Abstract:
A mobile radio telephone includes a transmitter output stage and an antenna. An impedance matching network is inserted between the transmitter output stage and the antenna. The matching network has a transformation factor (T) for adjusting the transmitter load impedance (Z PA ) to match with the antenna input impedance (ZA Ant ). The impedance matching network includes at least two impedance transformers that form at least two transformation factors by alternatively connecting in parallel the two impedance transformers.

Description:
GOVERNMENT LICENSE RIGHTS 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. F29601-99-C-0019 awarded by the United States Air Force. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a mobile radio telephone comprising at least a transmitter output stage, at least an antenna and at least an impedance matching network inserted between transmitter output stage and antenna, which matching network has a transformation factor for adjusting a transmitter load impedance to match with an antenna input impedance. 
     The invention further relates to a transmitter including at least a transmitter output stage, at least an antenna and at least an impedance matching network inserted between transmitter output stage and antenna which matching network has a transformation factor for adjusting a transmitter load impedance to match with an antenna input impedance. 
     BACKGROUND OF THE INVENTION 
     From disclosure DE 42 22 190 A1 is known such a radio telephone comprising an antenna switch for alternately operating in the transmitting and receiving states. For adjustment purposes is then arranged a λ/4 waveguide between transmitter output stage ( 31 ) and antenna ( 1 ). As a result, the special transformation properties for the initial and terminal resistance Z A  and Z E  respectively, of the λ/4 waveguide can be used. The terminal resistance Z E  of a λ/4 waveguide is transformed on the input of the λ/4 waveguide in dependence on its wave resistance Z. By means of the parameters of the waveguide range such as geometric dimensions and material constants (ε,μ), the wave resistance Z can be set for the wavelength λ to be used. The transformation of the resistance is generally effected in accordance with the relation Z A =Z 2 /Z E . In the transmitting state, the λ/4 waveguide ( 32 ) connected to the transmitter output stage ( 31 ) transforms the antenna impedance in accordance with the requirements of the transmitter output stage ( 31 ). In this manner, the transmitter output stage ( 31 ) in the transmitting state is adjusted to match with the antenna ( 1 ) and the output power of the transmitter output stage ( 31 ) is fully passed on to the antenna ( 1 ). As a result of, inter alia, possibilities to change the wave resistance Z, a suitable transformation factor may be set. The second λ/4 waveguide ( 32 ) may be discarded for the transformation of the resistance when the wave resistance of the waveguide is set equal to the antenna resistance (see above, transformation equation). 
     The load impedance of a power amplifier in a transmitter output stage is adjusted to match with the input of an antenna to achieve a maximum efficiency of the transmitter output stage (ratio of the power fed to the antenna to the overall power used). An adjustment by means of a state-of-the-art λ/4 waveguide, however, provides a good efficiency (fixed transformation factor) only for an operating state with a certain output power P A  and supply power U B  of the power amplifier. In mobile radio telephones, for example, for mobile radio in accordance with the GSM standard, however, various levels are determined for the output power, which are to be exactly set by means of a power control in dependence on the transmission properties. Furthermore, the supply voltage U B  varies in accordance with a discharge curve for the battery used. Consequently, as against the adjusted operating state, for example, with an increased supply voltage U B  and regulated output power P A , the efficiency drops because the voltage U A  on the output of the transmitter output stage remains constant. This also provides that the current I A  on the output retains the regulated constant value, so that the efficiency diminishes strongly. All in all, often only a reduced efficiency of the transmitter output stage is achieved. This is a considerable disadvantage, especially for battery-operated mobile radio telephones because of the consequent shortened service life. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the invention to provide a mobile radio telephone that has improved the efficiency of the transmitter output stage in different operating states. 
     The object is achieved in that at least two impedance transformers in the impedance matching network form at least two transformation factors by alternatively connecting them in parallel. In this manner, at least two different load impedances may be set for the transmitter output stage, whereas even more transformation factors are conceivable by suitable circuit formations. The arrangement in the impedance matching network may be adapted in a highly flexible manner to requirements especially by means of software-controlled switches, without the necessity to make changes in the hardware. This provides that a proper efficiency of the transmitter output stage can be achieved in different operating states, because the transmitter output stage has a load impedance from which the voltage amplitude develops that is the highest possible for the set transmitter power. 
     A preferred embodiment of the invention is that at least two λ/4 waveguides form the impedance transformers. The λ/4 waveguides may be arranged as coaxial conductors or as microstrips. Also a copy of a λ/4 waveguide by means of an LC network that has the respective transmission properties may be used. 
     In an advantageous further embodiment, a first λ/4 waveguide in the impedance matching network is coupled, on the one hand, to the output of the transmitter output stage and, on the other hand, to the input of the antenna and a second λ/4 waveguide is coupled, on the one hand, to the output of the transmitter output stage and, on the other hand, to at least one switch which establishes in a first position the connection to a ground terminal and in a second position to the input of the antenna. The switch may be formed by a PIN-diode high-frequency switch or a bistable mechanical relay. With such an arrangement, the control of the switches may be taken over by software, so that a fast and flexible control of the impedance matching network is possible. Furthermore, by arranging the switch on the side of the antenna, it is avoided that large losses arise in the switch. Since this side is high-ohmic, there is only a small current flowing in the transmitter output stage compared to the maximum transmitting current. 
     Besides, the object of the invention is further achieved by a transmitter in which at least two impedance transformers in the impedance matching network form at least two transformation factors by alternatively connecting them in parallel. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows a block diagram of the functional blocks of the mobile radio telephone, 
     FIG. 2 gives a diagrammatic representation of a part of the transmitting branch of the mobile radio telephone, and 
     FIG. 3 gives a detailed representation of an impedance matching network between a transmitter output stage and an antenna. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows in a block diagram the functional blocks of a transmitting and a receiving branch of a mobile radio telephone such as corresponds, for example, to a mobile telephone according to the GSM standard. An antenna  1  is coupled to an antenna duplexer  2  which connects the receiving branch and transmitting branch respectively, to the antenna  1  in a receiving and transmitting state. In the receiving state, the analog radio signals reach an A/D converter  4  via a receiving circuit  3 . The generated digital signals are demodulated in a demodulator  5  and subsequently applied to a digital signal processor (DSP)  6 . In the DSP  6  are performed in succession the functions of equalization, decryption, channel decoding and speech decoding which are not shown separately. Analog signals are generated by a D/A converter  7 , which signals are delivered through a loudspeaker  8 . 
     In the transmitting state, the analog speech signals captured by a microphone  9  are converted by an AID converter  10  and then applied to a DSP  11 . The DSP  11  carries out the functions of speech coding, channel coding and encryption which are complementary to the receiving state, so that all the functions are carried out by a single DSP. The binary coded data words are GMSK modulated in a modulator  12  and then converted into analog radio signals in a D/A converter  13 . A transmitter output stage  14  including a power amplifier generates the radio signal to be transmitted by the antenna  1 . 
     In the further description with reference to FIG. 2, only the power amplifier  15  of the transmitter output stage  14  is considered. The power amplifier  15  is connected to the antenna  1  via an impedance matching network  16  which has a transformation factor T. The antenna  1  has an input resistance Z Ant  which has a value of, for example, 75Ω. For obtaining optimum efficiency of the transmitter output stage  14 , the input resistance Z Ant  is to be adjusted to match with a required load Z PA  of the power amplifier  15 . For large transmitter powers (around 2 W) a load Z PA  of about 1-2Ω for the power amplifier  15  is to be set, whereas a more high-ohmic load is necessary for a required transmitting power that is smaller. As a result, the impedance matching network  16  transforms the input resistance Z Ant  of the antenna  1  by the transformation factor T.Z Ant =Z PA  to the output of the power amplifier  15 . 
     The power amplifier  15  is supplied with power by a battery (not shown) which has an operating voltage U B . On the output the power amplifier produces an output voltage U A  which is applied to the respective load Z PA . In a mobile radio telephone according to the GSM standard, a great many adjustable levels are determined for the transmitting power. The output power P PA  of the power amplifier  15  which power works on the real portion of the load Z PA , that is, is radiated as a transmitting power, is therefore always regulated to an exact value. According to P PA =½.U A   2 /Z PA  and with a constant load Z PA , the output voltage U A  also remains constant. As a result, according to Z PA =U A /I A , also the output current I A  remains constant, so that an excessive power consumption is frequently found. The load Z PA , that is, also the associated transformation factor T, is selected for a specific operating point of the output voltage U A . According to η=P PA /P Gesamt  and an operating voltage of U B  there is an efficiency η (typically η&gt;40%) for the power amplifier  15 . This efficiency degrades once the operating voltage U B  is increased compared with the set operating point, that is, in accordance with the typical discharge curve of the battery used. For optimizing the efficiency for various operating points of the operating voltage U B , the transformation factor T and thus the load Z PA  of the power amplifier  15  can be switched between two values. This will be described with reference to FIG.  3 . 
     The impedance matching network  16  is shown in more detail in FIG. 3. A first impedance transformer  17  arranged as a λ/4 waveguide is connected, on the one hand, to the output of the power amplifier  15  and, on the other hand, to the input of the antenna  1 . This λ/4 waveguide  17  is designed such that it has the impedance Z W =(Z Ant .Z PA ) with the transmitting frequency used, to adjust the connected load Z Ant  to match with the required load impedance Z PA  of the power amplifier  15 . The value results from the equation for a λ/4 transformer according to which for the input impedance Z EIN  of a λ/4 waveguide in dependence on the line impedance Z W  and the impedance on the output Z AUS  there holds: Z EIN =Z W   2 /Z AUS . By suitably selecting the parameters for the λ/4 waveguide  17 , a respective impedance Z W  and thus a transformation factor T=Z PA /Z Ant =(Z W /Z Ant ) 2  may be set. The λ/4 waveguide  17  may then be arranged as a stripline, coaxial cable or be formed by an LC network which copies such a λ/4 waveguide. The impedance may be set by means of the geometric dimensions and material constants. The waveguide may also be copied by copying the line with discrete building blocks. The copy of the line may then contain both inductive (L) and capacitive (C) elements which correspond to the respective propagation constant per unit length (L′=L/mm or C′=C/mm) of the waveguide to be copied. In all possible realizations, the effective length of the waveguide can be adjusted to match with the transmitting frequency used. 
     A second impedance transformer  18  is also realized by a λ/4 waveguide, which is connected in parallel to the first λ/4 waveguide  17  in the power amplifier  15 . At the other end, the second λ/4 waveguide  18  is connected to a switch  19  which connects in a first switch position to a ground terminal and in a second switch position to the antenna  1 . The short-circuit at the end in the first switch position is transformed into an open-circuit situation at the beginning by the λ/4 waveguide  18 . This causes the second λ/4 waveguide  18  to be inactive in this switch position. On the other hand, because of the parallel arranged λ/4 waveguides  17  and  18  in the second switch position, a new λ/4 waveguide arises. The impedance of the new λ/4 waveguide is the result of the parallel combination of the impedances of the λ/4 waveguides  17  and  18  with Z W =(Z 18 .Z 17 )/(Z 17 +Z 18 ). This also causes the transformation factor T of the impedance matching network  16  to change. 
     In this manner, two different transformation factors can be realized surprisingly simply and so can an adjustment of the power amplifier  15  to match with the antenna  1  at two different operating points. The fact that the switch  19  is switched to the antenna provides a switching to a high-ohmic position. The small load impedance Z PA  of the power amplifier  15  of about 1-2Ω is transformed up, so that the switch  19  is loaded with a small current. Therefore, a dimensioning of the switch to the maximum transmitting current is not required, and only slight losses occur in the switch  19 . Bistable mechanical relays or conventional high-frequency switches including PIN diodes can be used for the switch  19 . Mostly the adjustment of the transmitter output stage  14  to match with the antenna  1  can be effected in two transformation stages, the first stage being included in the transmitter output stage  14 . As a result, the impedance matching network  16  according to the invention need not convert Z PA =1Ω into Z Ant =75Ω. 
     For example, two waveguides are used as λ/4 waveguides  17  and  18 . The wide waveguide  17  has a wave resistance of Z 17 =53Ω and the narrow waveguide  18  of Z 18 =128Ω. With an antenna input impedance of Z Ant =75Ω there is a transformation to a value of 37.5Ω in the first switch position. This transformation is completed by a further stage which is included in the transmitter output stage  14  and realizes a transformation factor of n 2 =17. This additional transformation is developed by integrated transformers as well as by available parasitic inductivities and capacitances. This provides a load of Z PA =2.2Ω for the first switch position. In the case of second λ/4 waveguides  18  arranged in parallel, the active impedance of the new λ/4 waveguide is Z W =(Z 18 .Z 17 )/(Z 17 +Z 18 )=37.5Ω, so that the impedance matching network  16  transforms by the factor T=0.25 into 18.8Ω. By means of the additional transformation in the transmitter output stage  14 , there is a load of Z PA =1.1Ω. By turning the switch  19 , the load changes, so that the output current I A  of the power amplifier  15  is reduced with a constant output voltage U A . This reduces the power consumption of the transmitter output stage  14 , for example, with increased operating voltage U B  of a battery despite constantly controlled output power. A drop of the efficiency from the η&gt;40% to a value around 30% as a result of the mismatch may thus be avoided. Furthermore, when the load is switched over to, only the ohmic load is changed, whereas the resonance frequency of oscillator circuits set to the transmitting frequency is not off-tuned.