Abstract:
The bandpass filter has a comparatively large pass bandwidth, with, at the same time, comparatively steep edges up to the stop band and low attenuation in the passband. The bandpass filter contains three parallel LC elements, one of which is connected between the bandpass filter input and the bandpass filter output. The other two parallel LC elements each have one of their connections coupled to a fixed reference-ground potential.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending International Application PCT/DE98/01846, filed Jul. 3, 1998, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field Of The Invention 
     The invention lies in the electronics field. More specifically, the invention relates to a bandpass filter, particularly for use in circuits using RF technology. 
     In many circuit configurations using RF technology, for example in circuit configurations for contiguous division of a relatively large frequency range (e.g. the TV frequency range) into a plurality of smaller frequency bands, bandpass filters are required which have a comparatively large pass bandwidth, with, at the same time, comparatively steep edges up to the stop band and low attenuation in the passband. 
     French patent document FR 952403 discloses a bandpass filter circuit. There, a capacitor is connected in series with a parallel LC element. The filter four-terminal circuit has an additional parallel LC element at the output. 
     U.S. Pat. No. 5,483,209 discloses a circuit for changing over between different reception bands with variable attenuation control. The circuit has a plurality of bandpass filters tuned to the different input reception frequencies. In addition, the circuit has a plurality of variable attenuation diodes whose impedances are controlled by means of an AGC. A plurality of switching diodes are used to select the bandpass filter tuned-to the input signal. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a bandpass filter, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which satisfies the aforementioned requirements. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a bandpass filter, comprising:
         a bandpass filter input and a bandpass filter output;   a series circuit connected between the bandpass filter input and the bandpass filter output, the series circuit being formed of a first capacitor, a first parallel LC element connected to the first capacitor, a second capacitor connected to the first parallel LC element, and an inductor connected to the second capacitor;   a second parallel LC element having a first connection connected to a node between the first parallel LC element and the second capacitor and a second connection coupled to a fixed reference-ground potential via a third capacitor; and   a third parallel LC element having a first connection connected to a node between the second capacitor and the inductor and a second connection coupled to the fixed reference-ground potential, either directly or via a fourth capacitor connected between the second connection of the third parallel LC element and the fixed reference-ground potential.       

     In accordance with an additional feature of the invention, a fifth capacitor is connected to a node between the second capacitor and the inductor and to the fixed reference-ground potential. 
     With the above and other objects in view there is also provided, in accordance with the invention, a circuit configuration, comprising:
         an AC voltage input terminal and an AC voltage output terminal;   a plurality of frequency domain filter paths defined between the AC voltage input terminal and the AC voltage output terminal, and connected in parallel between a common first node and a common second node both coupled to a DC voltage connection;   each of the frequency domain filter paths containing at least one of the above-outlined bandpass filters connected in series with a first diode and a second diode connected in opposite forward direction from the first diode;   each of the frequency domain filter paths containing a switching unit for switching the first and the second diode in the frequency domain filter path during an operation of the circuit configuration for turning a respective one of the bandpass filters;   a third diode having a first terminal connected to the first node and a fourth diode having a first terminal connected to the second node of the frequency domain filter paths, such that a respective cathode of the third diode and of the fourth diode is connected to anodes of the first diodes and the second diodes, respectively;   a load-dependent DC voltage source having a first connection and a second connection; and   a third diode and the fourth diode each having a second terminal respectively connected to the first connection and the second connection of the load-dependent DC voltage source.       

     In accordance with another feature of the invention, the first, second, third, and fourth diodes are PIN diodes. 
     In accordance with a further feature of the invention, each of the switching units includes:
         a first electrical resistor and a second electrical resistor respectively connected, via a first terminal thereof, to the input and to the output of an associated one of the frequency domain filters, and to one another via a second terminal thereof;   an on/off switch having a first terminal connected between the first electrical resistor and the second electrical resistor, and a second terminal connected to a fixed reference-ground potential; and   a capacitor having a first terminal connected between the first electrical resistor and the second electrical resistor, and a second terminal connected to the fixed reference-ground potential.       

     In further summary, in the bandpass filter according to the invention:
         a) a series circuit, comprising a first capacitor, a first parallel LC element, a second capacitor and an inductor, is connected between a bandpass filter input and a bandpass filter output, the individual elements in the series circuit being connected one after the other in the aforementioned sequence;   b) a second parallel LC element, whose second connection is coupled to a fixed reference-ground potential via a third capacitor, is connected in a connection line between the first parallel LC element and the second capacitor; and   c) a third parallel LC element, whose second.connection is coupled to the fixed reference-ground potential directly or via a fourth capacitor, is connected in a connection line between the second capacitor and the inductor.       

     Optionally, the connection line between the second capacitor and the inductor is coupled to the fixed reference-ground potential (e.g. ground) via a fifth capacitor. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a bandpass filter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in,connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit schematic of an exemplary embodiment of the filter according to the invention; and 
         FIG. 2  is a schematic diagram of a circuit configuration with bandpass filters in accordance with the exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is seen a bandpass filter with a series circuit formed with a first capacitor C 1 , a first parallel LC element L p   1 /C p   1 , a second capacitor C 2 , and an inductor L. The series circuit is connected between a bandpass filter input IN and a bandpass filter output OUT. The circuit elements are connected in series, one after the other, in the aforementioned sequence. A first connection of a second parallel LC element L p   2 /C p   2 , whose second connection is coupled to a fixed reference-ground potential P via a third capacitor C 3 , is connected between the first parallel LC element L p   1 /C p   1  and the second capacitor C 2 . A first connection of a third parallel LC element L p   3 /C p   3 , whose second connection is coupled to the fixed reference-ground potential P directly or via a fourth capacitor C 4 , is connected between the second capacitor C 2  and the inductor L. 
     An optional fifth capacitor C 5  is illustrated in dashes. The capacitor C 5  may be provided to connect the fixed reference-ground potential P to the node between the second capacitor C 2  and the inductor L. 
     Optionally, a series circuit for the fixed reference-ground potential P can also be connected between the second capacitor C 2  and the inducer L. 
     While the drawings illustrate, and the above description refers to, inductors L, L p   1 , L p   2 , L p   3 , it is equally possible to use striplines as the inductances. 
     Reference will now be had to the circuit configuration shown in  FIG. 2 , where a parallel circuit, comprising a plurality of frequency domain filter paths FZ 1 , FZ 2 , . . . , FZi, is connected between an AC voltage input terminal IN RF  and an AC voltage output terminal OUT RF . Each of the frequency domain filter paths has a bandpass filter F 1 , F 2 , . . . , Fi in accordance with the exemplary embodiment shown in FIG.  1 . 
     A sixth capacitor C 6  is connected between the AC voltage input terminal IN RF  and a first node K 1  of the parallel circuit, and a seventh capacitor C 7  is connected between a second node K 2  of the parallel circuit and the AC voltage output terminal OUT RF . These two capacitors C 6  and C 7  are essentially used for DC voltage decoupling of the AC voltage connections IN RF  and OUT RF . 
     The nodes K 1  and K 2  each have a regulating voltage connection E R  coupled to them via the two inductor elements Dr 1 , Dr 2 , which is used to supply the nodes K 1  and K 2  with a direct current during operation. Instead of the inductor elements Dr 1  and Dr 2 , suitable nonreactive resistors can also be used. 
     In the exemplary embodiment, the regulating voltage connection E R  is connected to the emitter of a pnp transistor T whose collector is connected to the operating voltage input E B  and whose base is connected to a control voltage connection U AGC  via an electrical resistor RV. 
     Each frequency domain filter path FZ 1 , FZ 2 , . . . , FZi comprises a bandpass filter F 1 , F 2 , . . . , Fi connected between two diodes D 11 , D 21 ; D 12 , D 22 ; . . . ; D 1 i, D 2 i, preferably PIN diodes, in series with the latter. The two PIN diodes are connected with oppositely oriented forward directions. 
     Each bandpass filter F 1 , F 2 , . . . , Fi has a DC supply, comprising a first resistor R 11 , R 12 , . . . , R 1 i and a second resistor R 21 , R 22 , . . . , R 2 i, connected to it between the two associated diodes D 11 , D 21 ; D 12 , D 22 ; . . . ; D 1 i, D 2 i. The first resistor R 11 , R 12 , . . . , R 1 i and the second resistor R 21 , R 22 , . . . , R 2 i are respectively connected, on the one hand, to the input EF 1 , EF 2 , . . . , EFi and to the output AF 1 , AF 2 , . . . , AFi of the associated bandpass filter F 1 , F 2 , . . . , Fi and, on the other hand, to one another.  FIG. 2  indicates switching units SE 1 , SE 2 , . . . , SEi in dash-dotted boxes which each have a first electrical resistor R 11 , R 12 , . . . , R 1 i and a second electrical resistor R 21 , R 22 , . . . , R 2 i which are respectively connected between the input and the output of the associated frequency domain filter F 1 , F 2 , . . . , Fi and to one another. The respective connection line between the two resistors R 11 , R 21 ; R 12 , R 22 ; . . . ; R 1 i, R 2 i is blocked off to alternating current by means of a capacitor CF 1 , CF 2 , . . . , CFi in each case and is connected to a first switch terminal of an “on/off” switch S 1 , S 2 , . . . , Si (preferably an electronic switch, e.g. an open-collector switching output of an integrated circuit) in each case, whose second switch connection is connected to a fixed reference-ground potential. 
     Connected in parallel with the frequency domain filter paths FZ 1 , FZ 2 , . . . , FZi is a series circuit comprising a third diode D 3 , a first resistor R 1 , a second resistor R 2  and a fourth diode D 4 . The two diodes D 3  and D 4  are connected with oppositely oriented forward directions and are preferably likewise PIN diodes. In addition, the two resistors R 1  and R 2  are connected, on the one hand, to the diodes D 3  and D 4 , respectively, and, on the other hand, to a center tap M 1  of a voltage divider comprising a third resistor R 3  and a fourth resistor R 4 . 
     The voltage divider is, on the one hand, connected to an operating voltage connection E B , which is connected to the same voltage source as the regulating voltage connection E R , for example, and, on the other hand, to the fixed reference-ground potential P. The first resistor R 1  and the second resistor R 2  and the voltage divider which include the third resistor R 3  and the fourth resistor R 4  define a load-dependent DC-voltage source. The end of the first resistor R 1  that is connected to the third diode D 3  defines a first connection of the load-dependent DC voltage source and the end of the second resistor R 2  that is connected to the fourth diode D 4  defines a second connection of the load-dependent DC voltage source. 
     An eighth capacitor C 8  and a ninth capacitor C 9 , whose second connections are connected to the fixed reference-ground potential P, are connected between the third diode D 3  and the first resistor R 1  and between the fourth diode D 4  and the second resistor R 2 , respectively. These capacitors C 8 , C 9  serve to derive the AC voltage for the fixed reference-ground potential P (e.g. ground). 
     Changing over between the individual frequency domain filter paths FZ 1 , FZ 2 , . . . , FZi is performed by means of the diode pairs D 11 , D 21 ; D 22 ; . . . ; D 1 i, D 2 i, which are selectively switched on by the switches S 1 , S 2 , . . . , Si. 
     Advantageously, the respectively active diode pair D 11 , D 21 ; D 12 , D 22 ; . . . ; D 1 i, D 2 i is in this case simultaneously used as a series element of a regulatable π-attenuation element whose parallel elements are the third and the fourth diode D 3 , D 4 . In the high state, the latter have zero current (high resistance). If the regulating voltage U R  is reduced, current flows through them and they become less resistive, whereas the current through the respective active diode pair D 11 , D 21 ; D 12 , D 22 ; . . . ; D 1 i, D 2 i in the frequency domain filter path FZ 1 , FZ 2 , . . . , FZi which is operating falls with the regulating voltage, which means that these diodes become more resistive.