internal dimensions

II. FOLDED RECTANGULAR WG CAVITY FILTER

Fig.1 depicts the perspective view of the band-pass filter realized using WG cavity with SMA interface. It is an 8th order finite pole filter, incorporating VNC (Virtual Negative Coupling) between resonator 1 and resonator 8. Fig. 2 shows the top view and side view of the filter, whose internal dimensions are given in Table I. The filter casing is made from aluminium, which is omitted in Fig.1 and Fig.2, to depict the interior details. SMA centre pin and bullet is made from copper.

A. Design Methodology

Following specification is chosen for the filter design: centre frequency of 8310 MHz, bandwidth of 100 MHz (IL < 1.0 dB), rejection better than 40 dBc at 100 MHz offset on either sides from centre frequency. The above specification demands a very sharp roll-off for the filter. A traditional all pole filter design would require a 10th order filter. To reduce the filter order, a pair of TZ’s is introduced in the folded configuration (E plane), which in turn leads to compactness.

Table II compares the folded filter configuration and all pole (Chebyshev) configuration. It can be observed that the IL is improved by around 0.25 dB and volume is reduced by around 20% as well, in case of chosen finite pole filter when compared to all pole filter. Table III depicts the (N+2) × (N+2) Coupling Matrix (CM) derived for the chosen specification and configuration. An in-house developed CM generator & analysis tool is used. Well known, eigen solver based design is adopted for transforming CM to physical realization of the filter [4]. HFSS tool is used for complete 3D Electro-Magnetic simulation of the filter [9].

B. Virtual Negative Coupling

The required negative cross coupling between resonator 1 and resonator 8 is realized using a circular iris window in the common wall of the folded configuration, as depicted in Fig. 3. The figure also depicts the phase reversal between resonator 1 and resonator 8, which causes the coupling to be negative. Hence, the required negative coupling is realized virtually, with iris opening itself. This makes the entire filter structure robust from mechanical aspect, which is one of the key requirements for space application.

III. MEASUREMENT

Fig. 4 depicts the fabricated WG cavity filter. Aluminium is used for casing material, where as bullet is made from copper, which is soldered to the centre pin of the SMA connector. Fig. 5 depicts the SMA feed with bullet. Reflection and transmission co-efficient of the simulated and fabricated filter is plotted in Fig. 6 and Fig. 7, respectively. Simulated response is represented by red curve (solid curve), while blue curve (dashed curve) represents the measured response. We note that there is a good correlation between simulated and measured results of the filter. The designed and fabricated filter thus satisfies the desired performance. Table IV compiles the simulated and measured results of the WG cavity filter.

Fig. 3. Field distribution, depicting phase reversal between resonator 1 and 8

Fig. 4. Fabricated folded WG cavity band pass filter incorporating virtual negative coupling

Fig. 6. Reflection co-efficient (dB) : red solid curve denotes simulated data, blue dashed curve represents measured data

Fig. 7. Transmission co-efficient (dB) : red solid curve denotes simulated data, blue dashed curve represents measured data

Fig. 5. SMA feed used in the WG cavity filter, depicting the bullet mounted to the centre pin of the connector

IV. CONCLUSION

In this paper, the concept of VNC (Virtual Negative Coupling) has been successfully adopted in design & realization of a band pass filter with a stringent specification for space application. The WG cavity band pass filter is realized using folded configuration at 8310 MHz with a fractional bandwidth of 1.2%. Compared to traditional all pole filter, introduction of VNC has resulted in size reduction by around 20% and has an improved IL as well. Further, TZ’s resulted from VNC provides a sharp rejection better than 40 dBc at just 100 MHz offset from centre of the band. The key advantage of VNC especially for space application, is robustness of coupling mechanism to mechanical rigidity.

ACKNOWLEDGMENT

Authors acknowledge the support and encouragement of colleagues in Communication Systems Group, ISAC, ISRO, Bangalore, in particular friends of mechanical section for their valuable suggestions on fabrication aspects.

REFERENCES

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[8]Carceller, C.; Soto, P.; Boria, V.; Guglielmi, M.; Raboso, D., "New folded configuration of rectangular waveguide filters with asymmetrical transmission zeros," Microwave Conference (EuMC), 2014 44th European , vol., no., pp.183,186, 6-9 Oct. 2014

[9]ANSYS HFSS, www.ansys.com

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