диафрагмированные волноводные фильтры / 7df58d76-a15b-4dd2-b735-f11b4982b4cb

ABSTRACT— A new all-metal insert E-plane filter consisting of single, upper and lower ridge resonators placed alternatively along the waveguide housing is introduced. The proposed topology allows one, in large extent, to adjust the filter length, which can be particularly useful when designing manifold type diplexers of extracted pole variety. The concept has been validated by designing and manufacturing of a 6-pole filter with transmission zero placed on the upper side of the pass-band.

INDEX TERMS— Filters, resonator filters, band pass filters.

I. INTRODUCTION

Waveguide filters and diplexers are essential part of modern communication systems. In order to secure the commercial success these components need to be optimally designed in terms of performance and cost. It is understood that all-metal insert E-plane technology is one of the most suitable for mass production due to low cost and flexibility in design.

Typically, E-plane filter consists of a properly designed metal insert placed between two halves of the rectangular waveguide parallel to the E-plane. Since a filter function determines a topology of a metal insert, it is the insert which needs to be replaced whenever different filter function is required. This quality can be utilized in designing tune-free diplexers. A common waveguide housing and a set of allmetal inserts are enough to cover a complete receive-transmit frequency plan.

Despite many advantages all-metal insert E-plane technology has to offer there are also some drawbacks, namely, E- plane filters/diplexers exhibit a limited spurious free stopband and tend to be of a large size at lower frequencies. While the first issue has been addressed by many authors [1][2][3], relatively few publications covering the latter aspect are available. The size of the E-plane filters like any other type of filters can be reduced by realizing filter functions with finite frequency transmission zeroes. This is due to a fewer number of resonators required. A way of introducing transmission zeroes using extracted pole technique was demonstrated in [4][5]. In this case the selectivity of the filter is improved by adding stop-band cavity that provides a transmission zero at the given frequency. Folding the structure and introducing cross-coupling between non-adjacent resonators was demonstrated in [6]. The other approach [7] takes advantage of series and parallel coupled resonators to realize transmission zeroes on the upper side of the pass-band. In [8] the coupling arrangement was proposed allowing one to arbitrary place transmission zeroes at lower or upper sides of the pass-band. In both mentioned cases [7][8] the reduction of the filter lengths was reported, however, only relatively simple structures were

Fig. 1. Poposed resonator arrangement for an all-metal insert E- plane filter.

designed. It is believed that due to small size of the resonators the filter are more sensitive to the manufacturing tolerances. Furthermore, since the Q-factor degrades as the resonators get smaller in height the structures are also believed to be relatively lossy.

In this paper a novel topology of the all-metal insert E-plane filter is proposed that allows one to reduce the size of the structure. The filter consists of E-plane upper and lower ridge resonators placed alternatively along the waveguide housing. Such arrangement allows one to control the length of the filter in relatively large extent and leads to more than 20% of overall length reduction compared to the double ridge resonator filter [1] with the resonators of similar Q. The advantage of the topology is demonstrated by the example of 6th order filter with a transmission zero realized by extracted pole technique.

II. CONFIGURATION

The all-metal insert E-plane filter composed of upper and lower ridge resonators placed alternatively along the waveguide housing is shown in Fig. 1. The synthesis of the filters with extracted pole is extensively described in literature [9], [10] and will not be repeated here. However, to show the extent to which the length of a filter can be reduced utilizing single ridge E-plane resonators arranged as shown in Fig. 1, the normalized values of the coupling coefficients as a function of the distance between single ridge resonators and the double

Fig. 2. Coupling values as a function of a distance between sigle (solid line) and double (dashed line)ridge E-plane resonators.

Fig. 4. Change of the Q-factor for single, asymmetric double, and double ridge E-plane resonators.

Fig. 3. Scattering parameters of the designed reduced size filter with upper and lower ridge E-plane resonators placed alternatively along waveguide housing

ridge resonators are depicted in Fig. 2. The coupled resonators are synchronously tuned and have the same resonant frequencies. This is achieved by adjusting the length of the resonators whereas their height remains fixed. A careful scrutiny of the results suggests that in order to achieve the same coupling in both cases the distance between double ridge resonators needs to be about 20% longer compared to that of single ridge resonators (since the relation is not linear the length reduction depends on a selected coupling value). It is understood that the distance could be reduced even further by lowering the height of resonators. However, this should be done with caution as reducing the height of the resonators causes degradation of the Q factor. Securing the same conditions of the full-wave EM simulations it has been verified that half waveguide height

Fig. 5. Wideband response of the designed filter with upper and lower ridge E-plane resonators placed alternativly along waveguide housing (see inset)

E-plane resonator has 30% higher Q compared to that of a quarter waveguide height. Obviously, transition from a double ridge resonator to a single ridge resonator causes the change of the Q as it is indicated is Fig. 4. However, the difference in Q is marginal (≤ 6%). It is worth noting that whenever Q degradation can not be neglected the height of the resonators can be increased compensating for the loss of the Q. Obviously this leads to the increase of the filter size but the structure is still shorter than that of double-ridge resonators. A spurious pass-band of the single ridge resonator E-plane filter compared to that with double-ridge resonators is discussed in the next section.

Fig. 6. Wideband response of the designed double ridge E-plane resonator filter (see inset)

Fig. 7. Manufacured metal insert

III. REALIZATION AND RESULTS

In order to demonstrate the possibility of reducing the size of an all-metal insert E-plane filter by using single ridge E-plane resonators arranged as shown in Fig. 1 , the 6th order filter with the transmission zero located at the upper side of the pass-band has been designed and fabricated. The design has been carried out using general-purpose full-wave EM simulator HFSS. The center frequency of the filter is 14.64 GHz , pass-band 250 MHz and returns loss better than -23 dB. A transmission zero is realized as a band-stop cavity with a resonant frequency 14.9 GHz. The simulated response of the filter is shown in Fig. 3. The additional transmission zerois which can be observed (Fig. 5) results from the specific arrangement of the not full height resonators.

The subsequent E-plane resonators constitute trisections which like the extracted pole sections are able to realize transmission zeros [10]. The position of the aforementioned transmission zeros can be controlled by adjusting the height of the resonators, however, this issue falls outside the scope of this paper.

Fig. 8. Measured (solid -line) and simulated (dotted-line) responses of the single ridge E-plane resonator filter

Fig. 9. Measured (solid-line) and simulated (dotted-line) wideband responses of the single ridge E-plane rsonator filter

In order to demonstrate the advantage of the proposed solution a double ridge resonator filter fulfilling the same requirement has been designed. The length of the double ridge filter is 102mm whereas the length of the single ridge filter is 82mm. The insertion loss is higher by 0.05 dB in the latter case. The simulated out-of-band responses of the single and double ridge resonator E-plane filers are shown in Fig. 5 and 6 respectively. The spurious free band is very similar in both cases.

Finally the copper metal insert was manufactured using a wire erosion technique and placed in an aluminum waveguide housing. The picture of the metal insert is show in Fig 7. The measured and simulated responses are show in Fig. 8. The wideband measured and simulated reponses are depicted in Fig. 9 respectively. The return loss of the filter is below 21