Abstract:
A receiver for isolating a wanted signal in a received signal, the receiver comprising a downconverter for downconverting the received signal in frequency to produce a downconverted signal, a filter with a passband intended for isolating that part of the spectrum of the downconverted signal that contains the wanted signal and a controller that seeks to avoid or reduce the effect of passband intrusion in the form of a negative frequency representation of an interferer, appearing in the spectrum of the received signal, upconverted in frequency to the passband. The invention consists in corresponding methods also.

Description:
This application is a continuation of U.S. application Ser. No. 12/249,561, filed Oct. 10, 2008. 
    
    
     TECHNICAL FIELD 
     The present invention relates to schemes for isolating a desired communication signal that appears as part of a greater signal. The invention also relates to methods of determining configurations of signal receiver settings that are likely to be more useful for isolating a wanted communications signals that appears as part of a larger signal. The invention finds application in, for example, the field of mobile telephony. 
     BACKGROUND 
       FIG. 1  is a block diagram of a mobile telephone  10  viewed from the perspective of its role as a receiver of information from the network with which it communicates.  FIG. 1  shows only certain fundamental elements that are involved in the processing of signals that are received at the telephone  10 . As shown in  FIG. 1 , the telephone  10  comprises an antenna  12 , a quadrature downconverter  14 , a bandpass filter  16 , a demodulator  18  and an information sink  20 . The purposes of these elements is well known and therefore will now be only briefly discussed. 
     Wireless signals acquired by the antenna  12  are supplied to the quadrature downconverter  14 . The downconverter  14  shifts the acquired signals down in frequency from the RF (radio frequency) range to the IF (intermediate frequency) range. Hence, the acquired signals are said to be downconverted in frequency. In addition to downconverting the signals from the antenna  12  in frequency, the unit  14  also converts the acquired signals into a quadrature format. 
     The basic structure of the quadrature downconverter  14  is shown in  FIG. 1 . The signals that are received through the antenna  12  are supplied in parallel to mixers  22  and  24 . The quadrature downconverter  14  also comprises a local oscillator  26 , whose output is supplied to mixer  22  and, via 90° phase shifter  28 , to mixer  24 . The output of mixer  22  provides the in-phase component of the quadrature format downconverted signal  30  and the output of mixer  24  provides the quadrature phase component of the quadrature format downconverted signal. 
     The quadrature format downconverted signal  30  is then supplied to the bandpass filter (BPF)  16 . The bandpass filtered quadrature signal, indicated  32 , is then supplied to the demodulator  18 . The demodulator  18  recovers an information signal  34  from the bandpass filtered quadrature signal  32  and supplies it to the information sink  20 . The demodulator  18  can use various techniques to recover the information signal, as will be apparent to the skilled person. For example, the demodulator  18  could perform Viterbi equalisation on the bandpass filtered quadrature format signal  32 . The sink could be, for example, a display screen or a speaker forming part of the telephone  10 . 
     It will be apparent to the skilled person that the telephone  10  will comprise many other elements besides those shown in  FIG. 1 , for example an amplifier arranged to act on the signal from the antenna  12  before it reaches the quadrature downconverter  14  and an analog to digital converter to act on the bandpass filtered quadrature downconverted signal  32  before it is processed by the demodulator  18 . However, these and other elements are not described in this document for the sakes of both brevity and clarity, the description instead concentrating on those elements that are most closely connected to the invention. 
     The frequency of the output of the local oscillator  26  can be varied to adjust the part of the spectrum of the signal acquired by antenna  12  that is downconverted to lie at the passband of the bandpass filter  16 . However, the details of such channel selection schemes will be well known to readers skilled in this art. 
     Consider now the case where the signal acquired by the antenna  12  contains just a single active channel spanning a band of frequencies centred on an RF frequency f 1 . Mathematically, the spectrum of the signal acquired by the antenna  12  extending in the positive frequency domain can be regarded as reflected about 0 Hz to the negative frequency domain.  FIG. 2  illustrates the spectrum of the signal acquired by the antenna  12  comprising the signal  33  in the active channel centred on frequency f 1  and also its “reflection”  35 , being a complex-conjugated version of the signal  12  but at frequency −f 1 . 
     The complex-conjugation is shown as an asterisk in  FIG. 2  (and the same notation is used in those of the subsequent figures that illustrate spectra). 
     Consider also that the output of the local oscillator  26  is at frequency ω. The effect of the local oscillator signal on the frequency spectrum of  FIG. 2  is to convert each component of that frequency spectrum into two components, one shifted down in frequency by ω (and hereinafter referred to as the “downshifted component”) and one shifted up in frequency by ω (and hereinafter referred to as the “upshifted component”). This is shown in  FIG. 3 . The quadrature downconverter  14  converts the signal  33  at f 1  into a downshifted component  33   a  lying at f 1 −ω and an upshifted component  33   b  lying at f 1 +ω. Likewise, the quadrature downconverter  14  converts the signal at into a downshifted component  35   a  lying at −f 1 −ω and an upshifted component  35   b  lying at −f 1 +ω. Thus, each of the signals  33  and  35  at f 1  and −f 1  is converted into a pair of signals symmetrically disposed about the position of the original signal. 
     In each of these pairs, the lower frequency signal is regarded as the wanted signal and the other, unwanted, signal is regarded as an image signal (since it is symmetrically disposed beyond the original signal position). Accordingly, the quadrature downconverter  14  is designed to suppress these image signals and this suppression is apparent in  FIG. 3  since the upshifted component of each pair is at a much lower power than the downshifted component of the pair. The difference in power of the two components in such a pair is a measure of the image rejection ratio (IRR) of the quadrature downconverter  14 . However, in a practical downconverter, the IRR will never be perfect with the result that suppression of upshifted components will never be total. This imperfection in practical downconverters leads to certain problems as will now be discussed with reference to  FIG. 4 . 
       FIG. 4  pertains to the case where the spectrum of the signal acquired through antenna  12  contains a signal in a wanted channel, that is to be directed through the passband of the BPF  16 , and a signal in a channel adjacent to the wanted channel and having significantly higher power than the wanted channel.  FIG. 4  shows three power versus frequency spectra  36 ,  38  and  40 . The frequency axes of these spectra are aligned with one another, for ease of comparison of their frequency content, and the passband of BPF  16  is also shown. 
     Spectrum  36  shows the spectrum of the signal acquired by the antenna  12 . Again, the spectrum of the signal acquired by the antenna  12  can be considered mathematically as containing in the negative frequency region a “reflection” of what is contained in the positive frequency region. The signal in the wanted channel is indicated  42  and its negative frequency “reflection” is indicated  48 . The higher power signal in the adjacent channel is indicated  44  and its negative frequency “reflection” is indicated  46 . 
     Spectrum  38  shows, partially, the effect of the downconverter  14  on the positive frequency half of the spectrum  36 . The wanted signal  42  is downconverted to yield a downshifted component  42   a  which lies in the passband of the BPF  16  whereas the higher power adjacent channel signal  44  is downconverted to yield a downshifted component  44   a  which lies just below the passband. Of course, the downconverter  14  also produces upshifted components for signal  42  and  44  but these components are not shown since they do not bear on the passband (and in any event would lie off the right hand side of the diagram). 
     Spectrum  40  shows, partially, the effect of the downconverter  14  on the negative frequency half of spectrum  36 . The “reflections”  48  and  46  of the wanted and adjacent channel signals (respectively) are downconverted in frequency to yield respective downshifted components and upshifted components. The downshifted components do not bear on the passband and so are not shown (and in any event would lie off the left hand side of the diagram). The upshifted components, however, are shown. The upshifted component  46   b  of the “reflection”  46  of the adjacent channel signal appears in the passband and the upshifted component  48   b  of the “reflection”  48  of the wanted signal appears just below the passband. Of course, these upshifted components are suppressed in power to the extent possible given the design of the downconverter  14  (this extent is described by the downconverter&#39;s IRR). 
     It will be apparent that the downconverter  14  operates so as to place both the downconverted component  42   a  of the wanted signal  42  and the upconverted component  46   b  of the “reflection”  46  of the adjacent channel signal  44  in the passband of the bandpass filter  16 . Accordingly, the upconverted component  46   b  can hamper the demodulation of the downconverted component  42   a  in the demodulator  18 . It will also be appreciated that this problem will be worse the greater the power of the adjacent channel signal  44 . 
     BRIEF SUMMARY 
     According to one aspect, the present invention provides a receiver for isolating a wanted signal in a received signal, the receiver comprising a downconverter arranged to downconvert the received signal in frequency to produce a downconverted signal, a filter with a passband intended for isolating that part of the spectrum of the downconverted signal that contains the wanted signal and a controller arranged to control the operation of the downconverter, wherein the controller is arranged to respond to information specifying the location of an interferer in the frequency spectrum of the received signal by setting the frequency downshift that is applied by the downconverter to the received signal to avoid the interferer causing a passband intrusion in the form of a negative frequency representation of the interferer upshifted in frequency to the passband. 
     The invention also consists in a method for determining a downconverter setting to be employed in a receiver, wherein the downconverter is for downconverting in frequency a received signal acquired by the receiver so that a filter can isolate that part of the spectrum of the downconverted signal that contains a wanted signal and the method comprises obtaining information specifying the location of an interferer in the frequency spectrum of the received signal and responding to said information by setting the downconverter to apply a frequency downshift to the received signal that avoids the interferer causing a passband intrusion in the form of a negative frequency representation of the interferer upshifted in frequency to the passband. 
     The interferer may, for example, be a dominant interferer in the spectrum of the received signal. 
     Thus, the invention can avoid using a downconverter setting that might cause an interferer to mask a wanted signal to an unacceptable extent. 
     According to one aspect, the present invention provides a receiver for isolating a wanted signal in a received signal, the receiver comprising a downconverter arranged to downconvert the received signal in frequency to produce a downconverted signal, a filter with a passband intended for isolating that part of the spectrum of the downconverted signal that contains the wanted signal and a controller arranged to control the operation of the downconverter, wherein the controller is arranged to ascertain whether a frequency downshift that is or can be applied by the downconverter to the received signal causes or would cause a passband intrusion in the form of a negative frequency representation of an interferer, appearing in the spectrum of the received signal, upconverted in frequency to the passband. 
     Thus, the invention can be used to assess whether a downconversion setting will give rise to a passband intrusion. A passband intrusion can lead to poorer performance in the recovery of information conveyed by the wanted signal. 
     It can arise that a group of downconversion settings can each give rise to a passband intrusion. Certain embodiments can be arranged, upon encountering these circumstances, to select the downconversion setting from the group that has the least detrimental effect on the recovery of the payload of the wanted signal. It may transpire that this group of passband intrusion causing downconversion settings comprehends all available frequency settings that are usable for downconverting the wanted signal to the passband of the filter. 
     In certain embodiments, a frequency spectrum is deduced for the received signal and this spectrum is used in the assessment of whether or not passband intrusion occurs. The deduced frequency spectrum can be evaluated for features that will give rise to passband intrusions. The spectrum could be deduced using a fast Fourier transform (FFT) technique or by using a filter bank, for example. 
     In certain embodiments, use is made of information that is passed to the receiver about other active signals in the vicinity, besides the wanted signal. This information can be used to determine the presence of signals that could give rise to passband intrusions. 
     The downconversion process that is intended to direct the wanted signal to the passband of the filter may be a single or a multiple stage downconversion. A multiple stage downconversion process uses several mixing steps to provide the desired overall downshift in frequency whereas a single stage downconversion process uses just one mixing step to achieve the desired frequency downshift. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example only, certain embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of a mobile telephone; 
         FIG. 2  is a frequency spectrum of a signal entering the downconverter of the mobile telephone of  FIG. 1 ; 
         FIG. 3  is a frequency spectrum showing signals resulting from the operation of the downconverter of the telephone of  FIG. 1  on the signal shown in  FIG. 2 ; 
         FIG. 4  shows several spectra illustrating a problem that can arise with the downconverter of the mobile telephone of  FIG. 1 ; 
         FIG. 5  is a schematic block diagram of a variant of the mobile telephone of  FIG. 1 ; 
         FIG. 6  is a flowchart of a process performed by the controller within the mobile telephone of  FIG. 5  in order to avoid interference affecting the demodulation of a wanted signal; 
         FIG. 7  shows several spectra illustrating how the process outlined in  FIG. 6  avoids the problem illustrated in  FIG. 4 ; 
         FIG. 8  shows several spectra illustrating a downconversion process that can be performed by the mobile telephone of  FIG. 5 : and 
         FIG. 9  shows several spectra illustrating another downconversion process that can be performed by the mobile telephone of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  shows a mobile telephone  50 , which is a version of the mobile telephone  10  of  FIG. 1  that has been adapted to address the adjacent channel blocking problem that was discussed with reference to  FIG. 4 . In  FIG. 5 , elements of the telephone  10  that have been reused in the telephone  50  retain the same reference numerals and shall not be described again in detail. It is of course to be understood that the telephone  50  will comprise many other elements besides those shown in  FIG. 5 , which figure concentrates on those elements that are of greatest use in describing the present invention. 
     Above and beyond the elements of  FIG. 1 , the telephone  50  also includes a spectrum analyser unit  52  and a control unit  54 . A coupler  56  is associated with the line between the antenna  12  and the demodulator  14  for providing a fraction of the signal that is acquired through the antenna  12  to the spectrum analyser unit  52  via a signal line  58 . The spectrum analyser unit  52  analyses the signal that it receives through line  58  and provides the results of this analysis to the control unit  54  through signal line  60 . The nature of the analysis that is performed by the blocker detection unit will be described in detail later in this document. The control unit  54  uses the results of the analysis conducted by the spectrum analyser unit  52  in order to produce control signals that are delivered through signal lines  62  and  64  to the local oscillator  26  and to the bandpass filter  16 , respectively. The control signals delivered through line  62  control the frequency of the output signal of the local oscillator  26 . The control signals delivered through line  64  control the position of the passband of the BPF  16  in the frequency domain. 
     The spectrum analyser unit  52  will now be described in more detail. The function of the spectrum analyser  52  unit is to produce a frequency spectrum of the fraction of the signal that is acquired by the antenna  12  that is diverted by coupler  56  to the spectrum analyser unit  52 . The spectrum analyser unit  52  provides the control unit  54  over line  60  with a spectrum in the form of a series of frequency bins with a detected signal power value for each bin. The spectrum analyser unit  52  comprises filter bank composed of a plurality of filters, each filter having a passband matching a different one of the frequency bins of the spectrum, and the output of each filter being supplied to a respective power detector in order to produce the power values for the bins. 
     The operation of the control unit  54  will now be described in more detail. The function of the control unit  54  is to adjust the frequency of the output of the local oscillator  26  and the position of the passband of the BPF  16  such that the wanted signal, as downshifted to the passband, is overshadowed by other signals, hereinafter referred to as “blockers”, to the least extent possible. Having regard to the frequency that is set for the output of the local oscillator  26 , the control unit  54  evaluates the spectrum provided over signal line  60  to determine whether or not a blocker exists whose negative frequency “reflection” would be upshifted to the passband. If such a blocker exists, the control unit  54  attempts to adjust the frequency of the output of the local oscillator  26  and the position of the passband of the BPF  16  such that the wanted signal is not accompanied by a blocker in the passband. However, if it transpires that at all possible settings of the frequency of the local oscillator  26 , the passband of the BPF  16  would be affected by a blocker, then the control unit  54  is arranged to select the pair of local oscillator frequency and passband settings that would lead to the lowest level of blocker interference of the wanted signal in the passband. The operation of the control unit  54  will now be described in more detail with reference to the flow chart in  FIG. 6 . 
       FIG. 6  assumes that the frequency of the local oscillator  26  can assume only a number of discrete values, which hereinafter shall be referred to as the settings of the local oscillator  26 . Since the purpose of the BPF  16  is to provide just the wanted signal to the demodulator  18 , it will be apparent that the passband of the BPF  16  similarly has a number of settings, each setting corresponding to a respective one of the local oscillator settings. In other words, for each setting of the local oscillator, the passband of the BPF  16  must move to track the position in the frequency domain to which the wanted signal is downconverted. 
     In step S 1  of  FIG. 6 , an untried one of the local oscillator settings is selected. Initially, all of the local oscillator settings are untried. From step S 1 , the process moves to step S 2  in which the position of the passband of the BPF  16  is determined. The position of the passband is determined so that it spans the frequency domain position to which the wanted signal would be downconverted if the local oscillator setting chosen in step S 1  were to be applied to the local oscillator  26 . From step S 2  the process moves to step S 3 . 
     In step S 3 , the control unit  54  determines whether the local oscillator setting chosen in step S 1  would cause the upshifting into the passband determined in step S 2  of a deleterious amount of signal energy from a region in the spectrum supplied by the spectrum analyser unit  52  other than that occupied by the wanted signal. Accordingly, the control unit  54  takes the spectrum provided by the spectrum analyser unit  52  and calculates its negative frequency reflection. Then, the control unit  54  determines the upshifted components that correspond to the bins of the negative frequency part of the spectrum. The frequency positions of these upshifted components is readily determined from the knowledge of the local oscillator setting chosen in step S 1 . The power levels of these upshifted components are readily determined from knowledge of IRR of the downconverter  14 , which is made available to the control unit  54 . The control unit  54  then determines whether any of the upshifted components so calculated falls within the passband determined in step S 2 . The signal power of an upshifted component that is found to fall within this passband is compared with a threshold. If the signal power exceeds the threshold then there is deemed to be a passband intrusion. The signal power value at which this threshold is set will differ from one system design to another and the setting of an appropriate signal power value for this threshold will be apparent to the skilled person having regard to the circumstances in which the system that he or she is designing is to operate. 
     If a passband intrusion is detected in step S 3 , then the process moves to step S 4 . If no passband intrusion is detected in step S 3 , then the process moves to step S 5 . 
     In step S 4 , details of the passband intrusion detected in step S 3  are logged in a memory. Specifically, the signal power value of the intruding upshifted component and the local oscillator setting chosen in step S 1  are stored. From step S 4 , the process proceeds to step S 6  in which the control unit  54  determines whether or not there are any local oscillator settings that have not yet been tested for passband intrusion. If at least one untried local oscillator setting is available, then the process moves to step S 1  in which an as yet untried local oscillator setting is selected. If in step S 6  there are no untried local oscillator settings (such that the intrusion log is complete), then the process moves to step S 7 . 
     In step S 7 , the control unit  54  evaluates the data stored in the intrusion log and selects the local oscillator setting that would give rise to the lowest power intrusion into the passband. From step S 7 , the process moves to step S 5 , in which the frequency of local oscillator  26  is set to the local oscillator setting that was selected in step S 7  or, if step S 3  is exited with a negative result, step S 1 . The passband of the BPF  16  is then set so as to admit that part of the frequency spectrum to which the wanted signal will be downconverted given the local oscillator setting that has been applied. From step S 5 , the process moves to step S 8  and ends. 
     The frequency with which the controlled unit  54  runs through the process of  FIG. 6  will depend on the operating conditions. Clearly, the more often the process is run, then the less likely it becomes that a blocker will effect the demodulation of the wanted signal. Of course, the more often the process is run, the greater the burden will be on the processing resources that are tasked with performing the process. It will also be apparent that increased loading of processing resources also equates to increased power consumption, which is also undesirable. 
     An example of the operation of the process of  FIG. 6  will now be provided by reference to  FIGS. 4 and 7 . 
     It will be apparent to the reader that  FIG. 4  amounts to testing whether a particular setting of the local oscillator frequency would result in the intrusion of an upshifted component from the negative frequency region of the spectrum  36  into the passband. It is apparent from  FIG. 4  that upshifted component  46   b  arising from “reflection”  46  of adjacent channel signal  44  does indeed intrude on the passband. If signal  46   b  has sufficient power to transgress the threshold used in step S 3 , then the local oscillator setting to which  FIG. 4  pertains will be logged as giving rise to a passband intrusion (in step S 4 ). 
     Turning now to  FIG. 7 , the spectrum  36  is reproduced at the top of the diagram. However,  FIG. 7  pertains to a different local oscillator setting to  FIG. 4  and spectrum  66  shows, partially, the effect that the downconverter  14  would have on the positive frequency half of the spectrum  36  given the new local oscillator setting. The wanted signal  42  is downconverted to yield a downshifted component  42   c  and the higher power adjacent channel signal  44  is downconverted to yield a downshifted component  44   a . Of course, the downconverter  14  also produces upshifted components for signals  42  and  44  but these components are not shown since, as in  FIG. 4 , they are beyond the right hand edge of the diagram. Of course, the passband of BPF  16  needs to be adjusted to take into account the new local oscillator setting and the updated passband position is shown in  FIG. 7 . Spectrum  68  shows, partially, the effect that the downconverter  14  would have on the negative frequency side of spectrum  36 . The “reflections”  48  and  46  of the wanted and adjacent channel signals (respectively) are downconverted in frequency to yield respective downshifted components and upshifted components. The downshifted components, as in  FIG. 4 , fall beyond the left hand edge of the diagram and so are not shown. The upshifted components, however, are shown. The upshifted component of the “reflection”  46  of the adjacent channel signal is indicated  46   c  and the upshifted component of the “reflection”  48  of the wanted signal is indicated  48   c . The upshifted component  46   c  does not lie within the updated passband so the local oscillator setting to which  FIG. 7  pertains does not give rise to a passband intrusion. Accordingly, step S 3  of  FIG. 6  would be left by the negative path leading directly to S 5 . 
     In the system of  FIG. 5 , the spectrum analyser  52  uses a bank of filters followed by power detectors. Some other possibilities for the assessing the power versus frequency spectrum of the signal acquired by the antenna  12  will now be described. 
     In a first variant, the spectrum analyser  52  comprises digital signal processing hardware capable of calculating the spectrum of the signal arriving through line  58  using, for example, a fast Fourier transform technique. 
     In another variant, the spectrum analyser  52  employs a downconverter that mixes a local oscillator signal with the signal arriving through line  58 . The resulting signal is then band-pass filtered and supplied to a power detector (typically operating in the digital domain although an analogue version is possible). The frequency of the local oscillator signal may then be swept so that the power detector measures the power spectrum of the signal arriving on line  58 . 
     In a further but similar variant, several downconverters could act in parallel on the signal travelling along line  58 , each tuned to direct into the pass-band of a respective filter a relevant adjacent channel frequency band. The outputs of these filters are then subjected to power detection to measure the spectrum at relevant points relative to the wanted channel. 
     In yet another variant, the quadrature downconverter  14  and the BPF  16  can be used, with appropriate scanning of the frequency of local oscillator  26 , to probe the spectrum of the received signal. In such a case, the spectrum analyser unit  52  is redundant although sporadic diversion of the downconverter  14  and the BPF  16 , which are of course located in the main signal path, to the task of spectral analysis may result in the disruption of the signal provided to the sink  20 . 
     The embodiments described above use a power versus frequency spectrum to inform the setting of the local oscillator  26  and the passband of the BPF  16 . However, it is also envisaged that the control unit  54  could be provided with information about which parts of the frequency spectrum are likely to contain appreciable energy besides the wanted signal. This information could be communicated to the telephone  50  by, for example, the network in which the telephone operates. Such information could be used instead of, or in addition to, the power versus frequency spectrum deduced by the spectrum analyser  52 . 
     An algorithm for avoiding/reducing the intrusion of interferers into the signal processed by demodulator  18  was described with reference to  FIG. 6 . An alternative algorithm will now be described. 
       FIGS. 8 and 9  illustrate two different downconversion scenarios, as will now be discussed. These scenarios show a wanted signal and a dominant interferer (shown shaded), and their negative frequency “reflections”. The dominant interferer is the highest power interferer in the spectrum, for example as deduced by the controller  54  in assessing the output of the spectrum analyser  52 . Each of  FIGS. 8 and 9  shows three spectra with the frequency axes (horizontal) aligned for ease of comparison. 
     Referring now to  FIG. 8 , spectrum  69  shows the spectrum of the signal acquired by the antenna  12 . Again, the spectrum of the signal acquired by the antenna  12  can be considered mathematically as containing in the negative frequency region a “reflection” of what is contained in the positive frequency region. The signal in the wanted channel is indicated  70  and its negative frequency “reflection” is indicated  72 . The dominant interferer is indicated  74  and its negative frequency “reflection” is indicated  76 . The frequency, ω, of the signal to be applied by the local oscillator  26  in the downconversion process is also shown. 
     Spectrum  78  shows, partially, the effect of the downconverter  14  on the positive frequency half of the spectrum  69 . The wanted signal  70  is downconverted to yield a downshifted component  70   a  which lies in the appropriately placed passband of the BPF  16  whereas the dominant interferer  74  is downconverted to yield a downshifted component  74   a  which lies just below the passband. Of course, the downconverter  14  also produces upshifted components for signals  70  and  74  but these components are not shown since they do not bear on the passband (and in any event would lie off the right hand side of the diagram). 
     Spectrum  80  shows, partially, the effect of the downconverter  14  on the negative frequency half of spectrum  69 . The “reflections”  72  and  76  of the wanted signal and the dominant interferer (respectively) are downconverted in frequency to yield respective downshifted components and upshifted components. The downshifted components do not bear on the passband and so are not shown (and in any event would lie off the left hand side of the diagram). The upshifted components, however, are shown. The upshifted components  72   a  and  76   a  of the “reflections”  72  and  76  of the wanted signal and the dominant interferer (respectively) appear just above the passband of the BPF  16 . Of course, these upshifted components are suppressed in power to the extent possible given the design of the downconverter  14  (this extent is described by the downconverter&#39;s IRR). 
     Spectrum  82  shows the spectrum of the signal acquired by the antenna  12 . Again, the spectrum of the signal acquired by the antenna  12  can be considered mathematically as containing in the negative frequency region a “reflection” of what is contained in the positive frequency region. The signal in the wanted channel is indicated  84  and its negative frequency “reflection” is indicated  86 . The dominant interferer is indicated  88  and its negative frequency “reflection” is indicated  90 . The frequency, ω, of the signal to be applied by the local oscillator  26  in the downconversion process is also shown. 
     Spectrum  92  shows, partially, the effect of the downconverter  14  on the positive frequency half of the spectrum  82 . The wanted signal  84  is downconverted to yield a downshifted component  84   a  which lies in the appropriately placed passband of the BPF  16  whereas the dominant interferer  88  is downconverted to yield a downshifted component  88   a  which lies just above the passband. Of course, the downconverter  14  also produces upshifted components for signals  84  and  88  but these components are not shown since they do not bear on the passband (and in any event would lie off the right hand side of the diagram). 
     Spectrum  94  shows, partially, the effect of the downconverter  14  on the negative frequency half of spectrum  82 . The “reflections”  86  and  90  of the wanted signal and the dominant interferer (respectively) are downconverted in frequency to yield respective downshifted components and upshifted components. The downshifted components do not bear on the passband and so are not shown (and in any event would lie off the left hand side of the diagram). The upshifted components, however, are shown. The upshifted components  86   a  and  90   a  of the “reflections”  86  and  90  of the wanted signal and the dominant interferer (respectively) appear just below the passband of the BPF  16 . Of course, these upshifted components are suppressed in power to the extent possible given the design of the downconverter  14  (this extent is described by the downconverter&#39;s IRR). 
     From  FIGS. 8 and 9 , two rules can be deduced:
     a) if the dominant interferer is in a lower frequency channel than the wanted signal, then the frequency of the local oscillator  26  can be set higher than the frequency of the wanted signal (as in spectrum  69  of  FIG. 8 ), thereby ensuring that the wanted signal is not overlapped by the imperfectly suppressed upshifted version of the negative frequency “reflection” of the dominant interferer.   b) if the dominant interferer is in a higher frequency channel than the wanted signal, then the frequency of the local oscillator  26  can be set lower than the frequency of the wanted signal (as in spectrum  82  of  FIG. 9 ), thereby ensuring that the wanted signal is not overlapped by the imperfectly suppressed upshifted version of the negative frequency “reflection” of the dominant interferer.   

     The controller  52 , having made an assessment of, or having been provided with, the location of the dominant interferer can use these rules to configure the BPF  16  and the downconverter  14  appropriately to ensure that the dominant interferer does not interfere with the wanted signal in the input to the modulator  18 .