Abstract:
Electronic circuit including a frequency conversion device ( 4 ), an oscillator ( 6 ), a band-pass filter ( 8 ), and a controller ( 14 ), the oscillator ( 6 ) connected to the frequency conversion device ( 4 ), the frequency conversion device ( 4 ) connected to the band-pass filter ( 8 ),  
     the frequency conversion device ( 4 ) arranged to receive a first signal (S 1 ) at a frequency (f t ), to transform the first signal (S 1 ) into an intermediate signal (S 2 ) at an intermediate frequency (f i ) by applying a selection frequency (f loc ) from the oscillator ( 6 ),  
     the band-pass filter ( 8 ) arranged to receive the intermediate signal (S 2 ) and to perform a band-pass filtering at a centre frequency (f bpf ) and with a bandwidth (f w ), the centre frequency (f bpf ) equal to the intermediate frequency (f i ) at a predetermined working temperature of the band-pass filter ( 8 ), and the controller ( 14 ) capable to receive data indicative of a shift of the centre frequency (f bpf ) due to a temperature deviation of the band-pass filter ( 8 ) from the predetermined working temperature and to control the selection frequency (f loc ) of the oscillator ( 6 ) for changing the intermediate frequency (f i ) to follow the shift of the centre frequency (f bpf ).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to radio receiver circuitry.  
         PRIOR ART  
         [0002]    In communications systems, the receiver circuit employing Intermediate Frequency Band-Pass Filtering is known to have temperature dependent centre frequency characteristics. The temperature of the intermediate frequency band-pass filter (IF BPF) increases while the communications system is in use, and causes a shift of the centre frequency of the IF BPF. Generally, the temperature dependence is stronger for cheaper filters and/or filters with smaller dimensions.  
           [0003]    Under normal conditions, the bandwidth of an IF BPF is chosen slightly larger than the bandwidth of a selected signal (i.e., a signal in a specific selected channel) to separate only that signal (or: channel) from other signals, present in the medium. For example: in the 5 GHz ISM (Industrial, Scientific and Medical) band for wireless communications, the signal bandwidth for OFDM (Orthogonal Frequency Division Multiplexing modulation) is typically 16.6 MHz, the IF BPF has a bandwidth of 18 MHz, and the separation of the centre frequencies of adjacent transmission channels is 20 MHz.  
           [0004]    If the centre frequency of the IF BPF changes due to a change of temperature of the filter, the selected signal may be cut-off at one of the bandwidth boundaries of the intermediate frequency band-pass filter, which may cause a loss of signal strength. Also, since transmission channels are closely spaced to each other, a signal in an adjacent channel may enter into the bandwidth of the IF BPF and may distort the selected signal in the signal processing steps after passing the IF BPF.  
           [0005]    Disadvantageously, these operational changes of the IF BPF result in a condition where the received selected signal cannot be processed reliably anymore, causing the communications system to fail its specifications due to, e.g., a high Bit Error Rate, or a low reception sensitivity.  
         SUMMARY OF THE INVENTION  
         [0006]    It is an object of the present invention to provide an electronic circuit comprising an intermediate frequency band-pass filter, which compensates for the shift of the centre frequency of such a filter, due to a change in temperature.  
           [0007]    The present invention relates to an electronic circuit having a controller arranged to receive data indicative of a shift of the intrinsic centre frequency due to a temperature change of the band-pass filter from the predetermined working temperature and to control the first selection frequency of the local oscillator for changing the intermediate frequency of the intermediate signal to follow the shift of the intrinsic centre frequency.  
           [0008]    Thus, the present invention provides an electronic circuit which processes a selected signal reliably, irrespective of the operating temperature of the band-pass filter.  
           [0009]    Moreover, the present invention allows the use of a standard band-pass filter without the need for a more expensive band-pass filter with less temperature-dependent properties.  
           [0010]    Also, the present invention relates to a method to be carried out by a controller. The method carried out by the controller includes the steps of:  
           [0011]    receiving data indicative of a shift of the intrinsic centre frequency due to a temperature deviation of the band-pass filter from the predetermined working temperature, and  
           [0012]    controlling the first selection frequency of the local oscillator for changing the intermediate frequency of the intermediate signal to follow the shift of the intrinsic centre frequency. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    The present invention will be explained with reference to some drawings, which are intended for illustration purposes only and not to limit the scope of protection as defined in the accompanying claims.  
         [0014]    [0014]FIG. 1 shows a schematic diagram of an electronic circuit according to the present invention in a first preferred embodiment as part of a receiver of a communications system;  
         [0015]    [0015]FIGS. 2 a  and  2   b  show a schematic graph of an intermediate frequency spectrum and a corresponding base-band frequency spectrum, respectively, before temperature compensation;  
         [0016]    [0016]FIGS. 2 c  and  2   d  show a schematic graph of an intermediate frequency spectrum and a corresponding base-band noise frequency spectrum, respectively, after temperature compensation;  
         [0017]    [0017]FIG. 3 shows a schematic diagram of an electronic circuit according to the present invention in a second preferred embodiment as part of a receiver of a communications system. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    An electronic circuit according to the present invention as part of a receiver of a communications system is preferably part of a receiver and may be applied in a wireless communications system such as a mobile telephone for selecting and receiving wireless signals, broadcasted to that mobile phone. Also, the electronic circuit of the present invention may be applied as part of an interface card in a personal computer for receiving wireless signals from a wireless local area network (LAN). Also, the electronic circuit may be used in a receiver for receiving signals from a wired local area network (LAN).  
         [0019]    In the following description, the electronic circuit of the present invention will be described with reference to a such receiver.  
         [0020]    However, it will also be appreciated that the electronic circuit of the present invention may not only be employed in receivers but also in transmitters. Moreover, the electronic circuit of the present invention may be employed in any electronic device for signal processing that uses band-pass filtering techniques.  
         [0021]    [0021]FIG. 1 shows a schematic diagram of an electronic circuit according to the present invention in a first preferred embodiment as part of a receiver of a communications system. The receiver  1  comprises an antenna  2 , an intermediate frequency conversion device  4 , a tuneable local oscillator  6 , a band-pass filter  8 , a tuneable base-band oscillator  9 , a base-band conversion device  10 , signal processing means  11 , a (wireless) medium access controller (W-MAC)  14 , memory  16 , a temperature sensor  18 , and a current source  20 .  
         [0022]    The antenna  2  is connected to the intermediate frequency conversion device  4 , which is further connected to the tuneable local oscillator  6  and the band-pass filter  8 . The band-pass filter  8  is connected to the base-band conversion device  10 , which connects to the signal processing means  11 . In this first embodiment, the signal processing means  11  comprise a digital signal processing unit (DSP)  12 , an analog-digital converter  22  and a switching device  24 . Here, the base-band conversion device  10  connects to the switching device  24 , which connects to the analog-digital converter  22 . The analog-digital converter  22  is preferably embedded in the DSP  12 . Further, the base-band conversion device  10  is connected to the tuneable base-band oscillator  9 . The DSP  12  is connected to the W-MAC  14 . Also, the memory  16  is connected to the W-MAC  14 . The temperature sensor  18  is also connected to the switching device  24 . Further, the temperature sensor  18  is connected in a closed loop to the current source  20 . The W-MAC  14  is also connected to the tuneable local oscillator  6 , the tuneable base-band oscillator  9 , and to the switching device  24 .  
         [0023]    As known to persons skilled in the art, the memory  16  may comprise various kinds of memory such as random access memory (RAM), read-only memory (EEPROM, ROM or Flash), and hard disk.  
         [0024]    Reception of signals, present in the medium, is done by the antenna  2 , which passes all received signals SR to the intermediate frequency conversion device  4 . The selection of a particular signal S 1  at a broadcast frequency f t  from the received signals SR is done by setting the local oscillator  6  to such a frequency f loc  that after processing in the intermediate frequency conversion device  4 , the signal S 1  is transformed into a selected signal S 2  with an intermediate frequency f i  which substantially matches the centre frequency f bpf  of the band-pass filter  8 . The transformation by the intermediate frequency conversion device  4  comprises the synthesis of the intermediate frequency f i  as the difference of the broadcast frequency f t  of S 1  and the local oscillator frequency f loc  (f i =f t −f loc  or f i =f t +f loc ).  
         [0025]    Ideally, only the selected signal S 2  is within the bandwidth of the band-pass filter  8  for passing to the base-band conversion device  10 . The base-band conversion device  10  transforms the signal S 2  into the base-band signal S 3 , which is to be processed by the DSP  12 . It will be appreciated by persons skilled in the art, that here the conversion device  10  may transform the signal S 2  into a signal S 3  in a second intermediate frequency band, if desired.  
         [0026]    The temperature sensor  18  is located close to the band-pass filter  8  to sense the temperature of the band-pass filter  8  in an accurate manner. In this first embodiment of the present invention, the temperature sensor  18  provides a temperature dependent signal ΔT, related to the temperature of the band-pass filter  8 , to the switching device  24  for passing to the DSP  12  for further processing of ΔT. The temperature sensor  18  may be a resistor with a temperature dependent resistivity coefficient. Also, the temperature sensor  18  may a temperature dependent diode or another type of device which is capable of providing a temperature dependent signal ΔT. In the embodiment of FIG. 1, the temperature sensor  18  is shown as a temperature dependent resistor.  
         [0027]    Under control by the W-MAC  14  (wireless medium access controller), the analog-digital converter  22  on the DSP  12  measures a voltage drop ΔV over the temperature sensor  18 . To generate the voltage drop ΔV, the current source  20  supplies a small constant current to the temperature sensor  18  with minimal heat dissipation, preferably none. Thus, the voltage drop ΔV may be proportional to the change of the temperature ΔT of the band-pass filter  8 , relative to a given reference temperature.  
         [0028]    It is noted that the temperature sensor  18  may be a passive element such as a thermocouple, which generates a temperature-related voltage signal, as known to persons skilled in the art. In that case, current source  20  may be omitted.  
         [0029]    The temperature signal ΔT is passed to the switching device  24 . In order to avoid interference with the reception of the selected signal S 1 , the W-MAC  14  controls at which instant the switching device  24  may switch the voltage drop signal of the temperature sensor  18  to the DSP  12  for measuring the voltage.  
         [0030]    Preferably, the DSP  12  is arranged to capture and process the temperature signal ΔT (i.e., ΔV) at regular intervals without disturbing the reception and transmission of signals by the communications system. To measure the value of the voltage drop ΔV, the DSP  12  uses the embedded analog-digital converter  22 . The DSP  12  is arranged to pass the measured value to the W-MAC  14 . The W-MAC  14  is arranged to compare the measured value of ΔV with empirical data stored in the memory  16 .  
         [0031]    The change of the band-pass filter frequency f bpf  as a function of the temperature of the band-pass filter  8  can be empirically established in an experimental set-up and the results recorded as a function of the temperature change ΔT and/or the voltage drop ΔV. This empirical relationship between the shift of the centre frequency of the band-pass filter  8  and the temperature change ΔT is preferably stored in the memory  16 . (It is known that certain types of intermediate frequency band-pass filters comprising e.g., LiNbO 3  typically display a negative temperature coefficient for the shift of the centre frequency as a function of increasing temperature.)  
         [0032]    By comparison of the measured value of ΔV with these empirical data, the W-MAC  14  is capable of controlling the frequency f loc  of the local oscillator  6  to adjust the intermediate frequency f i  of the selected signal S 2  to remain within the bandwidth of the band-pass filter  8 . Simultaneously, the W-MAC  14  controls the frequency f bb  of the base-band oscillator  9  to produce the selected signal S 3  in the base-band domain in correspondence with the intermediate frequency f im  of the selected signal generated in the intermediate frequency conversion device  4 .  
         [0033]    The temperature compensation will now be explained below.  
         [0034]    [0034]FIGS. 2 a  and  2   b  show a schematic graph of an intermediate frequency spectrum and a corresponding base-band energy distribution frequency spectrum, respectively, before temperature compensation.  
         [0035]    [0035]FIG. 2 a  shows schematically the selected signal S 2  and the frequency window F of the band-pass filter  8  as a function of the frequency. The reception energy level of S 2  is plotted in the vertical direction as a function of the intermediate frequency (on the horizontal axis). The selected signal S 2  has a centre intermediate frequency f i . The band-pass filter  8  has a centre frequency f bpf  and a full-width 2f w .  
         [0036]    Due to a temperature change ΔT in operating temperature of the band-pass filter  8  as argued above, the frequency window of the band-pass filter  8  is shifted with respect to the selected signal S 2 . The centre frequency of the band-pass filter  8  f bpf  is at a lower frequency than the intermediate frequency f i  of the selected signal S 2 . The selected signal S 2  extends over the high frequency boundary of the band-pass filter  8 , with the high-frequency limit f i +f s  of the selected signal outside the bandwidth of the band-pass filter  8 . The low-frequency limit of the selected signal f i −f s  is within the bandwidth of the band-pass filter  8 .  
         [0037]    In FIG. 2 b  the energy distribution of the base-band signal S 3  is plotted in the vertical direction as a function of the base-band frequency (on the horizontal axis) in accordance to the plot of the selected signal S 2  in the band-pass frequency domain as depicted in FIG. 2 a.    
         [0038]    In FIG. 2 b , the dashed vertical line at f w  indicates the boundaries of the band-pass filter  8 . The centre frequency of the band-pass filter  8 , f bpf , corresponds to a base-band frequency of zero. The solid line depicts the energy level as a function of the band-pass filter&#39;s frequency spectrum transposed to base-band frequency. Due to the asymmetry of the selected signal S 2  with respect to the position of the (shifted) band-pass filter  8 , the reception energy levels of base-band signal S 3  will appear in the base-band domain as a two-level energy distribution with an inflexion lower limit frequency f bpf −(f i −f s ) of the base-band signal S 3 .  
         [0039]    The tail of the signal S 3  above frequency f w  is plotted as a dotted line. The tail extends to approximately the frequency f i +f s −f bpf . As known to those skilled in the art, the part of the signal S 3  above frequency f w  is outside the band-pass filter window and thus discarded for further processing by the DSP  12 .  
         [0040]    The W-MAC  14  receives the value of ΔT from the DSP  12 , and controls the adjustment of the intermediate frequency f i  of the selected signal S 2  by changing the frequency setting of the local oscillator  6 . The actual setting of the frequency f loc  of the local oscillator  6  is derived by the W-MAC  14  from the empirical data stored in memory  16 . The frequency f bb  of the base-band oscillator  9  is adjusted simultaneously by the W-MAC  14 .  
         [0041]    [0041]FIGS. 2 c  and  2   d  show a schematic graph of an intermediate frequency spectrum and a corresponding base-band energy level frequency spectrum, respectively, after temperature compensation.  
         [0042]    [0042]FIG. 2 c  displays schematically the selected signal S 2  and the frequency window F of the band-pass filter  8  as a function of the frequency, when the centre frequency f bpf  of the band-pass filter  8  is substantially equal to the centre frequency f i  of the selected signal. The reception energy level is plotted in the vertical direction as a function of the intermediate frequency (on the horizontal axis). The band-pass filter  8  has a centre frequency f bpf  with a full-width 2f w .  
         [0043]    In this situation, which occurs after temperature compensation, the modified intermediate frequency f im  of the selected signal S 2  has been reduced, relative to the intermediate frequency f i  of FIG. 2 a . (Also, in the ideal case, this situation occurs when the centre frequency of the band-pass filter  8  is temperature-independent.) The selected signal S 2  is within the bandwidth of the band-pass filter  8 .  
         [0044]    The measured energy distribution of the base-band signal S 3  is shown in FIG. 2 d , where the energy distribution level is plotted as a function of the frequency in the base-band domain. The measured profile appears basically as a single-level energy distribution (with some small tail close to f w ).  
         [0045]    The compensation of the temperature-related change of the centre frequency of the band-pass filter  8  is accomplished by changing the intermediate frequency of the receiver  1  as discussed with reference to the first embodiment shown in FIG. 1.  
         [0046]    [0046]FIG. 3 shows a schematic diagram of an electronic circuit according to the present invention in a second preferred embodiment as part of a receiver of a communications system.  
         [0047]    In FIG. 3, entities with the same reference numbers as used in FIG. 1, refer to the same entities as shown in FIG. 1. The receiver  31  of the wireless communications system of FIG. 3 is almost identical to the receiver  1  shown in FIG. 1, except for the temperature sensor  18 , the current source  20 , and the switching device  24 , which have been omitted in the second preferred embodiment.  
         [0048]    In the embodiment of FIG. 3, the shift of the centre frequency of the band-pass filter  8  as a function of temperature is monitored by the DSP  12  by measuring the energy level distribution of the base-band signal S 3  as a function of the base-band frequency.  
         [0049]    The base-band signal S 3  is derived from the selected signal S 2  as enclosed within the bandwidth of the band-pass filter  8 . Whereas the spectrum of the band-pass filter is centred around a centre frequency f bpf  and extends from f bpf −f w  to f bpf +f w , with f w  being the half-width of the band-pass filter, the base-band spectrum extends from a frequency of zero to a frequency of f w . Band-pass signal frequencies below f bpf  are transposed to frequencies above the base-band zero frequency, and the intensity of such frequencies is added to the intensity of the band-pass signal frequencies above f bpf .  
         [0050]    In the embodiment of FIG. 3, the DSP  12  probes the energy level within the bandwidth of the base-band as a function of the base-band frequency, as passed by the band-pass filter  8 , to establish the reception energy level of the band-pass filter  8  and it&#39;s position with respect to the position of a selected signal S 2  at the intermediate frequency f i . This will be explained in more detail below.  
         [0051]    The probing of the base-band spectrum by the DSP  12  is done in relatively small frequency increments compared to the bandwidth of the band-pass filter  8 . For example, the probing increment is about 100 kHz or less, at a bandwidth of 18 MHz.  
         [0052]    The probing procedure by the DSP  12  and it&#39;s result will now be explained with reference to FIGS. 2 a ,  2   b ,  2   c , and  2   d.    
         [0053]    When the centre frequency of the band-pass filter  8  shifts due to a change of operating temperature, the selected signal S 2  (centred at the intermediate frequency as set by the local oscillator  6 ) will be partly outside of the bandwidth of the band-pass filter  8  (see FIG. 2 a ).  
         [0054]    The DSP  12  probes the energy level distribution of the corresponding base-band signal S 3  and passes the probing result to the W-MAC  14 . When the W-MAC  14  detects a two-level distribution as shown in FIG. 2 b , correction of the position of the intermediate frequency signal S 2  with respect to the band-pass frequency f bpf  is required. Again, as in the first embodiment, by changing the intermediate frequency f i  of the selected signal S 2  (by means of the intermediate frequency conversion device  4 ) to match the shifted band-pass filter frequency f bpf , the selected signal S 2  can be shifted to be within the bandwidth boundaries of the band-pass filter  8 . The frequency of the local oscillator  6  is adapted to modify the intermediate frequency f i  of S 2 . At the same time, the frequency of the base-band oscillator  9  is changed accordingly.  
         [0055]    When the temperature change actually becomes so large that the selected signal may be partly outside of the bandwidth of the band-pass filter  8 , the W-MAC  14  controls the frequency f loc  of the local oscillator  6  to change the intermediate frequency f i  of the selected signal S 2 , as synthesised in the intermediate frequency conversion device  4 , to a new intermediate frequency value f im  in such a way that the selected signal S 2  is within the bandwidth of the band-pass filter  8  and the intermediate frequency f im  again substantially matches the centre frequency f bpf  of the band-pass filter  8  that shifted under thermal load (see FIG. 2 c ). The base-band frequency spectrum will comprise a single-level energy distribution as shown in FIG. 2 d.    
         [0056]    The profile measured by the DSP  12 , is evaluated by the W-MAC  14  (preferably by comparison with a reference profile). Similar to the situation for the first embodiment, as a response to the outcome of the measured profile, the W-MAC  14  controls the frequency f loc  of the local oscillator  6  to change the intermediate frequency f i  of the selected signal S 2 , as synthesised in the intermediate frequency conversion device  4 , to a new frequency value f im  in such a way that the selected signal S 2  remains within the frequency window of the band-pass filter  8 . Simultaneously, the W-MAC  14  controls the frequency f bb  of the base-band oscillator  9  to produce a base-band signal S 3  corresponding with the new intermediate frequency f im  of the selected signal S 2  generated in the intermediate frequency conversion device  4 .  
         [0057]    In the second embodiment of the present invention the probing procedure, as described above, is used to control the temperature compensation. By repeating the probing procedure by the DSP  12 , a measured base-band signal profile can be changed from a two-level energy distribution (shown in FIG. 2 b ) to a single level energy distribution as shown in FIG. 2 d  by modifying the intermediate frequency generated in the intermediate frequency conversion device  4 . Also, by repeated probing, the base-band signal profile can be preserved as a single level energy distribution during a change of temperature of the band-pass filter  8 .  
         [0058]    As described above, the temperature compensation control in the receiver  1 ; 31  is (conveniently) done by the W-MAC  14 . It is noted that a separate processing unit (e.g. a micro-controller) may be used in addition to the W-MAC  14  to perform this task, although this may increase the cost of such a receiver.  
         [0059]    Moreover, it will be appreciated by those skilled in the art, that in other embodiments that may readily be understood from the description given above, some of the components in the receiver  1 ;  31  may be replaced by other components with similar functionality: the wireless medium access controller may be replaced by a micro-controller. Also, the DSP  12  may be a signal processing circuit.  
         [0060]    Furthermore, it is noted that the temperature compensation control according to the present invention may also be used in receivers for wired (-LAN) applications that comprise comparable band-pass filters.  
         [0061]    It will also be appreciated that the filter centre frequency temperature compensation of the present invention may not only be employed in receivers but also in transmitters by adapting the appropriate frequency conversion devices in such transmitters. Thus, an input signal in a base frequency range which is to be transmitted, may be converted by one or more appropriate frequency conversion devices into a transmittable signal in a transmission band frequency. By application of any one of the methods according to the present invention any temperature-related frequency shift of a frequency conversion device may be corrected.  
         [0062]    Moreover, the filter centre frequency temperature compensation of the present invention maybe employed in any electronic device for signal processing that uses band-pass filtering techniques.  
         [0063]    It will further be appreciated that other embodiments may be provided in accordance with the present invention. These and other embodiments that may readily be understood from the description given above, will be within the scope of protection as defined in the accompanying claims.