FIELD OF THE INVENTION  
       [0001]    This invention relates to a low Intermediate Frequency (‘IF’) radio receiver and especially, but not exclusively, to Very Low IF (‘VLIF’) receivers. The expression “low IF” refers to intermediate frequencies that are comparable with the bandwidth of the resulting baseband signal and VLIF to intermediate frequencies that are much smaller than the bandwidth of the resulting baseband signal. 
       BACKGROUND OF THE INVENTION 
       [0002]    Very Low IF receivers are widely used for slot-based radio communications and, in particular Time Division Multiple Access (‘TDMA’) protocols such as the Global System for Mobile communications (‘GSM’), Digital Enhanced Cordless Telecommunications (‘DECT’) and Enhanced Data for GSM Evolution (‘EDGE’), in the General Packet Radio Service (GPRS), an extension to the GSM standard that provides higher speed access. These receivers need to reject interferer signals that fall as image frequencies on or very close to the wanted signal when the wanted signal is translated to the desired intermediate frequency. This is achieved through image cancelling mixers or poly-phase filters at low frequency. The success of these image cancelling techniques depends upon the balance achieved in terms of phase and gain of the I (in phase) and Q (quadrature phase) paths in the receiver. Patent specifications U.S. Pat. No. 6,597,748 and EP 1 058 378 describe receivers of this kind. 
         [0003]    The balance of I and Q in the receiver may be achieved by precision analogue design and by compensation in the form of a digital equaliser. This adds cost in terms of yield, software overhead, manufacturing overhead and extra hardware. Moreover, the image rejection varies with temperature and frequency band. 
         [0004]    The image channel is often one of the adjacent frequency channels or alternate adjacent channels to the wanted one. The other adjacent or alternate adjacent channel is not an image frequency and can be rejected adequately without the need for such accurate quadrature balance. Instead it is easily rejected by standard filter topologies. Selectivity is especially a problem for alternate channels on the ‘low side’ IF, that is to say where the local oscillator (‘LO’) frequency used to convert the carrier frequency down to IF is smaller than the carrier frequency, so that the IF is positive, regarding interferers at twice channel spacing (400 kHz in the example shown in  FIG. 2  of the accompanying drawings) since the receiver treats them like an image and part of the interferer spectrum falls in band. 
         [0005]    Patent specification US2002/0183030 describes radio receiver and transmitter apparatus that includes Local Oscillator (LO) frequencies Low LO  and High LO  selectable based on the frequency band of operation of the mobile communication device in the transmitter, whereas in the receiver the LO inputs Low LO  and High LO  are connected to different receiver paths via a sub-harmonic mixer. 
         [0006]    The specifications for low IF radio receivers include stringent requirements regarding the rejection of interferers and there is a need to meet or surpass these specifications at minimal cost. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a radio receiver as described in the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block schematic diagram of a hardware implementation of a receiver in accordance with one embodiment of the invention, given by way of example, 
           [0009]      FIG. 2  is a diagram of the frequency spectrum of signals appearing during a first time slot in operation of the receiver of  FIG. 1 , 
           [0010]      FIG. 3  is a diagram of the frequency spectrum of signals appearing during a subsequent time slot in operation of the receiver of  FIG. 1 , 
           [0011]      FIG. 4  is a diagram of the variation with time of parameters in operation of the receiver of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    The receiver shown in  FIG. 1  comprises an antenna  1  for supplying received radio signals to an amplifier  2 . A ‘High Side’ local oscillator  3  produces a local oscillator signal at a frequency f LO  higher than the wanted channel frequency f wanted , so that f LO =f wanted +f IF . The signal from the High Side local oscillator  3  is supplied to an I-channel mixer  4  and to a phase shifter  5  that shifts the phase of the signal from the High Side local oscillator  3  by 90° and supplies the phase-shifted signal to a Q-channel mixer  6 . The I- and Q-channel mixers  4  and  6  mix the received signal from the amplifier  2  with the local oscillator signals to produce low IF signals (in the present case VLIF signals) in phase quadrature and supply the low IF signals to I- and Q-low-pass-filters  7  and  8 . The filtered analogue signals from the filters  7  and  8  are then converted to digital signals by analogue-to-digital converters (‘ADCs’)  9  and  10  respectively and down-converted from VLIF to DC in a digital mixer stage  11  that mixes the signals from the ADCs  9  and  10  with a VLIF local oscillator signal from a VLIF local oscillator  12  and from a further 90° phase-shifter  13 . 
         [0013]    In operation, the wanted signal presents a bandwidth  14 , shown in  FIG. 2  as 200 kHz by way of example, corresponding to the GSM and EDGE channel widths. The frequency  15  of the local oscillator is higher than the centre frequency f wanted  of the wanted channel  14  by an amount f IF  equal in this example to 100 kHz. When mixed with the local oscillator signals, the wanted channel will be centred at a low IF frequency −f IF  and signals from an image region  16  will appear in a region centred at a frequency +f IF  and will be passed by frequency filters: this is the case for an interferer  17  from an adjacent channel or an alternate adjacent channel  18  centred at a frequency offset by f interferer  (=200 kHz or 400 kHz in the example shown in  FIG. 2  of the accompanying drawings) relative to the centre frequency f wanted  of the wanted channel  14 . 
         [0014]    As described in patent specification U.S. Pat. No. 6,597,748, the image rejection depends on the gain and phase imbalances of the I- and Q-channels. Digital gain and phase correction as described in patent specification EP 1 058 378, for example, enables significant compensation of these errors to be achieved and gives a high degree of image rejection. However, as described above, it is desirable to improve image rejection since the receiver treats them like an image and part of the interferer spectrum falls in the wanted band. In particular, in certain applications, selectivity is desired to be at detected carrier to interference (C/I) levels better than 10 dB for a received interferer (I) at 41 dB greater than the carrier (C) where carrier is the wanted channel, even at carrier frequencies greater than 2 GHz. 
         [0015]    In the embodiment of the invention shown in the drawings, the local oscillator means includes frequency alternation means for causing the local oscillator frequency to alternate in successive time-slots between first and second values one of which is greater and the other smaller than the desired carrier frequency of the input signal. More specifically, in the embodiment shown in  FIG. 1 , a Low Side local oscillator  19  is provided that produces a local oscillator signal at a frequency f LO  lower than the wanted channel frequency f wanted , so that f LO =f wanted −f IF . The signals from the High Side local oscillator  3  and the Low Side local oscillator  19  are supplied to a two-position switch  20 . The two-position switch  20  selects alternately the LO signals from the High Side and Low Side local oscillators  3  and  19  for alternate time slots of the received signals. The relationship between the frequencies of the wanted channel, the Low Side local oscillator  19 , the image region  15  and the same interferers  17  and  18  when the Low Side local oscillator  19  is selected is then as shown in  FIG. 3 . It will be appreciated that the interferers then fall outside the image region  15  and are rejected by the frequency filters  7  and  8 . 
         [0016]    In order to maintain the polarities of the VLIF I- and Q-channel mixer stage  11 , this mixer stage comprises first and second I-channel mixers  21  and  22  and first and second Q-channel mixers  23  and  24 . A VLIF two-position switch  25  synchronised with the LO two-position switch  20  provides phase alternation and alternately applies the VLIF LO signal from the VLIF local oscillator  19  to the first I- and Q-mixers  21  and  23  in one time-slot and the phase-shifted VLIF LO signal from the VLIF phase shifter  13  to the second I- and Q-mixers  22  and  24  in the next time-slot. The I-mixers  21  and  22  mix these signals with the signal from the I-channel ADC  9  and the Q-mixers  23  and  24  mix these signals with the signal from the Q-channel ADC  10 . The signals from the first I-mixer  21  and the second Q-mixer  24  are supplied to an adder  26 , which adds the mixed signals to produce a Q-output signal at a Q-output  27  and the signals from the second I-mixer  22  and the first Q-mixer  23  are supplied to a subtractor  28 , which subtracts the mixed signals to produce an I-output signal at a I-output  29 . 
         [0017]    The signal amplitudes and phases can be expressed as follows: 
         [0018]    In the case where the Local RF oscillator is higher than the Wanted channel frequency, the High Side Low IF, the low IF receiver I,Q vector output can be expressed as: 
         [0000]        I   out ( t )+ j·Q   out ( t )= A   w ( t )/2 ·e   −j·φw(t)   **H   wanted   +A   i ( t )/2 ·e   −j·(φi(t)+ 2·n·finterferer·t) **H   wanted  (term  a )+ A   w ( t )/2 ·e   +j·(φw(t)− 4·n·fIF·t) **H   image   +A   i ( t )/2 ·e   +j·(φi(t)+ 2·n·finterferer·t−4·n·fIF·t) **H   image  (term  b ) 
         [0019]    where A w (t) is the wanted channel amplitude signal, 
         [0020]    where φ w (t) is the wanted channel phase signal, 
         [0021]    where f IF  is the intermediate IF frequency, 
         [0022]    where A i (t) is the interferer channel amplitude signal, 
         [0023]    where φ i (t) is the interferer channel phase signal, 
         [0024]    where F interferer  is the offset of the interferer frequency versus the wanted channel frequency (e.g in GSM the +400 khz alternate channel interferer case is considered), 
         [0025]    where H wanted  is the wanted channel filter impulse response expressed as H wanted =(H I +H Q )/2 where H I  is the I path channel impulse response and H Q  is the Q path channel impulse response, and 
         [0026]    where H image  is the Image channel filter impulse response expressed as H image =(H I -H Q )/2 where H I  is the I path channel impulse response and H Q  is the Q path channel impulse response. 
         [0027]    Ideally if H I =H Q , ie perfect quadrature over frequency, then H image  is nullified. 
         [0028]    The term b represents the image portion that will fall inside the RX band based on H image  which is not nullified due to the non-ideal matching between I and Q paths, so for example if f interferer =2*f IF , the second portion of term b becomes: 
         [0000]      A i (t)/2·e +j·(φi(t)) **H image    
         [0000]    which is centered at 0 frequency, ie falling inside the wanted RX channel and the amplitude of this interference depends on the level of the interfence max(A i (t)) and the image rejection magnitude abs(H image ). 
         [0029]    In the case where the Local RF oscillator is lower than the Wanted channel frequency, ie fLO=fwanted channel−fFI, the Low Side Low IF, the low IF receiver I,Q vector output can be expressed as: 
         [0000]        I   out ( t )+ j·Q   out ( t )= A   w ( t )/2· e   −j·φw(t) **Hwanted+ Ai ( t )/2 ·e   −j·(φi(t)+ 2·n·finterferer·t)**Hwanted (term  a ′)+ A   w ( t )/2 ·e   +j·(φw(t)+ 4·n·fFI·t) **H   image   +A   i ( t )/2 ·e   +j·(φi(t)+ 2·n·finterfer·t+4·n·fFI·t) **H   image (term  b ′) 
         [0030]    The term b′ represents the image portion that will fall inside the RX band based on H image  which is not nullified due to the non-ideal matching between I and Q paths, so for example if f interferer =2*f IF  (same interferer location as in the High Side case), the second portion of term b′ becomes: 
         [0000]      A i (t)/2·e +j·(φi(t)+ 2·n·4fFI·t)**H image    
         [0000]    which is centred at 4·f IF  frequency, ie falling outside the wanted receiver channel thus resulting in removing this contribution term during the time where the receiver is operating in Low side Low IF Mode. 
         [0031]    Since the local oscillator frequency f LO  alternates between High Side and Low Side, the outputs alternate between the values indicated and the average impact of the interferers present is reduced. It is not necessary to know on which side of the wanted frequency band the interferer frequency occurs, which is often not even possible, provided that the alternation between High Side and Low Side occurs several times while the interferer is on the same side of the wanted frequency band. 
         [0032]    It will be appreciated that, instead of the implementation shown in  FIG. 1 , with two local oscillators  3  and  19 , it is possible in certain applications to combine the two oscillators in one, the switch  20  then switching the output of this local oscillator between the two LO frequencies. This implementation is particularly useful where reception time-slots are separated by inactive reception periods, during which the local oscillator is changed and has time to stabilise at the new frequency before the start of the next reception time-slot. It will be appreciated that the hardware switches  20  and  25  shown in  FIG. 1  may then be replaced by software switching of the local oscillator frequencies. 
         [0033]    It will also be appreciated that in certain applications, it is not necessary to alternate the local oscillator frequency between the two LO frequencies at each successive time-slot, provided that the alternation occurs often compared to the fluctuations of the interferers that is to say many times in the same block (of 2000 slots in the case of GSM and EDGE).
A slot-based low Intermediate Frequency ('IF') radio receiver comprises an IF local oscillator ( 3, 5, 19) for producing I and Q IF local oscillator signal components in phase quadrature, I and Q mixer channels ( 4, 6) for mixing the input signal with the I and Q IF local oscillator signal components to produce I and Q IF signal components. The IF local oscillator ( 3, 5, 19) includes frequency alternation means ( 20) for causing the IF local oscillator frequency to alternate a plurality of times during each frame between first and second values, one of which is greater and the other smaller than the desired carrier frequency of the input signal so as to reduce the effect of adjacent and alternate interferers. The phase of the baseband local oscillator ( 12, 13) is alternated in synchronism with the alternation of the IF local oscillator frequency.