DE1791017B1 - MICROWAVE FILTER - Google Patents

Description

and that the cavities are separated from one another by waveguide sections with wave resistances Z ₁₂ and Z _{23, where:}

IO

O ⁺ is) · * "^ t-

whereby

J ?, a load impedance, R _g a generator impedance, and g ₀ , g _u g ₂ and g _{4 are} normalized values of a lumped element filter having a band edge frequency W ₁ . With the filter according to the invention

• wünschtes behavior avoided, and a simple reduction in the height of the waveguide in a _^3/4 wavelengths long area between two given resonant cavities is sufficient if the reduction is dimensioned such that in this area a specific value of the characteristic impedance is formed which on the parameters of the Cavities themselves is based. No new discontinuities are added between the cavities, ie the only existing discontinuities (namely with the iris diaphragm coupling the cavity and with the capacitive adjusting screw that usually belongs to the iris diaphragm) coincide and are electrically matched to these.

If the principle of the invention is applied to a typical maximally flat (structurally symmetrical) filter with three cavities, the arrangement according to the invention is reduced to a single plate of conductive material, which is I ¹ Z ₂ wavelengths long and has a special thickness, and which is in the connecting rectangular conductor on one

»Extends wide wall and lies in the middle between the iris diaphragms of the resonance cavities. The principle of the invention can be extended to filters with any number of cavities, both can be symmetrical as well as asymmetrical.

The invention is described below with reference to the drawings. It shows

F i g. 1 is a cross-sectional view of an exemplary three-cavity filter according to FIG Invention,

F i g. 2 a typical schema filter with elements contracted at points with one for the Fig. 1 desired characteristic,

F i g. 3 to 5 successive equivalent circuit diagrams for transfer lines to ensure equivalence between the F i g. 1 and 2 to be derived.

In Fig. 1 a filter with three cavities was chosen to explain the principle of the invention. As can be seen from the cross-section, the filter consists of a section of a right- angled , conductively limited waveguide W and ~~ of three cavities 11, 12 and 13 about half a wavelength long, which penetrate a wide wall of the conductor 10 with the help of the iris diaphragms 14,15 and 16 are coupled at points which are three quarters of a wavelength apart in the conductor. The capacitive tuning screws 17, 18 and 19 are located in the wide wall opposite the iris diaphragms 14, 15 and 16. Up to this point the arrangement is conventional and known.

According to the invention, the characteristic impedance of the conductor 10 in the area that extends from the iris diaphragm 14 to the iris diaphragm 15 (or from the screw 17 to the screw 18) is reduced to a value _{denoted by Z 12} by the narrow cross-sectional dimension of the Conductor 10 is narrowed by the conductive plate 20, while it is reduced in the area between the iris diaphragms 15 and 16 (or the screws 18 and 19) by the conductive plate 21 to a value which is denoted _{by Z 23.} In the form shown, the screws 17, 18 and 19 pass through holes in the plates 20 and 21, the end of each plate being intended to coincide with the diameter of a screw.

In order to explain the principle of the invention and _{to determine the impedance values Z 12} and Z ₂₃ according to the invention, a method is used that is known in filter technology, in which a schematic low-pass filter with elements drawn together at points and with the desired transmission characteristic is used, then the transmission line equivalent of the schema filter is derived and finally the corrections are made which are necessary to describe the shape of the cavities with distributed parameters. For a further discussion of this solution and the justification for its validity, reference is made to the standard works, e.g. B. "Microwave Transmission Circuits" by GL R agan, Radiation Laboratory Series, Vol. 9, McGraw-Hill Book Co., New York 1951, or to published articles, e.g. B. "Microwave Band-Stop Filters with Narrow Stop Bands" by L. Young, GL Matthaei and EMT J ο η es, in the journal "IRE Transactions on Microwave Theory and Techniques", Vol. MTT-10 (November 1962) No. 6, pp. 416 to 427.

In Fig. 2 shows a typical low-pass basic circuit which consists of a series inductor 31, two parallel capacitors 32 and 33, a source impedance 34 and a load impedance 35. The values g ₀ , g _u g ₂ , g ₃ and g _A , which are referred to in the above literature as “<7” values, are the normalized element values which indicate the desired frequency dependence.

The transmission line equivalent of FIG. 2 is in FIG. 3, in which a short-circuited stub line 36 with the characteristic impedance Z ₂ and the length I replaces the inductive element 31, while the open stub lines 37 and 38 with the characteristic admittance I ^ and J ^ and the same length / the capacitive elements 32 and Replace 33, namely. using frequency transformation

ω = A tan ßl (1)

where ω is the normalized basic filter frequency variable, A is the bandwidth constant, I is the length of the stub lines and β is its propagation constant. If you according to the invention

selects, where / Ig _{0 is} the wavelength of the band center

frequency, and if one applies the transformation of equation (1) to the wave resistances, so will

Y ₁ = g _x A; Z ₂ =
whereby

and Y ₃ = g ₃ A,

A = W ₁ cot

2 w

(3)

(4)

Here, W _{1 is} the band edge frequency (e.g. the 3-db point of the schema filter) and A _{gl is} the desired band edge _{guide wavelength corresponding to W 1} .

In Fig. 4 was a transmission line section 39 with a length Z, now vacate FIG. 1. It is easy to adjust the characteristic impedance of a transmission stub so that the desired values of Z ₁ , Z ₂ and Z ₃ are obtained, but this is almost with a cavity arrangement not possible. On the other hand, the Q of a resonant cavity can be easily changed by changing the size of its coupling iris. Hence, it is obviously an advantage to have the parameters of the cavities 11, 12 and 13 of FIG. 1 to be expressed by the quality values Q in the loaded state. It is known that a half-wave cavity with a given quality value Q in the loaded state of a short-circuited three-quarter-wave transmission stub with the characteristic impedance Z is equivalent, if the following applies:

2Z '

(12)

and the same impedance g ₀ as the generator upstream of the stub lines 36 and 37 in FIG. 3 inserted, while a transmission line section 41 with the same length and the same impedance g ₄ as the load behind the stub lines 36 and 38 were inserted. None of the insertions change the amplitude transfer characteristic of the network. ■, - ■

It should be remembered that the Kuroda identity is an exchange of an open stub line with the impedance Y and a connecting line with the impedance Z for a short-circuited stub line with the impedance

This term is known to be sufficiently accurate for frequencies close to the resonant frequency, as in most waveguide filters. In the same way, it is more convenient to express the load impedance Ri and the generator impedance Jl ₉ directly and not in their normalized equivalents g ₀ and g _4. Therefore, by substituting equation (12) for each cavity impedance into equations (7) to (11) and multiplying all of the impedances

with the factor - * - the following design

So

equations for the construction of the F i g. 1.

Z ² Y

1 + 2Y

(5)

Table 1

35

and a connecting line with the impedance

1 + 2Y

(6)

40 Q ₂ =

3 π

3-π

go
Ag ₂

50

permitted. Given the identity of equations (5) and (6) to line 39 and stub 37 and then back to line 41 and the stub 38 of FIG. 4, one obtains FIG. 5, for which applies:.

3jrg ₀

Z ₁ =

1 +

Z ₁₂ -

go

goY 1 +

Z ₂ =. ^ g ₂ ;

Ag _3g4 .

_g4 Y ₃

(8th)

(9)

(10)

(H)

The network of F i g. 5 is now that of FIG. 1 equivalent, apart from the practical ones Differences between the resonance transmission stubs of FIG. 5 and the resonance hollow

45 These equations define the corresponding quality values Q of a series of cavities on the basis of the parameters of the schema filter for a given transmission characteristic; furthermore, according to the invention, they define the characteristic impedances Z ₁₂ and Z ₂₃ , which are to be established in the areas between the cavities with the plates 20 and 21 . While it may seem that these equations are complicated and difficult to apply to an actual arrangement, it can be seen that once numerical values are obtained for Z ₁₂ and Z ₂₃ , the dimensioning of an actual arrangement is only to be cause the height of the conductor restricted by the plates 20 and 21 to have the same ratio to the non-restricted height of the conductor 10, which have the desired impedances Z ₁₂ or Z ₂₃ to the characteristic impedance of the conductor 10.

If the desired filter is symmetrical, as in the case of a maximally flat filter, of the most desired type, then Q ₁ and Q ₃ and Z ₁₂ and Z _{23 are the} same. Then the plates 20 and 21 are on one

j 7 8

only plate made of conductive material with the required _ _ 3, -τ / 1 g "

l '/ wavelengths ^ ² ~ 2R \ 1 + 4g

\1

borrowed thickness and a length of l '/ ₂ wavelengths ^ ² ~ 2R _g \ 1 + / 4g _o gi Ag ₂ (I.

reduced. If only a filter with two cavities

is desired, define the values of Q ₁ , Q ₂ and _t

Z ₁₂ the required parameters if 5 Z ₂₃ = R g _Q (Ag ₂ H -)

V * H " AgogiJ

R ₁ = R ₉ L.

Sog3 3 ji J /

is ^ 3 ⁼ ~ 2ji An π ' ^{Μ =} ^ gSo [Ag ₄ . +

The principle of the invention can also be applied to filters with ίο ^{β SoS3 v}

t ^ 3 2ji An π ' ^{Μ =}

The principle of the invention can also be applied to filters with ίο ^SoS3

four or more cavities are expanded.

If one uses the same approximation that is given on the basis of q ^ ^71/1

i i

/ 1 g ^ \

\ 1 + Ag ₅ g ₆ Ag ₄ [I + Ag ₅ g ₆ f J

gg q ^

F i g. 2 and 5 respectively, e.g. B. for a filter with ^ 2 £ ₉ g ₀ \ 1 + Ag ₅ g ₆ Ag ₄ [I + Ag ₅ g ₆ f five cavities, it follows that the following is obtained

", _" _8l / ± ± ASlSt \

, g, g

Table »on relationships:", _ „_8l / ± ± ASlSt

Table 2 s

^3π (i ι ^l \ 7 - R ( ^{1 + Agog} i \ η - ^{3π g6}

I ^{2 +} > ^{Z Ä} U J &

1 sheet of drawings

Copy

209.523 / 335

Claims (4)

In the prior art, the basic patent claims: laying microwave filter has already been used extensively, which consists of a multitude of conductive-limited resonance 1. Electromagnetic microwave filter, which consists of cavities that are coupled to a conductively limited ches a conductively limited, rectangular waveguide 5 rectangular waveguide coupled at points and a plurality of resonant cavities, which has a spacing of an odd number, which with the waveguide in each case by times a quarter wavelength from each other, one of a multitude of spaced The setting of the resonance frequency and the quality iris diaphragms are coupled, thereby value Q of the cavities resulted in numerous differing characteristics that the waveguide (10) io dene band-pass and band-stop filters with numerous useful limit characteristics in the area of the iris diaphragms (14, 15, 16). In the first few years of cross-sectional dimension of such a development, the cavities had a dimension (Z ₁₂ ) reduced (by parts 20 and 21) stood at a quarter of a wavelength, ranging from the point of an iris diaphragm (14) to the effects of each section of the ver-place of an adjacent iris diaphragm (15) equal-15 binding waveguide between the cavities remains shaped. to neglect. 2. Filter according to claim 1, characterized later it was found that for physically feasible records that the iris diaphragms (14, 15, 16) have a band-stop filter in waveguides a distance of Distance three quarters of the wavelength of the. a greater multiple of a quarter wavelength Wave energy in the conductor and for the mid-20 should be used to prevent the frequency of the filter from each other. Coupling of higher order wave types between 3. Filter according to claim 1, characterized in the neighboring cavity see the operation of the draws that the cavities (11, 12, 13) deteriorated about the filter. The effects of the longer Half of the wavelength of the wave energy in the connecting sections will then be too large to be used for Conductor and for the passband of the filter on the current design norms, disregarded, let to be / M considering this difficulty 4. Filter according to claim 1, characterized by BM · S chiffma η η and GL records that three cavities with quality values in the Matthaei eg in the article »Exact Design loaded condition Q ₁ , Q ₂₅ Ö3 are present of band stop Microwave Filters "in the journal and that the cavities are separated from one another by waveguide sections" IEEE Transactions on Microwave Theory and sections with wave resistances Z ₁₂ and Z ₂₃ Techniques "Volume MTT-12 (January 1964) No. 9, where the following applies : Page 6 suggested placing a multitude of quarter-wave transformer sections between each hollow _ .. turn on the room. If this solution is also theoretically Q ₁ = - —fl + j ζ = R (1 H- Ag ₀ S ₁ ) ' ^{1 35 is} correct, its realization is impenetrable. 2 R _g V Ag _o g _t ) ' ^a feasible, since the complicated form is further discontinuous abilities in an arrangement conditional · which is already more has unavoidable discontinuities than easy 3 π g ₀ g ^ _, can be controlled. , Qz - -qjT ~ ΤΓ > ^Z 23 = ^R g - (! + ^A & Sa) 40 For waveguide couplers (The Bell System Technical ⁹ ■ ^So Journal, Vol. 33, May 1954, No. 3, pp. 661 to 719, in particular Fig. 50A) it is already known to the Cross section of the waveguide at the coupling point _ _3π | ο_ (^ ₊ _i __ \ Ji ₁ _-ß JJiL _5. This is supposed to be a difference between ³ 2jR ₃ g4 V Ag ₃ g4.J ' ^s go' 45 the phase constants of the two together coupled lines are created. The invention is based on the object of providing a microwave filter of the type specified at the outset . --- ._ ■ to train that the desired filter behavior without λ - _r , rr, t (^ ⁷¹ häX 50 a large number of discontinuities in the arrangement \ L A _gl / voltage is reached. The problem posed is achieved in that the R _{1 is} a load impedance, .R _{3 is} a generator waveguide in the region of the iris diaphragms with respect to impedance and g ₀ , g _x , g ₂ and g ^. normalized one cross-sectional dimension is reduced to such an Ab-values of a measurement with concentrated elements, which are from the location of an iris-guided filter, which has a band-edge diaphragm up to the point of an adjacent iris diaphragm frequency ω _τ . remains uniform. In a further development of the invention, the iris diaphragms have a distance of three quarters of the wave ——— Γ ~ 6o length of wave energy in the conductor and for the middle frequency of the filter from each other. In a second development of the invention The invention relates to an electromagnetic, the cavities have about half the wavelength Microwave filter, which limited a conductive, the wave energy in the conductor and for the pass band rectangular waveguide and a variety of 65 of the filter. Has resonance cavities, which with the wave A third development of the invention is thereby head in each case by one of a plurality of in that three cavities are coupled with the quality spacing arranged iris diaphragms. evaluate in the loaded state Q ₁ , Q ₂ , Q ₃ provided