DE2510854A1 - BANDPASS FILTER FOR MICROWAVES - Google Patents

Description

Liedl, Dr. Foniani, Nöih, Zeltler 2510854

° atentanv: old iOOO Munich 22 Steinsdorfstrasse 21 - 22 Telephone 089/29 84 62

D 7224

MICROWAVE ASSOCIATES INC. South Avenue, Burlington, Massachusetts 01803, U.S.A.

Band pass filters for microwaves

The invention relates to microwave cavity resonators and frequency sensitive and selective lines, such as wave filters, frequency meters, etc., which are used in such resonators. The invention can be used in particular in those resonators in which there is more than one resonance mode type within the frequency band of interest, for example in the case of such cylindrical hollow bodies which use the TE _Q11 mode. In this case, the TM ₁₁₁ modes are individually disruptive in that they have essentially the same resonance frequencies and

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can interfere with desired frequency-sensitive characteristics.

Cavity resonators have many uses in microwave systems because of their frequency sensitive characteristics. It is known that closed cavities with electrically conductive walls act as frequency-sensitive resonators, so that upon coupling on electric waves coming from outside the electromagnetic wave energy in the cavity at certain resonance frequencies is more strongly excited than at other frequencies. If such a cavity is connected to an input waveguide and an output waveguide is coupled, it acts as a frequency-sensitive wave filter and lets an energy out of one with little loss On the other hand, waveguides pass through at such coupled resonance frequencies and significantly dampen waves of other frequencies. For others Applications, a cavity can have only one coupling element to a single waveguide. Such singularly coupled resonators selectively absorb the energy of an incident wave at the resonance frequencies, but reflect the wave energy from other frequencies to a large extent.

The different resonance frequencies have characteristic images of the electric and magnetic field excitation within the cavity and each of these frequencies with the associated pattern or image is called the mode of the cavity. The field images are different for each mode and many modes are possible in all the cavities, which are essentially enclosed by electrically conductive walls. For a given frequency band, normally only one mode is desired and this situation is mostly achieved by dimensioning the cavity according to the known theory that the desired mode is the one with the lowest resonance frequency. The mode of the next higher

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Frequency can then be at a much higher frequency and outside the band of interest. For those cases, however, which require a very high degree of frequency selectivity, a higher order of the modes may be preferred, since this has a larger "Q" value, that is to say that energy losses are lower for a given amount of energy stored in the internal electromagnetic fields are. One of the most common higher order resonance modes is the mode commonly referred to as the TEq.j mode, which is present in a cylindrical cavity. Unfortunately, this low-loss, high-Q mode is ambiguous in a given band of frequencies. In a simple cavity designed for this mode, it has been found that there are two _{modes, generally designated TM 1} -.- mode, which _{resonate at the same frequency in the TE 011} mode and that there are other modes as well at nearby frequencies there. It has been found that the frequency sensitive characteristics of such a cavity or filter are distorted by the presence of more than one mode at the same frequency or at adjacent frequencies. It is known to provide a limitation of this problem to the effect that the coupling diaphragms are only arranged on the cylindrical side wall in order to only couple to the internal magnetic field in the axial direction H and a coupling to the circular

components of the magnetic field H _Q or the radial components H to avoid. Since the TM modes have no axial magnetic field components, coupling to the TM modes can theoretically be suppressed in this way. In fact, manufacturing inaccuracies do occur and there is generally some degree of coupling to TM modes. It is also known to use a disk-shaped end plate in the interior of the cylindrical cavity which does not touch the cylinder wall, with damping material being hidden in the area behind this plate. Of the

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Modus does not couple well in this area, causing small losses for this mode can be achieved. However, the TM modes couple strongly in this zone and suffer a loss as a result, the their adverse effects are diminished. However, the use of these techniques results in high manufacturing costs and is not always sufficient in effect.

For economical production it is sometimes useful to couple into a cylindrical cavity via an end walli) this means coupling to the radial magnetic field. In this case, both TE and TM modes can directly interfere with very undesirable Effects are coupled.

The aim of the invention is to produce a filter that is economical to manufacture of the type mentioned at the outset, in which these disadvantageous effects are eliminated by detuning the undesired TM modes by shifting their resonance frequencies out of the frequency band of interest.

Once this is done, endwall docking is practical and feasible and the desired TE ₀₁ mode is the only one that exists within a significant frequency band.

In the present invention, a wave energy is in a cavity resonator, in particular, a high Q cylindrical cavity coupled through one or both end walls and decoupled from this. Unwanted modes are selectively detuned from the desired frequency band in that in the Cavity elongated, conductive elements, for example pins, introduced are arranged and aligned such that a field distortion and detuning of the undesired mode or

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the undesired modes are caused, but which have only a small influence on the field image of the desired mode. Filters and resonators which have such cylindrical cavities with end-wall coupling, but essentially without mode interference effects use, allow a significantly more economical method of construction than was previously the case.

Embodiments of the invention are shown in greater detail in the drawing explained. Show it:

Fig. 1 is a longitudinal section through a single cavity resonator or filter, the basic principles of the invention are shown;

Fig. 1A is a section 1A-1A of Fig. 1;

Fig. 2 is a longitudinal section of a filter with two cavities;

FIG. 3 shows a cross section 3-3 from FIG. 2;

Fig. 4 shows a cross section 4-4 from Fig. 2; and

Fig. 5 is a longitudinal section through another filter in which the invention is provided with a series of cylindrical cavities is shown, which on a common coupling plate alternately on the opposite side are arranged.

In Fig. 1, a cavity resonator 1 is shown with a cylindrical shape, which is intended for resonance operation according to the TE .. ^ mode.

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The cavity is the air space within the cylindrical side wall and the flat end walls 4 and 5. Ih each end wall a diaphragm opening 8 is provided for coupling the cavity to the two waveguides 6 and 7 are used. Adjustment of the coupling degree is standard practice Orifice adjustment screws 9 used. A larger tuning screw 10 is used in the side wall for adjusting the resonance frequency of the cavity, which is also common.

Li Fig. 1 also shows four elongated mode detuning pins 2 which extend from each end wall 5, 6 in the axial direction into the cavity. These pins are arranged on a circle at an angular displacement of 90 °, the center of the circle lying on the cylinder axis and having a radius such that the pins are approximately in the middle between the side wall and the cylinder axis. Furthermore, four elongated mode detuning pins 3 are shown, which extend from the central plane of the cylindrical side wall 1 into the cavity. According to a well-known theory ("Fields and Waves in Modem Radio", by Simon Ramo and John R. Whinnery, published by John Wiley and Sons, 1944), the TE ₀₁ mode has an electric field image whose lines of force are circles that run around the cylinder axis and lie in planes perpendicular to this axis. These electric field lines are perpendicular to the longitudinal direction of the pins 2 and 3 and are therefore only slightly disturbed by their presence. However, the theory also states that the TML .. mode has a strictly axially oriented electric field on the end wall at a radius lying between the axis and the side wall. The pins 2, which are approximately in the middle between the axis and the side wall, therefore interfere with this field to a great extent, since they lie along or parallel to it. The consequence is that the resonance frequency of the cavity for the

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TM. j. -Modes is reduced. The TM- -. -Modes also have a strictly radially oriented, electrical field component in the middle area of the cavity to the side wall and you Si lit © ώ, which lie in this area and are aligned parallel to this component, have the same effect here. The effect of these mode detuning pins is to add capacitance to the equivalent circuit of the resonator in so far as it affects certain modes, and to decrease the resonant frequency of the cavity for any mode that has a high frequency electric field with the pins in their place collapses in the cavity. In this way there is the possibility of substantially reducing the resonance frequency of the cavity selectively for such modes, as a result of which the resonance frequencies for these modes are shifted out of the frequency band of interest. In the case of the cavity shown in Fig. 1, the high Q resonance for the TEq .. mode is retained in the frequency band of interest and the mode detuning _{pins 2 and 3 effectively serve to drive the TM 1} .. modes out of that band push.

In this given device, not all of these are mode shift pins 2 and 3 necessary. A single pin 2 or 3 in a single position can detune the cavity for one of the two TM. ^ - fashions will be sufficient. With a second pen

which is arranged at an angular offset of perhaps 90 ° to the first pin, both TM * jj modes can be detuned. Three Equally spaced pins 2 or 3 may also be sufficient. In most cases the results are satisfactory has been achieved when four pins 2 have been used in a 90 ° separation on only one end wall. Of course, five or more pens are used. In Fig. 1 there are more mode shifts (Detune) pens shown as necessary to give the designer the

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to show different options.

Ih Fig. 1, the two coupling panels 8 and two are coupled Waveguides 6 and 7 are shown, one of which is attached to an end wall 4 and 5, respectively. The cavity is too applicable when there is only one aperture and one waveguide, in which case the frequency selective characteristics become of the waves reflected from the single waveguide is used. With two waveguides, the device can serve as a bandpass filter, which allows a lossless transmission of frequencies near resonance from one waveguide to the other, other frequencies but very strong attenuates which are not in the resonance range or which are shifted in resonance.

In Fig. 1, the two waveguides are on the opposite End walls shown. The device behaves quite similarly, however, when both apertures 8 and both waveguides 6 and 7 are on a single end plate are arranged in a different angular position around the cylinder axis. The technique, wave energy over Coupling two diaphragms in a single end wall into and out of a cavity is used in the embodiment shown in FIG applied.

Filters of this type are usually made of metal, however Any other strong, hard material can also be used if the inner surfaces of the cavity and the bezels are covered with a highly conductive material are coated.

The structure shown in Fig. 1 is a single resonance filter. For many applications, however, multiple resonance filters are required, to obtain a higher degree of frequency selectivity. In the

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It is already known in practice to use such filters as a cascade arrangement to form of cavity resonators, in the coupling elements of each end cavity to a waveguide or another Transmission conductors and coupling elements are provided between each two adjacent cavities. The resonators are common tuned to approximately the same frequency and the coupling elements are designed according to known theoretical principles to to give a frequency selective "bandpass" characteristic. The application of this invention to multiple cavity filters will now be described described.

lh Figures 2, 3 and 4 are two cylindrical cavities 11 and shown with a common interstage coupling plate

13 are joined together. Figure 3 shows a side view of this filter. FIG. 4 is a cross section taken through the coupling plate 13. The plate 13 has mode shifting pins 14 which pass therethrough. These pins are arranged in positions that favor detuning the TM .. modes. A diaphragm opening 15 in the coupling plate is used to decouple the cavity 11 from the cavity 12. The cavities operate according to the TE «... cylinder mode. The diaphragm 15 lies outside the center point, preferably at the radius for the H maximum (magnetic field in the radial direction). The mode shifting pins 14 are arranged perpendicular to the E field of the TE _Q .. mode and cause a slight detuning for this mode. For the two TM of the undesired TM modes lying on the relevant end wall has a maximum. The pencils

14 effectively increase the capacity with respect to such TM modes, thereby the resonance frequency of the cavity for these modes reduces

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that is. In this sense, the pins cause a noticeable detuning of the cavities in which they are installed, relative to the TM modes, in particular to the TM ₁ ^ ₁ modes, by pushing the resonance frequency of these modes below the frequency band being operated.

The aperture 15 can be adjusted in its degree of coupling with a screw 16, which extends from the outside through the body of the Plate 13 extends into the diaphragm, with an adjustment arrangement on the outside 17 is provided. Tuning screws 18 and 19 for the respective cavities 11 and 12 are in the side walls of each cavity attached for tuning the resonance cavity.

At the outer ends of the cavities 11 and 12, plates 21 and 22 are provided which have diaphragms for coupling energy into and out of the filter. The input plate 21 has a coupling diaphragm 25 which is similar to the interstage coupling diaphragm 15 and has comparable tuning elements 26 and 27. The outer surface of the input plate is for coupling to an input waveguide 31, the end of which is coupled to the input panel 25. Mode shift pins 24 are arranged on the inner surface 23 and have a similar appearance to the mode shifting pins 14 for the wave energy propagating in the filter. the Output coupling plate 22 is functionally the mirror image of the Input coupling plate 21 is. It has the mode-Vers shift pins 34, which protrude into the second cavity 12, and an aperture 35 for coupling this cavity to a waveguide 36 and an aperture adjustment pin 37. As in the intermediate stage coupling plate 13, four mode shift pins are arranged in each end coupling plate; so there is in every end wall of everyone Cavity four mode shift pins. It is emphasized again

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that in the figure more mode shift pins are shown than are actually necessary.

So that the principle shown in Figures 2, 3 and 4 can be extended to three or more cavities, it is only necessary add further cylindrical elements such as the cavities 11 and 12 and further coupling plates corresponding to the plate 13 to be provided between two cylindrical elements. According to known principles, the cavities should be tuned very carefully and the degree of coupling at each diaphragm should be set correctly in order to get the desired bandpass characteristic.

In the devices shown in Figures 1, 2, 3 and 4 and their Extension to multiple cavities is a restriction to the effect that the tuning screws 10, 18, 19 in the cylinder side walls can only pass through a limited tuning range without a noticeable loss and noticeable disruption of the internal field image. The cylindrical elements 1, 11 and 12 must also close very precisely before they are assembled Lengths can be cut which are determined by the desired frequency. The tuning screws are best placed on one of the flat End walls attached. It is also desirable to allow adjustment of the length of the cylindrical cavity as measured on its axis. With such an arrangement, a single filter structure can be tuned over a much wider band of frequencies. the Above mentioned limitation is overcome in the device according to FIG. 5, in which the end plates 71, 72, 73, 74 and 75 in axial Direction can be adjusted in their position and tuning screws 89, 90, 91, 92, 93 are provided for fine trim adjustment.

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In FIG. 5, a number of cavities 51, 52, 53, 54 and are arranged together on a common coupling plate 41. This plate contains all coupling covers 42, 43, 44, 45, 46 and 47 as well as their tuning screws 42'-47 'for the filter. To simplify the description of this figure, the two sides of the coupling plate 41 are referred to below as the first side 61 and the second side 62. An input waveguide 63 is coupled to the first aperture 42 on the first side. The first cavity 51 is attached to the plate 41 on the second side and is ^{aligned for coupling according to the TE} _O ii mode with the first and second apertures 43 and 43 respectively. The second cavity 52 is attached to the plate on the first side and is _{aligned with the second aperture 43 and the third aperture 44 for coupling according to the TE 011} mode. In the same way, the third cavity 53 is attached to the second side 62 and aligned with third and fourth apertures 44 and 45 for TE _{011 mode coupling.} The fourth cavity 54 is on the first side 61 for coupling in the same mode with the fourth and fifth diaphragms 45 and 46 and the fifth cavity 55 is on the second side 62 for coupling with the fifth and sixth diaphragms 46 and 46, respectively. attached. An output waveguide 64 is coupled to the last aperture 47 on the first side 61.

In the embodiment shown in Fig. 5, each cavity has its entrance and exit aperture which lie in the same end wall. However, each diaphragm has the same properties as a coupling diaphragm according to FIG. 2 and is eccentric, specifically on a radius for the maximum H wave according to the TE _n ^ ₁ mode.

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The coupling plate 41 provides an end wall for each of the cavities through which an energy is coupled. The opposite end of each cavity is defined by an individual end plate 71, 72, 73, 74 or 75 closed. Each of these end plates has one

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Set of four Modus shift pins and a tuning screw for the respective cavity. These details are referred to in more detail on such a plate, namely the plate 73, where two Mode shift pins 84 and a tuning screw 89 are shown. However, each of these end plates has four mode slide pins 84. Similarly, the docking end plate 41 may have a set of four mode shifting pins in each cavity, wherein two pins 84 'of such a set are shown in the central cavity 53. In this embodiment of the invention the cavities are matched over the end walls and there is no need to put any means on the side walls of the Provide cylinder.

It should also be noted that if Figure 5 is also an embodiment with five cavities reproduces the principle of interconnection also with any other number of cavities in a cascade connection is possible. For a single cavity configuration, for example, the cylindrical elements 52, 53, 54 and 55 along with their separate end plates can be omitted and plate 41 can be cut off between members 51 and 53 will. The waveguide 64 is then attached to the plate 41 over the bezel 43. For two cavities, the two cavity elements would 51 and 52 remain and the waveguide 64 would have to be fastened above the diaphragm 44 on the bottom side of the plate 41.

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Claims (5)

Claims 1. / Band-pass filter for electric waves with one section a hollow tube, for example a cylinder, with electrically conductive walls for use as a cavity resonator for microwave energy according to a TE ₀₁₁ mode in a frequency band which is centered around a specific frequency, characterized in that the cavity (1; 11, 12; 51 - 55) has the property of resonance for other wave modes located in this, that an end wall (4, 5; 21, 22; 41) of this cavity (1; 11, 12; 51-55) transversely to the tube or cylinder axis is arranged and eccentrically, at a radial distance from the axis lying coupling elements (8; 25, 35; 42 - 47), whereby a coupling to the radial component of the magnetic field H for the TE _01N mode is favored, and that mode shifting elements ( 2; 14, 24; 84) are provided, which consist of at least one electrically conductive, elongated pin, the cross section of which is no greater than a fraction of its length, which extends into the cavity of at least one of its end walls (4, 5; 21, 22; 41) extends out and its longitudinal expansion runs in the axial direction into the cavity, whereby the cavity can be detuned for a wave energy according to the other modes by changing the resonance frequency of the cavity for such other modes from the frequency band with a very small increase in area, on the electrical Currents must flow, can be effectively shifted. 2. Filter according to claim 1, characterized by at least two hollow cylinders (11, 12) and a plate (13) between these hollow cylinders, which at least part of an end wall for each cavity 5 0 9 8 4 1 / O B O 2 - 13 - forms, the axes of the cylinders (11, 12) being directed essentially perpendicular to this plate (13) and extending from it and the coupling elements (15) are provided for each cavity (11, 12) in the plate (13). 3. Filter according to claim 2, characterized in that the coupling elements are formed by openings (15) in the plate (13). 4. Filter according to claim 2 or 3, characterized in that one Sequence of at least two and up to "n" number of cavities (51-55) is formed, in which alternately a first cylinder (51) on a first side (62) of an end plate (41), a second cylinder (52) on the second side (61) of the end plate (41), a third cylinder (53) on the first side (62) of the plate (41), possibly a fourth Cylinder (54) is arranged on the second side (61) of the plate (41), the plate (41) being elongated and the cylinders in one double arrangement in a first row of the first, third cylinder (51, 53 ...) etc. on the first side (62) or in a row of the second, fourth cylinder etc. (52, 54 ..,) on the second side (61) the plate (41) are aligned so that each cavity (51-55) has a first and a second coupling element (42-47) in each case has the same end wall (41), whereby the coupling elements of each cavity are diametrically opposite to the cylinder axis, that the first cavity (51) by means of one of its coupling elements (43) through the plate (41) with the second cavity (52) is coupled via a coupling element (43) of this second cavity (52) that the second cavity (52) via its second coupling element (44) through the plate (41) optionally with a third cavity (53) via a coupling element of this third cavity is coupled, and so on, up to an "n" .cavity attached to the plate (41) is provided, and that the remaining coupling • 5098A1 / 0602 element (42) of the first cavity (51) and that of the n.Hohlraums is available for input or output of the wave energy. 5. Filter according to claim 4, characterized in that the coupling elements (42-47) are provided in the plate (41) and the cylinders are attached to this plate, which form the end plate of each cylinder represents, wherein each cylinder axis runs at an equal distance from two adjacent coupling elements. 6. Filter according to claim 5, characterized in that the coupling elements (42-47) are a series of openings in the plate (41). 7. Filter according to one of claims 4 to 6, characterized in that that each cavity (51-55) has a second, own end wall (71-75) at the end of the cylinder opposite to the plate (41) and in each of the second end walls (71-75) elements (89-93) for tuning each cavity independently of the other cavities in the filter (51-55) are provided. 8. Filter according to claim 2 or 3, characterized in that the plate (13) has a common end wall for each of the two cavities (11, 12) and each cavity (11, 12) has a second end wall (21 and 22) with coupling elements (25 and 35) for the entry the wave energy in the first cavity (11) or for its exit from the second cavity (12). 9. Filter according to claim 8, characterized by mutually independent Tuning elements (18, 19) in each side wall of the cavities 12). ⁷²²⁴ 509841/0602 10. Filter according to one of claims 1 to 9, characterized in that that each mode shift pin (2; 14, 24; 84, 84 ') at a radial distance between the cylinder axis and the side wall of the respective cavity (1; 11, 12; 51-55) is arranged. 11. Filter according to one of claims 1 to 10, characterized in that that in each cavity (1; 11, 12; 51-55) there is at least one mode shifting pin (2; 14, 24; 84, 84 ') from each of the end walls (4, 5; 13, 21, 22; 41, 71-75) extends into it. 12. Filter according to one of claims 1 to 11, characterized in that that each cavity has at least one additional mode slide pin (3) extending into the cavity, wherein its longitudinal direction extends in the radial direction starting from the cylindrical side wall and this at a point between the Cylinder ends is arranged. 5 0 9 8 U 1/0802