Patent Publication Number: US-2009233570-A1

Title: Receiver front-end with low power consumption

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates to receivers, transceiver, and integrated circuits for information transmission. More particularly, this invention relates to a radio frequency (RF) receiver front-end for data communication and devices based thereon. 
     2. Description of the Related Art 
     The competition in the communication market is extremely fierce and business success relies increasingly upon the constant product innovation. For a semiconductor provider specialized in communications ICs and system solutions, this means innovations in both system architecture and circuit techniques in order to be able to realize low-cost, low-power, and small form-factor terminals. 
     Low power is a research topic both at universities and in the industries. For many applications, such as portable applications for example, the need for low power consumption is most important. The most power in a receiver is dissipated in the front-end, so logically it is the primary focus of low-power research activities. 
     In an RF communication system, an RF receiver is employed that extracts a baseband signal from an RF carrier. This process involves a frequency translation from the RF carrier to the baseband frequency. There are two types of receivers depending on the manner this frequency translation takes place: heterodyne receivers and homodyne receivers. The same is the case for optical communication systems, where heterodyne receivers and homodyne receivers are employed as well. 
     Conventional homodyne and heterodyne receivers comprise a voltage controlled oscillator (VCO) issuing a signal having a frequency f LO . To be more specific, the frequency f LO  in a receiver is normally generated by the VCO placed in a phase-locked-loop (PLL). The frequency f LO  is chosen either to be equal to the frequency of a radio frequency (RF) signal received at the transceiver&#39;s antenna (referred to as homodyne receiver), thus converting the RF signal directly down to the base band. Such homodyne receivers are by some manufacturers referred to as zero intermediate frequency receivers. Or, a frequency f LO  very close to the radio frequency f RF  is chosen in a so-called super heterodyne receiver, translating the RF signal in a first step to the intermediate frequency (IF), then to base band in a second or even third step. 
     A conventional homodyne RF receiver  10  with a VCO  15  placed in a phase-locked-loop (PLL) is illustrated in  FIG. 1 . The receiver  10  comprises an antenna  11 , a low-noise amplifier (LNA)  12 , and mixer  13 . A local oscillator signal f LO  is generated by a part of the receiver that comprises a reference oscillator  18 , a phase detector  17 , a divider  16 , a VCO  15 , a low-pass filter  19 , and a buffer/amplifier  14 . An input signal s(t) comprising a high frequency component (f RF ) and a local oscillator signal (f LO ) are applied to the inputs of the mixer  13  and the mixer provides for a down-conversion of the input signal s(t) to a signal r(t) at a lower frequency band. 
     In such a homodyne RF receiver but also in heterodyne RF receivers, the LO frequency (f LO ) is very high. In GSM, for example, f RF  lies in 900 MHz, in DCS in 1800 MHz, and in Bluetooth, it is even higher in the ISM band of 2450 MHz. In a receiver, the VCO  15 , the following buffer  14 , and the divider  16 , etc. operate roughly at such a high frequency, and hence consume quite a lot of power. Particularly the VCO  15 , which, quite often, is designed to operate at even doubled frequency in order to generate in-phase and quadrature signals of the desired LO frequency f LO . The current drawn by a LC VCO, for example, is proportional to squared oscillation frequency, i.e., 
       Idd VCO ≈f LO   2   (1) 
     So that switching from 900 MHz to 1800 MHz the power consumption of a VCO will be quadrupled. 
     In order to provide sufficient isolation and to provide an adequate amplitude at the mixer input, a buffer/amplifier  14  is almost always required between the VCO  15  and the mixer  13 . The buffer/amplifier circuit  14  has to be wideband in order to allow the mixer  13  for current commutating with fast switching. For this reason, practical VCO buffers/amplifiers  14  are designed to be wideband and, therefore, consume quite a lot of power. For MOSFET implemented buffers/amplifiers  14 , the relationship between its drain current Ids and the resultant unity-current gain frequency f T  is given by 
       Ids≈f T   2   (2) 
     The unity-current gain frequency f T  of the MOS transistor has to match the LO frequency f LO , so every doubling of the LO frequency f LO  requires the quadrupled power consumption of the buffer/amplifier  14  (equal to Idd BF *Vdd in  FIG. 1 ) 
     Frequency dividers (also referred to as prescalers) are essentially digital circuits using logic gates such as dFFs. The dynamic power consumption of a logic circuit is proportional to the clock frequency. So that every doubling of the clock frequency requires a doubling of the divider&#39;s dynamic power consumption. 
     The homodyne and heterodyne receiver front-end used in optical communication systems also consumes a substantial amount of power, since part of the signal processing is done electrically. An example of a homodyne receiver is described in the European patent application as published on 9 May 2001 under the publication number EP 1098459-A2. 
     One of the most straightforward techniques for reducing power consumed by RF receivers is to lower the voltage. This represents a reduction in power consumption. However, in order for a receiver to maintain a certain dynamic range and thus the ability to operate in poor RF conditions requires a certain voltage level at least for the analog receiver front-end. 
     There are several other obvious techniques for reducing power consumption. At the system level, one should make partitioning choices so that as much circuitry as possible can be turned off when it is not needed. In addition, functions should be allocated where they can be executed with the minimum power. A certain reduction of the power consumption can further be achieved if one reduces the clock rate of the digital part of a receiver, for instance. 
     In addition, great efforts have been made and almost everything has been tried to minimize the power of a receiver front-end in particular, since the receiver front-end is known to consume a substantial amount of power. These efforts include increasing the inductance of the LC VCO, decreasing the power supply voltage, as indicated above, and using half-swing logic, just to mention some examples. 
     These conventional approaches to a reduction of the power consumption are not going far enough. The reductions achievable are not sufficient. 
     BRIEF SUMMARY 
     It is thus an objective of the present invention to provide a scheme for reducing the power consumption of a receiver without impacting the receiver&#39;s performance. 
     In one embodiment, a receiver for receiving an RF signal comprising a high frequency component f RF  includes a local oscillator, at least one mixer and at least one filter. The local oscillator provides at least one square wave local oscillator signal. The at least one square local oscillator signal has a fundamental frequency component with a frequency of f LOnew , where f LOnew  is at least one-third (⅓) of the high frequency f RF , and the at least one square local oscillator signal has at least one odd harmonic frequency component that is an odd multiple of the fundamental frequency component of f LOnew . 
     The at least one mixer down-converts the RF signal to a desired output frequency. The at least one mixer has a first input to which the RF signal is applied, a second input to which the at least one square wave local oscillator signal is applied, and an output from which a composite output signal is provided. The composite output signal includes a desired output signal produced by multiplying the fundamental frequency component of the at least one square local oscillator signal with the RF signal, and an undesired output signal produced by multiplying the at least one odd harmonic frequency component of the at least one square local oscillator signal with the RF signal. The desired output signal has an amplitude and a frequency, and the undesired output signal has an amplitude greater than the amplitude of the desired output signal and has a frequency greater than the frequency of the desired output signal. 
     The at least one filter filters the composite output signal of the at least one mixer to select the desired output signal and reject the undesired output signal. 
     In one embodiment, a receiver for receiving an RF signal comprising a high frequency component f RF  includes an RF amplifier and upper and lower parallel signal processing branches which receive an amplified RF signal output from the RF amplifier. 
     The upper branch includes a first mixer having an output connected to a first filter, and the first filter has an output connected to a first limiter. 
     The lower branch includes a second mixer having an output connected to a second filter, and the second filter has an output connected to a second limiter; 
     The receiver also includes a local oscillator that provides a respective local oscillator signal to each of the upper and lower parallel signal processing branches. 
     The first mixer performs a multiplication with a first local oscillator signal S′ LOnew (t) from the local oscillator (LO) that is phase shifted by 90° and has a frequency f′ LOnew . 
     The second mixer performs a multiplication with a second local oscillator signal S LOnew (t) from the local oscillator (LO) that is not phase shifted and has a frequency f LOnew ; 
     The receiver also includes a detector that receives a respective output from both the first limiter and the second limiter and that extracts information from the received outputs. 
     The frequency of the first signal f′ LOnew  and second f LOnew  output by the local oscillator LO conforms with the following: 
       A f LOnew =A f′ LOnew ≦f RF , with A≧5. 
     It is an advantage of the present invention that it yields unprecedented power reduction in virtually all types of receiver front-ends and devices employing such receiver front-ends. 
     Immediate benefits of this invention are drastically reduced power consumption and thus improved competitiveness. The proposed receivers, transceivers and integrated circuits based thereon are simple and cheap. The receivers, transceivers and integrated circuits according to the present invention are reliable and can be expected to show a performance that is at least as good as the performance of conventional devices. 
     Other advantages of the present invention are addressed in connection with the detailed embodiments. 
     For a more complete description of the present invention and for further objects and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a schematic block diagram of a conventional receiver structure; 
         FIG. 2  shows three diagrams illustrating the conventional mixing process in the frequency domain; 
         FIG. 3  shows a schematic block diagram of a first receiver in accordance with the present invention; 
         FIG. 4  shows three diagrams illustrating the inventive mixing process in the frequency domain; 
         FIG. 5  shows a schematic block diagram of a transceiver in accordance with the present invention; 
         FIG. 6  shows a schematic block diagram of an optical receiver in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     So far it has never been considered to reduce the power consumption by changing the LO frequency f LO  of a receiver. Until now this parameter remained untouched although great potential exists, as suggested by the above equations given in the introductory portion of this specification. 
     One tends to presume that the LO frequency f LO  is either specified, or immediately fixed once f IF  is chosen, or determined beforehand and, therefore, cannot be changed. Contrary to this public belief, the present invention demonstrates that huge power reductions by a factor of at least 10 can be achieved by lowering the LO frequency f LO , while maintaining the specified f IF  and other advantages of the receiver. 
     Before demonstrating the feasibility of lowering f LO , the conventional receiver architecture  10  of  FIG. 1  is addressed in more detail.  FIG. 1  shows a typical receiver architecture  10  with various filters omitted. The very weak RF signal s(t) picked up by the antenna  11  is first amplified by the LNA  12  and then converted from the RF regime down to the IF regime. This down-conversion is done by the mixer  13 . 
     A mixer is a very critical building block and the overall performance of a receiver heavily depend on it. In principle, mixers can be viewed as multipliers. A mixer requires two inputs. In a receiver, the RF signal s(t) is applied to a first mixer input and the LO signal is applied to a second mixer input. The LO signal is generated by a VCO  15  in a frequency synthesizer. 
     Assuming the input signal coming from the LNA  12  is S RF (t)=sin ω RF (t), and the LO signal provided by the VCO  15  is a square wave of frequency ω LO , described as 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       LO 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       4 
                       π 
                     
                      
                     
                       [ 
                       
                         
                           sin 
                            
                           
                               
                           
                            
                           
                             ω 
                             LO 
                           
                            
                           t 
                         
                         + 
                         
                           
                             1 
                             3 
                           
                            
                           sin 
                            
                           
                               
                           
                            
                           
                             ω 
                             LO 
                           
                            
                           t 
                         
                         + 
                         
                           
                             1 
                             5 
                           
                            
                           sin 
                            
                           
                               
                           
                            
                           5 
                            
                           
                             ω 
                             LO 
                           
                            
                           t 
                         
                         + 
                         … 
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Viewing the mixer  13  as a multiplier yields the following result: 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       out 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           S 
                           RF 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                       · 
                       
                         
                           S 
                           LO 
                         
                          
                         
                           ( 
                           t 
                           ) 
                         
                       
                     
                     = 
                     
                       
                         2 
                         π 
                       
                        
                       
                         [ 
                         
                           
                             
                               
                                 
                                   cos 
                                    
                                   
                                       
                                   
                                    
                                   
                                     ω 
                                     IF 
                                   
                                    
                                   t 
                                 
                                 + 
                                 
                                   
                                     1 
                                     3 
                                   
                                    
                                   
                                     cos 
                                      
                                     
                                       ( 
                                       
                                         
                                           2 
                                            
                                           
                                             ω 
                                             RF 
                                           
                                         
                                         - 
                                         
                                           3 
                                            
                                           
                                             ω 
                                             IF 
                                           
                                         
                                       
                                       ) 
                                     
                                   
                                    
                                   t 
                                 
                                 + 
                               
                             
                           
                           
                             
                               
                                 
                                   cos 
                                    
                                   
                                     ( 
                                     
                                       
                                         2 
                                          
                                         
                                           ω 
                                           RF 
                                         
                                       
                                       - 
                                       
                                         ω 
                                         IF 
                                       
                                     
                                     ) 
                                   
                                    
                                   t 
                                 
                                 + 
                                 … 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where low-side injection is assumed, i.e., ω IF =ω RF −ω LO . The first term in the bracket of equation (5) is the down converted IF signal, and the rest of the term is unwanted products to be removed by a bandpass (or lowpass) filter, yielding: 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       outfiltered 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       2 
                       π 
                     
                      
                     
                       [ 
                       
                         cos 
                          
                         
                             
                         
                          
                         
                           ω 
                           IF 
                         
                          
                         t 
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     The mixing process in the frequency domain is illustrated in  FIG. 2 . The input signal S RF (f) comprising a high frequency component f RF  is depicted in the uppermost diagram. The LO signal S LO (f) with frequency f LO  is shown in diagram in the middle. The output signal S out (f) after mixing is illustrated in the diagram at the bottom. The desired peak  9  is the down converted IF signal with frequency f IF . The other peaks in the diagram at the bottom represent the unwanted terms. 
     The present invention is based on the recognition that the most efficient way to minimize the power consumption of a receiver VCO is to lower its frequency. In principle, the LO frequency f LO  can be lowered by any odd integer. As an example, the attainable power saving is considered by just lowering LO frequency f LO  by a factor A=3, resulting in a new LO frequency herein referred to as ω LOnew . 
     Like the previous LO signal S LO (t), the new LO signal S LOnew (t) with frequency f LOnew  is assumed to be a square wave with 50% duty cycle. Similarly, its Fourier series can be written as shown in equation (7): 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       LOnew 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       4 
                       π 
                     
                      
                     
                       [ 
                       
                         
                           
                             
                               
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 
                                   ω 
                                   LOnew 
                                 
                                  
                                 t 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 
                                   1 
                                   3 
                                 
                                  
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 3 
                                  
                                 
                                   ω 
                                   
                                     LO 
                                      
                                     
                                         
                                     
                                      
                                     new 
                                   
                                 
                                  
                                 t 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 
                                   1 
                                   5 
                                 
                                  
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 5 
                                  
                                 
                                   ω 
                                   LOnew 
                                 
                                  
                                 t 
                               
                               + 
                               … 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Applying the new LO signal S LOnew (t) to the same mixer  13 , and for the same RF input signal ω RF (t), again, one obtains the output of the mixer  13  by multiplying both signals: 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       out 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       2 
                       π 
                     
                      
                     
                       [ 
                       
                         
                           
                             
                               
                                 
                                   1 
                                   3 
                                 
                                  
                                 
                                   cos 
                                   IF 
                                 
                                  
                                 t 
                               
                               + 
                               
                                 
                                   1 
                                   5 
                                 
                                  
                                 
                                   cos 
                                    
                                   
                                     ( 
                                     
                                       
                                         2 
                                          
                                         
                                           ω 
                                           RF 
                                         
                                       
                                       - 
                                       
                                         5 
                                          
                                         
                                           ω 
                                           IF 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                                  
                                 
                                   t 
                                   3 
                                 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 cos 
                                  
                                 
                                   ( 
                                   
                                     
                                       2 
                                        
                                       
                                         ω 
                                         RF 
                                       
                                     
                                     - 
                                     
                                       ω 
                                       IF 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   t 
                                   3 
                                 
                               
                               + 
                               … 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In order to come to the above result, the relationship shown in equation (6) and ω IF =ω RF −ω LO  have been used. It can be shown that now one obtains after filtering: 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       outfiltered 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       2 
                       
                         3 
                          
                         π 
                       
                     
                      
                     
                       [ 
                       
                         cos 
                          
                         
                             
                         
                          
                         
                           ω 
                           IF 
                         
                          
                         t 
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Comparing this new result in equation (9) with the previous one of equation (5), it is interesting to note that 
     Both results contain the same desired IF term, at the same IF frequency, meaning that the signals at the output in both cases are exactly the same except a difference in signal level, that is amplitude. 
     The desired IF term in equations (8) and (9) is smaller, indicating a lower conversion gain. In a Gilbert mixer for example, two circuit portions contribute directly to the conversion gain: One is a transconductance stage which converts the input RF voltage S RF (t) to a current, and the other are the loads, which convert the commutated currents back to a voltage signal S out (f). Note that the transconductance stage and its contribution to the conversion gain have not been altered after ω LO  is replaced by ω LOnew . As far as the desired IF term is concerned, nothing has been changed with ω LOnew  except that now it is the 3rd harmonic of the LO signal that mixes with the input signal S RF (t) and results in the desired IF. Therefore, the gain reduction is purely due to the smaller amplitude of the 3rd harmonic instead of the fundamental. This lower gain is not deemed to be a problem because immediately after down-conversion, it can be easily compensated. 
     The ratio Θ of the frequency of the next closest unwanted term to the frequency f IF  of the desired IF term is an indication of the requirement on the bandpass or lowpass filter that follows the mixer. In a homodyne receiver, this filter selects the desired channel by allowing the IF term to pass without any attenuation while sufficiently rejecting all unwanted terms. Larger ratio Θ means more relaxed requirement. With the f LOnew , frequency, the ratio Θ is slightly reduced but still large enough for any filters. 
     A first inventive embodiment of a receiver  20 , in accordance with the above theory, is illustrated in  FIG. 3 . The receiver  20  comprises an antenna  21  for receiving an RF signal s(t). This signal is amplified by a low noise amplifier  22  and fed to the first input of a mixer  23 . The amplified signal is referred to as S RF (t). The receiver  20  further comprises a local oscillator unit  30 . This local oscillator unit  30  comprises a reference oscillator  28 , e.g. a quartz oscillator, a phase detector  27 , a divider  26 , a VCO  25 , and a buffer  24 . According to the present invention, the local oscillator unit  30  provides a LO signal S LOnew (t) with a frequency f LOnew . This LO signal S LOnew (t) is applied to a second input of the mixer  23  and the mixer  23  provides for a down-conversion of the signal S RF (t) to a lower frequency band defined by the frequency f IF . The receiver  20  comprises a low-pass filter (LPF) for filtering the mixer&#39;s output signal S outnew (t). 
     The local oscillator unit  30  provides a reference signal S LOnew (t) required for mixer injection to facilitate frequency translation in the receiver  20 . The LO signal S LOnew (t) is a large signal that drives the mixer diodes or transistors into a nonlinear region, thereby allowing the mixer  23  to generate a signal S outnew (t) with fundamental frequencies along with harmonics and mixing terms at the output. 
     The VCO  25  is locked in phase to a high-stability reference oscillator  28  (for example a crystal oscillator). The phase detector  27  compares the phase of a divided VCO frequency output to that of the precise reference oscillator  28  and creates a correction voltage at the output  27 . 1  for the VCO  25  based on phase differences between the reference and the VCO. The correction voltage is fed via a LPF  27 . 2  to the VCO  25 . 
     The local oscillator unit  30  is designed to provide a LO signal S LOnew (t) with a frequency f LOnew ≦A f RF , with A≧3. That is, the frequency f LOnew  of the LO signal S LOnew (t) is at least 3 times lower than the frequency f RF  of the input signal s(t). Due to the fact that a low-frequency LO signal S LOnew (t) is employed instead of a LO signal S LO (t) equal or very close to the frequency f RF , the local oscillator unit  30  consumes less power than a conventional local oscillator unit. 
     The mixer design uses nonlinear devices, such as diodes or transistors. Using diodes, the mixer is passive and has a conversion loss. Using active devices, such as transistors, a conversion gain is possible. A variety of circuit topologies exist for mixers. A single-ended mixer is usually based on a single Schottky diode or transistor. A balanced mixer typically incorporates two or more Schottky diodes or a Schottky quad (four diodes in a ring configuration). A balanced mixer offers advantages in third-order intermodulation distortion performance compared to a single-ended mixer because of the balanced configuration. Any of these kinds of mixers are suited for being employed in receivers according to the present invention. 
     The currents flowing into the various building blocks of the local oscillator unit  30  are shown in  FIG. 3 . If one assumes that all building blocks of the local oscillator unit  30  operate at a reduced frequency, the overall current consumption of the local oscillator unit  30  is 
         Idd   new   =Idd   Gnew   +Idd   PDnew   +IDD   PSnew   +Idd   VCOnew   +IDD   BF   (10) 
     To achieve this kind of reduction in current consumption, a reference oscillator  28  may be employed issuing a signal with a lower frequency f refnew . It is also possible to keep the reference oscillator  28  at the usual frequency f ref , but to employ a divider  26  that reduces the frequency in accordance with the present invention. 
     The mixing process, according to the invention, is illustrated in  FIG. 4 . The input signal S RF (f) comprising a high frequency component f RF  is depicted in the uppermost diagram. The new LO signal S LOnew (f) with a low frequency f LOnew  is shown in the diagram in the middle. The output signal S outnew (f) after mixing is illustrated in the diagram at the bottom. The desired peak  9  (cf.  FIG. 4 ) is the down converted IF signal with frequency f IF . The other peaks in the diagram at the bottom represent the unwanted terms. 
     Another embodiment is depicted in  FIG. 5 . In this Figure, the schematic block diagram of a transceiver  40  is shown. The transceiver comprises a transmitter  51  and a receiver  53 . The transmitter  51  and the receiver  53  both use the same antenna  41 . There is a unit  51  that discriminates incoming and outgoing signals. The receiver  53  is a zero-IF (homodyne) receiver providing for a narrow baseband filtering with integrated low-pass (LP) filters  49 . 1 ,  49 . 2 . At the input side of the receiver  53  there is an RF amplifier  42 . There are two parallel signal processing branches. The upper branch comprises a mixer  43 . 1 , the filter  49 . 1 , and a limiter  44 . 1 . The mixer  43 . 1  of the upper branch performs a multiplication with a LO signal S′ LOnew (t) having a frequency f′ LOnew . This LO signal S′ LOnew (t) is phase shifted by 90°. The phase shifting is carried out by a phase shifter  46 . The lower branch comprises a mixer  43 . 2 , the filter  49 . 2 , and a limiter  44 . 2 . The mixer  43 . 2  of the lower branch performs a multiplication with a LO signal S LOnew (t) having a frequency f LOnew . This LO signal S LOnew (t) is not phase shifted. A detector  45  is provided at the receiver&#39;s output side. The detector  45  extracts information from the signal received. 
     According to the embodiment depicted in  FIG. 5 , both branches receive a LO signal from one local oscillator unit  50 . One LO signal S′ LOnew (t) is shifted by 90° with respect to the other signal S LOnew (t). The local oscillator unit  50  is designed to provide the two LO signals with a frequency f LOnew =f′ Lonew ≦A f RF , with A≧5. That is, the frequency of the two LO signals is at least 5 times lower than the frequency of the input signal s(t). 
     According to another embodiment of the invention, the gain loss caused by the fact that a LO signal S LOnew (t) with low frequency is employed for the mixing process, is compensated by means of an amplifier. This amplifier may be positioned after the low-pass filter (LPF). That is, the amplifier amplifies the baseband signal after down-conversion and after filtering. 
     The present invention may also be used to reduce the power consumption of an optical receiver front-end  60  illustrated in  FIG. 6  or in other optical receivers. The receiver front-end  60  is part of a homodyne receiver. An optical light wave e s (t) received by the receiver front-end  60  is superposed in a 180°-hybrid with the light wave e l  of a local oscillator laser  58 . The front end  60  comprises two photo-diodes  58  and  59  which are coupled via a capacitor C to an amplifier  70 . The receiver-front end  60  provides for a down-conversion of the signal e s (t). At the output  61  a baseband signal u(t) is provided. The receiver front-end  60  further comprises a loop filter  54  and a signal generator  55  in a phase locked loop arrangement. The loop filter  54  and the signal generator  55  control the frequency of the local oscillator laser  58 . In a conventional optical receiver, the frequency of the local oscillator laser  58  is identical to the carrier frequency of the optical light wave e s (t). According to the present invention, the local oscillator&#39;s frequency is reduced by at least a factor of 3. This allows to safe a substantial amount of energy, like in the RF receivers proposed herein, since the laser as well as the electronic components of the feedback loop would consume less power. 
     Although simple mixers are shown and described, the proposed power saving method is applicable to quadrature mixers as well. Given quadrature signals in square-wave, it can be shown that their harmonics of any order are also in quadrature. 
     In yet another embodiment, the mixer design is altered since the mixer now just needs to perform a multiplication at lower frequencies. 
     Yet another embodiment is characterized in that the whole receiver front-end, except for the antenna, is realized on one chip. The present invention is well suited for realizing a fully integrated receiver comprising a low noise amplifier, a resistive FET mixer, and a voltage controlled oscillator. 
     The invention can be used in homodyne receivers, super heterodyne receivers, double super heterodyne receivers, and so forth. 
     In this specification, receivers were described where the emphasis in development has been to minimize the DC power consumption without sacrificing any key performance parameters. 
     There are many direct benefits when using a lower LO frequency in accordance with the present invention: 
     Less self-mixing: One of the major drawbacks in a homodyne or direct-conversion receiver is the so-called self-mixing. Due to capacitive or substrate coupling, a finite amount of feed-through exists from the VCO/LO port to the input of the LNA or the input of the mixer. As a consequence, the leakage signal appearing at the inputs of the LNA and the mixer is mixed with the LO signal, thus producing a DC component at the mixer&#39;s output. As in a receiver, the gain from the antenna to an analog-to-digital converter (ADC) situated after the mixer is typically around 80 to 100 dB. A very small DC offset at the mixer&#39;s output may saturate some of the circuit in the receive chain. This leakage is frequency dependent. With a lower LO frequency, as proposed herein, the self-mixing effect is greatly reduced. 
     Lower noise: It has been found that the noise due to the tail capacitance of a standard mixer is proportional to ω LO . So with ω LOnew , a better noise performance is obtained without an increase of power consumption and without any change of the mixer circuit. 
     Lower intermodulation distortion: The intermodulation distortion is reduced when the LO frequency is decreased. 
     Less power dissipation by the VCO buffer/amplifier and the prescaler: A lower LO frequency f LO  means that a smaller division ratio is required by the frequency divider. 
     In the above, a mixer example is shown where the LO frequency is lowered by 3 and another mixer example is shown where the LO frequency is lowered by 5. As pointed out, the lowering factor A can also be any other odd integer numbers greater than 3. The choice obviously depends on the specific application, and one has to make a trade-off between attainable power reduction and the conversion gain, also the ratio in order to achieve the best overall performances. 
     The key specifications for a local oscillator (note that a receiver may have more than one local oscillator, depending upon the number of IFs and the system architecture) include tuning range, frequency stability, spurious output levels, lock-time, and phase noise. Most of these specifications determine an LO&#39;s suitability for a particular wireless receiver application The spurious and phase-noise performance also impact sensitivity and dynamic-range performance. Nobody so far has considered to reduce the power consumption of the receiver front-end by reducing the frequency f LO  of the LO, as proposed herein. 
     It is appreciated that various features of the invention which are, for clarity, described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable sub combination. 
     In the drawings and specification there has been set forth preferred embodiments of the invention and, although specific terms are used, the description thus given uses terminology in a generic and descriptive sense only and not for purposes of limitation.