EP2517299A1

EP2517299A1 - Frequency-tunable microwave bandpass filter

Info

Publication number: EP2517299A1
Application number: EP10793260A
Authority: EP
Inventors: Bernard Meuriche
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2009-12-22
Filing date: 2010-12-17
Publication date: 2012-10-31
Anticipated expiration: 2030-12-17
Also published as: WO2011076698A1; EP2517299B1; AU2010335206A1; AU2010335206B2; US8975985B2; FR2954596A1; MY167198A; FR2954596B1; US20130169384A1

Abstract

The invention relates to a frequency-tunable microwave bandpass filter that comprises, altogether, at least the following elements: a wave guide having a rectangular cross-section and including a stationary first conductive portion I and a movable second conductive portion II. Said first portion includes a plurality of first conductive partitions (105₁, 105₂ and 105₄, 105₅) having one or more conductive obstacles related to the complementary openings Oi in the section of the guide that forms capacitive irises. Said first partitions are transversely mounted, at the propagation of the wave in the guide and defining a plurality of cavities Ki (106₁, 106₂) in the longitudinal direction of the guide, and are rigidly connected to the first portion I and the second conductive partitions (105₃) that have one or more openings defining capacitive irises and that, in combination with the adjacent guide lengths (L3, L4), moreover form imitance inverters Ji. Said first partitions (105₁, 105₂ and 105₄, 105₅) form a series of resonating cavities Ki that are coupled by the imitance inverters Ji. A means makes it possible to ensure electrical contact between the conductive portions I and II.

Description

MICRO-WAVE FILTER PASS BAND TUNABLE IN FREQUENCY

The invention particularly relates to a frequency tunable bandpass microwave filter produced by the waveguide technique.

Microwave transmissions require the use of transmit and receive filters to select the frequency band in which the signal is transmitted. At microwave frequencies, it is possible to use guide filters which make it possible to obtain low losses and great selectivity.

In some applications, it is interesting to be able to tune the filter within a frequency band in order to be able to configure the hardware or the device at any time and until operation according to the frequency of the signal to be transmitted.

There are several ways to make the filters in a guide. Some use transverse partitions forming irises of inductive or capacitive type, others longitudinal partitions (septum). The narrowest filters may have a relative bandwidth of a fraction of a percent of the center frequency.

US Patent 5,808,528 [4] discloses a band pass filter which comprises a waveguide having a plurality of conductive walls and a movable wall defining the "large" dimension "a" of the waveguide. In this patent, the discontinuities created by a septum T with self-limiting obstacles in the vicinity of the axis of symmetry of the guide are used to define the cavities and the couplings of the filter (FIG. 1 a).

The equivalent diagram of a guide cavity shown in FIG. 1b according to the prior art [2] page 697 in which L is an admittance line section Yo and an end-of-line admittance.

Devices according to the prior art comprise cavities with end inductive admittances (made by means of iris or septum), for which the values of the equivalent inductive admittances (jB) at the ends of the cavities:

B / Yo ~ - (A _g ) / a ^* cot ² (π of l {2 a))

(where "a" and "d" are defined in Figure 1c) depend directly on the size of the long side of the guide "a" and vary considerably when "a" varies when the small side "b" of the guide is moved parallel to itself to set "a". FIG. 1c represents an example of an inductive iris according to the prior art.

The object of the present invention relates to a microwave frequency tunable band pass filter comprising in combination at least the following elements:

A rectangular section waveguide comprising a first fixed conductive portion I and a second movable conductive portion II,

Said first fixed part I comprising three longitudinal conducting partitions forming three sides of the waveguide G, the section of the guide having a large side "a" defined by the position of the movable conductive part II when it is inserted in part I and a small side "b",

Said first part I comprising a plurality of first conductive partitions with one or more conductive obstacles associated with the complementary openings in the section of the waveguide forming capacitive-type irises, said first partitions being mounted transversely to the propagation of the wave in the guide and defining a plurality of cavities Ki in the longitudinal direction of the guide and integral with the first portion I, and a plurality of second conductive partitions with one or more openings i defining capacitive-type irises and which form, in combination with the adjacent guide lengths of the inverters of immitance Ji, said first partitions forming a succession of resonant cavities Ki coupled by the immitance inverters Ji,

Said mobile conductive part II comprising a wall, parallel to the short side "b" of the guide, forming the fourth face of the waveguide G, said wall defining the dimension value "a" of the long side of the guide and thus the central frequency of the filter, the second part II comprising a plurality of slots receiving the partitions of part I which form the capacitive-type irises, the cavities Ki thus being formed when the part I and the part II are nested,

Means for ensuring electrical contact between the first fixed conductive portion I and the second movable conductive portion II. The capacitive type irises used to form the cavities Ki have, for example, an opening "d (x)" variable as a function of the abscissa x according to the side "a" which makes it possible to maintain the bandwidth of the filter constant when " a "varies. In one possible embodiment, the variable "d (x)" aperture as a function of the abscissa x along the long side "a" may be a linear function to give this aperture a trapezoidal shape.

In a possible embodiment, the filter comprises to ensure electrical continuity along the small side "b" movable guide a sliding metal contact spring copper alloy.

We can also ensure electrical continuity along the short side

"B" moving the guide by means of a sliding contact with conductive elastomer loaded seals.

The filter may comprise to provide electrical continuity along the short side "b" movable guide a trap bringing a short circuit to sliding contact points "C" for a selected guided wavelength.

The filter may comprise means for moving the partitions of the capacitive irises of the cavities parallel to the short side "b" of the waveguide to vary the opening "d" identically to the ends of each cavity and thus simultaneously change the value the overvoltage coefficient Q for all the cavities Ki.

The filter may also comprise means making it possible to vary the opening "d" of the capacitive irises of the cavities when the narrow adjustable side "b" of the guide moves with the movable side II using one of the two ways described above. after:

· By a separate motorized control or not, and common to all the cavities,

• by pushing the irises of the cavities upwards parallel to the small side "b" of the waveguide to increase the value of "d" when the value "a" of the long side is decreased by a compensated device in the direction reverse.

The movable partition associated with the movable conductor II side of the guide is moved mechanically parallel to itself by one or more rotary or linear motors or piezoelectric motors. Other features and advantages of the device according to the invention will appear better on reading the description which follows of an example of embodiment given by way of illustration and in no way limiting attached to the figures which represent:

FIG. 1 has an example of a cavity using an inductive obstacle septum according to the prior art, FIG. 1 b the equivalent diagram of a waveguide cavity according to the prior art, FIG. inductive iris used according to the prior art to limit such a cavity,

FIG. 2, a rectangular section waveguide bandpass filter composed of two conductive parts I and II,

FIG. 3, the manner in which the two parts I and II of FIG. 2 are nested to form the waveguide filter,

FIG. 4, a sectional view of a transverse section of the waveguide,

FIG. 5 is a sectional view of a longitudinal section of the guide filter composed of a succession of resonant cavities and immitance reversers,

FIG. 6a several examples of embodiments of capacitive irises,

FIG. 6b, an example of an iris having a trapezoidal opening making it possible to maintain the bandwidth of the filter substantially constant when the central frequency of the filter varies with "a",

FIGS. 7a, 7b and 7c, several embodiments for making a contact providing electrical continuity between the two parts I and II, and

• Figure 8, a schema recall used for the calculation of immitance inverter.

The description relates to a waveguide filter having stability in bandwidth when tuned to frequency. According to the last proposed embodiment, the bandwidth is practically insensitive to the change of frequency agreement. FIG. 2 depicts a portion of a rectangular section waveguide pass filter which comprises, for example, the following elements:

Three longitudinal conducting partitions 101, 102, 103 forming three sides of a fixed part I of the waveguide G,

• conductive partitions integral with the part I: 105-1, 105 ₂ and 105 ₄ , 105 ₅ with one or more conductive obstacles associated with complementary openings Oi in the section of the guide forming iris capacitive type (ref. ] and [7]), partitions mounted transversely to the propagation of the wave in the guide of E vers

S. The partitions 105i and 105 ₂ define the cavity (K-) 106i of length L1 and the partitions 105 ₄ 105 ₅ the cavity (K ₂ ) 106 ₂ of length L2. The partition 105 ₃ with an aperture defining a capacitive-type iris which has an opening "i" (Figure 5), and forms, with the two adjacent guide lengths L3 and L4, an immitance reverser J between the two cavities 106 ₁ and 106 ₂ ,

• a side wall 104 forming the face of II which constitutes the fourth side of the guide and which is in front of the partition 102. Part II is embedded or inserted in part I on the small side "b" of the guide , to define the value of the dimension of the long side of the guide

"a", passing through slots 108i having dimensions selected to receive the partitions of Part I which form the capacitive-type irises, and closing the guide on the fourth side 104 of the inner dimension guide "b". The reference 107 corresponds to the outer movable partition of the waveguide G when the first fixed part I and the second movable part II are nested one inside the other.

The values of the parameters "a", "b" and "d" are chosen according to the frequency of the filter and the dimensions of the guide which are functions of this frequency. In practice we have, for example, "a" ~ "b" / 2 and "d" must allow the realization of the opening.

The two conductive parts I and II are nested as described in FIG. 3. The signal propagates between the inputs E and S of the waveguide G passing through the openings Oi of height "d" capacitive irises through cavities Ki thus formed by the transverse partitions and the walls of the guide and through the openings "i" of the walls whose particular function is to perform a function of inversion of immitance.

The shape of the capacitive apertures and the dimension "d" or "i" of their opening under each transverse partition (iris) is determined to obtain the desired frequency response and selectivity for the filter (see FIG. 4).

The side of the movable portion II of the waveguide, which closes all the cavities, is manually or mechanically adjusted by means of a single adjustment to move the frequency filter in the desired band. The filter is therefore tuned throughout the band to be covered by this unique adjustment of the dimension of the "a" side. The moving conductive partition II of the guide can be moved mechanically parallel to itself by one or more rotary motors or linear or piezoelectric or other. Mechanical movement can be controlled by software. These displacement means are known to those skilled in the art and will not be represented for reasons of simplification.

Operation of the tunable waveguide filter according to the invention

In a rectangular guide of long side "a" and of small side "b", the guided wavelength "A _g " of a signal at frequency f is equal to:

A _g = 1 / (f ² / c ² -1 / (2 a) ² ) ^A ½

where "a" is the dimension of the long side of the rectangular section guide.

By varying "a", it is possible to have the same guided wavelength A _g in the filter for different frequencies f ₁ and f ₂ with sides respectively "a- and" a ₂ ":

A _g = 1 -1 / (2 a) V 2 = 1 / (f ₂ ² / c ² -1 / (2 to ₂ ) ² ) ^A ½

In cavity filters, for a rectangular guide, a cavity has a length L close to "A _g 72, and its Q load overvoltage coefficient is a function of the openings of the end irises (couplings jB) (ref. [2] ).

The overvoltage coefficients (Q) of each resonator being determined, the architecture or design of the filter is obtained by an association of resonators in series and parallel.

If two different frequencies have the same guided wavelength, "A _g ", in the guide, as the cavities keep the same length L, and if the couplings between cavities remain equal, the response of the filter is similar to the two frequencies.

According to the approximate formulas given in ref. [1], we see that the capacitive type couplings jB depend only on the height of the opening "d" and the dimension of the small side "b" which do not vary when "a" varies (see Figure 4) . For example, for an iris with a single rectangular opening of height "d" in a small side guide "b", we have:

B / Yo ~ 8b / (A _g ) * LN (csc (nd 12b))

With B admittance of the iris, Yo reference admittance, LN for Népérien logarithm.

This independence property of the value of B with respect to "a" remains valid for all capacitive iris types ([7] pages 21 8-221 or 248-255 or 404-406 depending on their shape and thickness) .

Thus, a rectangular waveguide filter with capacitively coupled cavities whose large side "a" is varied by means of a movable partition on the short side "b" and for cavities with "A _g " fixed, goes :

• have its central frequency f which varies,

• keep its couplings and Q roughly constant,

and therefore the bandwidth of the filter will remain approximately constant.

The dimensions and shape of the capacitive irises of cavities having dimensions (opening "d") achievable are determined for example in the manner described below. The "design" is obtained by fixing overvoltage values Q of the cavities Ki which make it possible to have reasonable iris openings and by coupling the cavities by means of immitance inverters. These immitance inverters of value J must also use capacitive-type irises so that their value is independent of "a" when the short side of the guide "b" is moved.

An example of an immitance inverter adapted to this application is known to those skilled in the art and in accordance with the diagram of FIG. 8 according to the example on page 63 of reference [5].

The design or architecture of the tunable filter according to the invention is obtained for example using methods known to those skilled in the art, as it is explained in [5] page 59 or in [6] page 559. By For example, the structure of a filter of order 4 is obtained by placing the four cavities (Ki) between the immitance reversers Ji:

J1 K1 J2 K2 J3 K3 J4 K4 J5

(see part of this filter in FIG. 5: L-, and L ₂ represent the lengths of the cavity portion K1 and K2, and J the inversion immitance portion)

The capacitive irises used may be thin or thick. The formulas that make it possible to calculate their respective equivalent schemes are known from the prior art, for example, [7] (pages 218-221 or 248-255 or 404-406 depending on their shape and their thickness).

Capacitive irises may include one or more transverse conductive obstacles associated with one or more corresponding, complementary openings in the section of the guide.

Figure 6a, non-limiting, represents several possible embodiments of this type of iris. For example, the conductive obstacle 51 associated with the two complementary openings 01 in the section of the waveguide forms such an iris. Similarly, the conductive obstacles 52 and 53 associated with the complementary central opening 02 in the section of the guide constitute an iris of this type. The two conductive obstacles 55 and 56 associated with the three complementary openings 03 also form a capacitive iris.

The transverse conductor obstacle 54 associated with its complementary opening 04 in the section of the guide is a capacitive iris similar to that shown in the guide of FIG. 4.

A more precise analysis of the overvoltage coefficient of a cavity shows that it is a function of:

the susceptance jB of the iris (jB ₀ has the same value at the center frequency f ₀ of the cavity whatever its value in the band to be covered when "a" varies),

the guided wavelength "A _go ", constant at the center frequency f ₀ of the cavity when "a" varies,

the length of the cavity L, constant,

the wavelength in the air at the transmitted frequency f _0, we have, as a first approximation: Q = kf ₀ ²

k (B _0, A _go, L) constant when the center frequency of the cavity varies.

Like BW ', the bandwidth of the resonator is equal to f ₀ / Q, we obtain as a first approximation:

BW '= k' / f ₀

We see that for a frequency shift of +/- 5% (for example +/- 300 MHz at 6 GHz), the bandwidth of the resonator will vary by - / + 5% because fo varies (for example - / + 1 MHz for BW = 20MHz when f ₀ varies by +/- 5%).

According to another embodiment and in order to compensate for the variation of the bandwidth BW of the filter as a function of the center frequency f ₀ , one solution consists in varying B ₀ by changing the opening "d" of the capacitive irises of the cavities .

One way is to make this iris mobile parallel to the short side of the guide while maintaining electrical contact with the fixed and movable parts forming the cavity. One possible adjustment is to move the iris partition parallel to the small side "b" of the waveguide to vary the opening "d" identically to the ends of each cavity and thus simultaneously change the value Q to all the cavities. The change of "d" in practice is small, of the order of a few tenths of a millimeter. On the other hand, it is not necessary to change the values of the openings "i" of the capacitive irises used in immitation inverter J so that these inverters retain the same value J.

The frequency response of the filter thus adjusted, and regardless of the number of poles of the filter is then exactly the same throughout the covered band and requires only two settings "a" and "d" in all for the filter.

The variation of the aperture "d" of the capacitive iris can be obtained when the narrow adjustable side "b" of the guide (partition 107) moves with the moving side II, for example, using one of two ways described below: • by a separate motorized control or not, and common to all cavities, • by pushing the irises of the cavities upwards to increase the value of "d" when the value "a" of the long side is decreased by a compensated device in the opposite direction, for example by a spring.

According to another exemplary embodiment, shown in FIG. 6b, it is possible to increase the apparent value "d" of the capacitive irises used for the resonant cavities when "a" decreases.

To obtain this result, the opening "d" has a slightly variable value along the dimension x of the large side "a". When the small side of the guide associated with the movable conductive part II moves by increasing the value of "a", the apparent opening of the iris "d (x)" decreases, which makes it possible to slightly vary the coefficient of overvoltage Q to compensate for the variation of the bandwidth of the filter BW when f ₀ varies.

An approximate form of the opening is for example obtained from the calculation of "d (x)" at the two extreme points in frequency of the band to be covered by f ₀ . This shape of the iris shown in FIG. 6b is then a rectangle trapezium whose large base 20 is on the wall "b" of the fixed part I of the guide, the smallest side 21 being on the opening side receiving mobile part II.

By increasing the number of calculation points in the frequency band covered by the filter when its center frequency f ₀ is varied while keeping its bandwidth BW constant or substantially constant, a more precise form is obtained for the opening of the filter. (x) "as a function of the abscissa x along the long side" a ".

Spurious responses

The parasitic responses of the filter, close to the cut-off frequency of the guide which is a function of "a", are suppressed, for example by putting in series with the tunable filter a suitable length of guide under the cut at these frequencies.

Sliding contacts

The tunable filter according to the invention can use at least three types of sliding contacts C to ensure electrical continuity along the small moving side of the guide. The first possibility is to use a copper alloy spring metal part 30 fixed to the movable partition and providing a spring action to maintain the relative position of the movable partition and conductive walls (see Fig. 7a).

The second uses a sliding contact to (see Figure 7b). The movable partition has one or more grooves along the movable partition in which conductive elastomeric seals 32 make it possible to maintain the ohmic contact. The third solution is to ensure the contact according to the trap technique used to ensure good electrical continuity at the junction between the guides (see ref. [3]). It consists in bringing back by means of a trap (33) a short circuit at the sliding contact points ("C") for a selected guided wavelength (see FIG. 7c). The trap consists of the complete cut schematized by hatching. This solution seems interesting taking into account that "A _go " is constant in the guide when "a" varies, for any central frequency f ₀ of the filter.

Thus, a rectangular waveguide filter with capacitively coupled cavities whose large side "a" is varied by means of a movable partition on the short side "b" (and for cavities with "A _g " fixed ), goes :

• have a central frequency f that varies

• keep its couplings and Q nearly constant, and

• so the bandwidth of the filter will remain approximately constant.

This is not the case for filters using inductive iris or septum for which the equivalent admittances at the end of cavities jB depend directly on the large width of the guide "a" and vary considerably when "a" varies. References

[1] "Design of tunable resonant cavities with constant bandwidth" L.D.Smullin Technical Report No. 106 RLE / MIT 1949.

[2] "Maximally flat filters in waveguide", W.W.Mumford, BSTJ October 1948, p 684-713.

[3] "Circuits for ultrashort waves" E.Roubine ESE 1966. [4] US 5,808,528.

[5] "Microstrip Filters for RF / Microwave Applications" Jia-Sheng Hong and M.J.Lancaster, John Wiley 2001.

[6] "Handbook of Filter Synthesis" Anatol I Zverev, Wiley-Interscience.

[7] Marcuvitz's "Waveguide Handbook", Radiation Laboratory Series No. 10, McGraw-Hill, 1951.

Claims

1 - Frequency-tunable bandpass microwave filter comprising in combination at least the following elements:

A waveguide G of rectangular section comprising a first fixed conductive part I and a second movable conductive part II,

Said first fixed part I comprising three longitudinal conducting partitions (101), (102), (103) forming three sides of the waveguide G, the section of the waveguide having a large side "a" defined by the position of the moving part II when inserted in part I and a small side "b",

Said first part I comprising several first conductive partitions (105-1, 105 ₂ and 105 ₄ , 105 ₅ ) constituted by one or more conductive obstacles which associated with complementary openings Oi in the section of the waveguide form irises of capacitive type, said first partitions being mounted transversely to the propagation of the wave in the guide and defining several cavities Ki (106i, 106 ₂ ) in the longitudinal direction of the guide and integral with the first portion I and several second conductive partitions (105 ₃ ) with one or more apertures "i" defining capacitive-type irises and which, together with the adjacent guide lengths (L3, L4) of the immitance reversers Ji, form said first partitions (105-1, 105 ₂ and 105 ₄ , 105 ₅ ) forming a succession of resonant cavities Ki coupled together by the immitance inverters Ji,

Said second movable conductive part II comprising a wall (104), parallel to the small side "b" of the guide, forming the fourth face of the waveguide G, said wall (104) defining the dimension value "a" of the long side of the guide, and thus the central frequency of the filter, the mobile part II comprising a plurality of slots (108i) receiving the partitions of Part I which form the capacitive-type irises, the cavities Ki thus being formed when Part I and Part II are nested,

Means (30, 32, 33) for providing electrical contact between the first fixed conductive portion I and the second movable conductive portion II. 2 - tunable filter according to claim 1 characterized in that the capacitive type irises (105-i, 105 ₂ and 105 ₄ , 105 ₅ ) used to form the cavities Ki have an opening of dimension "d (x)" according to " b "variable according to the abscissa x according to the side" a "which makes it possible to maintain the bandwidth of the filter constant when" a "varies.

3 - tunable filter according to claim 2 characterized in that the opening of dimension "d (x)" according to "b" variable depending on the abscissa x along the long side "a" is a linear function to give this opening a trapezoidal shape.

4 - tunable filter according to claim 1 characterized in that it comprises a metal piece (30) fixed on the movable partition and providing a spring action to maintain the relative position of the movable clolison and conductive walls.

5 - tunable filter according to claim 1 characterized in that said movable partition has one or more grooves along the movable partition in which conductive seals (32) charged elastomer to maintain the ohmic contact.

6 - tunable filter according to claim 1 characterized in that it comprises to ensure the electrical continuity between the part I and the part along the short side "b" mobile guide a trap (33) bringing a short circuit points sliding contact "C" for a chosen guided wavelength.

7 - tunable filter according to claim 1 characterized in that it comprises means for moving the partitions capacitive irises cavities parallel to the small side "b" of the waveguide to vary the opening "d" in the same way at the ends of each cavity and change thus simultaneously the value of the overvoltage coefficient Q for all the cavities Ki.

8 - tunable filter according to claim 1 characterized in that it comprises means for varying the opening "d" capacitive irises of the cavities when the narrow adjustable side "b" of the guide moves with the movable side II in using one of the 2 ways described below:

• by a separate motorized control or not, and common to all the cavities,

"By pushing the irises of the cavities upwards parallel to the small side" b "of the guide to increase the value of" d "when the value" a "of the long side is decreased by a compensating device in the opposite direction. 9 - tunable filter according to claim 1 characterized in that the movable partition (107) associated with the movable side II of the guide is moved mechanically parallel to itself by one or more rotary motors or linear or electric piezo.