EP1689019A1 - Balanced coaxial filter - Google Patents

Abstract

The filter has an inner conductor (1) coupled to another inner conductor (2) on the same axis. A third inner conductor coupled to the inner conductors (1, 2) is axially displaceable with respect to the inner conductors. Series capacitances are formed between the inner conductors in a coaxial technique without any electrically conductive contacts. Engaging regions of the conductors are separated by an insulating sleeve.

Description

The invention relates to a coaxial tunable filter, comprising at least one stage which comprises an outer conductor, a first inner conductor and a second inner conductor, which is capacitively coupled to the latter and has the same axis.

Although the coils and capacitors for filters for frequencies above a few hundred MHz can be simulated by line pieces of suitable length and with corresponding characteristic impedances. While inductances with the respectively required value can be realized relatively easily by means of short line sections with high characteristic impedances, it is virtually impossible in terms of production technology to maintain the capacitance values of a few pF sufficiently accurately by capacitively coupled internal conductors. Production-related tolerances of the capacitance values, however, considerably impair the frequency response. 1 illustrates the diagram in FIG. 2. Curve 1 shows the frequency-dependent profile of the reflection factor for the nominal values of the inductances and the capacitances. Curve 2 shows the course of the reflection factor when the capacitance values deviate by 10% from the setpoint. Because of this difficulty, multicircular filters are usually produced in stripline technology and with adjustable trimming capacitors. For insertion eg in a coaxial antenna feed cable corresponding transitions are therefore required at the input and the output.

Although a single-stage filter in coaxial design according to the aforementioned type is also known. The facing surfaces of the first and second inner conductors are dimensioned to form a longitudinal capacitance of a predetermined value, optionally in conjunction with a dielectric other than air. If this value is to be adjustable, one of the inner conductors may be formed telescopically. The displaceable part of this inner conductor is electroplated with its fixed part, e.g. via spring blades, contacted. However, at such contact points, intermodulation products arise, which, given high intermodulation distance requirements, e.g. in mobile communications are not tolerable.

The invention has for its object to provide a contrast manufacturing simpler, coaxial filter that can be adjusted, but does not give rise to intermodulation products.

This object is achieved by a third inner conductor, which is capacitively coupled to both the first and the second inner conductor and axially displaceable.

The filter thus has tunable longitudinal capacitances in coaxial technology, but does not require galvanic contacts and is therefore intermodulation-free.

In a first embodiment, the first inner conductor and the second inner conductor are surrounded by an insulating sleeve in the region of their capacitive coupling, and the third inner conductor is designed as a metal sleeve which is axially displaceable on the insulating sleeve.

The first and the second inner conductor can telescopically mesh in the region of their capacitive coupling, but remain galvanically separated from one another.

The telescopically engaging portions of the first and second inner conductors may be separated by an insulating sleeve. This has the advantage that it is possible to influence the overall length by choosing an insulating material with a suitable relative dielectric constant.

In another embodiment, the first inner conductor and the second inner conductor are at least tubular in area of their capacitive coupling and enclose a common insulating sleeve in which the third inner conductor is axially displaceable, e.g. via a longitudinal slot in the first and / or the second inner conductor and the insulating sleeve.

In addition, the first and the second inner conductor may be axially displaceable relative to each other.

In order to carry out the adjustment, the outer conductor (with a circular or polyonal inner cross-section) can be divided longitudinally or, for example, in the longitudinal direction. be equipped with a removable lid. Instead, the outer conductor can also be provided at the adjustment points with a closable opening.

In the matching position found, the third inner conductor can be fixed with any suitable HF-compatible means, for example by gluing or by PTFE rings.

It has been shown that it is usually sufficient in series production, the adjustment to a sampling of the filter and transfer the found positions of the inner conductors to the other filters of the same series without renewed electrical adjustment.

Fig. 3

ein erstes Ausführungsbeispiel,

Fig. 4

das gleiche Ausführungsbeispiel mit eingezeichneten Längskapazitäten und Ersatzschaltbild zu Fig. 3,

Fig. 5

einen Ausschnitt aus einer Weiterbildung der Ausführungsform gemäß Fig. 3,

Fig. 6

ein zweites Ausführungsbeispiel,

Fig. 7

das gleiche Ausführungsbeispiel wie Fig. 6, ergänzt um die Längskapazitäten und das Ersatzschaltbild.

Simplified and in particular only single-stage embodiments of the filter according to the invention are shown in the further figures in partial longitudinal sections. It shows:

Fig. 3

a first embodiment,

Fig. 4

the same embodiment with drawn longitudinal capacitances and equivalent circuit diagram to Fig. 3,

Fig. 5

a detail of a development of the embodiment of FIG. 3,

Fig. 6

a second embodiment,

Fig. 7

the same embodiment as FIG. 6, supplemented by the longitudinal capacitances and the equivalent circuit diagram.

In FIG. 3, a first inner conductor 1 and a second inner conductor 2 are arranged in an outer conductor 5. The end face of the inner conductor 1 faces that of the inner conductor 2 at a given distance. On a calculated length, the inner conductors have 1 and 2 sections 1.1 and 2.1 with a reduced diameter. Sections 1.1 and 2.1 are seated in a common insulating sleeve 4. The insulating sleeve 4 is surrounded by a third, tubular inner conductor 3, which is axially displaceable by a distance a in the direction of the inner conductor 1 or the inner conductor 2. The inner conductors 1 and 2 including their sections 1.1 and 2.1 and the inner conductor 3 act in conjunction with the outer conductor. 5 as inductances whose values can be calculated, for example, by means of the commercial software APLAC from the corresponding diameters and lengths.

worin C den Kapazitätswert, A die Fläche, d den Abstand, ∈₀ die absolute Dielektrizitätskonstante und ∈_r die relative Dielektrizitätskonstante bedeuten. Der Wert der Gesamtlängskapazität beträgt dann $${\mathrm{C}}_{\mathrm{ges}}={\mathrm{C}}_0+\frac{{\mathrm{C}}_1\cdot {\mathrm{C}}_2}{{\mathrm{C}}_1+{\mathrm{C}}_2}$$

und ist somit von der Position des Innenleiters 3 relativ zu dem Innenleiter 1 und dem Innenleiter 2 abhängig, weil die Fläche A in (1) proportional zu L1 bzw. L2 ist. Daraus folgt auch, dass die Reihenschaltung von C1 und C2 ihren maximalen Kapazitätswert hat, wenn L1 = L2 ist.4 illustrates the longitudinal capacitances between the galvanically separated inner conductors and the corresponding block diagram. Neglecting the marginal capacities, the respective capacity values result from the known formula $$C={\mathrm{<figure-callout\; id="0"\; label="\epsilon "\; filenames="imgf0001.png"\; state="\{\{state\}\}">\epsilon </figure-callout>}}_0\cdot {\epsilon }_r\cdot \left(\frac{A}{d}\right)$$

where C is the capacitance value, A the area, d the distance, ∈ ₀ the absolute dielectric constant and ∈ _r the relative dielectric constant. The value of the total longitudinal capacity is then $${\mathrm{C}}_{\mathrm{ges}}={\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}}_0+\frac{{\mathrm{<figure-callout\; id="1"\; label="C"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">C</figure-callout>}}_1\cdot {\mathrm{C}}_2}{{\mathrm{<figure-callout\; id="1"\; label="C"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">C</figure-callout>}}_1+{\mathrm{<figure-callout\; id="2"\; label="C"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">C</figure-callout>}}_2}$$

and thus depends on the position of the inner conductor 3 relative to the inner conductor 1 and the inner conductor 2, because the area A in (1) is proportional to L1 and L2, respectively. It also follows that the series connection of C1 and C2 has their maximum capacitance value when L1 = L2.

Fig. 5 shows a modification of the embodiment according to Figures 3 and 4. The sections 1.1 and 2.1 of the inner conductors 1 and 2 are no longer with their end faces in a predetermined distance opposite but engage telescopically in one another. Between the section 1.1 of the inner conductor 1 and the section 2.1 of the inner conductor 2, a Isolierstoffbüchse 6 is arranged. Both the immersion depth of the inner conductor section 1.1 as well as the thickness and the material of the insulating material 6, the value of C0 can be influenced. The fine adjustment takes place as in the case of FIG. 3 by displacing the third inner conductor Third

Fig. 6 shows another embodiment based on the same principle. The inner conductors 1 and 2 each end in tubular or hollow cylindrical sections 1.2 and 2.2. These sections enclose a common insulating sleeve 4.1. In this, a third inner conductor 3.1 is disposed axially displaceable by a distance a. In the region of their opposite end edges, the sections 1.2 and 2.2 of the inner conductors 1 and 2 and the insulating sleeve 4.1 have a common longitudinal slot S, via a transverse bore 3.1.1 of the third inner conductor 3 is accessible to the latter by means of a suitable tool for the purpose of balancing to move axially.

Fig. 7 shows the corresponding longitudinal capacitances in connection with the equivalent circuit diagram.

The value of the total longitudinal capacity between the inner conductor 1 and the inner conductor 2 is consequently $$C_{\mathit{ges}}={\mathrm{<figure-callout\; id="0"\; label="C"\; filenames="imgf0001.png"\; state="\{\{state\}\}">C</figure-callout>}}_0+\frac{\left(C_{01}+C_1\right)\cdot \left(C_{02}+C_2\right)}{C_{01}+C_{02}+{\mathrm{<figure-callout\; id="1"\; label="C"\; filenames="imgf0002.png,imgf0003.png"\; state="\{\{state\}\}">C</figure-callout>}}_1+{\mathrm{<figure-callout\; id="2"\; label="C"\; filenames="imgf0001.png,imgf0002.png"\; state="\{\{state\}\}">C</figure-callout>}}_2};$$

Claims (6)

Coaxial tunable filter comprising at least one stage comprising an outer conductor (5), a first inner conductor (1) and a second inner conductor (2) capacitively coupled to the latter, characterized by a third inner conductor (3, 3.1) is capacitively coupled to both the first and the second inner conductor and axially displaceable. Filter according to claim 1, characterized in that the first inner conductor (1) and the second inner conductor (2) in the region of their capacitive coupling of a Isolierstoffhülse (4) are surrounded and that the third inner conductor as on the Isolierstoffhülse (4) axially displaceable metal sleeve (3) is formed. Filter according to claim 1 or 2, characterized in that the first and the second inner conductor (1, 2) telescopically engage in the region of their capacitive coupling. Filter according to claim 3, characterized in that the telescopically intermeshing regions of the first and the second inner conductor are separated from each other by a Isolierstoffbuchse. Filter according to claim 1, characterized in that the first inner conductor (1) and the second inner conductor (2) at least in the region of their capacitive coupling tubular (1.2, 2.2) are formed and a common insulating sleeve (4.1 enclose, in which the third inner conductor ( 3.1) is axially displaceable. Filter according to one of claims 1 to 5, characterized in that the first and the second inner conductor (1, 2) are axially displaceable relative to each other.

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