Abstract:
The invention refers to a diplexer, particularly for use in microwave devices, comprising a low-pass filter and a high-pass filter, both having inductors and capacitors. It is suggested that at least one inductor of each of said filters is provided as an active inductor.

Description:
[0001]    The invention is based on a priority application EP 01 440 41 2.3 which is hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a diplexer, particularly for use in microwave devices, comprising a low-pass filter and a high-pass filter, both having inductors and capacitors.  
           [0003]    Generally, a diplexer is a three-port frequency-dependent device that may be used for separating or combining signals. The diplexer comprises a low-pass filter for extracting signals in a low frequency band and a high-pass filter for extracting signals in a high frequency band. Both, the low-pass filter and the high-pass filter are designed as passive units using capacitors and inductors for realising the mentioned functions.  
           [0004]    A diplexer as mentioned above is for example known from U.S. Pat. No. 5,793,265. This diplexer also comprises a high-pass filter and a low-pass filter and serves the function of separating an incoming television signal from an incoming telephony signal, so that each of these signals can be processed by separate receivers when in a private home. Each of the filters is constructed in the form of a ladder network having series branches and parallel branches. In the high-pass filter, the series branches comprise capacitors and the parallel branches comprise inductors, with one of the parallel branches having a series inductor-capacitor circuit. In the low-pass filter, the series branches comprise inductors and the parallel branches comprise capacitors, one of the parallel branches having an inductor-capacitor series circuit.  
           [0005]    In this document, it is proposed that all of the inductors of both filters be constructed as toroids. However, the problem is that these passive components have a physical size which makes it difficult to further minimize the diplexer. Moreover, the inductors have a bad quality factor in a frequency range above 1 GHz.  
           [0006]    It has also turned out that using active filters instead of the passive filters shown for example in the above-mentioned prior art document is not an appropriate approach to the mentioned problem due to the relatively poor characteristics of such active filters. These characteristics make them insufficient for use in diplexers.  
           [0007]    In view of the above, it is the object of the present invention to provide for a diplexer as mentioned in the outset which overcomes the problems mentioned above. Particularly, it is an object of the present invention to provide for a diplexer having a compact design, a high quality factor and stability, and being less expensive than prior art diplexers.  
         SUMMARY OF THE INVENTION  
         [0008]    This object is solved by a diplexer particularly for use in microwave devices, comprising a low-pass filter and a high-pass filter, both having inductors and capacitors, wherein at least one inductor of each of said filters is provided as an active inductor.  
           [0009]    That means in other words that spiral inductors, like toroids, are replaced with active components being constituted by capacitors, resistors and transistors and having the electrical characteristic of an inductor. The advantage of the diplexer according to the present invention is a substantial reduction of size since an active inductor builds smaller than a corresponding spiral inductor. Preferably, a reduction of about 90% may be achieved.  
           [0010]    A further advantage of the diplexer according to the present invention is a cost reduction of the diplexer due to smaller packaging. Hence, the diplexer according to the present invention is less expensive compared to prior art diplexers.  
           [0011]    A further advantage of the diplexer according to the present invention is an approvement of the quality factor and the stability.  
           [0012]    The diplexer according to the present invention may be designed using GaAs technology. However, it is preferred to implement the diplexer using SiGe technology since the consumption of the diplexer is lower.  
           [0013]    In a preferred embodiment, said active inductor having an input and an output terminal, said terminals being connected by a first branch comprising a first resistor, a first capacitor, a drain-source path of a first transistor and a second capacitor; a second branch comprising a gate-source path of a second transistor, a source-gate path of a third transistor, a third capacitor, a gate-source path of said first transistor and said second capacitor; a third branch comprising a gate-drain path of said second transistor, a fourth capacitor and a second resistor; a fourth branch comprising said gate-source path of said second transistor, a source-gate path of said third transistor, a fifth capacitor and said second resistor; and a fifth branch comprising said gate-source path of said second transistor, said source-drain path of said third transistor, a sixth capacitor and said second resistor.  
           [0014]    This design has turned out as advantageous in view of its electrical characteristics. However, it is to be understood that other designs may be possible and the present invention is not limited thereto.  
           [0015]    In a further embodiment, the low-pass filter comprises series and parallel branches, the series branches having inductor elements and active inductors alternately arranged, and the parallel branches having inductor elements and capacitor elements serially connected. Preferably, said high-pass filter comprises series and parallel branches, the series branches having capacitor elements, and the parallel branches having either inductor elements or active inductors, and capacitor elements serially connected.  
           [0016]    It is to be understood that other inductor elements of the low-pass and high-pass filters may be replaced with active inductors.  
           [0017]    Further features and advantages can be taken from the following description and the enclosed drawings.  
           [0018]    It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the scope of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    An embodiment of the invention is shown in the drawings and will be explained in more detail in the description below with reference to same. In the drawings:  
         [0020]    [0020]FIG. 1 is a schematic block diagram of a diplexer;  
         [0021]    [0021]FIG. 2 is a schematic diagram of a low-pass filter having active inductors;  
         [0022]    [0022]FIG. 3 is a schematic diagram of a high-pass filter having active inductors; and  
         [0023]    [0023]FIG. 4 is a schematic diagram of a preferred design of an active inductor used in the low-pass and high-pass filters shown in FIGS. 2 and 3. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]    In FIG. 1, a diplexer is shown and indicated with reference number  10 . The diplexer has three terminals  12 ,  14 ,  16 , two of which  12 ,  14  serving as input terminals and the other terminal  16  serving as an output terminal in one operating mode. In a second operating mode, the function of the terminals is reversed, i.e. terminal  16  serves as input terminal and the other two terminals  12 ,  14  serve as output terminals.  
         [0025]    The diplexer  10  comprises a low-pass filter  20  and a high-pass filter  40 . The low-pass filter is coupled between the terminal  12  and the terminal  16 , whereas the high-pass filter  40  is coupled between the terminal  14  and the common terminal  16 .  
         [0026]    In a first operating mode, the function of this diplexer is to combine two signals supplied to the terminals  12  and  14  and carrying information in a low frequency band and a high frequency band, respectively, to one signal carrying both information. This combined signal is supplied to terminal  16 .  
         [0027]    In a second operating mode, the function is reversed. I.e., a common signal carrying both information in a low and a high frequency band is separated by the low-pass filter  20  and the high-pass filter  40  in a first signal carrying the information in the low frequency band and a second signal carrying the information in the high frequency band, respectively. The first signal is supplied to terminal  12 , while the second signal is supplied to terminal  14 .  
         [0028]    The field of application of such diplexers is broad. For example, diplexers are used in current dual-band mobile telephone devices in order to separate the frequency ranges 880 MHz-960 MHz and 1710 MHz-1880 MHz. However, it is to be understood that the present diplexer is not limited to this application.  
         [0029]    Referring to FIG. 2, the structure of the low-pass filter  20  is shown. The low-pass filter  20  is constructed as a four stage ladder network, wherein the first stage comprises an inductor  21 . 1  and an inductor  23 . 1  connected in series with a capacitor  25 . 1 . The second, third and fourth stages are constructed similarly to the first stage and also comprise an inductor  21  and an inductor  23  connected in series with a capacitor  25  (the stage number follows the reference number delimited by  11 . 11 ). The series circuits of inductors  23  and capacitors  25  constitute parallel branches of the ladder network, and the inductors  21 . 1 - 21 . 4  constitute series branches of the ladder network. The low-pass filter  20  is terminated with a further inductor  27  coupling an output terminal  17  of the low-pass filter  20  with the common node of the series branch and the parallel branch of the fourth stage of the ladder network.  
         [0030]    In FIG. 2, the inductors  21 . 2  and  21 . 4  of the series branch of the second stage and the fourth stage, respectively, are provided as active inductors  30 . The design of the active inductor will be described in detail below with reference to FIG. 4. The remaining inductors  21 . 1 ,  21 . 3 ,  27 ,  23 . 1 - 23 . 4  are designed as spiral inductors, for example as coils.  
         [0031]    In FIG. 3, the design of the high-pass filter  40  is shown. It is also provided as a four stage ladder network comprising series and parallel branches between the terminal  14  and a second terminal  19  being connected with terminal  16  in the diplexer. Each of the series branches comprises a capacitor  41 . 1 ,  41 . 2 ,  41 . 3  and  41 . 4 , respectively. The corresponding parallel branches each comprise a series circuit of an inductor  43 . 1 - 43 . 4  and a capacitor  45 . 1 - 45 . 4 . The ladder network is terminated by a capacitor  47  coupling the terminal  19  with the common node of the series branch and the parallel branch of the fourth stage. Here, the digit of the reference number following the point indicates the stage number of the ladder network.  
         [0032]    In the high-pass filter  40 , the inductors  43 . 2 ,  43 . 3  of the second and third stage, respectively, are provided as active inductors  30 .  
         [0033]    With reference to FIG. 4, the design of an active inductor  30  will now be described in detail. The active inductor is constructed as a component using resistors, capacitors and transistors but no inductors. However, the electrical characteristic of this active inductor is very similar to that of a passive inductor like a toroid.  
         [0034]    The active inductor  30  comprises a first resistor  31 . 1  connected in series with a first capacitor  33 . 1 . This series circuit is coupled at its first end to a terminal  34  and at its second end to the drain of a first field effect transistor  35 . 1 . The source of this transistor  35 . 1  is coupled to a second capacitor  33 . 2  which is in turn coupled to a further terminal  34 . 2 .  
         [0035]    The gate of the first transistor  35 . 1  is coupled to a third capacitor  33 . 3  which in turn is coupled to the drain of a second field effect transistor  35 . 2 . The source of the second transistor  35 . 2  is coupled to the source of a third field effect transistor  35 . 3 , the gate of which is coupled to the first terminal  34 . 1 .  
         [0036]    The drain of the third transistor  35 . 3  is connected to a node  36  via a fourth capacitor  33 . 4 . The gate of the second transistor  35 . 2  is connected to said node via a fifth capacitor  33 . 5 . Finally, the drain of the second transistor  35 . 2  is connected to said node  36  via a sixth capacitor  33 . 6 .  
         [0037]    A second resistor  31 . 2  connects said node  36  to said terminal  34 . 2 .  
         [0038]    It is to be understood that the described design is only a preferred design for an active inductor. A person skilled in the art will contemplate variations and modifications of this design without departing the scope of the present invention as defined in the claims. One major aspect of the present invention is to avoid the employment of passive inductors, like coils. According to the present invention, at least one such passive inductor is replaced with an active inductor showing substantially the same characteristic as a passive inductor, but using active elements, like transistors.