диафрагмированные волноводные фильтры / 94563204-229d-4633-8305-3a091cd0090b

Abstract—Economical compact filters using posts and shorting stubs have been proposed in this paper. The concept of capacity-loaded shorting stubs has been suggested to minimize the bulky and complicated shorting stubs. CAD simulations and tested results from a sample filter will be presented in this paper. Compared with conventional direct-coupled waveguide filter, the filter length can be decreased by a factor of more than 3.

. INTRODUCTION

Waveguide band-pass filters are widely used in radar, counter-interference and communication systems.

However, the applications of conventional direct-coupled cavity filters are limited by their high fabrication costs and/or big sizes. In order to lower down the fabrication cost, waveguide filters with inductive circular posts [1, 2] have been proposed. To reduce the sizes of the filters, capacity-loading [3] and shorting stubs [4-6] have been proposed.

Shorting stubs offer a freedom to introduce transmission zeros at any pre-specified frequencies. In practical applications, filters with such asymmetric transmission characteristics are more demanding. To the authors’ view, the study on the applications of shorting stubs to filters has attracted little attention considering the excellent selectivity they can bring about. One factor which might have limited the use of shorting stubs is their sizes and fabrication complexity.

Here in our group, a systematic study is going on to use shorting stubs together with other techniques, to build filters with high selectivity, compact size and low cost. To reflect such an effort, we report the design and measurement of an economical compact filter, with special effort to minimize the size and fabrication complexity of the shorting stubs. The concept, CAD simulation results, and the tested results will be presented in this paper.

II. COMPACT SHORTING STUBS

A shorting stub is usually a transmission line with one end shorted and the other end coupled to a main transmission line. To introduce a transmission zero at a certain frequency, the length of the shorting stub should be about quarterwavelength. This will cause the shorting stub to be too long and difficult to fabricate, especially when the frequency is relatively low. Here through derivation, we will show that the length of the shorting stub can be considerably shortened by capacitively loading the shorting stub at the coupling area between the shorting stub and the main transmission line.

Z 0

Zin

(b)

Fig.1 (a) A waveguide loaded with a shorting stub on its E plane and

(b) Its equivalent circuit.

Fig.1 (a) shows a waveguide with impedance Z0 loaded with a shorting stub with impedance Z b and length l on its E

plane. The small coupling slit will provide a capacity loading to the shorting stub. Fig.1 (b) shows the equivalent circuit, with the admittance B representing the capacity of the coupling slit. The input impedance is

Zin = Z0 + 1 ( jB + Z1in' )

Where Zin ' = jZ0tg(βl) is the input impedance of the

shorting stub without the capacity loading and β is wave number.

The reflection coefficient is,

Where n=0, 1, 2… and ∆ω the 3-dB angular frequency bandwidth of the reflection zero. The Q-factor of the reflection zero is defined as

From formula (1) and (2), we know that:

1.Increasing capacity (i.e., increasing B), the length of the shorting stub can be considerably reduced.

2.The Q-factor will usually be higher and the bandwidth of the stop band will be narrower with capacity loading.

3.When there is no capacity loading, the stop band will be narrower with the decrease of Zb .

4.Using harmonics in the shorting stubs, we can realize very high Q and very narrow stop bands.

III. DESIGN AND OPTIMIZATION OF A FILTER

As an example, a waveguide filter with shorting stubs will be designed. The target filter will have a center frequency of 8 GHz and bandwidth of 600 MHz. The reflection coefficient in the pass band should be lower than -15dB and the elimination at 600MHz above the pass-band should be lower than -45dB. To meet this specification, we choose the order of the filter to be 4 and use two shorting stubs on the E-plane. Both shorting stubs are the same for best selectivity at the upper boundary of the pass band.

To lower down the fabrication cost, we use 5 pairs of inductive posts instead of inductive irises to form the filter. Each pair of inductive posts is located symmetrically with respect to the centre line of the waveguide. To reduce the size of the filter, capacity-loading technique is used by introducing a capacitive post at the centre of each resonator of the 2 outer ones. All the posts with the same diameter are fixed by screws and are convenient to be changed, which may low down the fabrication cost. For compact shorting stubs, narrow slits are used to increase the capacity loading near the coupling area between the filter and the shorting stubs. The shorting stubs and the coupling slits have the same width as the waveguide, with the coupling slits aligned with the neighboring walls of the two shorting stubs. Fig.5 is the configuration of the filter.

Shorting stub1 Shorting stub2

Port 1

Capacitive Post

Fig.2. order-4 waveguide filter with two compact shorting stubs

Then, with the configuration chosen, design goal is achieved by simulation and optimization using CST Microwave Studio. The simulated results are shown in Fig.3 and the value of configuration parameters are shown in Table , with a detailed description of each parameter.

Fig.3. Simulated S-parameters of the sample filter.

TABLE I. The configuration parameters of the sample filter

IV. TESTED RESULT

The photo of the sample filter fabricated according to TABLEI is shown in Fig.5. Considering the fabrication errors, tuning screws are used to adjust the centre frequency of each resonator, coupling coefficient between resonators, and the centre frequency of each shorting stub. The S-parameters tested from the fine-tuned compact filter are shown in Fig.6. The tested suppression is lower than -51dB ranging from 8.48GHz to 10GHz which is the upper limit for single-mode operation in the waveguide. The insertion loss in pass-band, with the loss of the two coaxial-waveguide

324

transitions excluded, is 0.55dB and return loss in the pass band is lower than -19.5dB. A transmission zero is successfully introduced at 8.53GHz.

Fig.5. photo of the filter according to table I

freq, GHz

Fig.6. Tested result of the filter, together with 2 coaxial-waveguide transitions.

V. DISCUSSIONS AND CONCLUSION

The sample filter demonstrated very good selectivity at the upper side of the pass-band. From Fig.6, we can see that there is a transmission zero at 8.53GHz, and the suppression at 8.48GHz, which is 180MHz away from the passband, is higher than 51dB. The total length of the filter without flanges is only 75mm.

By looking up curves in Matthaei’s book [7], the order to realize such selectivity with a Chebyshev bandpass filter at centre frequency of 8 GHz would be 10, corresponding to a total filter length of 248mm, i.e., the length of the sample filter is less than 1/3 of a conventional waveguide filter.

In conclusion, in this paper we reported our effort on systematic study of using shorting stubs together with other techniques to realize economical compact filters with high selectivity. Formulae have been derived to show that the length of a shorting stub can be considerably shortened by introducing capacity loading to the coupling area. A waveguide filter with inductive posts, capacitive posts and shorting stubs has been designed, simulated, optimized and tested. The sample filter revealed excellent Characteristics:

centre frequency=8GHz, bandwidth=600MHz, passband insertion loss≈0.55dB, and the elimination is better than 51dB from 8.48GHz up. There is no spurious pass-band up to 10GHz, which is the upper limit for singlemode operation in the waveguide. Without the flanges, the filter is only 75mm long, less than 1/3 of a conventional direct-coupled waveguide filter. The height of the shorting stubs is 7.55 mm and is easy to be fabricated.

REFERENCES

[1]P. Meyer, “The design and analysis of waveguide E-plane filters with multiple round inductive posts using a moment-method approach,” IEEE AFRICON Conf., 1996, 1, p 532-535.

[2]E. I. Lavretski, “Efficient method for design and analysis of rectangular waveguide band-pass filters with multiple inductive circular posts,” IEEE Proc.-Micrw. Antennas Propaga., 2005, 152, (3), pp.171-178.

[3]C. Rauscher, “Design of Dielectric-filled cavity filters with ultrawide stopband characteristics,” IEEE Trans., MTT, 2005, 53, (5), pp. 1777-1786.

[4]J.D.Rhodes, “Explicit design formulas for waveguide single-sided

filters,” IEEE Trans., MTT, 1975, 23(8), pp.681-689.

[5]I.I.Ryzkov, A.R.Sorkin, “Cell of waveguide band-pass filter with poles of attenuation on finite frequencies,” 1999 IEEE-Russia Conference: MIA-ME’99, Section 2, pp.24-29.

[6]J.R.Montejo-Garai, et al., “Synthesis and design of in-line N-order filters with N real transmission zeros by means of extracted poles implemented in low-cost rectangular H-plane waveguide.” IEEE Trans., MTT, 2005, 53(5), pp. 1636-1642.

[7]G.Matthaei, L.Young, and E.M.T. Jones, “Microwave filters, impedancematching networks, and coupling structures,” McGraw-Hill, New York , 1964, p.90.