EP0255068A1 - Filter for short electromagnetic waves having the shape of comb filters or interdigital filters - Google Patents

Abstract

Microwave filters which have the best electrical characteristics for small volumes are required in radio communications particularly in traffic broadcast communication links and the invention provides filters formed as comb line or interdigital line filters in which the inner resonator conductors are formed as flat spirals.

Description

The invention relates to a type of comb line or. Interdigital line filter designed filter according to the preamble of claim 1.

Filters of the aforementioned type are from the literature reference "Band-Pass and Band-Stop Microwave Filters using λ / 4 Circular Cylindrical Real Resonators", Fujitsu Scientific Technical Journal, Vol. 3, pp. 29 to 52, (authors Dy Juhio Ito, Takeshi Meguro).

With mobile radio, directional radio and satellite radio, i.a. Transceivers and ZF bandpasses with high selectivity and low losses required.

In addition to the demand for high resonator quality, especially with mobile radio, such as for car phones, small volume, light weight and inexpensive manufacturing processes for mass production.

So far, such filters have been used with helix resonators according to the literature reference BK Dube "The Design of Filters Using Helical Resonators in VHF-Band, J. Instn. Electronics Telecom. Engrs., Vol. 22, No. 2, 1976, pp. 77 bis 79 ". or constructed with resonators in the form of metal rods, for example as comb or interdigital filters in accordance with the literature reference mentioned at the beginning, ceramic as a dielectric, for example according to US Pat. No. 4,431,977, is used, which reduces the metal rod length and the volume by the factor √ε, if ε is the dielectric constant of the ceramic. Filters are also known in which planar spiral coils with discrete capacitors are added to series circuits on ceramic substrates and interconnected to form a bandpass filter. In this technology, neither high resonator qualities nor cost-effective production are achieved.

Helix filters also require a relatively large production outlay and many individual parts. The filters with air dielectric built with metal rods are voluminous, those with ceramic dielectric are relatively heavy, which is particularly undesirable in portable devices.

The invention has for its object to provide implementation options of filters in the manner of comb line or interdigital line filters which have high-quality electrical properties and which can be produced as inexpensively as possible in a small size.

This object is achieved according to the invention for filters according to the preamble of patent claim 1 according to the characterizing part of patent claim 1.

Advantageous refinements are specified in the subclaims.

The invention is explained in more detail below on the basis of exemplary embodiments.

• Fig. 1a die Draufsicht auf ein bekanntes Filter das als Kammleitungsfilter ausgebildet ist,
• Fig. 1b das Filter nach Fig. 1a im Aufriß,
• Fig. 2a ein Spiralresonatorfilter mit vier planaren Resonatoren,
• Fig. 2b den Aufriß und den Seitenriß eines Filters nach Fig. 2a,
• Fig. 3 ein vereinfachtes Ersatzschaltbild eines Filters nach Fig. 2 mit vier Schwingkreisen,
• Fig. 4a die Draufsicht und den Seitenriß eines Spiral­resonatorfilters mit vier planaren Resonatoren auf einem Trägermaterial T mit einer Überkopp­lung Ü,
• Fig. 4b den Aufriß eines Filters nach Fig. 4a,
• Fig. 5 den Aufriß und den Seitenriß eines Spiralreso­natorfilters mit vier planaren Resonatoren auf einer doppelt-kaschierten Leiterplatte L,
• Fig. 6 ein vereinfachtes elektrisches Ersatzschaltbild der Filter nach den Fig. 4a, 4b und 5,
• Fig. 7 eine fünfkreisige Spiralresonatoranordnung in rechteckiger Ausführung der Spiralen,
• Fig. 8 den Aufriß und die Seitenansicht eines fünfkrei­sigen Spiralresonatorfilters, dessen Resonatoren gegenüber den Fig. 2 bis 7 um 90° gedreht sind,
• Fig. 9 ein fünfkreisiges Spiralresonatorfilter mit 90° gedrehten Einzelresonatoren und einer Innen-Mas­sung M der Spiralen,
• Fig. 10 eine vierkreisige Spiralresonatoranordnung mit planaren Einzelresonatoren und einer Innenmas­sung der einzelnen Resonatoren,
• Fig. 11 die Betriebsdämpfung a_B und die Reflexions­dämpfung a_r eines Vierkreisfilters nach Fig. 4a, b als Funktion der Frequenz f.

It show in the drawing

• 1a the top view of a known filter which is designed as a comb line filter,
• 1b the filter of FIG. 1a in elevation,
• 2a shows a spiral resonator filter with four planar resonators,
• 2b shows the elevation and the side elevation of a filter according to Fig. 2a,
• 3 shows a simplified equivalent circuit diagram of a filter according to FIG. 2 with four resonant circuits,
• 4a the top view and the side view of a spiral resonator filter with four planar resonators on a carrier material T with a coupling U,
• 4b shows the elevation of a filter according to Fig. 4a,
• 5 shows the elevation and the side elevation of a spiral resonator filter with four planar resonators on a double-laminated circuit board L,
• 6 shows a simplified electrical equivalent circuit diagram of the filter according to FIGS. 4a, 4b and 5,
• 7 shows a five-circle spiral resonator arrangement in a rectangular design of the spirals,
• 8 shows the elevation and the side view of a five-circuit spiral resonator filter, the resonators of which are rotated by 90 ° with respect to FIGS. 2 to 7,
• 9 shows a five-circuit spiral resonator filter with 90 ° rotated individual resonators and an internal dimension M of the spirals,
• 10 shows a four-circuit spiral resonator arrangement with planar individual resonators and an internal ground of the individual resonators,
• 11 shows the operational attenuation a _B and the reflection attenuation a _{r of} a four-circuit filter according to FIG. 4a, b as a function of the frequency f.

In the exemplary embodiment of FIG. 1, the state of the art is shown again for rapid understanding, as is given, for example, in the above-mentioned literature reference "Fujitsu Scientific Technical Journal, Vol. 4, No. 3, pages 29 to 52". As an example, a comb line filter is shown, which with the so-called ten interdigital filters are known to have the same effect. In the case of the comb line filter, the inner conductors are arranged in the manner of a comb and open out on the same housing surface, while in the interdigital filter the inner conductors alternately open out on opposite housing surfaces. In the example of Fig. 1a and Fig. 1b, four resonators R₁ to R₄ are provided. They have approximately the length λ / 4. The resonators R₁ to R₄ are arranged in the housing G and on their faces the capacitances CV₁ to CV an can be seen, which can either actually be switched or which also symbolically represent the stray capacitances of the inner conductors R₁ to R₄. The resonators R₁ to R₄ have the diameter d. At the first resonator R₁ opens an input line E, which is usually designed as a coaxial line. The inner conductor of this coaxial line is firmly connected to the resonator R 1, the outer conductor is firmly connected to the housing G. Correspondingly, the output line A can be seen on the resonator R₄, the inner conductor of which is connected to the resonator R₄, while the outer conductor is also connected to the housing G. It can also be seen from the reference numerals K₁, K₂ and K₃ that the coupling between the resonators acts as a line coupling, as is the case with interdigital filters.

However, this type of filter implementation has the disadvantage that it takes up a relatively large amount of space and may also be relatively difficult.

In the embodiment of FIGS. 2a and 2b spiral resonators SpR₁ to SpR₄ are now used, which are designed as flat, flat spirals and which are also housed in the housing G. Between these spirals there is a line coupling K₁ K₂ and K₃. The input line E and the output line A can also be seen. The elevation of Fig. 2b shows that there Tuning screws A₁ to A₄ are provided, which in the special embodiment are perpendicular to the planes of the spirals and their longitudinal axis goes approximately through the center of the spirals.

3 shows the electrical equivalent circuit diagram, which therefore contains four resonant circuits 1, 2, 3 and 4. Input E and output A are shown as tapped coils in order to symbolically represent the transformer effect of the tapping.

The main advantage of planar spiral resonators, however, is that the entire resonator set of a filter can be manufactured precisely and inexpensively using punching, form-etching or casting technology, as well as on laminated circuit boards, which is basically not possible, for example, with filters with helical resonators. All design methods for line filters (e.g. Fujitsu Scientific Technical Journal, Vol. 4 No. 3, pp. 29 to 52) can be used to design, whereby the coupling distance K₁-K₃ between the spirals depends on the chosen spiral shape and the winding sense and experimental must be determined. A slight shortening of the spiral length compared to an extended resonator is also necessary because of the additional capacitance C _w occurring between the spiral windings.

Fig. 2 shows an undivided filter lying between input E and output A with an etched or punched or spark-eroded compact resonator set SpR₁-SpR₄, installed in a housing G and surrounded by a dielectric D₁, which is here, for example, air. Frequency tuning is possible with the screws A₁-A₄. The already explained FIG. 3 shows the simplified equivalent circuit with four resonant circuits.

4a, 4b and 5 show further advantageous embodiments. In these exemplary embodiments, parts having the same effect are also designated with the same reference notes as in the previous figures, so that there is no longer any need to go into them in detail. 4a, 4b and 5 are shown in elevation, in side view and in Fig. 4a also the top view spiral resonator filter with a coupling Ü₁ or Ü₂. The associated electrical equivalent circuit diagram is drawn in FIG. 6. The overcoupling U₁ leads from the input E to a connection point S₁, the overcoupling Ü₂ which is shown as an example and which is not realized in the exemplary embodiment - leads from a connection point S₂ to the output A. If such overcouplings do not lead directly from the input to the first resonator SpR₁ or analogous to this, overcoupling Ü₂ does not lead directly to output A, then, as is known, such measures can produce damping poles in the filter characteristic. In particular, in the embodiment of FIG. 5 two sets of resonators SpR₁ to SpR₄ are connected in parallel. The two sets of resonators have the same geometry and the parallel connection of the individual conductor parts reduces the losses and thus increases the quality of the resonators. In Fig. 6, the individual resonators are again labeled 1 to 4, the associated inductors with L₁ to L₄ and the associated capacitances C₁ to C₄. The coupling-in capacitance is denoted by C _K1 and the coupling-out capacitance by C _K2 . Between the individual resonance circuits there are inductors in the longitudinal branch of the circuit, which are also identified by L _K1 and L _K2 . A capacitive overcoupling C _ü , which is connected from the input to the resonant circuit 2, illustrates the effect of the overcoupling Ü₁.

In the exemplary embodiment in FIG. 4, the complete set of resonators to avoid mechanical vibrations was additionally installed in the housing G on a low-loss, for example, Teflon carrier plate T. In the support plate T are also holes for the tuning elements A₁ to A₄ and the coupling bases S₁ and S₂ incorporated.

ersieht man eine endliche Polstelle, die durch die Überkopplung C_ü (Fig. 6) bzw. Ü₁ realisiert wird.In FIG. 5, the resonator set was implemented as an example on a double-laminated, low-loss printed circuit board L. Depending on the type of dielectric used, this solution is expected to have a lower quality than that of a pure air dielectric.
The equivalent circuit diagram for the implementations according to FIGS. 4 and 5 is shown in FIG. 6. You can see some other advantageous details. From the characteristic function belonging to FIG. 6

you can see a finite pole, which is realized by the coupling C _ü (Fig. 6) or Ü₁.

Another pole would be possible, for example, by coupling Ü₂ from SpR₄ to SpR₃ (Fig. 4).
When designing filters made of λ / 4 resonators, for example with an air dielectric, the following points should be considered.

The conductor length of the spiral including the effect of a shortening factor is λ / 4. The corresponding frequency is related to the center of the pass band.

The characteristic impedance Z is expediently chosen to be 50 ... 150. With a rectangular cross section of the conductor, Z is known to depend on the conductor width and thickness and on its distance from the metal housing and can be calculated using known methods such as in the strip-line technique.

The resonator qualities depend essentially on the nature and conductivity of the surface and the filter volume. Two resonator arrangements of the same geometry (according to FIG. 5) which are approximately parallel at a distance from the conductor width bring quality improvements of up to 30%.

7 to 10 further possible design variants are only shown schematically, since the mode of operation has already been described in the foregoing.

For example, the geometry of the resonators need not be limited to spirals with a continuous course. If necessary, the resonators can also be realized in a rectangular shape as shown in FIG. 7 or with a different line cross section - adapted to the current occupancy of the resonator. Likewise, a 90 ° rotation of the spirals SpR₁ to SpR₅, as shown in Fig. 8 or Fig. 9, is possible. The spiral center points M as in FIGS. 9 and 10 can also be selected as the common base point of the spirals. In the example of FIG. 10, a carrier plate 6 is used for receiving the ground connections M and the resonators SpR₁ to SpR₄.

FIG. 11 shows the measured curve of the operating attenuation a _B and the reflection attenuation a _r as a function of the frequency f of a filter according to FIG. 4 implemented at 900 MHz. The pass band lies approximately between 935 MHz and 970 MHz. In the low-frequency blocking range, that is to say around 910 MHz, a damping pole of the operating damping a _b occurs, so that it can be seen that the operating damping curve can be increased at any time.

In addition, the filters described above, especially in the frequency range of traffic radio, require a relatively small volume with good electrical properties. The resonators designed as spiral resonators have a shortening of the electri length, which is particularly advantageous in mobile systems.

Claims (10)

1. In the manner of comb line or interdigital line filters designed filter for short electromagnetic waves, in which the resonators are arranged such that their coupling acts as a line coupling (K₁ ... K₃),
characterized,
that the inner conductor of the resonators (R₁ ... R₄) is designed as a flat spiral (SpR₁ ... SpR₄) (Fig. 2). 2. Filter according to claim 1,
characterized,
that tuning elements (A₁ ... A₄) are provided which plunge into the field space of the spiral resonators (SpR₁ ... SpR₄). 3. Filter according to claim 2,
characterized,
that the tuning elements (A₁ ... A₄) are designed as tuning screws, the longitudinal axis of which is perpendicular to the plane of the spiral resonators (SpR₁ ... SpR₄) and penetrates the spiral approximately in the center. 4. Filter according to one of the preceding claims,
characterized,
that the shape of the spiral (SpR₁) deviates from the continuous shape. 5. Filter according to claim 5,
characterized,
that the spiral (SpR₁) is reproduced as a rectangular line. 6. Filtering according to one of the preceding claims,
characterized,
that the conductor cross-section of the spiral (SpR₁) changes continuously or suddenly. 7. Filter according to one of the preceding claims,
characterized,
that the spiral resonators (SpR₁ ... SpR₄) are arranged such that the planes formed by the spirals are in the same plane (Fig. 2). 8. Filter according to one of claims 1 to 6,
characterized,
that the spiral resonators (SpR₁ ... SpR₅) are arranged such that the planes formed by the spirals run parallel to each other (Fig. 8). 9. Filter according to one of the preceding claims,
characterized,
that the coupling (E) or the coupling (A) is designed such that at least one resonator (SpR₁) is bridged (Fig. 4). 10. Filter according to one of the preceding claims,
characterized,
that two resonator sets (SpR₁ ... SpR₄) of the same geometry are connected in parallel (Fig. 5).

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