DE1108823B - Bandpass filter with high slope - Google Patents

Description

GERMAN

PATENT OFFICE

J 6618 Vma / 21g

A N M E L D E T A G: NOVEMBER 25, 1952

NOTICE
THE REGISTRATION
AND ISSUE OF THE
EDITORIAL: JUNE 15, 1961

The invention relates to a band-pass filter with a high slope, the pass band of which is in Area of the meter, decimeter or centimeter waves and its attenuation poles by multipath coupling of filter components.

It is known that filter networks, the transmission impedance of which has maximum values and zeros in the Has finite, can be designed so that they have a higher slope than filter networks, whose transmission impedance has no finite zeros. For example, assign filters with so-called m-semicircles have a greater slope than a constant K filter. In the VHF and the microwave range, as is known, bandpass filters with very narrow passbands are required. Such filters have the prerequisite that the reactance values of the various components at the band center frequency are based on tight tolerances. Only then is it guaranteed that the The resonance frequencies of the resonators and the couplings between the resonators are prescribed Transmission behavior result.

Attempts have been made to date to use VHF and microwave bandpass filters which, due to poles in the damping curve have a high slope, to be realized by known Formations and terms of filter theory used for filter circuits from components with concentrated inductance and capacitance was developed. However, this results in circles in favor of such training and have such reactance ratios that they are practically unrealizable.

The invention aims to provide a bandpass filter with attenuation poles in the finite range, whose reactances in the VHF and microwave range can be practically produced. Using Resonators with a coaxial inner conductor or cavity resonators, which are analogous to a low-frequency resonant circuit are effective as well as by coupling elements that each resonator with the following Coupling the resonator either inductively or capacitively, a bandpass filter is created which can be used in the VHF and microwave range. The dimensions of the cavity resonator are like this selected that it oscillates at the desired frequency, or tuning means are provided to To produce resonance at the desired frequency. The dimensions of the coupling openings between adjacent resonators are decisive for the coupling coefficient. By exact Adjustment of the resonator resonance frequency, the coupling coefficient between neighboring reso band-pass filters high slope

Applicant:

International Standard Electric Corporation, New York, N.Y. (V. St. A.)

Representative: Dipl.-Ing. H. Ciaessen, patent attorney, Stuttgart-Zuffenhausen, Hellmuth-Hirth-Str. 42

Claimed priority: V. St. v. America December 1, 1951

Milton Dishal, Nutley, N.J. (V.St.A.) has been named as the inventor

nators as well as the coupling of the generator and the output-side terminating resistor with the first or last resonator and finally by a special form of the Multipath coupling, a transmission characteristic with attenuation poles is achieved that is the prescribed Attenuation curve is largely approaching.

Bandpass filter with a high slope, the pass band of which is in the range of meters, decimetres or Centimeter waves and their damping poles are created by multipath coupling of filter components, are known per se. For example, RF bridge circuits are used in known RF networks in the form of ring forks or transmission lines consisting of two conductors or also subsequently Provided on an existing line attachable conductor loops which are dimensioned in this way are that a high damping and edge steepness is achieved. The conductor loop represents with respect to their connection points represent two parallel paths of different lengths.

Furthermore, band-pass filters for IF channels of superheterodyne receivers are known that additionally have so-called detour couplings, which are dimensioned in such a way that two voltages arise, which is in phase at a frequency which at least approximately corresponds to the band center frequency are, while at each of the cutoff frequencies they are the

109 617/363

3 4

have opposite phase. With these acquaintances by suitable choice of dimensions and In addition, means are provided for the th arrangements, by means of which the oscillation shape of the resonators is precisely determined an output voltage arises, which are provided by the.

vectorial sum of the first and second voltage. Embodiments of the invention are shown in FIG

depends. The filter is so constructed that explained in more detail in the 5 drawings.

Cutoff frequencies the first voltage exactly the second Fig. 1 is a longitudinal section through a bandpass filter

Voltage at the output circuit of the filter cancels, so according to the invention for VHF and microwaves;

that at these frequencies an infinitely high FIG. 2 is a cross-section along line 2-2 of FIG

Attenuation is achieved. Fig. 1;

The bandpass filter according to the invention is therefore FIG. 3 shows the attenuation curve of a dashed line opposite characterized in that an odd bandpass filter without attenuation poles in the finite Number of resonators showing the damping poles and, as a solid line, the damping curve cause in the transfer curve, by coupling a bandpass filter according to the invention, the elements are connected in a chain and the multipath attenuation poles on both sides of the forward coupling due to the additional coupling to one area;

other odd numbered resonators by means of Figs. 4, 5, 6 and 7 are longitudinal sectional views

two lines is brought about, of which the other embodiments of the bandpass filter according to one line the electric fields and the other the invention.

Conduction the magnetic fields of the successive The band-pass filter shown in FIGS. 1 and 2

The chain circuit 20 consists of five coaxial resonators 1, 2, 3, 4 and 5 which are coupled to the odd-numbered resonators. with voting stamps 6, 7, 8, 9 and 10. All reso-

In a band-pass filter according to the invention, nators are similar to each other, so that the description of the chain circuit of the individual exercise of a resonator is sufficient.
Coupling elements mediating resonators through The resonator 1 is due to the tuning

Coupling openings between adjacent 25 stem feed 6 in its interior a coaxial line resonators educated. An advantageous embodiment resonator. The tuning die 6 behaves in a namform manner the bandpass filter according to the invention results in a similar way to the center conductor of a coaxial line, further, when the coupling openings between while the walls of the resonator 1 as the successive resonators arranged in such a way with respect to outer conductors of such a coaxial line are that electrical and magnetic coupling 30 of the electrical field present therein behave, alternate within the derailleur system. Apart from the one provided in a coaxial line resonator

In accordance with the idea of the invention, there is an existing electric field as well as an additional magnetic field Coupling path between two over-the-counter fields, which alternate with the electric field Odd-numbered resonators form- is coupled. The lines of force of the magnetic field The two lines are coaxial lines, the length of which runs perpendicular to the lines of force of the elekmit the odd multiple of a quarter tric field. To develop such fields is corresponding to the filter band center frequency, it is necessary that the dimensions of the cavity Wavelength coincides and whose coupling is precisely dimensioned for a desired frequency, probes are designed and arranged so that the For example, a filter for 1500 MHz could be used one line coupling the electric fields of 40 approximated 37 mm cubes with an edge thickness of approximately and the other line the coupling of the magnet 2 mm and a tuning punch of about 10 mm tables fields of two consecutive odd-diameters can be used. The dimensions causes numerous resonators. In further development of the cube are for the resulting resonance The idea of the invention causes the filter frequency of importance, while the thickness of the The coupling opening in the first Reso 45 walls and the diameter of the tuning die nator of the filter can be an electrical coupling of the more or less arbitrary and from Filters to the previous network and the skin effect and the filter output in the passband Mediating coupling opening in the desired degree of coupling with adjacent last resonator of the filter depends on a magnetic off resonators. The voting stamp 6 is for coupling of the filter or, conversely, the filter 50, the frequency selection is adjustable. The opening 11, magnetically to the preceding network, which is adjacent to the opening 12, determines the coupled and the filter decoupling is carried out by coupling coefficients to the neighboring resonator, electrical coupling means. where the approximate design equation for the

The number of cavity resonators is arbitrary, coupling coefficients between neighboring resonators, however, with the restriction that the previous resonators
mentioned coupling lines alternating hollow BW3db

spaces when a resonator is skipped Kb _ena chbart = QJ 7—

have to couple in the specified way and no J °

Coupling lines energy after other coupling is.

transmission lines. An explanation for 60 here BW 3db means the bandwidth of the filter, the creation of the attenuation poles is that between those points at the two ends of the at some frequencies outside the transmission range, where the attenuation range is a 180 ^ phase difference between the at 3 db has increased; with / ₀ the band center of the cavity is denoted by the additional coupling frequency of the filter,
The coaxial resonators 1, 2, 3, 4 and 5 can be the result of the hollow space jammed by the two coupling lines by soldering, bolting or screwing or cracking, whereby the two by any other suitable means ver-streams cancel each other out when the coupling is tied. If desired, the walls can

of the individual cavities are made from one piece. The coupling between neighboring resonators is effected through the openings already mentioned. The arrangement shown in Fig. 1 is constructed so that in a known manner, the magnetic and electrical coupling alternately from Advance resonator 1 to resonator 5 of the filter. This coupling through alternating fields reduces the direct feed-through, as the magnetic field tends to reduce the edge selectivity of the Filters to decrease. In other words, by alternating the coupling of neighboring resonators by means of magnetic and electric fields, it is necessary that the energy in the magnetic Field of a first resonator converted into the electric field by a second resonator before it can go to the third resonator, so that the energy selectively from one cavity to it must go to another, which results in a generally optimal selectivity. Under application of these principles, the inlet opening 13 and the outlet opening 14 should follow the same teaching. If the arrangement of the openings 13 and 14 does not correspond to this teaching, the far selection is not as big as possible.

The location of the strongest electric field is in the zone 15 near the end of the tuning die 6 and the location of the strongest magnetic field in zone 16, far from the end of the tuning die 6. For maximum results it is useful to use the coupling openings such as B. 11 and 12 as well as 13 and 14 to be arranged in these zones of maximum field strength. The design of the filter is, however not dependent on the location of the openings in the zone of the strongest fields as long as the coupling means are arranged adjacent to the fields. The openings 11 and 12, which are adjacent to the magnetic field can be oval or rectangular in shape, the longer being the longer Dimension should be parallel with the magnetic lines of force. If such openings are adjacent placed in the electric field, they may be circular, although different Shapes can be used.

A filter of the type described so far has - similar to a constant-K filter made of components with concentrated inductance and capacitance - an attenuation curve which, although it has zero points, has no attenuation peaks. The attenuation curve of such a VHF and microwave band-pass filter corresponds to curve A in FIG. 3. This arrangement is not as good as is often desired, since the slope is not as steep as is usually necessary. Only the area covered by the teachings of the invention, additional coupling of the odd-numbered resonators of FIG. 1 provides the attenuation curve B. A feedback line 17 shown in Fig. 3 with its Ankoppelschleifen 21 and 21 a is used to an inductive coupling between the resonators 1 and 3 to generate, while a coupling line 18 with its coupling probes 22 and 22 α causes a capacitive coupling between the resonators 1 and 3. The coupling lines 19 and 20 and their end probes give the same function with respect to the resonators 3 and 5. Since, as mentioned above, the electrical length of the coupling lines 17, 18 and 19, 20 depends on the probe combinations thereof, these are generally a multiple of a quarter the operating wavelength must be long.

One of the four different combinations of coupling probes and line lengths is shown in the resonators 1 and 3 of Fig. 1, where the inductive coupling resulting probes 21 and 21 a, which are connected to a coupling line 17 of the length of a quarter of the operating wavelength λ , and the capacitive coupling causing probes 22 and 22 a, which are connected to a Jt / 4 coupling Ieitungl8, are designed in such a spatial arrangement that any current introduced into the resonator 3 as a result of the voltage across the resonator 1 with respect to the Voltage across the resonator 1 is shifted by - 90 ° and correspondingly by + 90 ° in phase. This is one of the basic requirements that must be satisfied by a coupling line pair and its end probes. Another basic requirement is that one of the aforementioned 90 ° currents increase in size in direct proportion to the frequency and the other 90 ° current decrease in size in direct proportion to the frequency.

There is also a further basic condition for the couplings causing the damping poles, which can be described as follows. The phase relationships between the current introduced into the resonator by the alternating resonator coupling and the current introduced into the resonator 3 by the resonator coupling giving rise to the formation of damping poles must be such that the currents are in antiphase at the desired damping pole frequency. When the phase of the waves having 180 ° phase difference at this frequency, which is possible by accurate adjustment of the amount of the two couplings, it is achievable that the attenuation poles provided in Fig. 3 of the drawings by the reference numeral 23 are formed in the curve B. The above equation gives the approximate required value for the adjacent couplings to obtain a desired pass bandwidth, and the following equation gives the approximate required value for the alternate couplings

"BWsdi

₀₀ herein, BW frequency distance indicated in Fig. 3 with Af ^ between the attenuation poles on both sides surrounding the passband during _db under BW _2, in turn, the. 3 Aj ₃₁ Ij, designated frequency spacing between those dots at the both ends of the in Figure Transmission range is to be understood in which the attenuation has reached the 3-db limit.

The other points of infinite attenuation (24 in curve B) are created in a similar manner using the coupling lines 19 and 20 which cooperate with the coaxial resonators 3, 4 and 5; the different phase relationships are basically the same as described above. Across the pass band, the capacitive coupling coefficient of the coupling line 18 and the inductive coupling coefficient of the coupling line 17 are substantially constant and equal in size.

Kr,

■ alternating - '/ "

Fig. 4 of the drawing shows another embodiment of the invention, in which the individual Resonators are identical to the coaxial resonators 1, 2, 3, 4 and 5 of FIG. The difference between the exemplary embodiments is that the coupling openings 25, 26, 27, 28 are arranged alternately in an electric field zone and a magnetic field zone, however Both the inlet opening 29 and the outlet opening 30 of the filter are in a magnetic field zone are neighbors. The results obtained with this arrangement are essentially identical to FIG Arrangement according to Fig. 1 with the possible exception that the attenuation is far from the passband as a result the spatial position of the coupling opening 28 and the outlet opening 30 in the zone thereof Energy field is somewhat reduced. This apparent disadvantage of the coupling arrangement in the coaxial resonator 31 can easily be removed by shifting the opening 30 in the vertical direction, namely in such a way that it is close to the electric field, so there is the possibility of direct feed-through to reduce the unwanted frequency in the magnetic field. The tensions in the Coupling lines 32 and 33 or 34 and 35 generate in interaction with the voltage over the resonators 2 and 4 the points of infinite attenuation, as with reference to FIGS. 1 and 2 of the Drawing was described.

The combination of the magnetic probes 36 and 36 α and the electrical probes 37 and 37 a with their corresponding coupling lines 33 and 32 are spatially arranged so that they meet the phase conditions as set in the embodiment shown in FIG. On the other hand, the coupling line 32 is as long as the coupling line 33, and the total length of each line is an odd multiple of a quarter wavelength.

Another embodiment of the invention is shown in Figure 5 of the drawings. This example works on the same principle as the previous embodiment for generating zeros. The cavity resonators are rectangular waveguides 38, 39, 40 and 41 which oscillate in the H ₀₁ mode. The individual resonators are approximately 0.707 / long and 0.707 /. wide, where / is the wavelength in free space, and the height is chosen so that it meets the requirements of the electric field strength. As is known, in a waveguide resonator of this type, the electric field is concentrated between points 43 and 44, while the magnetic field is circular in cross-section and is concentric with the concentrated electric field.

The filter is entered by means of a coupling loop 45 through the opening 46 in the wall adjacent the magnetic field zone of the resonator 38. The voltage oscillates between the magnetic field and between the electric field, with part of the electric field passing through the circular opening 44 in FIG the resonator 39 is coupled. A resulting magnetic and electrical field oscillation occurs in the waveguide 39, and part of the magnetic field is coupled into the waveguide 40 via a rectangular slot opening 48. This alternating application of the electrical and magnetic coupling continues over the entire filter until the energy of the transmitted frequency band is coupled out via the opening 49 in the electrical field of the waveguide 42. The attenuation poles in the transmission characteristic are generated by using coupling lines 50, 51, 52 and 53 and the associated probes. In the probes shown, lines 50 and 51 must be equal to an identical odd number of quarter wavelengths, and lines 52 and 53 must also be identical and an odd number of quarter wavelengths.

Another embodiment is illustrated by FIG. 6 which has three waveguide resonators 54, 55, 56 shows, the design of which has already been described in connection with FIG the coupling of the successive resonators by quarter-wave coaxial lines 58 and 59 is effected.

The attenuation poles in this case are explained by using the third of the four previously explained Coupling probe combinations reached. With regard to the different ways of attaching the Coupling loops 64 and 64 a have the coupling lines 62 and 63 with their associated Probes 64 and 64a and 65 and 65a differ in length from one another by half a wavelength. In the bandpass filter according to FIG. 6, the coupling line 62, which carries the magnetically acting probes connects to be half an operating wavelength longer than the coupling line 63. This arrangement requires in accordance with the basic phase conditions that the coupling loops 60 and 61 are bent in the opposite direction.

A fourth coupling probe combination can be achieved in that the coupling line 63, connecting the capacitive probes is half a wavelength longer than the line 62. Diea requires a coupling loop 60 or 61 to be reversed from the position shown to achieve the exact phase relationship.

7 shows another embodiment in which the resonators 64, 65 and 66 are adjacently coupled by quarter-wave waveguides 67 and 68. The resonator coupling leading to the formation of damping poles is achieved by the coupling line 69 and 70 with the inductively acting coupling probes 71 and 71α and the capacitively acting coupling probes 72 and 72a. The spatial arrangement of the inductive probes

71 and 71a are opposite with respect to the corresponding capacitively effective coupling probes

72 and 72 a, so that the coupling lines 69 and 70 must differ in length by half a wavelength. Because of the coupling waveguides 67 and 68, both of which are positive Have mutual inductance, it is necessary to use the coupling line 70, which is the capacitive Probes 72 and 72 a connects to form half a wavelength longer than the coupling line 69.

Claims (9)

PATENT CLAIMS: 1. Bandpass filter with a high slope, whose pass range is in the area of meter, decimeter or centimeter waves and whose attenuation poles are created by multipath coupling of filter components, thereby indicates that an odd number of resonators, which cause the attenuation zeros in the transmission curve of the band filter, are connected in a chain by coupling elements and the multipath coupling is brought about by the additional coupling of successive odd-numbered resonators by means of two lines, one line of which the electric fields and the other line couples the magnetic fields of the successive odd-numbered resonators of the chain circuit. 2. Band filter according to claim 1, characterized in that the chain circuit of the individual Coupling elements mediate resonators through coupling openings between adjacent ones Resonators are formed. 3. Bandpass filter according to claim 2, characterized by such an arrangement of the coupling openings between successive Reso- nators that electrical and magnetic coupling within the chain circuit is mutual alternate. 4. Bandpass filter according to claim 2 and 3, characterized in that the electrical coupling of adjacent resonators effecting coupling openings formed circular and that the magnetic coupling between successive resonators producing coupling openings are contactors whose longitudinal axis is parallel to the magnetic Lines of force of the resonators to be coupled runs. 5. Bandpass filter according to claim 1, characterized in that the chain circuit of the individual Coupling elements causing resonators, coaxial line sections or hollow pipes are the length of which is a quarter or an odd multiple of a quarter of that corresponding to the filter band center frequency Wavelength coincides. 6. Bandpass filter according to claim 1, characterized in that the one additional coupling path between two successive odd-numbered resonators forming two lines Coaxial lines are the length of which is an odd multiple of a quarter of the the wavelength corresponding to the filter band center frequency and the coupling probes are designed and arranged so that one line the coupling of the electric fields and the other line the coupling of the causes magnetic fields of two successive odd-numbered resonators. 7. Bandpass filter according to claim 1, characterized in that the mediating the filter input Coupling opening in the first resonator of the filter, an electrical coupling of the filter to the preceding network and the coupling opening mediating the filter output in last resonator of the filter causes a magnetic decoupling of the filter or vice versa the filter is magnetically coupled to the preceding network and the filter output coupling takes place by electrical coupling means. 8. band-pass filter according to claim 1, characterized in that the resonators vibrating pots with a coaxial center conductor in the form of a cylindrical tuning punch. 9. Bandpass filter according to claim 1, characterized in that the resonators are waveguide resonators are rectangular in cross-section. Considered publications:
British Patent No. 491,359;
U.S. Patent No. 2,156,137;
Bulletin of the Swiss Electrotechnical Association, Vol. 35, No. 5, pp. 109 to 111 (March 8, 1944). 1 sheet of drawings © 109 617/363 6.61