WO2000022692A1

WO2000022692A1 - Microwave filter

Info

Publication number: WO2000022692A1
Application number: PCT/GB1999/003336
Authority: WO
Inventors: Charles William Turner; Djuradj Budimir
Original assignee: Btg International Limited
Priority date: 1998-10-09
Filing date: 1999-10-08
Publication date: 2000-04-20
Also published as: AU6113999A

Abstract

A microwave device (1) (shown in top cross sectional view) in the form of a narrow passband waveguide filter has a tubular, rectangular cross-section waveguide (1), and dielectric slabs (2) are arranged to provide periodic array structures (3) and (4), which themselves are spaced apart to form a resonator cavity (5). The dielectric slabs have a thickness equivalent to μ/4 in the dielectric medium, where μ is the chosen central frequency of passband of the filter, and the spaces between the dielectric slabs (2) are μ/4 in air. The spacing between the array structures (3) and (4) is μ/2 in air, providing a cavity resonator. The arrangement creates a bandpass device where the individual periodic array structures (3) and (4) would otherwise be bandstop devices.

Description

MICROWAVE FILTER

The present invention relates to microwave devices, and in particular, though not exclusively, to microwave, and millimetre-wave, waveguide and microstrip filters.

As a preliminary remark, it is noted that in the art the terms "microwave" and "millimetre" are not always applied consistently to defined frequency bands. Therefore, hereinafter, the single term "microwave" will be used to encompass electromagnetic energy in the frequency band from 500 MHz to 100 GHz.

The steady growth in commercial interest in microwave wave systems, especially in wireless communications, security and sensor applications, GPS location systems, and military and transportation electronics, has provided a significant challenge to conventional microwave circuits and their design methodologies. High performance wide-band and narrow-band bandpass filters having both a low insertion loss and a high selectivity are important for modern microwave communication systems.

Devices using microwaves are used in a range of consumer and commercial market products. These products range from Satellite Television receiver modules, satellite telephones, PCNs (Personal Communication Networks) and VSAT (Very Small Aperture Satellite) systems, and devices for commercial application in emerging uses in transportation and automobile projects, such as sensors in traffic management schemes and vehicle anti-collision devices.

At present most filters at microwave frequencies are produced either in waveguide (air-filled rectangular and nonradiative dielectric) with high associated machining costs, or using planar technologies (microstrip, suspended substrate stripline and coplanar waveguide). Planar microwave circuits are commonly used in communication systems at frequencies from around 1 GHz to 100 GHz. These circuits are easily mass-produced and are light and compact. In microwave transmission and reception narrowness in bandwidth is important. Planar microwave circuits are not generally capable of very high performance, and when such high performance is required hollow waveguide circuits are generally used.

Generally microstrip and coplanar waveguide filters are not able to achieve high selectivity characteristics because loss and radiation limit the Q values. Although in principle highly selective characteristics can be achieved by using elliptic function filters, this technique becomes less viable at microwave frequencies.

Many different prior art microwave filter designs are known. One type of microwave filter is described in research papers by Robert and Town, Page 739, IEEE Transactions on Microwave Theory and Techniques vol. 43, No.4, April 1995. and Le Roy et al Page 639 1997 IEEE MTT-S Digest (WE3A-5). This type of microwave filter consists of a non-uniform microstrip, with a designed continuously varying width. The length of microstrip of continuously varying width is designed to be selectively transmissive at chosen frequencies. In these filters it is the continuously varying impedance of the transmission line that creates the filter response.

According to the invention there is provided a microwave device, comprising: at least one periodic array of at least one first element, and at least one second element, the elements being arranged so that at at least one interface between first and second elements, reflection of incident microwave energy can occur, and the periodicity of the array can allow constructive interference of the reflected microwave energy, and wherein the at least one periodic array is disposed in such a manner that at least one frequency within the bandwidth that would otherwise be reflected, is transmitted. In one preferred arrangement, two periodic arrays are arranged to interact in use to provide transmission at a desired frequency. Preferably the two periodic arrays are arranged with a resonator between the arrays to provide in combination the selected frequency transmission.

In an alternative arrangement a single periodic array is arranged with a reflector of microwave energy so that, in use, incident microwave energy is transmitted, at the selected frequency through the periodic array, reflected by the reflector, and passes again through the periodic array.

According to another aspect of the invention there is provided a microwave bandpass filter which comprises a hollow waveguide in which there are, sequentially: a) a first component consisting of a plurality of elements comprising at least one first element and at least one second element arranged alternately, and the linear dimensions of which elements are substantially the same, b) a second component comprising a resonator, and c) a third component substantially the same as the first component.

According to another aspect of the invention there is provided a coupler for use in microwave circuits which comprises a first component which comprises a plurality of elements which have one of two different impedances with the elements arranged alternately so that each element has an impedance different from that of the adjacent element, a second component which comprises a resonator and a third component which comprises a plurality of elements which have one of two different impedances with the elements arranged alternately so that each element has an impedance different from that of the adjacent element,

The resonator can be any conventional resonator such as a planar metal strip, hollow waveguide or free space resonator. Preferably the length of the elements in the first and third component, in the direction of transmission, is substantially one quarter wavelength and the second component is substantially one half a wavelength long, the wavelength being measured at the central operating frequency of the device. Alternatively the lengths of the first and third elements could be submultiples of one quarter wavelength, such as one eighth or one sixteenth of a wavelength etc. and the second component can be one half a wavelength long. Alternatively the lengths of the first and third component could be arbitrary.

Preferably the first component comprises from 2 to 9 elements and more preferably 3 to 7 elements. The ratio of ZI : Z2 is preferably between 1 :1 and 3: 1. and more preferably from 1.5 to 2.5:1, where ZI is the impedance of one type of element and Z2 is the impedance of the other type of element. For planar circuits ZI is preferably in the range of 25 to 75 ohms and Z2 is in the range of 30 to 60 ohms, and the resonator impedance Z3 is preferably from 20 - 80 ohms. For a hollow waveguide ZI , Z2 and Z3 are preferably 200 to 500 ohmns. For any particular application the values of ZI, Z2 and Z3 can be selected to obtain the desired characteristics of the transmitted radiation, such as the bandwidth and the insertion loss of the passband and stopband bandwidth and rejection.

The principles embodied in the invention can be incorporated in various forms, such as metallic or dielectric waveguide, in planar form, such as microstrip, stripline or coplanar waveguide and in nonradiative dielectric (NRD) waveguide structures. Where the hollow waveguide is used, preferably one of the array elements is air.

The invention enables the production of narrow band pass filters. This kind of structure is useful in many applications particularly in low phase noise microwave oscillators, highly selective microwave filters, diplexers, and multiplexers, narrowband bandpass and notch filters, frequency selective surfaces and antennas. Preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings wherein:

Figure la is a diagram illustrating the principle of reflection at an interface of impedances Zi + Z₂;

Figure 2a is a diagram illustrating the reflection of microwaves by a grating structure;

Figure 2b is a schematic diagram illustrating a generalised microwave component embodying the invention;

Figure 3 is a generalised microwave component according to a second group of preferred embodiments;

Figure 4 is a generalised microwave component according to a third group of preferred embodiments;

Figure 5 is a plot of the ideal behaviour of a narrow bandpass filter;

Figure 6 is a top cross sectional view of a waveguide filter according to a first detailed embodiment of the invention;

Figure 7 is a top cross sectional view of a waveguide filter according to a second detailed embodiment of the invention;

Figures 8 and 9 are plots of the results obtained from the circuit of Figure 6 and 7 respectively; Figures 10a and 11a illustrate simulated microstrip devices according to third and fourth detailed embodiments of the invention and the graphs in Figures 10b and l ib show a computer simulated plot of transmitted and reflected power against frequency;

Figure 12a and 12b show transverse and longitudinal cross-sections of a waveguide filter according to a fifth detailed embodiment of the invention;

Figures 13, 14, 15, 16 and 17 contain graphs which show the simulated insertion loss of different forms of the waveguide E-plane resonator of the fifth embodiment, at 9.455 GHz, 36.80 GHz, and 42 GHz ;

Figure 18 shows the simulated insertion loss of the waveguide E-plane three-channel multiplexer at 42 GHz;

Figure 19 shows the measured transmission loss of a fabricated waveguide resonator at 36.8 GHz;

Figure 20a shows transverse and longitudinal cross-sections of a waveguide filter according to a sixth detailed embodiment of the invention;

Figure 20b shows a computor simulated response of the sixth embodiment.

The general concepts underlying the invention will now be described with reference to the drawings, initially with reference to Figures 1, 2a and 2b.

It is well known in microwave waveguide theory that each homogeneous section of the waveguiding structure has a unique value of wave impedance. Commonly, the wave impedance is determined by geometrical factors and material parameter values. In the case of a discontinuity between two sections of waveguide with different wave impedances (Zi and Z₂, say) wave reflection takes place at the interface, see Figure 1 The (complex) reflection coefficient R can be expressed in terms of its magnitude, (R), and its phase angle, θ.

In the case of a succession of such interfaces a wide range of behaviour can be observed as a function of the physical separation of the interfaces and the relative values of Z, and Z₂.

In particular, see figure 2a, if an alternating sequence of waveguide sections of differing impedance, Zi, and Z , is arranged in a periodic array structure, such that each section is equal in length to one quarter of a wavelength (ie the separation between the interfaces of Zi, and Z_2> is λ/4) virtually complete reflection of the incident wave is observed. (Only in the theoretical case where an infinite number of sections are cascaded is 100% reflection achieved). For a finite number, for example 3, 4 or 5 say, strong reflection takes place but a small amount of the incident wave energy is transmitted. This phenomenon is known as "Bragg reflection^" and the periodic array structure is known as a "Bragg grating". However, one aspect of the present invention is based on the novel effect that such overtly non-transmissive structures can be employed in microwave devices in a transmissive mode.

If two such structures (designed to operate at a chosen frequency) are separated by a gap of arbitrary length, only a very small percentage of the incident wave energy will pass through to the output after traversing the two gratings. Referring to figure 2b which illustrates schematically one type of microwave device embodying the invention, at microwave frequencies where the gap is adjusted to be equal to one half wavelength long, virtually complete transmission takes place. There is a surprisingly large sensitivity to any change in the gap region length or physical character at this half wavelength setting.

The present invention is embodied in a number of microwave applications of this novel effect, notably filters with narrow bandwidth (see figure 5). Also, microwave oscillators (signal sources) often need to have very low phase noise generation: this can be achieved with narrow bandwidth filters in the feedback path. Other applications, such as sensors, would utilise the sensitivity of the signal transmission to external influences such as temperature.

The practical realization of devices employing the periodic structures described above is determined by a number of factors including frequency and power level. Hollow pipe waveguides in which the periodic structures are enclosed generally give good circuit performance especially at high microwave frequencies. Where hollow waveguides are too bulky or expensive, planar circuits are often be preferred. Other possible types include coaxial line and striplines.

Although structures embodying the Bragg principle of a one quarter wavelength separation between reflecting interfaces within the grating, are the principal focus of the discussion above, other periodic structures have been found to possess similar properties. In particular, periodic structures comprising elements with a length of one eighth wavelength produce similar properties to those with elements which are one quarter wavelength in length.

Microwave devices would normally have fixed elements arranged in a rigid structure. Referring to figure 3 some applications envisaged for the grating-coupled resonator could use moving parts, for example, such that the gap (resonator) length would vary as the mechanical motion within a system alters the relative position of component parts. Applications in the fields of automotive engineering, avionics and mechatronics are seen to be feasible.

Another generic variation of the application of the novel effect of the invention replaces a structure with two periodic array structures, with just one array structure, see figure 4. In the type of device embodying the invention illustrated in figure 4, a reflecting metal plane and a single periodic array structure spaced apart from a reflecting metal plane are arranged to produce a device in which the reflection coefficient is controlled through the properties of the resonator formed by the metal plane and the inter array structure-metal plane spacing. Applications where the reflection of a beam of microwaves from a moving object (e.g. anti-collision radars) or the identification of a target are involved might also be able to take advantage of the properties of such devices.

A number of detailed embodiments of the invention will now be described, illustrating the application of the invention to different microwave device technologies.

Referring to figures 6 and 8, figure 6 shows a top cross sectional view of a narrow passband waveguide filter embodying the invention. In a tubular, rectangular cross- section waveguide 1, dielectric slabs 2 are arranged to provide periodic array structures 3 and 4, which themselves are spaced apart to form a resonator cavity 5. The waveguide 1 is conventional, copper, stainless steel, brass etc. The dielectric slabs have a thickness equivalent to λ/4 in the dielectric medium, where λ is the chosen central frequency of passband of the filter, and the spaces between the dielectric slabs 2 are λ/4 in air. The spacing between the array structures 3 and 4 is λ/2 in air, providing a cavity resonator. The device illustrated in figure 6 was fabricated in copper with dielectric slabs made from PTFE. The device was tested using microwaves with a central frequency of 11.3 GHz. The plot of the transmitted signals for the device is shown in figure 8 and, as can be seen the results are very close to the ideal of figure 5.

Referring to figures 7 and 9 is a view showing the invention embodied in another narrow passband waveguide filter embodying the invention. In a tubular, rectangular cross-section waveguide 10, indentations 12 in the side walls 11 are arranged to provide periodic array structures 13 and 14, which themselves are spaced apart to form a resonator cavity 15. The waveguide 10 is conventional, copper, stainless steel, brass etc. The indentations have a width equivalent to λ/4 in air, and create alternating spaces of width a and a' with differing impedances. The spacing between the array structures 13 and 14 is λ/2 in air, providing cavity resonator 15. The device illustrated in figure 7 was fabricated in copper The device was tested using microwaves with a central frequency of 11.3 GHz over a 2.65 GHz bandwidth. The plot of the transmitted signals for the device is shown in figure 9 and, as can be seen the results are very close to the ideal of figure 5.

Referring again to the drawings, figures 10a and 11a illustrate passband filters, embodying the invention, implemented in microstrip. A conventional microstrip material, eg copper clad alumina, is configured with the transmission line formed as illustrated in the figures to provide periodic array structures 23 and 24, on either side of a resonator 25. The periodically changing width of the microstrip line provides alternating regions of impedance along the transmission line. Typical dimensions for a filter operating at 9.5 GHz for the figure 11a arrangement are a length of 4.93mm (λ/2) for the resonator section 25, and a length of 3.18 mm for each element of the two periodic array structures 23, 24. Computer simulations of the response of device designed according to figures 10a and 11a to operate at 6 GHz are shown in figures 1 Ob and 11 b respectively.

It should be noted that the results of the circuit shown in fig. 7a also demonstrate that conducting strips with periodically varying width within a periodic array and resonator structure can achieve a similar result. Further, the resonator can be a combination of resonant elements in order to produce a specific type of frequency response for the system.

Referring to figures 12a and 12b, in a fifth embodiment, an waveguide E-plane resonator 30 has metallic septa 31 arranged in two 3-section periodic arrays 33, 34 separated by a resonator cavity 35. The septa have a length L1,L3,L5 and are spaced apart (by air) at a spacing L2,L4. The length of the resonator cavity 35 is Lr. The graphs of the simulated insertion loss of the waveguide E-plane resonator using periodic array stuctures at 9.455 GHz, 36.80 GHz, and 42 GHz are shown in Figures 13, 14, 15, 16 and 17. The specific arrangement of the metal septa, their dimensions and the dimensions of Lr are given in the sub figures 13a and 13b, etc., and/or on the corresponding graphs of the simulated insertion loss. The devices of figures 13, 14 and 15 have single resonator cavities, while the devices of figures 16 and 17 have two cavities and multiple cavities respectively. Figure 18 shows the graph of the simulated insertion loss of the waveguide E-plane three-channel multiplexer at 42 GHz using the configuration of metal septa shown in sub- figures 18a and 18b. An E-plane waveguide was fabricated to the configuration of figure 17, operating at 42 GHz, using brass for the waveguide housing and copper for the metal insert and the graph in figure 19 shows the measured transmission loss of the fabricated waveguide resonator at 36.8 GHz, made using a HP 851 OB vector network analyzer. A further embodiment is shown in Figure 20a and its computer simulated response is shown in Figure 20b. In this embodiment a microstrip line, similar to that of Figures 7 and 8, is designed with a continual varying and periodic form

Claims

1. A microwave device, comprising: at least one periodic array (Fig 3; Fig 4; 3, 4; 13, 14; 23, 24; 33, 34) of at least one first element (ZI, 2, a) and at least one second element (Z2, 2, a'), the elements being arranged so that at at least one interface between first and second elements, reflection of incident microwave energy can occur, and the periodicity of the array can allow constructive interference of the reflected microwave energy, and wherein the at least one periodic array is disposed in such a manner that at least one frequency within the bandwidth that would otherwise be reflected, is transmitted.

2. A microwave device according to claim 1 wherein, two periodic arrays (Fig 3; 3, 4; 13, 14; 23, 24; 33, 34) are provided and arranged to interact in use to provide transmission at a desired frequency.

3. A microwave device according to claim 2 wherein there is provided a resonator and the two periodic arrays (Fig 3; 3, 4; 13, 14; 23, 24; 33, 34) are arranged with the resonator (L3; 5; 15; 25; 25) to provide the frequency selective transmission.

4. A microwave device according to claim 1 wherein, a single periodic array (Fig. 4) is arranged with a reflector of microwave energy so that, in use, incident microwave energy is transmitted at the selected frequency through the periodic array, reflected by the reflector, andean pass again through the periodic array.

5. A microwave bandpass filter which comprises a hollow waveguide in which there are, sequentially: a) a first component (3, 13, 23, 33) consisting of a plurality of elements comprising at least one first element and at least one second element arranged alternatively, and the linear dimensions of which elements are substantially the same, b) a second component comprising a resonator (5, 15; 25; 25) and c) a third component (4, 14, 24, 34) substantially the same as the first component.

6. A device for use in microwave circuits, which comprises: a first component (3; 13; 23; 33) which comprises a plurality of elements which have one of two different impedances, with the elements arranged alternately so that each element has an impedance different to that of the adjacent element; a second component which comprises a resonator(5; 15; 25; 35) and a third component (4; 14; 24; 34) essentially the same as the first component.

7. A microwave device as claimed in claim 6 wherein the effective length of the elements (3, 4; 13, 14; 23, 24) in the first and third component, in the desired direction of transmission of microwave energy, is substantially one quarter wavelength and the second component is effectively one half a wavelength long, the wavelength being measured at the central operating frequency of the device.

8. A microwave device as claimed in claim 6 wherein the effective length 1 of the first and third elements (3, 4; 13, 14; 23, 24) in the medium of the element is a submultiple of one quarter wavelength and the second component can be one half a wavelength long.

9. A microwave device as claimed in claim 8 wherein the submultiples are selected to be one eighth or one sixteenth of a wavelength, etc..

10 A microwave device as claimed in wherein the device is formed from a hollow waveguide and the elements (3, 4; 13, 14; 23, 24) are made from one or more of the following: air; dielectric material; metal;

11 A microwave device as claimed in any preceding claim wherein the device is formed from a hollow waveguide (lθ)and the walls (l l)are indented (12) to form at least some of the elements.

12. A microwave device according to any preceding claim wherein for the periodic array structure of elements, there are 2 to 9 elements or 3 to 7 elements.

13. A microwave device as claimed in any preceding claim wherein the device is formed as a microstrip transmission line.

14. A microwave device as claimed in claim 13, wherein adjacent elements have impedances ZI and Z2, and the ratio of ZI : Z2 is preferably between 1 : 1 and 3: 1 or from 1.5 to 2.5:1 or ZI is of 25 to 75 ohms and Z2 is in the range of 30 to 60 ohms.

15. A microwave device as claimed in claims 13 or 14 wherein there is a resonator and the resonator impedance is from 20 to 80 ohms for a microstrip or planar resonator.

16. A microwave device is claimed in any of claims 1 - 12 wherein the device is formed from a hollow waveguide.

17. A microwave device as claimed in claim 16 wherein adjacent elements have impedance 21 and and 21 and 22 are in the range 300 to 500 ohms.

18. A microwave device as claimed in claim 16 or 17 wherein there is a resonator and the impedance of the resonator is 300 to 500 ohms.

19. A microwave system having a microwave device is claimed in any preceding claim.