EP0281773B1 - Adjustable microwave filter - Google Patents

Description

The present invention relates to a tunable filter, in particular in the band from 800 to 1800 MHz.

In applications such as TVRO ("Television Reception Only"), DATARO ("Data Reception Only") ... the signal received at the antenna of the earth station is in the bands 10.95-11.7 GHz or 12.25-12.75 GHz or 11.7-12.5 GHz etc ... A first stage transposes these bands to the RF (radio frequency) bands 950-1700 MHz, 950-1450 MHz, 950-1750 MHz, respectively.

A second transposition brings these frequencies around an intermediate frequency, or IF, of a few hundred MHz, for example 450 MHz.

Each channel in the radio frequency band, or RF, is a few tens of MHz wide. The role of the tunable filter is to transmit only the channel chosen in the total RF input band. Thus the rejection of the local oscillator frequencies, or OL, of the last transposition, of the image frequencies and of the mixing products are clearly improved.

The document EP-0138438 describes a tunable microwave filter which comprises two branches of the same type which, at each end, are connected to ground and which each comprise, connected in series, a fixed inductance, a first adjustable inductance, a first diode with variable capacitance, a filter for high frequencies formed by a series resonance circuit comprising a second diode with variable capacitance and a second variable inductance, and a coupling element comprising a series inductance between the input point and the output point of the filter.

Due to the existence of adjustable inductors, such a filter is not inexpensive.

The object of the invention is to overcome this drawback.

To this end, it proposes a tunable microwave filter comprising two branches of the same type each connected at the two ends to ground and each comprising, connected in series: the second end, grounded, a first and a second diode with variable capacity, a connection line, and the first end, grounded, the diodes being controlled by a DC signal, said filter being arranged on the first face of a planar substrate, characterized in that the line of connection is a microstrip line, and in that the microwave input and output are connected to the common points between the microstrip line and the second variable capacity diode of the first and second branch respectively by a symmetrical adapter
Such a filter has the great advantage of being able to be easily produced in a reproducible manner and therefore of allowing automatic mounting. The cost price of such a filter is therefore very low; it can therefore be used in the case of reception for satellite links, intended for a large audience.

Advantageously, the filter of the invention is produced on a substrate, the second face of which is completely metallized and forms a ground plane, the ground points being produced by metallized holes.

et par la résistance du filtre RC, et à l'extrémité à la masse de la première diode à capacité variable par la capacité du filtre RC; une seconde ligne microruban, reliant les deux branches à la source de tension continue, telle que la longueur totale des première et seconde ligne soit $\text{(2k' + 1)λ/4}$

; k et k' étant des entiers quelconques.More precisely, the filter of the invention is such that the DC voltage supply for each branch is produced by means of an RC filter, this DC voltage supply being connected to the common point of the two variable capacity diodes by a first microstrip line in length $\text{(2k + 1) λ / 4}$

and by the resistance of the RC filter, and at the ground end of the first variable capacitance diode by the capacity of the RC filter; a second microstrip line, connecting the two branches to the DC voltage source, such that the total length of the first and second line is $\text{(2k '+ 1) λ / 4}$

; k and k 'being any integers.

Advantageously, the variable capacitance diodes are arranged in holes drilled in the substrate; which allows for great positioning accuracy.

The characteristics and advantages of the invention will become apparent from the description which follows, by way of nonlimiting example, with reference to the appended figure in which a top view of the filter according to the invention is shown.

This figure represents a substrate 8 on the first face 9 of which is the filter of the invention. This consists of two symmetrical parts or branches 10 (10A) with respect to an axis △ located on the face 9 of this substrate 8.

• une ligne 11 (11A) de direction parallèle à l'axe △ réalisée par une métallisation à la surface du substrat;
• deux diodes à capacité variable, ou varicaps, 12, 13 (12A, 13A) en série, disposées entre la ligne 11 (11A) et la masse 24;
• une ligne 14 (14A) de longueur $\text{(2k + 1)λ/4}$
  
  , λ étant la longueur d'onde du signal hyperfréquence considéré, relié à une source V de courant continu qui permet de polariser inversement les deux varicaps 12, 13 (12A, 13A) par l'intermédiaire d'une résistance 15 (15A) et d'une capacité d'isolation 16 (16A);
• une ligne 17 (17A) d'adaptation 50 Ω reliée au point commun entre la ligne 11 (11A) et la première varicap 12 (12A) par l'intermédiaire d'un adaptateur symétrique 18, 19 (18A, 19A) formé par deux stubs 18 (18A) symétriques de direction parallèle au bras principal 19 (19A).

Each part (10, 10A) of the filter of the invention comprises:

• a line 11 (11A) of direction parallel to the axis △ produced by metallization on the surface of the substrate;
• two variable capacity diodes, or varicaps, 12, 13 (12A, 13A) in series, arranged between line 11 (11A) and earth 24;
• a line 14 (14A) in length $\text{(2k + 1) λ / 4}$
  
  , λ being the wavelength of the microwave signal considered, connected to a direct current source V which makes it possible to reverse polarize the two varicaps 12, 13 (12A, 13A) via a resistor 15 (15A) and an insulation capacity 16 (16A);
• a 50 Ω adaptation line 17 (17A) connected to the common point between line 11 (11A) and the first varicap 12 (12A) via a symmetrical adapter 18, 19 (18A, 19A) formed by two symmetrical stubs 18 (18A) parallel to the main arm 19 (19A).

This adaptation line 17 (17A) constitutes in the first part (10), the input to which the microwave signal E is connected. It constitutes for the second part (10A) the output on which the microwave signal S is obtained in output of the inventive filter.

The two lines 11 and 11A of the two parts 10 and 10A of the filter of the invention are coupled together and connected at their end by a section of line 20 itself connected to the ground 22.

.The midpoint of the two varicaps 12A and 13A of the second part is connected to the DC voltage source V via a line 21 such that the length 21 + 14A is equal to $\text{(2k '+ 1) λ / 4}$

.

The substrate is completely metallized on its second face. The ground points are therefore metallized holes 22 and 24.

Varicaps 12 (12A) and 13 (13A) are arranged in positioning holes 220 (220A) and 23 (23A).

The structure of the filter of the invention is produced in microstrip technique and the substrate used is for example made of epoxy glass; which is a factor in reducing the cost price.

The filter is tuned by modifying the DC voltage V applied to varicaps 12, 13, 12A and 13A. These varicaps are for example therefore cheaper than varicaps in AsGa.

The insulation capacities 16 (16A) as well as the resistors 15 (15A) have the role of bringing to the level of the junction of the two varicaps 12 and 13 (12A and 13A) an infinite impedance for the microwave frequencies considered: these are "surface mount components" (CMS); which allows automatic serial production of the filter of the invention and therefore a reduction in the cost price.

The varicaps are housed in circular holes drilled in the substrate to allow better reproducibility of the assembly, so the lengths of the connection tails are not very variable from one assembly to another.

The filter of the invention is therefore a two-pole filter in which each resonator is composed of a line, two varicaps, and chokes due to the connection tails of the varicaps. The coupling between the two resonators takes place at the level of the lines: the coupling between the two lines 11 and 11A depends on their thickness, their spacing and their length.

By modifying the dimensions of the filter, it is therefore possible to design filters tunable in different bands, in particular 950-1750 MHz.

The coupling between resonators, by its nature, depends on the operating frequency. Due to the impedance inversions, the necessary filtering function would not be broadband, which would introduce the deterioration of the adaptation in the tuning band according to the voltage applied to the varicaps. The purpose of the adapters 18, 19 and 18A, 19A is to overcome this drawback: the elements of these symmetrical adaptations in fact make it possible to obtain a broadband adaptation which is preserved practically throughout the frequency band considered.

Varicaps can be used whose capacity can, for example, vary from 18 pF to 2pF for a voltage V varying from 0 to 24 Volts (at 500 KHz). The capacity ratio which is then 9, limits the band in which the filter is adjustable. It is possible, by increasing this ratio, to obtain filters adjustable over a wider band.

It is understood that the present invention has only been described and shown as a preferred example and that its constituent elements can be replaced by equivalent elements without, however, departing from the scope of the invention.

Claims (6)

1. A tuneable microwave filter comprising two branches of the same type each of which is connected to ground at each end (22, 24) and includes a series connection of: the second end (24) connected to ground, a first (13, 13A) and a second variable capacitance diode (12, 12A), a connection line (11, 11A) and the first end (22) connected to ground, the diodes being driven by a d.c. signal (V), said filter being disposed on the first face of a plane substrate (8), characterized in that the connection line (11, 11A) is a micro-strip line, and in that the microwave input (E) and the microwave output (S) are respectively connected via a symmetrical impedance matcher (18, 19 and 18A, 19A) to the point which is common to the micro-strip connection line (11, 11A) and the second variable capacitance diode (12, 12A) of the first and second branch respectively.
2. A filter according to claim 1, characterized in that the second face of the substrate (8) is completely metallized and constitutes a ground plane, with the ground connection points (22, 24) being in the form of plated-through holes.
3. A filter according to claim 1 or 2, characterized in that the direct voltage feed (V) to each branch is provided via an RC filter (15, 16 and 15A, 16A).
4. A filter according to claim 3, characterized in that the direct voltage feed (V) is connected to the common point of the two variable capacitance diodes (12, 13 and 12A, 13A) via a first micro-strip line (14, 14A) of length $\text{(2k+1)λ/4}$

   

   and via the resistance (15, 15A) of the RC filter, and to the grounded end (24) of the first variable capacitance diode (16, 16A) via the capacitance of the RC filter; a second micro-strip line (21) (21) connecting both branches to the direct voltage source (V), such that the total length of the first and second lines (14 or 14A and 21) is equal to $\text{(2k'+1)λ/4}$

   

   , with k and k' representing any possible integer number and λ being the wavelength of the microwave signal under consideration.
5. A filter according to claim 1, characterized in that each matcher is constituted by a central arm (19, 19A) and by two stubs (18, 18A) forming two arms parallel thereto.
6. A filter according to any preceding claim, characterized in that the variable capacitance diodes (12, 12A and 13, 13A) are disposed in holes (22, 22A and 23, 23A) drilled through the substrate in such a manner as to enable these diodes to be positioned with great accuracy.

Images