диафрагмированные волноводные фильтры / 1f4670ec-0d52-46b8-8cfd-ee8467a31fb7

Abstract— A rectangular waveguide e-plane fin-line filters compared with metal insert filters at Ka-band is introduced. Optimized design dates for fourto five-resonator type filters are given for mid-band frequency of about 35.6 GHz. Emulational passband insertion losses of prototypes are 0.02dB and 0.5dB respectively. A type of waveguideto microstriptransition on a single dielectric substrate for millimeter wave band-pass filters is proposed. The insertion losses of the two filters connected with transition are 0.5dB and 0.9dB respectively, which satisfy engineering.

I. INTRODUCTION

The pace of development of modern millimeter-wave integrated circuit has put a lot of pressure on low insertion-loss band-pass filters. The E-plane fin-line filter (Fig. 1) and metal insert filter (Fig. 2) have been present.

The structure of fin-line filters is similar to the one metal insert, which is described detailed in [1]-[3]. It is made up of rectangular waveguide and the bilateral finline bridges based on the technology of photomechanical process on the substrates shown in Fig.1. The length of resonators determine essentially the mid band frequency f0 , and the widths of the

metal bar determine essentially the ripple of the passband insertion loss. The threeand five-section Ku-band inductive strip filters described in [4], [5] are calculated by an equivalent-circuit approach. From Fig.2, we know that pure metal inserts placed in the E-plane of rectangular waveguides is made up of metal bar and empty waveguide without any substrates, which size of metal bar are indicated in Fig.2 too.

Design examples at two different Ka-band of Chebyshev

E-plane band-pass filters with less than 1-percent bandwidth at Ka-band have been accomplished.

Fig.1 Bilateral e-plane fin-line filter

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978-1-4244-2193-0/08/$25.00 ©2008 IEEE

Fig.2 Metal insert filter without supporting dielectrics

To connect with low-noise amplifiers or low-noise converters easily in millimeter-wave bands, and to make the e-plane circuits compatible with standard waveguide accesses, transitions with good performance are required.

The coplanar transmission lines present some advantages to mount or integrate active components, especially at the higher microwave frequencies and millimeter wavelengths [6]. This letter presents a transition structure waveguideto- finlineto- microstrip transmission line. Using Ansoft-HFSS, filters and transitions are analyzed, calculated, optimized and emulated. The design of waveguide to microstrip transition is shown in Section .

II. DESIGN OF WAVEGUIDE FILTERS

A four-resonator metal insert filter and a five-resonator fin-line filter for mid-band frequencies of about 35.6 GHz are chosen for design examples. The corresponding waveguide housing is WR28 (Ka-band, a=7.112mm, b=3.556mm).

A. Metal Insert Filter

The thickness of the strips impacts tremendous on filter. The thicker the copper bar, the greater frequency drift, and it will lead to reduced bandwidth and increased insertion loss. The thinner the copper bar, the higher technics the processing require. A thickness of 0.5mm has been chosen for the design.

The design process is started with filter dimensions calculation through theory. An optimizing computer program, Ansoft HFSS is useful software. Through analyzing, optimizing and emulating to calculation in theory, a typical design for four-resonator filter has the following dimensions:

Freq[GHz]

Fig.3 Emulated insertion-loss results of metal insert filter.

The emulational insertion-loss in decibels as a function of frequency for four-resonator Ka-band metal insert filter is shown in Fig.3. The calculated minimum insertion losses in passband (35.45-35.75GHz) are about 0.02dB.

B. E-plane Fin-line Filter

RT/duroid 5880( r =2.22) has turned out to be a relatively

cheap substrate material with sufficiently good electrical properties. The finite thickness of the metallization is also included in the computations. The thinner of the copper cladding thickness, the less influence of the metallization would mainly lead to incorrect calculations of the midband frequency. A thickness of 17.5µm was commercially available. The chosen substrate thickness t commercially available is 0.02in and the copper cladding thickness h=17.5µm.

The corresponding insertion losses values of a Ka-band fin-line filter are given in Fig.4. The calculated minimum insertion loss in passband is about 0.5dB, again, a better

and photomechanical technology, which more suits to integrated circuit and mass fabrication.

III. DESIGN OF WAVEGUIDE-MICROSTRIP TRANSITION

Broad band transitions of waveguideto- finlineusually adopt finline taper to obtain. In this fabrication of taper in the form of circular arcs which are used to match a rectangular waveguide and a homogeneous finline. An experiential taper function present by Mirshekar has been adopted in this design [7].

Length of taper should not be too short, or else, reflectance in wave port will be too high. A normal project use a

where 0 is the operating wavelength in middle frequency.

The energy in waveguide transmits to finline, and then couples to microstrip, which can be connected to other active component. Metalized in the slot’s substrate, we can get the microstrip line, and the slot’s conductive face equals to microstrip’s ground face. Energy is coupled by interleaving the groove and microstrip line. To obtain good performance in the operating frequency band, the following parts need to be optimized:

-Length of the transition tapers L

-Length of the open circuit microstrip line Ls -Distance between short-circuit slot and open circuit

microstrip line Lm

Freq[GHz]

Fig.4 Emulated insertion-loss results of e-plane fin-line filter.

In e-plane circuit supporting dielectrics cause additional losses, compared to a metal insert filter, the integrate fin-line filter structures can be produced by metal etching techniques

Fig.5 Configuration of finlineto- microstrip transition model

A transition structure waveguideto- finlineto- 50 microstrip transmission line is modeling by HFSS, shown in Fig.5, which is matching to 50 microstrip by T-type impedance convertor finally.

The length of transition is a major parameter in design, and its influence on performance is extraordinary significant. Normally the performance of longer transition is better than the short one. Because the return loss (S11) will deteriorate

with the length of the transition shortens. We analyze the length of finline transition by simulation. The L must be

frequency in the operating frequency band). Its insertion loss (S21) simulated by commercial software HFSS is shown in Fig 6.

Freq[GHz]

Fig.6 Transmit characteristic when L=9mm, 10mm, 11mm and 12mm

The parameter Lm determines the performance in a certain extent, the longer the Lm, the better the diversion character is. We design a transition at Ka-band with L-10mm Ls-1.2mm Lm-0.8mm. Its insertion loss (S21) and return loss (S11) are shown in Fig 7.

Freq[GHz]

Fig.7 Simulation performance of finline transitions at Ka-band

It can be seen that the transition’s return loss performance is smaller than 22dB and non obvious resonance points in the frequency range of 33GHz to 37GHz. Its insertion loss is quite small too.

IV. ANALYSIS, OPTIMIZATION AND OVERALL DSIGN

It is very difficult fixed copper bar, prone to middle frequency drift. Copper bar is not good to integrate with the solid state circuits. So it is hard to satisfy the requirement of high integration and miniaturization. Using photographic printing technology makes integrated finline filters more suits to integrated circuits and mass fabrication, but its insertion loss and rectangular coefficients are slightly worse.

While frequency is increasing, the influence of dimension to results is great, therefore, inhibit of high-end is always not good than the low-end. With the increasing number of sections, bandwidth of passband increase, and performance of filters meliorate. However, the corresponding increase in length, dimension of circuit element increases.

Connecting metal insert filter and transition, model and

simulation performance are shown in Fig.8. Its insertion loss is very small, only 0.5 dB, and return loss is bigger than 20 dB. Unfortunately, passband drift nearly 150 MHz to left.

Freq[GHz]

(b) Simulation performance

Fig.8 Metal insert filter connected with transition

Connecting finline filter and transition, model and simulation performance are shown in Fig.9.

(a)Model

Freq[GHz]

(b)Simulation performance

Fig.9 E-plane finline filter connected with transition

The corresponding insertion loss and return loss in the passband are about 0.9dB and 13dB, respectively. Unfortunately, passband drift more than 100MHz to left.