Abstract:
The invention pertains to a method and an arrangement for linearizing a radio receiver ( 20 ). The invention is advantageously applied in the reception circuits of mobile stations. A receiver according to the invention monitors the signal strength on the receive channel and its neighboring channels. In addition, the receiver can determine the quality of the detected signal by calculating its bit error ratio or signal-to-noise ratio. In normal conditions, i.e. when the signal strength is satisfactory on the receive channel and normal on the neighboring channels, the supply currents of the front-end amplifiers (A 1 , A 2 ) and at least the first mixer (M 1 ) of the receiver are kept relatively small. If the signal strength on the receive channel goes below a predetermined value or, on a neighboring channel, exceeds a predetermined value, said supply currents are increased. This may happen also if the bit error ratio or signal-to-noise ratio reaches a predetermined limit. The supply currents may be adjusted in one or more steps.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and an arrangement for linearizing a radio receiver. The invention can be advantageously applied in the reception circuits of mobile stations. 
     2. Description of the Related Art Including Information Disclosed Under 37 C.F.R 1.97 and 1.98. 
     The quality of a signal in a radio receiver is degraded by noise and interference added to the signal on the transmission path, noise in the reception circuitry, as well as by noise and interference indirectly caused by other radio signals. The latter refers to a situation in which a signal on a neighboring channel, which momentarily is considerably stronger than the signal to be received, shifts the operating point of the high-frequency amplifier and/or mixer towards non-linearity and saturation. This degrades the signal-to-noise ratio at the amplifier output. In the mixer the increase of non-linearity will intensify intermodulation products and, thus, increase the overall noise and interference level. 
     A basic method for reducing noise and interference is filtering, which takes place in all receivers in many stages of operation and which will not be discussed further in this description. Rather, we will discuss the widening of the linear range of amplifiers and mixers as a means of reducing noise and interference. The linear range gets wider as the operating voltage is increased or, if the operating voltage remains unchanged, as the supply current is increased. So, the signal quality can be improved by using more energy in the receiver. In the prior art, said circuits are specified such that the signal quality requirements will be met even in poor noise and interference conditions. A disadvantage of this method, as regards portable devices, is that it requires a relatively large supply of energy and, therefore, shortens the battery life. 
     From the prior art it is also known a method in which the strength of the received signal is monitored and the supply current of the mixer is increased if the strength of the signal drops. An advantage of the method is that it improves the quality of the signal and reduces intermodulation products if the field received is weak. A disadvantage, however, is that the supply current will be increased even if the receiver received no other, interfering, signals. Moreover, the amplifiers have to be rated for the worst case, which results in a relatively large continuous current consumption in the amplifiers. 
     The object of the invention is to reduce the aforementioned disadvantages related to the prior art. 
     BRIEF SUMMARY OF THE INVENTION 
     The basic idea of the invention is as follows: A receiver monitors the signal strength on the receive channel and neighboring channels. In addition, the receiver can determine the quality of the detected signal by calculating its bit error ratio or signal-to-noise ratio. In normal conditions, i.e. when the signal strength is satisfactory on the receive channel and ordinary on the neighboring channels, the supply currents of the receiver&#39;s front-end amplifiers and at least the first mixer are kept relatively low. If the signal strength goes below a certain value on the receive channel or exceeds a certain value on a neighboring channel, said supply currents are increased. This may happen also if the bit error ratio or signal-to-noise ratio deteriorates down to a certain limit. Adjustment of the supply currents may take place in one or more steps. 
     The advantage of the invention is that the energy consumption of the receiver can be reduced without degrading the signal quality. This means longer life for the battery or, if the life of the battery is kept unchanged, that a smaller battery can be used. If the mean energy consumption is kept the same as in receivers of the prior art, the advantage is a better signal quality. 
     The method according to the invention for linearizing a radio receiver in a signal chain in which 
     a signal received from an antenna is amplified and the amplified signal is detected, and 
     the received signal is processed by at least one linear unit is characterized in that 
     at least one quality factor of the detected signal is measured, and 
     the supply current of said at least one linear unit is controlled on the basis of the measurement results. 
     The arrangement according to the invention for linearizing a radio receiver comprising means for detecting a received signal and at least one linear unit with current supply circuits for processing the received signal is characterized in that it comprises 
     means for measuring at least one quality factor of the detected signal, and 
     means for controlling the linear unit&#39;s supply current on the basis of the measurement results. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
     Preferred embodiments of the invention are disclosed in the dependent claims. 
     The invention will now be described in detail. Reference will be made to the accompanying drawings wherein 
     FIG. 1 is a block diagram of a typical radio receiver according to the prior art, 
     FIG. 2 shows the structure of FIG. 1 supplemented with a functional block according to the invention, 
     FIG. 3 shows an example of supply current control according to the invention, 
     FIG. 4 shows a second example of supply current control according to the invention, 
     FIG. 5 shows a third example of supply current control according to the invention, 
     FIG. 6 shows an example of amplifier supply current control according to the invention, and 
     FIG. 7 shows an example of mixer supply current control according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a simplified example of a radio receiver without the low-frequency components. An antenna ANT is coupled to a duplex filter F 1 , which is needed e.g. in mobile phones. Filter F 1  is coupled to a low-noise pre-amplifier A 1 . Amplifier A 1  is coupled to a channel selector filter F 2  in the tuner, and filter F 2  is coupled to an amplifier A 2 . The output of amplifier A 2  is coupled to the first input of a first mixer M 1 . The second input of mixer M 1  is connected with a local oscillator O 1 . The output of mixer M 1  is coupled to an amplifier A 3 , and the output of the latter is coupled to a first intermediate frequency (IF) filter F 3 . The output of filter F 3  is coupled to the first input of a second mixer M 2 . The second input of mixer M 2  is connected with a second local oscillator O 2 . The output of mixer M 2  is coupled to a second IF filter F 4  and the output of the latter to a detector DET. Detector DET provides a baseband signal s b . The amplifiers and mixers in the receiver are in this description and particularly in the claims called “linear units”. In the ideal case they operate in a totally linear manner in all situations. In practice, non-linearity is present in their operation which causes signal degradation. The higher the signal levels to be processed, the more there is non-linearity in the operation of the linear units. 
     FIG. 2 shows a similar signal path structure as FIG.  1 . Unit  21  includes the linear units, or amplifiers A 1 , A 2  and A 3  as well as mixers M 1  and M 2 . What is additional to FIG. 1 is a control unit  22  according to the invention for controlling the supply currents of the linear units. Detector DET provides information about the signal strength (RSS) on the channel to which the receiver is tuned. In the example of FIG. 2 the transmitted signal is digital. Unit  23  calculates the bit error ratio (BER) that describes the quality of the received and detected signal s b . The operation of unit  23  may be realized in the form of a computer program. Signals RSS and BER are taken to the control unit  22  the outputs of which are coupled to the linear units. Control signal c A1  sets the supply current of amplifier A 1 . Similarly, signal c A2  sets the supply current of amplifier A 2 , signal c M1  that of mixer M 1 , signal c A3  that of amplifier A 3 , and signal c M2  that of mixer M 2 . 
     Alternatively, the quality of the detected signal can be determined by calculating its signal-to-noise ratio. In that case, unit  23  provides signal SNR (signal-to-noise ratio). 
     FIG. 3 relates to a simple example of supply current control for the linear units. A control unit  32  only receives the receive signal strength RSS information which has threshold values S 31  and S 32 . The control unit  32  produces one one-bit control signal c, which is taken to all linear units A 1 , A 2 , M 1 , A 3  and M 2 . The supply current of a given linear unit has two values. Supply currents are controlled with signal c according to the table below. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 RSS 
                 c 
                 I i   
               
               
                   
                   
               
             
             
               
                   
                 ≦S31 
                 0 
                 I ih   
               
               
                   
                 &gt;S31, &lt;S32 
                 1 
                 I il   
               
               
                   
                 ≧S32 
                 0 
                 I ih   
               
               
                   
                   
               
             
          
         
       
     
     Subindex I refers to a linear unit. I il  stands for the lower supply current value of the linear unit and I ih  for the higher supply current value. When the signal strength RSS is not more than S 31 , signal c is 0 and the linear unit supply currents are at their higher values. The relatively large supply currents help reduce the effects of possible interference from outside the receive channel. When the signal strength RSS is between S 31  and S 32 , signal c is 1 and the supply currents are at their lower values. The relatively small supply currents help reduce energy consumption. When the signal strength is at least S 32 , signal c is 0 and the supply currents are again at their high values. Increasing the supply currents this way prevents the operating points of the linear units from shifting into the non-linear region when there is an exceptionally strong signal on the receive channel. 
     FIG. 4 relates to a second example of supply current control for the linear units. A control unit  42  receives from detector DET an indication about either the receive channel signal strength RSS or the strength of any signal on the reception band. Signal a from the receiver&#39;s tuner control indicates the channel. Signal RSS has one threshold value S 4  which corresponds to a relatively low receive signal strength. Signal RSS n  also has one threshold value Sn, which corresponds to a relatively high signal strength on the channel. The control unit  42  provides two one-bit control signals c A  and c M . Signal c A  is 0 when signal RSS is greater than S 4  and signal RSS n  is at the same time smaller than Sn. Otherwise, signal c A  is 1. Signal c A  is taken to amplifiers A 1  and A 2 . Signal c M  is 1 when signal RSS is not greater than S 4  and signal RSS n  is at the same time at least Sn. Otherwise, signal c M  is 0. Signal c M  is taken to mixer M 1 , amplifier A 3  and mixer M 2 . If signal c A  or signal c M  is 0, the supply current in the linear unit in question is lower, and, conversely, if signal c A  or c M  is 1, the supply current is higher. Thus, the linear unit supply currents are controlled according to the table below. RSS n  stands for the signal strength of the neighboring channel in which the signal strength is greater. Subindex A refers to linear units A 1  and A 2 , and subindex M to linear units M 1 , A 3  and M 2 . Subindex I refers to the lower supply current of the linear unit, and subindex h to the higher supply current. For example, I M1  means that the control current in mixers M 1  and M 2  and in amplifier A 3  is set to the lower value. 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 RSS n   
                 RSS 
                 I A   
                 I M   
               
               
                   
                   
               
             
             
               
                   
                 &lt;Sn 
                 &gt;S4 
                 I Al   
                 I Ml   
               
               
                   
                 &lt;Sn 
                 ≦S4 
                 I Ah   
                 I Ml   
               
               
                   
                 ≧Sn 
                 &gt;S4 
                 I Ah   
                 I Ml   
               
               
                   
                 ≧Sn 
                 ≦S4 
                 I Ah   
                 I Mh   
               
               
                   
                   
               
             
          
         
       
     
     When the signal strength on the receive channel is normal or relatively high, and on the neighboring channels normal or relatively low, all linear unit supply currents are set to the lower values. When the signal strength on the receive channel drops relatively low, the supply currents of the front-end amplifiers A 1  and A 2  are increased. When the signal strength on the receive channel is at least normal, but relatively high on a neighboring channel, the supply currents of amplifiers A 1  and A 2  are set to the higher values and the supply currents of the other linear units to the lower values. When the signal strength on the receive channel is relatively low and on a neighboring channel relatively high, the supply currents of all linear units are set to the higher values. 
     FIG. 5 relates to a third example of supply current control for the linear units. A control unit  52  receives a receive channel signal strength indication RSS and receive channel bit error ratio indication BER. Signal RSS has two threshold values S 51  and S 52 . Signal BER also has two threshold values E 1  and E 2 . The control unit provides three supply current control signals c A1 , c AM1  and c AM2 . Signal c A1  has two bits and is taken to amplifier A 1 . So, the supply current of amplifier A 1  may take four values. Signal c AM1  also has two bits and is taken to amplifier A 2  and mixer M 1 . So, the supply currents of these units may take four values each. Signal c AM2  has one bit and is taken to amplifier A 3  and mixer M 2 . So, the supply currents of these units may take two values each. The logic operation of the control unit  52  is arranged such that the supply current control for the linear units is in accordance with the table below, for example. For simplicity, supply current values are denoted by just numbers on the right-hand side of the table. Number  1  means the lowest supply current values, number two the second lowest values, number  3  the third lowest values and number  4  the highest supply current values. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                 RSS 
                 BER 
                 I A1   
                 I A2  and I M1   
                 I A3  and I M2   
               
               
                   
               
             
             
               
                 &gt;S51 
                 &lt;E1 
                 1 
                 1 
                 1 
               
               
                 &gt;S51 
                 &gt;E1, &lt;E2 
                 3 
                 2 
                 1 
               
               
                 &gt;S51 
                 &gt;E2 
                 4 
                 4 
                 2 
               
               
                 &lt;S51, &gt;S52 
                 &lt;E1 
                 2 
                 1 
                 1 
               
               
                 &lt;S51, &gt;S52 
                 &gt;E1, &lt;E2 
                 3 
                 2 
                 1 
               
               
                 &lt;S51, &gt;S52 
                 &gt;E2 
                 4 
                 4 
                 2 
               
               
                 &lt;S52 
                 &lt;E1 
                 2 
                 2 
                 1 
               
               
                 &lt;S52 
                 &gt;E1, &lt;E2 
                 3 
                 3 
                 2 
               
               
                 &lt;S52 
                 &gt;E2 
                 4 
                 4 
                 2 
               
               
                   
               
             
          
         
       
     
     When the signal strength RSS on the receive channel is normal, i.e. greater than S 51 , and the bit error ratio BER relatively low, the supply currents of all linear units are kept relatively small. When the bit error ratio is relatively high, i.e. greater than threshold value E 2 , the supply currents of all linear units are set to the maximum values regardless of the signal strength value. If the bit error ratio is high even though the signal strength on the receive channel is normal, the probable cause is an exceptionally strong signal on a neighboring channel, so that there is a special reason to improve the linearity of the linear units. The rest of the nine cases shown on the table fall between these cases. For example, row  5  of the table depicts a case in which the signal strength is somewhat smaller than normal and the bit error ratio is somewhat greater than normal. The supply current of the first amplifier A 1  is thereby set to the second highest value, the supply currents of amplifier A 2  and mixer M 1  are set to the second smallest values, and the supply currents of amplifier A 3  and mixer M 2  are set to the smallest values. Naturally, suitable lag is arranged in the operation of the control unit  52  lest oscillatory phenomena occur in the control process. 
     FIG. 6 shows an example of a linear unit&#39;s supply current control. In this example the linear unit  61  is an amplifier realized by a differential pair. The supply current control circuit  62  comprises transistors Q 1  and Q 2 , resistors R 1 , R 2  and R 3 , as well as switches k a  and k b . Switch k a  is in series with resistor R 2  and switch k b  is in series with resistor R 3 . Both series connections are coupled in parallel with resistor R 1 . One end of the whole parallel connection, which thus has three branches, is coupled to the higher power supply voltage V CC  terminal of the voltage supply and the other end to the collector of transistor Q 1 . The collector and base of transistor Q 1  are coupled together and the emitter is coupled to the lower power supply voltage V EE  terminal of the voltage supply. The collector of transistor Q 2  is coupled to amplifier  61  so that the total current I of the transistors in the amplifier is brought to the collector. The base of transistor Q 2  is coupled to the base of transistor Q 1  and the emitter of transistor Q 2  is coupled to the emitter of transistor Q 1 . Thus, transistor Q 2  has the same control voltage V BE  as transistor Q 1 . If the transistors were identical, their collector currents would be equal as well. From the point of view of conservation of energy, transistor Q 1  is advantageously manufactured such that its collector current kI is considerably smaller than the collector current I of transistor Q 2 . So, the transistor pair Q 1 , Q 2  serves as a current amplifier, the current gain 1/k being  10 , for example. In the aforementioned three-branch parallel connection the current of resistor R 1  is I 1 , the current of resistor R 2  is I 2  and the current of resistor R 3  is I 3 . Thus the current kI of transistor Q 1  is the sum I 1 +I 2 +I 3 . Current I 2  is zero if switch k a  is open, and current I 3  is zero if switch k b  is open. Switches k a  and k b  are controlled with a two-bit digital signal c=AB. When bit A is 0, switch k a  is open, and when bit A is 1, switch k a  is closed. Correspondingly, bit B determines the status of switch k b . The control for the total current kI is thus in accordance with the table below. The supply current I of amplifier  61  follows current kI according to the description above. 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 B 
                 A 
                 kI 
                 kI/I 1   
               
               
                   
                   
               
             
             
               
                   
                 0 
                 0 
                 I 1   
                 1 
               
               
                   
                 0 
                 1 
                 I 1  + I 2   
                 2 if R1 = R2 
               
               
                   
                 1 
                 0 
                 I 1  + I 3   
                 3 if R1 = 2 · R3 
               
               
                   
                 1 
                 1 
                 I 1  + I 2  + I 3   
                 4 if R1 = R2 = 2 · R3 
               
               
                   
                   
               
             
          
         
       
     
     In FIG. 6, the supply current control circuit  62  is realized with bipolar transistors. A corresponding structure may naturally be realized using MOS (Metal Oxide Semi-conductor) transistors, for example. The circuit realized with transistors Q 1  and Q 2  as well as other circuits operating on the same principle are called “current mirrors” in the claims. 
     FIG. 7 shows another example of controlling a linear unit&#39;s supply current. In this case, the linear unit  71  is an analog multiplier used as a mixer: It receives a RF or IF signal and a subcarrier from a local oscillator. The mixer produces a signal the spectrum of which includes the spectrum of the input signal shifted both upwards and down. The control circuit  72  for the supply current I of the analog multiplier  71  comprises transistors Q 1  and Q 2 , resistors R 1  and R 2  as well as switch k. The collector of transistor Q 1  is coupled to the first branch of the analog multiplier  71 , and the collector of transistor Q 2  is coupled to the second branch of the analog multiplier. The emitter of transistor Q 1  and the emitter of transistor Q 2  are both coupled to the negative voltage V EE  terminal of the voltage supply, the first via a resistor the resistance of which is relatively low, and the latter via resistor R 4 . The bases of transistors Q 1  and Q 2  are coupled to one another. When the resistances of resistors R 3  and R 4  are identical and transistors Q 1  and Q 2  are identical, the collector currents of the transistors are also identical, i.e. half of the analog multi-plier&#39;s  71  supply current I. Resistor R 2  and switch k are coupled in series, and this series connection is coupled in parallel with resistor R 1 . One end of the parallel connection is coupled to signal ground and the other end to the bases of transistors Q 1  and Q 2 . Switch k is controlled with a one-bit digital signal c. When signal c is 0, switch k is open and the control current I B  of transistors Q 1  and Q 2  is V B /R 1 , where V B  is the transistors&#39; base voltage. When signal c is 1, switch k is closed and the control current I B  is V B /R 1 +V B /R 2 . Thus, the control current I B  increases, causing the supply current I to increase as well. Desired values for the supply current I can be achieved by choosing suitable resistances for resistors R 1 , R 2 , R 3  and R 4 . Naturally, a current mirror structure could also be used for controlling the mixer supply current. 
     Above it was described techniques according to the invention for controlling the supply currents of linear units in a radio receiver. The invention is not limited to the cases described. The number of linear units to be controlled may vary. Direct sampling does not have any mixers as such. In that case the supply current of analog-to-digital converter can be controlled. 
     In that case, the supply current of the detector can be controlled. The number of values set for a particular supply current may vary. The manner in which the degradation of the received signal is taken into account in the control of supply currents may vary greatly. The inventional idea can be applied in numerous ways within the scope defined by the independent claims.