Abstract:
An arrangement for processing the antenna signal of a radio receiver and for leading it to low-noise amplifiers LNA of parallel amplifier branches. On the transmission path of the receiver from the antenna to the amplifiers LNA, functionally different elements are combined into physically united elements, such as the conductors ( 432, 433 ) of the low-passing part of the antenna filter and the division conductors of the Wilkinson divider ( 430 ), and the conductor ( 441 ) of the phase-shifter and the inductive part (L 1 ) of the LNA matching circuit. Each physically united element is a conductor, which is insulated from the ground plane by air or a low-loss dielectric material. The arrangement reduces the number of lossy parts between the antenna and the amplifiers, and placing these parts on an ordinary circuit board is also avoided. For these reasons, inferior noise values compared to the prior art can be allowed for each LNA. In addition, the matching of the input impedance of the LNA becomes more accurate when no discrete coil is needed in it.  
     FIG.  4

Description:
[0001]     The invention relates to an arrangement for processing the antenna signal of a radio receiver and leading it to low-noise amplifiers. The arrangement is suitable for use on the receiving side of base stations of the mobile communication networks and in satellite receivers, for example, the low-noise amplifier unit consisting of two parallel and phased amplifier branches.  
       BACKGROUND OF THE INVENTION  
       [0002]     In all radio receivers, the first amplifier after the antenna when entering the receiver should be especially low-noise type, because the signal level at the input of this amplifier is very low, and the additional noise caused by the amplifier is amplified in all the following amplifier stages. An abbreviation LNA is generally used of such a low-noise pre-amplifier. So in this description and the claims, too. Some allowed maximum value is generally specified in receivers for the total noise figure of the LNA and its input and output circuits. Losses on the transmission path cause attenuation in the signal, directly increasing the noise figure by the same amount. Therefore, for example, if the antenna filter of the receiver is very low loss, the noise figure of the LNA can be correspondingly a little higher.  
         [0003]      FIG. 1  shows a block diagram of a common structure of the antenna side part of a receiver. In addition to the antenna and a possible antenna switch, the structure includes an antenna filter, signal divider, two parallel amplifier branches and a signal combiner. In the example of the figure, the antenna filter RXF has two parts: starting from the antenna, there is first a bandpass filter  110  and then a low-pass filter  120 . The former attenuates frequency components outside the receiving band of the radio system, and the latter further cleans up the range above the receiving band. The signal E in  coming from the low-pass filter  120  is divided in the divider  130  into two mutually identical parts E 11  and E 21 . The first division signal E 11  is led to the first amplifier branch, where its phase is changed 90 degrees in a phaseshifter  140  and then amplified in the first LNA  170 . The input impedance of the amplifier must naturally be matched, for which reason there is the first matching circuit  150  in its input. The first LNA outputs the signal E 12 . The second division signal E 21  is led to the second amplifier branch, where it is amplified in a second LNA  180 , in the input of which there is the second matching circuit  160 . The phase of the signal is then changed 90 degrees in the second phase shifter PSC, which outputs the signal E 22 . Again, the in-phase signals E 12  and E 22  are summed in a combiner CMB, the output signal of which, E out , continues towards the mixer of the receiver, In addition,  FIG. 1  shows also amplifier output matching circuits, which do not fall within the scope of this invention, as blocks M. Compared to a single LNA, in the arrangement described above especially the impedance matching of the antenna filter towards the amplifiers is easier. In addition, a wider dynamic and linear area and a better stability are achieved. On the other hand, the divider, the phase shifter and the additional wiring required by them cause more attenuation in the signal, which, as mentioned, directly impairs the noise figure of the LNA.  
         [0004]     In this description and the claims, the name “front stage” is used for the parts of the receiver from the antenna to the low-noise amplifiers, including these.  
         [0005]      FIG. 2  shows an example of a known input arrangement of an amplifier pair according to  FIG. 1 . It comprises a circuit board  201 , the lower surface of which, not visible in the figure, is conductive and functions as the signal ground GND. The integrated antenna filter RXF comprises resonators, and its output is connected through a connector  225  on its end wall to a coaxial cable  229 , which has a characteristic impedance of  50 Ω. The conductive cable sheath is connected to the signal ground at both ends. The cable  229  continues on the circuit board  201  as a transmission line, which consists of a micro strip  231  on the upper surface of the board, a ground conductor on the lower surface and dielectric material between them. The transmission line is dimensioned so that its characteristic impedance is 50Ω. It belongs to the divider  230  as its input line. The divider is of the Wilkinson type, which means that the above mentioned input line branches into two transmission lines, which are called division lines here. Their length is λ/4 on the operating frequency, and their characteristic impedance is √2·50 ≈71Ω. The first division line is formed of the first division conductor  232  on the upper surface of the board, the ground conductor on the lower surface and dielectric material between them, and the second division line correspondingly of the second division conductor  233  on the upper surface of the board, the ground conductor on the lower surface and dielectric material between them. A Wilkinson divider is formed when the tail ends of the first and the second division conductor have been connected together by a resistor  234  of the value of 2·50 =100Ω. In that case, if both transmission line branches have been terminated by an impedance of 50Ω, the energy coming from the filter is divided into them half and half, and theoretically without losses. Thus the divider does not consume energy in spite of the resistor in it. Only if the matching on the transmission paths continuing forward is inadequate, the resistor  234  causes losses. In addition, a good isolation between the branches is achieved. The first division line continues as a phase shifter, which has been implemented with a quarter-wave long transmission line. A micro strip  241  of this transmission line, which is a continuation of the first division conductor  232 , is seen in  FIG. 2 . That micro strip ends in the first matching circuit  250  including an air core coil L 1  and a chip capacitor C 1  in series. The latter functions as a decoupling capacitor at the same time. The matching circuit is connected at its tail end with a short micro strip to the input pin of the first LNA  270 . The second division conductor  233  is connected at its end on the side of the resistor  234  to the second matching circuit including a coil L 2  and a capacitor C 2  in series in the same way as in the first matching circuit. The second matching circuit is connected at its tail end with a short micro strip to the input pin of the second LNA  280 .  
         [0006]     The arrangement according to  FIG. 2  has the drawback of losses that occur in it in practice: the circuit board material causes dielectric losses both in the divider  230  and the phase shifter, the value of the losses being typically 0.2-0.5 dB in the former and 0.1-0.3 dB in the latter. The transmission line  229  from the filter to the divider and its connectors cause additional losses, the value of which can be several tenths of a decibel, naturally depending on the length of the line. The losses of the matching circuits on the input side of the amplifiers are also significant. In addition, the coil of the matching circuit causes a production problem, because the variation of its inductance is so wide in practice that the impedance matching on the operating band may be insufficient. This means additional losses in the divider. Attenuation corresponding to all losses directly increases the noise figure of the amplifier unit by the same amount. Then the requirements for the LNA itself correspondingly increase, if the total noise figure must remain as low as possible.  
         [0007]      FIG. 3  shows another example of a known input arrangement of an amplifier pair according to  FIG. 1 . This differs from the arrangement of  FIG. 2  only for the low-pass filter, otherwise the circuit is similar. In this example, the low-pass filter  320  consists of a conductor area on the upper surface of the circuit board  301  and the planar signal ground of the lower surface. The conductor area consists of a straight and relatively narrow micro strip  321 , which extends from the input of the filter to its output and in which the substantial characteristic is its inductance. The micro strip  321  has transverse enlargements on, such as an enlargement  322 , the substantial characteristic of which is their capacitance in relation to the ground plane. The structure thus corresponds to an LC chain implemented by discrete components, with coils in series, and a capacitor connected to the ground between each two coils. In the example of  FIG. 3 , there are four “coils” and three “capacitors”, in which case the order of the low-pass filter is seven. The values of the inductances and the capacitances naturally depend on the dimensioning of the parts of the conductor area, which dimensioning thus determines the filter response. The micro strip  321  of the filter  320  continues as micro strip  331 , which together with the ground on the lower surface of the circuit board and the dielectric material between them forms the input line of the Wilkinson divider  330 . In order to improve the mutual matching of the filter  320  and the divider  330 , there is a capacitor  307  at their junction, between the micro strip on the upper surface of the circuit board and the ground.  
         [0008]     Because of the filter solution, the arrangement of  FIG. 3  is more compact than the arrangement of  FIG. 2 . The cabling does not cause losses in this case, but a new drawback is caused by the dielectric losses that arise at the low-pass filter in the circuit board. Here, like in the example of  FIG. 2 , the losses can be reduced by selecting a low-loss material, such as teflon, instead of a generally used circuit-board material. However, in that case there is a flaw of a significant increase in production costs.  
       SUMMARY OF THE INVENTION  
       [0009]     It is the objective of the invention to reduce the above mentioned drawbacks of the prior art. The arrangement according to the invention is characterized in what is set forth in the independent claim  1 . Some preferred embodiments of the invention are presented in the other claims.  
         [0010]     The basic idea of the invention is the following: On the transmission path of the front stage of a receiver from the antenna to the low-noise amplifiers, functionally different elements are combined into physically united elements. In this way, the low-passing part of the antenna filter can be united with the Wilkinson divider and the phase shifter with the matching circuit of the LNA. Each physically united element is a conductor, which is insulated from the ground plane by air or some low-loss dielectric material.  
         [0011]     The invention has an advantage that the losses of the front stage of a receiver before the low-noise amplifiers are reduced, i.e. the attenuation caused by the transmission path is reduced. This is due to that the transmission path from the antenna to the low-noise amplifiers is formed of a smaller number of lossy parts and also to that placing these parts on an ordinary circuit board is avoided. The reduction of the losses means that the noise figure of the front stage improves, in which case inferior noise values can be allowed for its both LNAs, which further means saving of costs in amplifiers. In addition, the invention has the advantage that no discrete coil is needed for the matching of the input impedance of the LNA, and the matching thus becomes more accurate. Furthermore, the invention has the advantage that it simplifies the structure of the front stage, which means saving of costs in production. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     In the following, the invention will be described in more detail. Reference will be made to the accompanying drawings, in which  
         [0013]     FIG. 1  shows as a block diagram of a common structure of the antenna side part of a receiver,  
         [0014]      FIG. 2  shows an example of a known input arrangement of an amplifier pair according to  FIG. 1 ,  
         [0015]      FIG. 3  shows another example of a known input arrangement of an amplifier pair according to  FIG. 1 ,  
         [0016]      FIG. 4  shows an example of an input arrangement of an amplifier pair according to the invention,  
         [0017]      FIG. 5  shows another example of an input arrangement of an amplifier pair according to the invention,  
         [0018]      FIG. 6  shows a third example of an input arrangement of an amplifier pair according to the invention,  FIG. 7  shows an example of coupling losses of the divider in an arrangement according to the invention,  
         [0019]      FIG. 8  shows an example of the return attenuation in the output ports of the divider in an arrangement according to the invention, and  
         [0020]      FIG. 9  shows an example of the attenuation in a low-pass filter combined with the divider according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]      FIGS. 1, 2  and  3  were already explained in connection with the description of the prior art.  
         [0022]      FIG. 4  is an example of the input arrangement of an amplifier pair according to the invention. This implements the same functions as the arrangements of the previous figures, but with a different structure. The filter corresponding to the bandpass filter  110  in  FIG. 1  is of the resonator type, of which the inner conductor  411  of its output resonator RES is seen. The input conductor  431  of the divider  430  extends to the cavity of the output resonator. The part of the input conductor  431  in the cavity has an electromagnetic coupling to the output resonator, through which the energy of the signal coming from the antenna is transferred to the divider. Alternatively, the input conductor could be galvanically coupled directly to the inner conductor  411 . The divider is of the Wilkinson type, and in addition to the input conductor  431 , the first division conductor  432 , the second division conductor  433  and a resistor  434  connected between the tail ends of the division conductors are seen in  FIG. 4 . Said three conductors are fairly rigid strip conductors. They form a united piece, which is fastened and supported on the conductive frame of the device as insulated therefrom. The frame is not shown in  FIG. 4 ; only screw heads are shown of the fastening. The frame functions as a signal ground GND, at the same time. The distance of the strip conductors from the ground is such that the impedance of the input line formed by the input conductor and the ground is about 50Ω in this example, too, and the impedance of the division lines formed by the division conductors and the ground is about 71Ω as “viewed” from the end of the line.  
         [0023]     The low-pass filtering of the signal takes place in the divider according to the invention so that its both division lines also function as filters, at the same time. The division conductors have been shaped in the same way as the conductor area of the low-pass filter  320  seen in  FIG. 3  and described above. Thus there is a relatively narrow central part  421  in the first division conductor  432 , and transverse enlargements thereof, such as an enlargement  422 , so that the conductor together with the signal ground corresponds to an LC chain made by discrete components. The filters formed by the first and the second division line are identical.  
         [0024]     The first division line continues as a phase shifter, which has been implemented with a quarter-wave long transmission line, which is formed of the conductor  441  seen in  FIG. 4  and the ground conductor, or signal ground, or ground. Here and in the claims as well, the pair conductor of the ground conductor of the phase shifter is called the “upper conductor”, where the qualifier “upper” does not limit the position of the device in any way. The upper conductor  441  ends in the first matching circuit  450  including a conductor L 1  with a certain inductance and a chip capacitor C 1  in series. The tail end of the conductor LI extends to the circuit board  401  of the device, on which circuit board the capacitor C 1  is. This is connected with a short micro strip to the input pin of the first LNA  470 . The second division conductor  433  is connected at its tail end, or the end on the side of the resistor  434  to the second matching circuit  460 , which is identical to the first matching circuit. The tail end of the inductive conductor L 2  of the second matching circuit also extends to the circuit board  401 , where its serial capacitor C 2  is. The second matching circuit  460  is at its tail end connected with a short micro strip to the input pin of the second LNA  480 .  
         [0025]     The upper conductor  441  of the phase shifter, the inductive conductor L 1  of the first matching circuit and the inductive conductor L 2  of the second matching circuit are in this example similar fairly rigid, air-insulated strip conductors as the strip conductors of the divider  430 . The strip conductors  441  and L 1  form a united strip. The strip has a point of discontinuity where the phase shifter proper ends, and the relation of the strip conductor L 1  to the ground differs from the relation of the strip conductor  441 . In spite of these matters, the phase shift function and the matching function are not strictly separate with regard to the location, but overlapping. As can be seen, no discrete coil is needed in the matching circuit, which means an improvement in the accuracy of the matching. The same naturally also applies in the second matching circuit  460 . Another significant advantage as compared to the structure of  FIG. 3  is that the losses of the low-pass filter and the divider are substantially smaller. This is due to the air insulation of the conductors and that the filter is combined with the divider.  
         [0026]      FIG. 5  shows another example of the input arrangement of an amplifier pair according to the invention. The figure shows a metal housing HO with its cover removed. The housing contains the bandpass part  510  of the antenna filter, the divider  530  and the circuit board  501 . The bandpass filter  510  is formed so that the inner space of the housing HO is divided by conductive partition walls into resonator cavities, between which there are coupling holes. Each resonator cavity includes an inner conductor of a coaxial-type resonator, such as the inner conductor  511  of the output resonator. Two of the cavities confined by partition walls do not serve as resonators; one of them contains the divider  530  and another one the circuit board  501 . The cavity of the divider is beside the output resonator. The input conductor  531  of the divider extends through an opening in the partition wall to the output resonator, a coupling element  512  therein. In this example, the coupling element is a cylindrical conductor parallel with the inner conductor of the resonator and galvanically connected to the bottom of the resonator. The coupling element  512  has an electromagnetic coupling to the output resonator, through which coupling the energy of the signal coming from the antenna is transferred to the divider. The divider is of the Wilkinson type, and the parts seen of it in addition to the input conductor  531  are the first division conductor  532 , the second division conductor  533  and a resistor  534  connected between the tail ends of the division conductors. These three conductors are strip conductors, and they are supported to the bottom defining the cavity, as insulated therefrom, like the corresponding conductors of the divider in  FIG. 4  to the frame mentioned in the description of  FIG. 4 . The distances of the strip conductors from the housing that functions as the signal ground are also in this case such that the impedance of the division lines formed by the division conductors and the ground is about √2 times the impedance of the input line formed by the input conductor and the ground.  
         [0027]     The low-pass filtering of the signal takes place like in the example of  FIG. 4  so that both division lines of the divider function as filters, at same time. Both division conductors  532 ,  533  together with the signal ground thus correspond to a lowpassing LC chain made by discrete components. The upper conductor  541  of the phase shifter is, unlike the conductor  441  in  FIG. 4 , a micro strip on the surface of the circuit board  501 .  
         [0028]     For this reason, the phase shifter is in this example lossier than in the example of  FIG. 4 . The first and the second LNA, or LNA 1  and LNA 2  are also seen on the circuit board  501 .  
         [0029]      FIG. 6  shows a third example of the input arrangement of an amplifier pair according to the invention. The figure shows a metal housing HO with its cover removed. The housing contains the bandpass part  610  of the antenna filter, strip conductors belonging to the divider, phaseshifter and matching circuits, and a circuit board  601 . The low-pass part of the antenna filter is not visible in  FIG. 6 . The bandpass filter  610  is formed so that the inner space of the housing HO is divided by conductive partition walls into resonator cavities, between which there are coupling holes. Each resonator cavity includes an inner conductor of a coaxial-type resonator, such as the inner conductor  621  of the output resonator. Of the cavities confined by the partition walls, two do not serve as resonators, one of them contains the divider  630  and the phaseshifter  640  and another contains the circuit board  601 . The cavity of the divider is beside the output resonator. The input conductor  631  of the divider extends through an opening in the partition wall of the cavities to the output resonator, a coupling element  622  therein. The coupling element is a cylindrical conductor parallel with the inner conductor of the resonator, galvanically connected to the bottom of the resonator, like in  FIG. 5 . In the same way, the coupling element  622  has an electromagnetic coupling to the output resonator, through which coupling the energy of the signal coming from the antenna is transferred to the divider. In addition to the input conductor  631 , the first division conductor  632 , the second division conductor  633  and a resistor  634  connected between the tail ends of the division conductors are seen of the Wilkinson divider. These three conductors are strip conductors, and they are supported on the bottom confining said cavity, insulated therefrom like in the divider of  FIG. 5 . Because the low-pass filter is made by coaxial resonators, in this example the division conductors  632  and  633  serve only the signal dividing function. Instead, the upper conductor  641  of the phase shifter and the inductive part L 1  of the first matching circuit are integrated into a united strip conductor in accordance with the invention. The conductor L 1  extends at its tail end to said circuit board  601 , where the amplifiers LNA 1  and LNA 2  are. Correspondingly, the inductive part L 2  of the second matching circuit is a strip conductor, which extends from the tail end of the second division conductor  633  to the circuit board  601 .  
         [0030]     In the structure of  FIG. 6 , circuit board losses have been eliminated in the same way as in the structure of  FIG. 4 . Similarly, the need for a discrete coil in the matching circuits has also been eliminated, which means an improvement in matching accuracy.  
         [0031]      FIG. 7  shows an example of the coupling losses L co  of a divider according to  FIGS. 4 and 5  on the receiving band. Here the coupling losses mean an attenuation that exceeds the attenuation of 3.03 dB inevitably caused by halving the signal. Curve  71  shows the coupling losses in the first branch of the divider, which continues to the phaseshifter. The losses are approx. 0.1 dB. Curve  72  shows the coupling losses in the second branch of the divider. In it the losses vary in the range 0.02-0.07 dB being thus even smaller than in the first branch.  
         [0032]      FIG. 8  shows an example of the return attenuation L ret  in the output ports of the divider in the arrangement according to the invention on the receiving band. Here the return attenuation describes the quality of the matching as viewed forward from the divider; the higher return attenuation, the better. Curve  81  shows the return attenuation at the tail end of the first branch of the divider. The attenuation varies from 21.7 to 23.2 dB in the range 1.7-2.2 GHz. Curve  82  shows the return attenuation at the tail end of the second branch. There the attenuation varies from  23  to  25  dB , being thus even better than at the tail end of the first branch. The results were gained from a prototype piece, and they can naturally be improved by optimising the dimensioning.  
         [0033]      FIG. 9  is an example of the transmission coefficient S 21  of a low-pass filter combined with the divider according to the invention, i.e. its attenuation. The purpose of the low-pass filter is to attenuate frequency components that possibly occur at such high frequencies at which the stopband attenuation of the band-pass filter is not sufficient. The cut-off frequency of the filter of the example is about 7 GHz. The peak attenuation, the value of which is approx. 52 dB , is arranged at the frequency 8.9 GHz. Upward from this the attenuation decreases, but remains at almost  30  dB . On the receiving band, which is not seen in the figure, the attenuation is very close to zero.  
         [0034]     Examples of the arrangement according to the invention have been described above. The invention is not limited to them only. For example, the low-pass filter can also be united with the input line of the divider in a similar manner as it is in  FIGS. 4 and 5  united with the division lines. Instead of air-insulated strip conductors, the conductors of the divider and the phase shifter can also be micro strips on the surface of a low-loss dielectric board. Low-loss material is more expensive than ordinary circuit board material, but on the other hand the size of the board required is relatively small, The inventive idea can be applied in many ways within the limits set by the independent claim  1 .