WO1990013943A1

WO1990013943A1 - A high-frequency bandpass filter

Info

Publication number: WO1990013943A1
Application number: PCT/FI1990/000121
Authority: WO
Inventors: Erkki Juvonen
Original assignee: Telenokia Oy
Priority date: 1989-05-02
Filing date: 1990-04-27
Publication date: 1990-11-15
Also published as: FI892094A; FI89991C; FI892094A0; FI89991B; AU5527290A

Abstract

The invention relates to a high-frequency bandpass filter comprising at least one parallel resonator stage formed by an inductive component (L1-L5) and a capacitive component (VD1-VD6) connected in parallel with it. The invention provides a filter which is adjustable within a wide frequency range and which can be mounted without mechanical tuning, the filter being characterized in that the inductive component is formed by a generally U-shaped microstrip pattern (1, 2, 3) on a circuit board.

Description

A high-frequency bandpass filter

The invention relates to a high-frequency band¬ pass filter comprising at least one parallel resonator stage formed by a parallel connection of an inductive component and a capacitive component.

The transmitter and receiver parts in radio devices comprise high-frequency bandpass filters having their bandpass tuned to the frequency range used by the radio device. The bandpass filter is typically formed by using a parallel resonator type filter in which the inductive components consist of coils or helix filters connected in parallel with a capacitor of suitable valve. The filter is tuned mechanically by varying the inductance of the coil or the helix resonator. Due to the high tolerance of the inductance of the coils and the helix resonator and the limited accuracy associated with their mounting on circuit boards, the subsequent tuning always has to be carried out separately for each filter circuit. Another problem associated with prior art band¬ pass filters is that a single filter covers only a relatively narrow frequency band. However, fre¬ quencies and frequency bands deviating greatly from each other are used in different countries and dif¬ ferent systems e.g. in cellular and radio telephone systems, and as a consequence different filters have to be designed and produced for the different applications. This increases the production cost in addition to which it is impossible in practice to use a cellular telephone designed for one system in any other system.

The object of the invention is to provide a high-frequency bandpass filter which avoids the problems associated with prior art filters. This is achieved by means of a high-frequency bandpass filter which according to the invention is characterized in that the inductive component is formed by a substantially U-shaped microstrip pattern formed on a circuit board.

The basic idea of the invention is that an inductive component of a very accurate value is obtained when it is formed by a microstrip pattern formed on the same circuit board on which the other components of the filter will be mounted. This in¬ ductive component is reproducible with a very high precision also in mass production, and it does not require any subsequent mechanical tuning. The microstrip pattern is a very advantageous inductive component also in view of the manufacturing tech¬ nique, because the separate components and their mounting are omitted. In a preferred embodiment of the inventionthe frequency band of the bandpass fil¬ ter is adjusted electrically by means of capacitance diodes connected in parallel with the microstrip com¬ ponent. In this way a bandpass filter adjustable within a very wide frequency band is obtained. In addition, the frequency band of the bandpass filter may be automatically adjustable in accordance with other preset frequency values of a cellular or radio telephone. By means of the bandpass filter of the invention an adjusting range as wide as 130 MHz has been obtained, which in prior art solutions would require 3 to 4 different helix filters. Accordingly, a single bandpass filter type would suffice for both a 450 MHz and a 900 MHz frequency band.

In a preferred embodiment of the invention, comprising two parallel resonator stages, the filter comprises two generally U-shaped microstrip patterns positioned side by side with each other on a circuit board. A microstrip conductor projects perpendicular¬ ly in the sideward direction from the neighbouring branches of the microstrip patterns towards the other microstrip pattern in such a way that the microstrip conductors pass close to and in parallel with each other, thus forming a capacitive coupling between the two microstrip patterns. The very low capacitance required between the parallel resonator stages can in this way be effected very simply, whereas the low capacitance is very difficult to achieve with a dis¬ crete component. Furthermore, this kind of microstrip capacitor is reproducible with a very high precision in mass production. As to the manufacturing tech¬ nique, this kind of microstrip capacitor provides the same advantages as the above-mentioned microstrip inductor.

In a further embodiment of the invention a image frequence wave trap is formed by means of a earthed conductive film located on the opposite sur- face of the circuit board.

The invention will now be described in greater detail by way of example by means of specific embodi¬ ments, referring to the attached drawing, in which

Figure 1 shows a circuit diagram of a bandpass filter of the invention, comprising two parallel resonator stages;

Figure 2 shows a circuit diagram for a bandpass filter of the invention, comprising three parallel resonator stages; and Figure 3 shows microstrip patterns realizing the inductive components 1 and 2 of Figure 1 and the coupling capacitor C6 between the resonator stages. The bandpass filter of Figure 1 comprises two paral¬ lel resonator stages, of which the first is formed by an inductor L3 connected in parallel with a series connection of two capacitance diodes VD1 and VD2. The cathodes of the capacitance diodes VD1 and VD2 are connected together, whereby the anode of the capaci¬ tance diode VD1 is connected to one end terminal of the inductor L3 and the anode of the capacitance dio¬ de VD2 to the same potential as the opposite end ter¬ minal of the inductor L3, preferably to the earth potential of the circuit. A control voltage VI deter¬ mining the capacitance of the capacitance diodes is applied to the cathodes of the capacitance diodes VD1 and VD2 through a resistor Rl. A tap terminal of the inductor L3 forms an input IN to the filter. A capa¬ citor is connected between the input IN of the filter and the anode electrode of the capacitance diode VD1. The capacitor keeps the input impedance of the filter at a substantially constant value, e.g., 50 Ohm, within a wide frequency range. Correspondingly, the second parallel resonator stage of the filter is formed by an inductor 4 connected in parallel with a series connection of capacitance diodes VD3 and VD4. The cathodes of the capacitance diodes VD3 and VD4 are also in this case connected to each other and through a serial resistor R2 to the control voltage VI, which determines the capacitance of the capacitance diodes VD3 and VD4. A tap terminal of the inductor L4 forms an output OUT in the filter. A ca¬ pacitor C5 is connected between the output OUT and the anode of the capacitance diode VD3. The capacitor keeps the output impedance of the bandpass filter at a substantially constant value, preferably at 50 Ohm, within^" a wide frequency range. A connection point between the inductor 3 of the first parallel resonance circuit and the anode of the capacitance diode VD1 is connected by means of a decoupling capa- citor C6 to a connection point between the inductor L4 of the second parallel resonance circuit and the capacitance diode VD3. The capacitance of the capa¬ citor C6 is very low, e.g., of the order of 0.2 pF.

According to the invention, the inductors L3 and L4 are formed by microstrip patterns 1 and 2, respectively, wherefore they are indicated with dia¬ gonal lines in Figure 1. Correspondingly, the de¬ coupling capacitor C6 is also formed by microstrip conductors extending apart from each other. The microstrip conductors are indicated with darkened areas 4 and 5 in Figure 1. This kind of realization of the capacitor C6 is advantageous both in view of the manufacturing technique and the resolution of the filter as the microstrip patterns 4 and 5 on the cir- cuit board have good reproducibility in mass prod¬ uction, whereby the required low capacitance is ob¬ tained with an accuracy many times greater than that obtained with a capacitor C6 formed by a discrete component. Figure 3 shows the microstrip patterns 1, 2 4 and 5, which form the bandpass filter with two parallel resonator circuits in the preferred embodi¬ ment of the invention, shown in Figure 1. The circuit diagram of Figure 3 is drawn on a scale 2:1 for a bandpass filter intended for a frequency range 350 to 500 MHz. A generally U-shaped microstrip pattern 1 forming the inductor L3 comprises a longer branch 1A, a shorter branch IB and an arch 1C connecting the branches. A generally U-shaped microstrip pattern 2 forming the inductor L4 is positioned at a distance from the microstrip pattern 1 side by side with it. The second microstrip pattern similarly comprises a longer branch 2A, a shorter branch 2B and an arch 2C connecting the branches. The microstrip patterns 1 and 2 are positioned on the circuit board side by side and reversely symmetrically in such a way that the sides of the shorter branches IB and 2B face each other. A substantially straight microstrip conductor 4 projects from the shorter branch IB of the micro- strip pattern 1 perpendicularly in the sideward direction toward the microstrip pattern 2 at a dis¬ tance from it. Correspondingly, a substantially straight microstrip pattern 5 projects from the shorter branch 2B of the microstrip pattern 2 perpen- dicularly in the sideward direction toward the micro- strip pattern 1 at a distance from it. The microstrip patterns 4 and 5 extending side by side and in parallel but apart from each other form the capacitor C6 shown in Figure 2. The ends of the longer branches 1A and 2A of the microstrip patterns 1 and 2, respectively, are connected to earth potential. The input IN of the filter is connected to the longer branch 1A of the microstrip pattern 1 at a point between the earthed end of the branch 1A and the arch IC, preferably closer to the arch IC. The output of the filter is connected to a corresponding point on the longer branch 2A of the microstrip pattern 2. The capacitor C4 is preferably positioned on the opposite side of the circuit board and connected between the branches 1A and IB substantially at the input of said filter. Correspondingly, the capacitor C5 is positioned on the opposite side of the circuit board and connected between the branches 2B and 2A substantially at said output. The anodes of the capacitance diodes VD1 and VD2 shown in Figure 1 are intended to be connected to the ends of the branches IB and 2B.

By varying the mechanical dimensions of the patterns 1 and 2, the bandpass filter of the inven- tion can, in principle, be used within the frequency range of 0.05 to 30 GHz. The size of the patterns mainly decreases with increasing operating frequency; the capacitors C4 and C5, for instance, can be re¬ placed at very high frequencies with capacitances between the branches 1A and IB and the branches 2A and 2B, respectively.

A earthed conductive film 8, shown by dashed line in Figure 3, is formed on the opposite surface of the circuit board in a location between the U-sha- ped microstrips 1 and 2 and aligned with microstrips 4 and 5 of the capacitor C6. Firstly, the earthed film 8 reduce the direct coupling between the neigh¬ bouring stages of the filter. Secondly, the earthed film 8 has a capacitive coupling with the microstrips 4 and 5 through the dielectric material of the cir¬ cuit board, whereby an image frequence wave trap is formed for the image frequence which above reception frequence in the frequence domain. The absorbtion frequence of the image frequence wave trap (the loca- tion of trap in the frequence domain) can be adjusted to the desired value by changing the size or width of the earthed film portion 8 and thereby the capacitive coupling with the microstrips 4 and 5. Since the image frequence depends on the intermediate frequence of the receiver and the receivers may different IF frequences depending on the frequence band used, it important to be capable to easily adjust the absorb¬ tion frequence.

Figure 2 shows another bandpass filter of the invention, comprising three parallel resonance stages'. The first two stages correspond substantially to the parallel resonator stages of Figure 1, except that the output of the second stage is taken directly from a connection point between the inductor L4 and the capacitance diode VD3 instead of the tap point, because the output impedance of the stage need not be adjusted to 50 Ohm in this case. The output of the second stage is connected through a capacitor C7 formed by microstrip conductors 6 and 7 to a third parallel resonance circuit formed by an inductor L5 connected in parallel with capacitance diodes VD5 and VD6 substantially similarly as the inductor L4 and the capacitance diodes VD3 and VD4 in Figure 1. The interconnected cathodes of the capacitance diodes VD5 and VD6 are supplied with the control voltage VI through a resistor R3. The output of the filter is formed by a tap from the inductor L5 and a capacitor C9 is connected between the output and the anode of the capacitance diode VD5. The inductor L5 is formed by a microstrip pattern as described above in con¬ nection with the inductors L3 and L4. By means of the third resonance stage, it is possible to increase or decrease the band width. The attenuation of the filter, however, increases simultaneously. The figures and the description related to them are only intended to illustrate the present inven¬ tion. In its details, the high-frequency bandpass filter of the invention may vary within the scope of the attached claims.

Claims

Claims :

1. A high-frequency bandpass filter comprising at least one parallel resonator stage formed by a parallel connection of an inductive component (L1-L5) and a capacitive component (VD1-VD6), c h a r a c ¬ t e r i z e d in that the inductive component is formed by a generally U-shaped microstrip pattern (1, 2, 3) formed on a circuit board. 2. A high-frequency bandpass filter according to claim 1, c h a r a c t e r i z e d in that the end of one branch (1A) of the U-shaped microstrip pattern (1,

2, 3) is connected to earth potential, and that said capacitive component (VD1-VD6) is bet- ween the other branch (IB) and the earth potential.

3. A high-frequency bandpass^' filter according to claim 1 or 2, c h a r a c t e r i z e d in that the capacitive component is formed by two series- connected capacitance diodes (VD1-VD2, VD3-VD4, or VD5-VD6) the cathodes of which are interconnected so that the capacitances of the capacitance diodes are adjustable by a dc voltage applied to their cathodes.

4. A high-frequency bandpass filter according to claim 3, c h a r a c t e r i z e d in that the input of the filter is connected to the earth- connected branch (1A) of the U-shaped microstrip pattern (1) of the first parallel resonator stage at a point which is at predetermined distances from the end of the branch (1A) in question and from an arch (IC) connecting the branches (1A, IB).

5. A high-frequency bandpass filter according to claim 4, c h a r a c t e r i z e d in that a capacitor (C4) is connected between the input and the output of the first resonator stage.

6. A high-frequency bandpass filter according to any of the claims 1 to 5, c h a r a c t e r ¬ i z e d in that the filter comprises two generally U-shaped microstrip patterns (1, 2) positioned side by side on the circuit board, the microstrip patterns being capacitively coupled at the neighbouring branches.

7. A high-frequency bandpass filter according to claim 6, c h a r a c t e r i z e d in that a microstrip conductor projects from each one of the neighbouring branches of the two U-shaped patterns toward the other pattern, the microstrip conductors extending close to and in parallel with each other, thus forming said capacitive coupling, and that a earthed conductive film is formed on the opposite surface of the circuit board in a location between said U-shaped patterns and aligned with said said projecting microstrip conductors, the size of the earthed film determining the frequence of the image frequence wave trap.