диафрагмированные волноводные фильтры / a3b088b0-6983-4b93-99cb-be7c9cedcffe

I. INTRODUCTION

All metal insert E-plane filters provide a cost-effective solution for point-to-point radio systems. Since E-plane filter frequency characteristic is determined mainly by a metal insert, it is metal insert that needs to be replaced whenever filter requirements are redefined. This means that a set of metal inserts and common waveguide housing is usually sufficient to cover the frequency plan defined for the radio system operating in a given frequency band. However, despite the advantages related to the ease of fabrication and assembly all-metal insert filters exhibit drawback, among others, complicated realization of asymmetric filter functions. The transmission zero at the finite frequency can be introduced by folding the structure and introducing a cross coupling between non-adjacent resonators [1], however, this comes at the expense of a more complicated structure and the necessity of using more than one metal insert. Alternatively, the pseudo-elliptic filter response in an inline configuration can be obtained by using a pair of parallel coupled resonators [2]. However, this requires reducing the height of the resonators and shifting their position towards the upper and lower walls of the waveguide. This in turn leads to reduced value of Q-factor and subsequently to increased insertion losses. The selectivity and stop-band attenuation of the filter can also be improved by utilizing the extracted pole principle [4] [6]. The transmission zero is realized by implementing band-stop resonator in the filter structure. An E-plane band-stop resonator usually has a form of a T-junction with one port short-circuited. However, when extracted cavities are implemented on either side of the main part of the filter the length of the insert needs to be adjusted to fit into the space defined by the distance separating these cavities. This presents a difficulty and potentially restricts the possibility of having the same waveguide housing and a set of the metal inserts to realize various filter characteristics.

Fig. 1. All metal insert filter configuration with the extracted pole resonator.

In this paper a possibility of realizing asymmetric filter response using all-metal insert E-plane technology without the need of modifying waveguide part of the filter is presented. The solution is based on extracted pole principle and though it requires reducing the height of the band-stop resonator the impact on the insertion loss is reduced compared to [2]. The proposed solution utilizes a modified structure reported in [5]. It simplifies the filter structure described in [3] and by introducing the ridge section over the structure acting as an extracted pole resonator provides additional degree of freedom for controlling the external QE factor. The proposed modification facilities the design and makes practical implementation more feasible. In order to validate the concept four all-metal E-plane filters all of order 4 with asymmetric frequency characteristics were designed and one of them manufactured and measured.

Fig. 2. All metal insert filter configuration with the ridge extracted pole resonator.

Fig. 3. Frequency response of the metal insert E-plane extracted pole resonator

II. CONFIGURATION

The structures having a stop-band characteristic and used throughout the discussion in this and following Sections are depicted in Figs 1 and 2. The frequency response of the allmetal insert placed between two halves of the rectangular waveguide (see inset in Fig 3) operating at Ku-frequency band is shown in Fig. 3.

It is worth mentioning that while the resonant frequency of the extracted pole resonator is determined by the length (l) the

Fig. 5. External QE as a function of ridge height (Hr) (see Fig. 2)

external QE factor is controlled mainly by its height h (Fig. 1 and 2 - the dimension H −h is kept constant). Fig 4 shows the dependence of external QE factor on the resonator height (h). It is evident that the higher the resonator, the lower external QE . However, for the filters with transmission zero located very close to the pass-band the height of the stop-band resonator usually has to be reduced to the extent that it becomes impractical. In order to counteract the aforementioned undesired effect a ridge is placed over the structure of extracted pole resonator (Fig. 2). The ridge provides an additional degree of freedom for controlling the external QE factor. Fig 5 shows the dependence of external QE factor on the variation of the ridge height (H r) while the height (h as well as H ) of the extracted pole resonator is kept constant. It is seen that the value of external QE can be, in relatively large extent, controlled by changeing the height of the ridge. This effect is shown and discussed in the next Section.

III. DESIGN AND VERIFICATION

In order to demonstrate the possibility to position transmission zeroes independently at the finite frequencies utilizing the structre show in Fig. 1 two filters of order 4 with transmission zeros located on the upper and lower side of the pass-band were designed. The low-pass prototype for (4-1) filter with an extracted pole is shown in Fig. 6. The extraction of the low-pass parameters is extensively described in literature [6] and for this reason it will not be repeated here. However, to relate the changes in the all-metal insert shape to low-pass model parameters the values of the inverters and frequency invariant reactive elements are summarized in Table I. The second column of the Table I lists the values corresponding to the asymmetric filter response with a transmission zero location on the upper side of the pass band at s = j2.5 whereas the third column lists the values corresponding to the filter response with a transmission zero located on the lower side of the pass band at s = −j2.5. Moving transmission zeros for one side of the pass band to the other requires changing the sign of the phase shit and the signs of the frequency independent reactive elements (FIR). In practice to realize a negative phase shift the half-wavelength section is added, which is shown in practical implementation in the insets of Figs 7 and 8. Two aforementioned filters were designed at the center frequency 15 GHz with the bandwidth 200 MHz, return loss better than 24 dB and the transmission zeros located at lower side of the band-pass at 14.752 GHz (−j2.5) and upper side of the pass-band at 15.252 GHz (j2.5) respectively. The design was carried out using commercially available CAD tool uWaveWizard and verified using Ansis HFSS. The simulated responses of the filters are shown in Fig 7 and Fig 8 respectively.

Two additional filters satifying the same electrical specifi-

Fig. 7. Frequency response of the extracted pole resonator filter (see inset) with the transmission zero on the lower side of the bass-band.

Fig. 8. Frequency response of the extracted pole resonator filter (see inset) with a transmission zeros on the upper side of the pass-band.

cation as previous two but having a ridge structre over the extracted pole resonator (Fig. 2) were also designed. In both cases the height of the extraced pole resonators was almost doubled. In the case of the filter with a transmission zero located on the upper side of the pass-band the height of the extrated reonators was increased from 1.1 mm to 2 mm and in the case of the filter with a transmissino zero located on the lower side of the pass-band the change was from 0.9 mm to 1.8mm. The simulated charactersits of the filters are show in Figs. 9 and 10.

In order to verify the design the filter with the transmission zero located on the upper side of the band-pass was fabricated and measured. The copper metal insert of thickness 0.1mm was manufactured by electrical discharge machining (EDM) and placed in aluminum waveguide housing. The photo of the

Fig. 9. Frequency response of the ridge extracted pole resonator filter (see inset) with a transmission zero on the lower side of the pass-band.

Fig. 11. Measured (soli-line) and simulated (dashed-line) response of the realized filter.

Fig. 10. Frequency response of the ridge extracted pole resonator filter (see inset) with a transmission zero on the lower side of the pass-band.

metal insert is shown in Fig. 12. Both, measured and simulated frequency characteristics are shown in Fig. 11. It can be seen that the measured return loss is better than 23 dB and minimum insertion loss is 0.6 dB. However it needs to be stressed that the filter housing was not silver plated. Very good agreement between simulation and measurement is achieved.

IV. CONCLUSION

An E-plane extracted pole resonator suitable for being directly integrated with the main part of the all-metal insert E-plane filter has been introduced. A pseudo-elliptic response of the metal insert E-plane filter has been obtained without the need of modifying straight waveguide section. A ridge section placed over the extracted pole resonator has provided an additional degree of freedom for controlling external QE making

the practical implementation more feasible. The concept has been validated by designing and manufacturing of 4 order filter with transmission located on upper side of the pass-band.

REFERENCES

[1]E. Ofli, R. Vahldieck, and S. Amari, “Novel E-plane filters and diplexers with elliptic response for millimeter-wave applications”, IEEE Trans. Microwave Theory & Tech., vol. 53, no. 3, pp. 843 - 851, March 2005.

[2]R. Lopez-Villarroya, G. Goussetis, J.S. Hong and J.L. Gomez-Tornero, “E-plane Filters with Selectively Located Transmission Zeros”, 38th European Microwave Conf., 2008 , pp. 733 - 736, 2008.

[3]D. Young, Ahmad, I.C. Hunter, “Integrated E-Plane Filters with Finite Frequency Transmission Zeros”, 24th European Microwave Conf., 1994

, pp. 460 - 465, 1994.

[4]J. Bornemann, “A new class of E-plane integrated millimeter-wave filters”, IEEE MTT-S, 1998. Vol. 2. , pp. 599 - 602 , 1989.

[5]A. S. Omar and K. Schunemann, “Realizations and Design of Fin-Line Bandstop Filters”, 13th European Microwave Conf., 1983 , pp. 157 - 162, 1983.

[6]R. Cameron, C. Kudsia, and R. Mansour, Microwave Filters for Communication Systems Fundamentals, Design and Applications, New York: J. Wiley & Sons, 2007.