диафрагмированные волноводные фильтры / 19a58781-69e4-4257-aee3-4feaef3216da

Abstract — A rectangular waveguide compatible NRD guide E-plane bandpass filter is proposed for millimeter wave OFDM applications. The NRD guide E- plane bandpass filter is constructed by inserting a metal foil array in the E-plane of NRD guide. Simulation, fabrication, and handling of the filter are not difficult because each resonator is constructed by a couple of metal foils of a simple shape. A Chebyshev response 5-pole bandpass filter with a very narrow bandwidth of 550MHz is designed and fabricated at 55GHz. Simulated and measured filter performances agree well with the design specifications. Insertion loss and temperature stability of the fabricated filter are found to be quite practical.

Index Terms — bandpass filter, NRD-guide, NRD guide E-plane resonator, millimeter wave

I. INTRODUCTION

With the rapid advance of information and communication technology, millimeter wave applications have attracted much attention to realize high-speed and wideband communication systems.

Recently, wireless transmission systems of uncompressed high definition television (HDTV) have been developed in the license-free 60GHz band [1]. On the other hand, the 55GHz band (54.25-55.78GHz) was allocated for broadcasting services in Japan. TV- studio-use wireless HDTV cameras, which adopts the MIMO-OFDM system, have been developed as one of promising applications in the 55GHz band [2]. To realize such a system, characteristics required for the millimeter wave filters are: 1) narrow bandwidth, 2) high selectivity, 3) low insertion loss, 4) low cost, 5) productivity, and 6) small sizes. Several types of millimeter wave bandpass filters have been reported so far [3] [4]. However, they have one or two difficul-

ties to satisfy the above-mentioned requirements.

In this paper, a 55GHz Chebyshev response 5-pole NRD guide E-plane bandpass filter (BPF) is proposed for millimeter wave OFDM applications. For such an application, a filter having an extremely narrow passband of 1% or less is needed. This immediately means that very small coupling coefficients between constituent resonators have to be realized. Waveguide, for example, is very difficult to make fine adjustment to obtain small coupling coefficients, since it is a closed structure. On the other hand, NRD guide is promising since it is inherently semi-open in structure.

The NRD guide E-plane BPF is constructed by inserting a metal foil array in the E-plane of NRD guide. Simulation and fabrication of this filter is relatively simple because any field concentrations never take place in the filter structure and any discrete ceramic or dielectric resonators are not involved. Moreover, the filter is designed to provide waveguide input/output ports to make system construction easy. Simulated and measured frequency responses agree well with the design specifications.

II. NRD GUIDE E-PLANE RESONATOR

First of all, the NRD guide E-plane resonator, which is the key element of the filter, is shown in Fig.1, where upper parts of the conductor plates and dielectric strip are removed to show the inside structure clear. NRD guide consists of PTFE strip with height of 2.50mm, width of 2.80mm, and relative permittivity of 2.04, and is inserted in a below cutoff parallel plate waveguide [5]. This NRD guide used here has the single mode frequency range from 50 to

Dielectric strip

Metal foil

Conductor plate

s

L

s

2.80

Fig.1 NRD guide E-plane resonator.

L (mm)

Fig.2 Resonant frequency f0 versus spacing L.

60GHz. The NRD guide E-plane resonator is composed of a couple of rectangular metal foils located in the mid E-plane of the dielectric strip, with the transverse size equal to the full dielectric strip width, length s in the longitudinal direction and spacing L. Since NRD guide becomes below cutoff above and below the inserted metal foils, an NRD guide E-plane resonator can be constructed by the technique described here. For reference, material of the metal foil is copper and that of the NRD guide conductor plates is aluminum.

The resonant frequency f0 of the NRD guide E- plane resonator is computed by using “Ansoft HFSS Ver.8.5”, and shown as a function of the spacing L in Fig.2.

Since the midband frequency is 55.02GHz for the present application, the foil spacing L is found to be 3.95mm according to Fig.2. In addition, unloaded Q (Qu) is also computed to be Qu=2100 by assuming the following loss data: conductivity of copper foil Vcu=58x106 S/m, conductivity of aluminum plate Val= 36.5x106 S/m, loss tangent of PTFE tanG=2x10-4.

III. FILTER DESIGN

The filter to be designed here is a 5-pole Chebyshev BPF with midband frequency of 55.02GHz, passband ripple of 0.05dB, and 1dB passband width of 550MHz. The configuration of the BPF with waveguide input/output ports is shown in Fig.3, and its equivalent circuit in Fig.4. The transition between NRD guide

Fig.3 Structure of a 5-pole BPF with waveguide transition

Fig.4 Equivalent circuit for a 5-pole BPF.

Metal foil

Dielectric strip

3.95

s

3.95

3.00

1.25

2.80

Fig.5 Longitudinal sectional view of the coupling structure between two resonators.

s (mm)

Fig.6 Coupling coefficient ki,i+1 versus foil length s.

and rectangular waveguide (WR-15) is designed based on reference [6] and the CAD optimization, and the obtained dimensions are also given in Fig.3. The coupling coefficient ki,i+1 and the external Q (Qe) can be calculated by using the well-known formulas [7]. From the specification mentioned above, the required element values are found to be k=k12=k45=8.081x10-3, k23=k34=5.971x10-3, and Qe= Qei=Qeo=105.5, respectively.

The coupling coefficient ki,i+1 between the adjacent resonators can be controlled by changing the metal foil length s in the structure shown in Fig.5. The variations of ki,i+1 against the length s are shown in

Lower conductor plate

Metal foil

Dielectric strip

3.00

4.40

we se

1.25

2.80

Fig.7 Longitudinal sectional view of coupling structure between resonator and input/output line.

Fig.8 External Qe versus length se with gap width we as parameter

Freq. (GHz)

Fig.9 Simulated frequency responses for the 5-pole BPF with waveguide transitions.

Fig.6, where the foil spacing is kept to be L=3.95mm so that f0=55.0GHz is satisfied. In the present case, s1=0.70mm and s2=0.80mm are obtained the required

ki,i+1.

The required external coupling between the first/last resonators and the input/output lines, however, is too small to be adjusted by changing only the metal foil length se. In order to overcome such a difficulty, a gap of width we is introduced at the center of the metal foil as shown in Fig.7. The external Qe is shown as a function of se with we as a parameter in Fig.8, where the foil spacing is determined to be L=4.40mm. The longer L than the previous one, that is, 3.95mm, is due to the presence of the gap at the center of the metal foil. To obtain the required Qe, se=0.70mm and we=0.30mm are found to be appropriate. Thus, the initial design parameters are

guide input/output ports, (b) metal foil array, and

(c) upper half of dielectric strip.

Freq. (GHz)

Freq. (GHz)

(b)

Fig.11 Measured frequency responses of the 5-pole BPF.

(a) Narrow band, and (b) wide band responses.

L1=L5=4.40mm, L2=L3=L4=3.95mm, s1=0.70mm, s2= 0.80mm, se=0.70mm, and we=0.30mm.

However, the simulated frequency response of the designed filter does not necessarily agree with the specified one. The reason for the discrepancy may be due to the presence of resonators at both sides, one of which is ignored at the initial stage of design.

In order to improve the filter response designed so far, diagnosis and then minor adjustment are carried out using the circuit simulator “Agilent ADS2004A” provided with the optimization function. The final dimensions are obtained to be L1=L5=4.40mm, L2= L3=L4=3.92mm, s1=0.66mm, s2=0.82mm, se=0.64mm, we=0.30mm.

The simulated frequency response of the filter with waveguide transitions is compared with the ideal response calculated by the equivalent circuit method in Fig.9. They agree very well with each other.

Freq. (GHz)

Fig.12 Temperature dependence of frequency response for the fabricated filter.

Fig.13 Temperature dependence of midband frequency and insertion loss of the fabricated filter.

IV. FABRICATION AND MEASUREMENT

The designed 5-pole BPF is fabricated by using a numerically controlled processing machine, photolithograph technique, and wet etching process. The total length of the fabricated BPF is 50.8mm. The photograph of it is shown in Fig.10, together with the longitudinal sectional view.

The filter is tested by using a vector network analyzer. The frequency response measured at 25ºC is shown in Fig.11(a), exhibiting an excellent agreement with the simulated one, which takes into account the material losses and the transition loss between NRD guide and waveguide. It can be found that the midband frequency is 55.052GHz, 1dB passband width is 535MHz, and insertion loss is 2.0±0.3dB. The wideband performance is also shown in Fig.11(b). It can be seen that the first spurious resonance due to the presence of higher order modes appears around 59GHz, as predicted by simulation.

In addition, the temperature dependence of the filter response is also measured in the temperature range from 10 to 40ºC, and is shown in Fig.12. The temperature dependence of the midband frequency and the insertion loss are also summarized in Fig.13. It is found that the insertion loss slightly increases as the temperature increases. The midband frequency changes about 20MHz over the whole range of meas-

urement, showing the inflection point around 20ºC due to the phase transition of PTFE. Although some temperature variation is observed in the filter performance, it is possible to adjust the design of filter in order to keep the midband frequency stable within an acceptable tolerance. The steep slope of the filter leads to efficient up/down-stream separation in the 55GHz band OFDM application.

V. CONCLUSIONS

The Chebyshev 5-pole NRD guide E-plane bandpass filter has been designed and fabricated in the 55GHz band. The fabricated filter satisfies well the design specification. Moreover, the insertion loss and temperature variation of the filter are also found to be within manageable level. The adoption of new dielectric material may be another solution for the temperature stability improvement. It is expected that the NRD guide E-plane filter can be applied for millimeter wave OFDM applications.

ACKNOWLEDGEMENT

This work was supported by MEXT.HAITEKU(2002).

REFERENCES

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