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(11) | EP 1 492 194 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Bandpass filter and diplexer |
| (57) Filters, diplexers and methods of filtering to provide a pass band and broadband
stop band from a single filter are described. A waveguide filter has a pass band with
a pass band frequency range and a broadband stop band away from the pass band frequency
range. The filter includes a plurality of resonator cavities and at least one hybrid
coupling element between adjacent cavities. The hybrid element acts as a coupling
element at pass band frequencies and provides a band stop response at frequencies
away from the pass band frequency range. A diplexer can include one or two such filters. |
[0001] The present invention relates to filters, and in particular to waveguide filters having a passband and a broadband stopband.
[0002] As is well known filters are used to pass desired electromagnetic signals and to stop undesired electromagnetic signals. One area in which filters are commonly used is telecommunications. Some telecommunications schemes have channels in the electromagnetic spectrum, such as the radio wave and microwave parts of the spectrum, in which telecommunications signals are broadcast. A number of channels closely spaced in frequency are often used as bandwidth can be scarce. Hence it is important that noise associated with a signal in a first channel, such as harmonics of the broadcast signal or other sources of noise, does not impinge on the adjacent or neighbouring channels so as to interfere with the signal in those channels.
[0003] Irrespective of whether channels are used or not, it is generally undesirable for the noise generated by a broadcast signal to impinge on other parts of the spectrum. For example, the ETSI in Europe and the FCC in the United States of America have provided a requirement for noise prohibiting the transmission of up to the 2nd harmonic of a broadcast signal, and beyond in some instances.
[0004] One way of addressing this has been to provide a second filter, in conjunction with a filter for the broadcast signal, in order to remove the unwanted noise generated by the broadcast signal. The second filter is often a second device just 'bolted on' to the first filter. Hence this approach is costly, and results in a bulky device which is time consuming to manufacture, as two separate filters are required and attached together. The two filters can also be made as a single unit, but again, this results in a bulky unit. comprising the two separately operating filters.
[0005] US-6,232,853 (Goulouev) describes a waveguide filter similar to the above described type in which two separate low-pass filter units are attached to each end of a band-pass filter unit, although the different filter units are illustrated as being provided as integral parts of the same filter device. The low pass filter sections have symmetric slots which provide the high-pass and low-pass filter properties and the band-pass filter unit has asymmetric slots which provide the band-pass filter property.
[0006] EP-A1-0320268 (NEC Corporation) describes a waveguide band-pass filter. The filter has at least two resonators cascade connected along a waveguide. Each of the resonators consists of a high impedance section and a low impedance section. The resonators are cascade connected at intervals of λg/4 in the waveguide and the resonators act to block the 2nd harmonic while still acting as a band-pass filter. Hence in this filter, the resonators of the filter act to provide a narrow stop band at the 2nd harmonic frequency.
[0007] However, the first of these filters is essentially the same as joining filters together, even though provided as a unitary assembly, and the second of these filters only provides a narrow stop band at the second harmonic. Therefore noise at other frequencies will still be passed by the second filter. Therefore there is a need for a compact, single filter device having a pass band and a broad band stop band.
[0008] According to a first aspect of the present invention, there is provided a waveguide filter having a pass band with a pass band frequency range and a broadband stop band away from the pass band frequency range, the filter including a plurality of resonator cavities and at least one hybrid element between adjacent cavities, wherein the or each hybrid element acts as a coupling element at pass band frequencies and provides a band stop response at frequencies away from the pass band frequency range.
[0009] Hence the filter of the present invention has a single filter part with resonators and suitably configured hybrid elements that between them provide both the pass band and a broad band stop band. As the same filter device provides both the pass band and stop band behaviour, two separate filters are not required and so the filter of the present invention is compact, can be fabricated more simply and is cheaper. Also, the filter has a broad band stop band meaning that more noise can be filtered out and so the filter has a greater range of application. An element between resonators can be considered to be a hybrid element as the same physical entity combines two different behaviours: it acts as a coupling element at pass band frequencies; and it acts to provide a band stop response away from pass band frequencies.
[0010] The filter can have at least one hybrid element, preferably at least two hybrid elements and more preferably at least three hybrid elements. Three hybrid elements perform better than two hybrid elements and provide a good trade off between device complexity and performance. More than three hybrid elements can be used depending on the performance requirements of the filter.
[0011] The band stop properties of the or each hybrid element can be realised by any suitable structure. For example the structure can comprise or include waffle pegs, ridges or corrugations.
[0012] The filter can include at least one coupling element. The or each coupling element can be a capacitive iris or an inductive iris coupling element. The or each coupling element can also be a waveguide window coupling element.
[0013] The structure can be a trough. The structure can include a corrugation in a wall of the waveguide filter. The structure can includes at least one wall extending at least partially across the waveguide filter. The or each wall can have a height less than a height of the waveguide filter. The or each wall can extend from a wall of the waveguide filter and include a notch. The notch can be in a distal part of the wall.
[0014] The filter further can include at least one iris coupling element between adjacent cavities. Other coupling elements can be provided between adjacent cavities which allow the filter to provide the desired pass band.
[0015] The broadband stop band can be over frequencies higher than the passband frequency range. In this way signals at frequencies higher than the pass band are attenuated. The broadband stop band over frequencies lower than the pass band frequency range. In this way signals at frequencies lower than the pass band are attenuated.
[0016] The filter can be a microwave frequency filter. The filter can be a telecommunications filter.
[0017] According to a further aspect of the invention, there is provided a diplexer including at least one filter according to the first aspect of the invention. The diplexer can include a further filter according to the first aspect of the invention.
[0018] The diplexer can be an E-plane diplexer or an H-plane diplexer. The diplexer can be a T-junction, Y-junction of tuning fork diplexer.
[0019] According to a further aspect of the invention there is provided a method for filtering electromagnetic signals using a waveguide filter, comprising providing a plurality of resonators which generate a pass band over a pass band frequency range and providing at least one hybrid element between adjacent resonators acting as a coupling element at pass band frequencies and providing a band stop response at frequencies away from the pass band frequency range. The filtering method can be used in a first and/or second part of a diplexer. The filter can be used to filter microwave signals. The filter can be used to filter telecommunications signals.
[0020] An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:
Figure 1 shows a perspective view of a waveguide diplexer without a lid and including filters according to the present invention;
Figures 2 shows a perspective view of an unmodified coupling element of the filters shown in figure 1;
Figure 3 shows a perspective view of a hybrid element of the filters shown in figure 1;
Figures 4A and 4B respectively show cross sections along lines AA' an BB' of the hybrid element of figure 3;
Figure 5 shows a graph illustrating the measured S31 for a diplexer similar to that shown in figure 1, but without the hybrid elements of figure 3;
Figure 6 shows a graph illustrating the measured S31 for the diplexer of figure 1;
Figure 7 shows a graph illustrating the measured S32 for the diplexer of figure 1; and
Figure 8 shows a graph illustrating the measured S33 for the diplexer of figure 1.
[0021] Similar items in different Figures share common reference numerals unless indicated otherwise.
[0022] Figure 1 shows an E-plane T-junction waveguide diplexer 100, with a cover or lid removed therefrom. The diplexer has a first lower frequency input port 102, a second higher frequency output port 104 and a common port 106. A first filter 108 extends between the first port and the common port. A second filter 110 extends between the output port 104 and the common port 106. The waveguide has a short wall 116 with length 112 and a long wall 118 with length 114.
[0023] The first filter 108 includes seven resonator cavities 120. The first resonator cavity and the input port 102 have a hybrid element 124, as shown in Figure 3, between them. The first and second resonators and the second and third resonators also have a similar hybrid element between them. The third and fourth, fourth and fifth, fifth and sixth, sixth and seventh and seventh and common port have inductive iris coupling elements 128 between them, as illustrated in Figure 2.
[0024] The second filter 110 similarly has seven resonator cavities with four inductive iris coupling elements between the last five resonator cavities and three hybrid elements between the first three resonator cavities and the output port.
[0025] The diplexer 100 is made from WR42 (referred to as WG20 in the United Kingdom). The first filter has a centre frequency of 23.373GHz and a passband bandwidth of 600MHz. The second filter has a passband centre frequency of 22.365GHz and a passband band width of 600MHz. The short wall 116 has a length 112 of approximately 4.32mm long and the long wall 118 has a length of 114 is approximately 10.67mm long. In use, a lid is secured to the waveguide body by soldering. The lid can be secured to the waveguide in other ways. Mounting holes 128 can be provided in the walls of the waveguide to allow the diplexer to be mounted in use. The waveguide can be fabricated from a single piece of material or can be made of separate components which are soldered or brazed together. The waveguide can be made from silver plated aluminium depending on the performance required of the diplexer.
[0026] Figure 2, shows a standard inductive iris coupling element 128 which is provided between some of the resonator cavities of the filters. The inductive iris includes a first wall 132 extending from the short wall of the waveguide part way across the width of the waveguide. A second wall 134 extends from the opposite short wall of the waveguide extending partway across the width of the waveguide.
[0027] Figure 3 shows a hybrid element 124 which is essentially a modified coupling element having a structure 140 which acts as a broadband band stop resonance at frequencies higher than the passband frequencies, and which reflects unwanted signals. At passband frequencies the structure merely acts as a coupling element by providing a section of waveguide below the cut off frequency of the fundamental TE10 mode of the waveguide. An embodiment of a particularly suitable structure is described in greater detail below, but it will be appreciated that the present invention is not limited to the structure shown and that alternatives will be apparent to a person of ordinary skill in the art in view of the foregoing and following description of the invention.
[0028] Figures 4A and 4B respectively show cross sectional views along lines AA' and BB' of the hybrid coupling element 124. The structure of the hybrid element has a generally trough-like configuration and includes a single section 142 of a corrugated filter. A first wall 146 extends from the long bottom wall 114 and between the short walls 116 of the waveguide and to one side of a raised step 145 in the bottom wall 118 of the waveguide. A second wall 148 similarly extends from the long wall 144 and between the short walls 112 of the waveguide on the other side of the raised step 145. The walls 146, 148 and step 145 define between them a trough, including a single corrugation. Each of walls 146 and 148 include a notch 150', 150" in a top edge distal to the long wall 114. Walls 146 and 148 act as two capacitive irises at passband frequencies.
[0029] The structure of the hybrid coupling element 124 provides stopband elements at frequencies away from the passband frequencies. At passband frequencies the structure acts as a coupling element by providing a section of waveguide below the cut off frequency of the TE10 mode of the waveguide. At higher frequencies the structures act as a broadband stopband resonance which reflects the unwanted signals.
[0030] A number of the dimensions of the hybrid element structure can be varied in order to change the performance of the filter. There are two main design requirements. Firstly that the hybrid coupling element structure provide coupling into the filter in the passband, and secondly that the hybrid coupling element structure provide a broadband stopband resonance to clean up the filter stopband at frequencies away from the passband. The elements exhibit a low pass broadband stop response the properties of which depend on the dimensions of the structure. The coupling between resonators can be adjusted by varying the width of notches 150',150" and/or the depth of notches 150', 150". The depth of the trough 147 is a factor in controlling the resonant frequency of the structure. The length of the trough 149 (along the longitudinal axis of the waveguide) is a factor in controlling the bandwidth as is the height of the step 145. The width of the trough (transverse to the longitudinal axis of the waveguide) is a factor which influences the band stop resonant frequency.
[0031] A single trough hybrid coupling element is effective to reduce the out of pass band noise. Increasing the number of hybrid coupling element structures in the filter improves the band stop filtering performance of the filter.
[0032] The hybrid element has a number of advantages compared to other filters addressing the same problem, e.g. that described in EP-A1-0320268. The insertion loss of the filter is low as the band pass resonance of the device is within the main waveguide cavity and so has a higher unloaded Q. The hybrid element structures provide the band stop filter elements and are low Q. The band stop filter elements are provided by the hybrid elements in the invention, rather than in the resonators, as is the case in other filters e.g. that described in EP-A1-0320268. Further, the coupling element structures result in a broadband stop band response rather than merely filtering out a narrow range of frequencies, such as a second harmonic.
[0033] It will be appreciated that a variety of different structures could be used in place of the corrugated trough structure as described and shown in Figures 3 and 4A and 4B. For example a low pass ridged waveguide section could be used. A part of a corrugated waveguide section could be used. Waffle pegs could be used to provide the hybrid element structure.
[0034] Although Figure 1 shows a diplexer including two filters, 108 and 110, it will be appreciated that the invention is not limited to diplexers and that a filter or filters alone can be provided. Further, the invention is not limited to the particular diplexer shown in Figure 1. Diplexers including filters according to the present invention can be H-plane or E-plane filters. Further, the diplexer geometry can be any suitable type of diplexer geometry, such as T-junction, Y-junction or tuning fork junction configurations.
[0035] In the following, the subscript 1 refers to the input port 102, the subscript 2 refers to the output port 104 and the subscript 3 refers to the common port. Figure 5 shows a plot 150 of the measured S13 response as a function of frequency for a diplexer similar to that shown at Figure 1 but having only iris coupling elements 128 and no hybrid coupling element 124. As can be seen, the measured performance includes a signal 160 in the passband, and significant noise 162 at higher frequencies away from the passband. By way of comparison, Figure 6 shows a plot 165 of the measured S13 response of the diplexer 100 as a function of frequency over the same range as Figure 5. Again, there is a signal 170 in the passband, but, in addition, significant attenuation of the out of passband noise over a broad range of higher frequencies away from the passband. Hence the broadband stop band performance of the filter section 108 of diplexer 100 is evidenced.
[0036] Figure 7 shows a plot 175 of the measured S23 response as a function of frequency for the diplexer 100. Again, there is a significant signal 180 in the passband, and significant attenuation of the out of passband noise frequencies away from the passband. The measured performance of filter section 110 again evidences the good combination of passband and broadband stop band performance of the filter section 110. Figure 8 shows a plot 182 of the measured S33 performance as a function of frequency of the diplexer 100 illustrating the 600MHz passband of filter 110, centred on 22.365GHz, and the 600MHz passband 188 of filter 108 centred on 23.373GHz.
[0037] Various modifications to the filters and diplexers described in the foregoing will be apparent to a person of ordinary skill in the art in light of the above, and the invention is not intended to be limited to the specific embodiments described in the foregoing. It will be appreciated that the present invention provides a single filter, in which the same elements provide a good passband response over a first frequency range and a broadband stop band response over a different frequency of range away from the passband.
[0038] Although the filters of diplexer 100 have the stop band at higher frequencies, it will be appreciated that the stop band can be provided at lower frequencies, with respect to the passband frequencies. It will be appreciated that the centre frequency and width and filter characteristic of the passband can be changed by altering the dimensions and parts of the filter responsible for the passband response. Similarly, it will be appreciated that the broadband stop band response can also be altered using different structures and/or changing the dimensions of the hybrid element structures and/or the number of the hybrid coupling elements.
1. A waveguide filter having a pass band with a pass band frequency range and a broadband
stop band away from the pass band frequency range, the filter including a plurality
of resonator cavities and at least one hybrid element between adjacent cavities, wherein
the or each hybrid element acts as a coupling element at pass band frequencies and
provides a band stop response at frequencies away from the pass band frequency range.
2. The filter as claimed in claim 1, in which the or each hybrid element has the structure
of a trough.
3. The filter as claimed in claim 1, in which the structure of the or each hybrid element
includes a corrugation in a wall of the waveguide filter.
4. The filter as claimed in claim 1, and further including at least one coupling element
between adjacent resonators.
5. The filter as claimed in claim 4, wherein the or each coupling element is an inductive
iris coupling element.
6. The filter as claimed in claim 1, in which the structure of the hybrid element includes
at least one wall extending at least partially across the waveguide filter.
7. The filter as claimed in claim 6, wherein the or each wall has a height less than
a height of the waveguide filter.
8. The filter as claimed in claim 6, in which the or each wall extends from a wall of
the waveguide filter and includes a notch.
9. The filter as claimed in any preceding claim, the filter further including at least
one iris coupling element between adjacent cavities.
10. The filter as claimed in any preceding claim, in which the broadband stop band is
over frequencies higher than the passband frequency range.
11. The filter as claimed in any preceding claim, in which the broadband stop band over
frequencies lower than the pass band frequency range.
12. A diplexer including at least one filter as claimed in any preceding claim.
13. The diplexer as claimed in claim 12, and a including a further filter as claimed in
any of claims 1 to 10.
14. The diplexer as claimed in claim 12 or 13 in which the diplexer is an E-plane diplexer
or an H-plane diplexer.
15. The diplexer as claimed in any of claims 12 to 14, in which the diplexer is configured
as a T-junction, Y-junction or tuning fork diplexer.
16. A method for filtering electromagnetic signals using a waveguide filter, comprising:
providing a plurality of resonators which generate a pass band over a pass band frequency range; and
providing at least one hybrid element between adjacent resonators acting as a coupling element at pass band frequencies and providing a band stop response at frequencies away from the pass band frequency range.