Abstract:
A method and device for frequency conversion is disclosed in which a first signal with a first frequency is converted into a second frequency through mixing with a divided oscillator signal and wherein the frequency of the divided oscillator signal stands in a fractional rational ratio to the frequency of the undivided oscillator signal. The method is characterized in that the oscillator signal is divided such that an average value of the divided oscillator signal over time corresponds to half the separation between the extreme values of the divided oscillator signal.

Description:
[0001]     This nonprovisional application claims priority under 35 U.S.C. § 119(a) on German Patent Application No. DE 102004 005 503 filed in Germany on Jan. 30, 2004, which is herein incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method for frequency conversion in which a first signal with a first frequency is converted into a second frequency through mixing with a divided oscillator signal and wherein the frequency of the divided oscillator signal stands in a fractional rational ratio to the frequency of the undivided oscillator signal.  
         [0004]     In addition, the present invention relates to a receiver, having at least a mixer, an oscillator, and a switchable frequency divider for dividing a frequency of the oscillator by various natural numbers n, m, wherein the mixer mixes a first signal that has a first frequency with a divided signal from the oscillator and thus converts it to a second frequency, and having a control unit that, in a first operating state, periodically switches the switchable frequency divider from a division by the number n to a division by the number m according to a predefined time-slot pattern.  
         [0005]     2. Description of the Background Art  
         [0006]     A conversion of a first frequency to a second frequency is customary in, for example, receiving systems for radio frequencies. To this end, both the signal at the first frequency and the output signal of the phase-locked loop are supplied to a mixer that outputs as a result the signal at the second frequency (intermediate frequency).  
         [0007]     In special applications it is desirable to be able to set a division factor of 1.5. Division by 1.5 corresponds to multiplication by a factor of 2/3, which means that two output pulses are generated from every three input pulses.  
         [0008]     It is known to achieve frequency conversion with a fractional rational frequency ratio by division using the fractional-N principle. This principle is used, for example, in phase-locked loops with fractional rational division ratios to convert an oscillator frequency to a reference frequency. Conventional fractional-N dividers generate a fractional rational frequency ratio by periodically removing pulses from a periodic pulse sequence. Conventional fractional-N frequency dividers thus ultimately suppress output pulses in order to express the fractional rational frequency ratio. This creates an asymmetry in the time behavior of the output signal of the phase-locked loop that is associated with a DC component in the output signal. In other words, the average value over time of the output signal then does not correspond to half of the separation of its extreme values. However, a signal that has a DC component is not suitable for operating a mixer. In the prior art, the DC component must therefore be removed by filtering, which makes the preparation of a signal for the mixer complicated.  
       SUMMARY OF THE INVENTION  
       [0009]     It is therefore an object of the present invention to provide a method for preparing a divided signal that stands in a fractional rational ratio to the undivided signal and that is suitable for a mixer without complicated further processing.  
         [0010]     In addition, the object of the invention is to specify a receiver that prepares and utilizes such a signal.  
         [0011]     This object is attained with a method of the type initially mentioned in that the oscillator signal is divided such that the average value of the divided oscillator signal corresponds to half the separation between the extreme values of the oscillator signal. This object is further attained with a receiver of the aforementioned type in that the various natural numbers and the time-slot pattern are predetermined such that the average value of the divided oscillator signal corresponds to half the separation between the extreme values of the oscillator signal.  
         [0012]     This object is further attained with a receiver of the aforementioned type in that the various natural numbers and the time-slot pattern are predetermined such that an average value of the divided oscillator signal over time corresponds to half the separation between the extreme values of the divided oscillator signal.  
         [0013]     The object of the invention is attained in full through these features. In particular, a divided signal of an oscillator with the required properties is produced without the necessity of further processing to remove a DC component.  
         [0014]     In a preferred embodiment, the frequency of the oscillator signal is periodically divided by various natural numbers n, m in accordance with a time-slot pattern.  
         [0015]     As a result of this embodiment, no pulses need to be removed from a regular pulse sequence. Instead, the frequency of the divided signal, which has a fractional rational relationship to the undivided signal, results from alternating division by various natural numbers, which produces pulses of differing lengths in the divided signal.  
         [0016]     In a further embodiment, the natural number m is twice the natural number n, and for a fixed time-slot pattern with a pulse duty ratio of 50% to be used as the time-slot pattern. It is also preferred that m be equal to 1.  
         [0017]     The combination of this pulse duty ratio for all pairs n, m where n=2 m provides an output signal from the phase-locked loop that has the desired characteristics. The combination of n=2 and m=1 results in especially short period durations in the output signal of the phase-lock loop, which accelerates transient response after a switchover of the division factor.  
         [0018]     Another preferred embodiment is characterized by an FM radio signal as the first signal and an intermediate-frequency signal as the second signal.  
         [0019]     These features characterize a preferred area of application. In this area, the invention—using a division factor of 1.5 with the capability of switchover to other division factors (for example, 2 and/or 3)—opens up the possibility of covering a variety of different regional and application-specific receive frequency ranges with a universal receiver having a simplified construction.  
         [0020]     With regard to embodiments of the receiver, it is preferred that the oscillator provides a frequency between 170 MHz and 236 MHz.  
         [0021]     This frequency corresponds to approximately twice the oscillator frequency of the automobile radios that are customarily used. The use of such a comparatively high frequency opens up a variety of options for division, which permit simple adaptation to regional and/or application-specific requirements. As a general rule, multiple receivers are operated at the same time in automobile radios. For example, one receiver serves as an audio receiver, while another receiver continuously monitors the quality of reception on alternative frequencies for the station being listened to, in order to facilitate timely switchover. The FM band is approximately 20 MHz wide, and as a general rule a frequency of 10.7 MHz is chosen as the intermediate frequency because inexpensive ceramic filters are commercially available for this frequency. Oscillators are commonly used which oscillate at a frequency that is approximately one intermediate frequency higher than the first frequency, or receive frequency, that is to be received. If the receive frequency is at the lower end of the FM bandwidth, the oscillator oscillates within the receive band. Thus, due to ultimate decoupling of the receiver, an additional receiver will be interfered with if it happens to be operated at this frequency.  
         [0022]     In a preferred embodiment, the oscillator frequency is higher than a frequency of the FM radio signal.  
         [0023]     As a result of such a constellation, referred to as “high side injection,” the range of possible first frequencies that can be converted to an intermediate frequency with few division factors is increased.  
         [0024]     It is also preferred that, in a second operating state, the oscillator frequency is continuously divided by 2.  
         [0025]     As a result of such a division factor, the FM frequency band between 88 and 108 MHz (first frequency) used in Europe and the USA can be mixed with an output frequency from a phase-locked loop that is approximately one intermediate frequency of 10.7 MHz higher (98 MHz to 118 MHz) in order to convert the first frequency to a second frequency or intermediate frequency of 10.7 MHz.  
         [0026]     A further preferred embodiment is characterized in that the oscillator frequency is lower than the first frequency.  
         [0027]     Such a constellation, referred to as “low side injection,” also increases the range of possible first frequencies that can be converted to an intermediate frequency with few division factors.  
         [0028]     It is further preferred that, in a third operating state, the oscillator frequency is continuously divided by 3.  
         [0029]     As a result of such a division factor, the FM band between 78 and 98 MHz (first frequency) that is used in Japan can be mixed with an output frequency from a phase-locked loop that is approximately one intermediate frequency of 10.7 MHz lower (68 MHz to 88 MHz) in order to convert the first frequency to a second frequency or intermediate frequency of 10.7 MHz.  
         [0030]     Moreover, low side injection in combination with a division factor of 1.5 and an oscillator frequency of up to 236 MHz also permits conversion of a first frequency of approximately 168 MHz to an intermediate frequency of 10.7 MHz, and thus permits reception in this range of first frequencies. This is especially significant for use in the USA because weather information, in particular storm warnings, are broadcast there on a weather band with a narrow bandwidth at approximately 168 MHz.  
         [0031]     Overall, the features described provide both a method and a universal receiver that can permit reception in an important weather band in the USA in addition to reception in a normal FM band in the USA and Europe, and also reception in a normal FM band in Japan with a simple change of division factors. In doing so, the method and also the receiver allow the omission of further processing to remove the DC component in the divided signals.  
         [0032]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:  
         [0034]      FIG. 1  is a flow chart of a preferred embodiment of the present invention;  
         [0035]      FIGS. 2   a - e  are timing diagrams of divided and undivided signals;  
         [0036]      FIG. 3  is a schematic illustration of a receiver according to an embodiment of the invention; and  
         [0037]      FIG. 4  is a program flow chart illustrating the method according to a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0038]      FIG. 1  shows a sequence of steps  10 ,  12 , and  14 , where an oscillator signal is produced in step  10 , is divided in step  12 , and is combined in step  14  with a first signal (receive signal) in a mixer.  
         [0039]     In a first step  10 , an oscillator signal f_O is continuously produced. Such an oscillator signal can, for example, be produced by a phase-locked loop with a voltage-controlled oscillator. The oscillator signal can, for example, be a square-wave signal with a symmetrical pulse duty ratio of 50% and thus has a periodic sequence  16  of square-wave pulses  18 , as shown in  FIG. 2   a.    
         [0040]     In step  12 , a division of the output signal of the oscillator takes place. A division can, for example, be accomplished by generating, from the periodic sequence  16 , a new square wave signal in which a signal level change is triggered by every nth edge (rising or falling) of the oscillator signal.  
         [0041]     In this way, a halving of the oscillator frequency results when n=2. When n=3, the oscillator frequency is divided by 3. The corresponding result for n=2 is shown in  FIG. 2   b  as a periodic sequence  20 , and the result for n=3 is shown in  FIG. 2   c  as a periodic sequence  22 . The values n=2 and n=3 represent examples of integer relationships between the oscillator frequency and each one of the divided frequencies. In comparison to the period duration of the oscillator signal, the pulse sequences resulting from the division are periodic, with twice the period duration when n=2 and three times the period duration when n=3. Moreover, their average value over time  24  corresponds to half the distance between their extreme values  26 ,  28 , which are defined here by the low and high levels.  
         [0042]     To generate fractional rational ratios, conventionally, individual pulses are removed from the periodic pulse sequences using a so-called modulo divider. This is shown in  FIG. 2   d  using the example of a ratio of 1.5. With such a pattern  30 , as is shown in  FIG. 2   d , the average value over time  25  no longer corresponds to half the distance between the two signal levels  26 ,  28 , but instead is one third of the distance above the lower level  26  and thus is separated from the average value  24  of the two levels  26 ,  28  by one sixth. This one sixth represents the DC component, which in a subsequent combination with another signal in a mixer has an interfering effect.  
         [0043]     In accordance with a preferred embodiment of the invention, the fractional rational ratio is already set by the method of division in step  12  such that the interfering DC component does not appear. This is shown in  FIG. 2   e . To this end, the signal is initially divided by a factor n during a first time slot  32 , where n is a natural number and the division takes place in a manner analogous to the examples described above for values of 2 and 3.  
         [0044]     Then, in a second time slot  34 , a division by a factor m takes place, where m is also an element of the natural numbers. In the example in  FIG. 2   e , n=2 and m=1. The division here by the factor n takes place in each case until the divided signal encompasses a full period  32  associated with the factor 2. This period  32  then constitutes a first partial period  32  of the complete divided signal. Subsequently, division by the factor 1 is performed until the resulting divided signal encompasses a corresponding full period  34 . This period  34  constitutes a second partial period  34  of the complete divided signal. The sum of the two partial periods  32 ,  34  then results in a full period  36  of the complete divided signal. The number  38  designates the time slot pattern in which switching from division by the number n to division by the number m, and back again, takes place.  
         [0045]     The divided signal  40  generated in this way has two pulses  42 ,  44  for every three pulses  18  of the undivided oscillator signal and thus, like the signal  30  from  FIG. 2   d , corresponds to a fractional rational division ratio of 1.5 (multiplication by 2/3). However, in contrast to the signal  30  from  FIG. 2   d , the signal  40  visibly has an average value over time  24  that is one half of the distance between its signal levels, which here, too, constitute the extreme values  26 ,  28  of the signal  40 . The signal  40  thus has no interfering DC component and can be used in a subsequent step  14 , without any additional process steps such as filtering, to convert a receive signal with a first frequency to a second frequency (intermediate frequency).  
         [0046]     This sequence of steps  10 ,  12 , and  14  thus represents an example embodiment of a method for frequency conversion in which a first signal with a first frequency is converted to a second frequency by mixing with a divided oscillator signal  40 , and where the frequency of the divided oscillator signal  40  has a fractional rational ratio to the frequency of the undivided oscillator signal  16 , and where the oscillator signal  16  is divided such that the average value of the divided oscillator signal  40  corresponds to half the distance between the extreme values  26 ,  28  of the divided oscillator signal  40 .  
         [0047]     Although the method has been explained for numbers n=2 and m=1, it is a matter of course that the method is not restricted to these particular values, but rather can be used with any desired natural numbers for which a complete respective period results in a vanishing DC component. These conditions are always met, for example, when the larger number is twice the smaller number and a pulse duty ratio of 50% is maintained for each one. In this regard, the pulse duty ratio is understood to be the ratio of the time with high signal level  28  to the total duration of a signal segment being examined. A pulse duty ratio of 50% automatically results whenever division by a specific factor is always executed for a duration such that integer multiples of partial periods  32  of the divided signal  40  are present, and division by the other factor is executed for the same number of partial periods. In the example in  FIG. 2   d , the integer number is 1.  
         [0048]     The invention is preferentially used for conversion of an FM radio signal (FM=frequency modulation) to an intermediate frequency in an FM receiver. An example embodiment of such an FM receiver is shown in  FIG. 3 .  
         [0049]     The basic task of a receiver is to select a portion of a frequency spectrum and demodulate the signal voltage contained therein. A distinction is drawn in this regard between direct-detection receivers and superheterodyne receivers. In the direct-detection receiver, demodulation takes place at the frequency of the received signal. The receive frequency is selected by one or more bandpass filters. Adequate adjacent-channel selectivity requires multiple filter circuits, which sharply increases the expense for reception of different frequencies.  
         [0050]     The superheterodyne receiver avoids this disadvantage by converting different receive frequencies to one intermediate frequency. Using a mixer, different frequency spectra can be converted to a uniform intermediate frequency of, for example, 10.7 MHz by varying the divided oscillator frequency. Demodulation takes place at the intermediate frequency stage.  
         [0051]      FIG. 3  shows a receiver  46  with an antenna  48  that receives an FM signal  50  at a first frequency. If necessary, the received signal is amplified by a low-noise amplifier  52  and is fed to a mixer  54 . A divided oscillator signal  20 ,  22 ,  40  or an undivided oscillator signal  16 , which is to say a signal such as is qualitatively depicted in  FIG. 2 , is also fed to the mixer  54 .  
         [0052]     To this end, an undivided oscillator frequency is first generated in a local oscillator  56  and is divided by a subsequent programmable frequency divider  58 . The local oscillator  56  has, for example, a voltage controlled oscillator (VCO) that outputs a signal with a frequency f_O. This oscillator frequency LO depends on a DC voltage with which the oscillator can be controlled. To set a stable frequency f_O, the output signal with frequency LO is tapped by a programmable frequency divider, for example as part of a phase-locked loop that is not explicitly shown, and is compared to a reference signal in a phase/frequency detector. The reference signal can be generated by a quartz oscillator, for example. Differences in phase position generate correction pulses which, after filtering by a loop filter, change the control voltage for the oscillator. Deviations in the frequency of the divided output signal from the reference frequency thus produce a control intervention that causes the divided output signal to settle at the reference frequency. If the signals are in phase, then their frequencies also match. The phase-locked loop is then locked at the frequency f_O. The local oscillator  56  then supplies a signal  16  corresponding to the schematic representation in  FIG. 2   a  with a frequency f_O to the frequency divider  58 , for example.  
         [0053]     The frequency divider  58  is designed such that it implements at least one fractional rational division ratio. In the example in  FIG. 3 , the frequency divider  58  has a control unit  60 , which in each case selects one of three possible dividers  64 ,  66  or  68  by, for example, a switch  62 . The divider  64  has the value n, the divider  66  has the value m and the divider  68  has the value k. Similarly, the switch  62  can have three possible switch settings a, b and c, where switch setting a is associated with divider  64 , switch setting b is associated with divider  66 , and switch setting c is associated with divider  68 . In the discussion below, it is assumed that k= 3 , m=2, and n=1. However, it is understood that k, m and n can also take on other natural number values. It is further understood that the dividers  64 ,  66 ,  68  and the switch  62  can be implemented not only as circuit structures, but also preferably as program modules of a control program.  
         [0054]     An FM signal  50  with a first frequency, which is received through the antenna  48 , is first amplified by the amplifier  52  and is then converted to an intermediate frequency in a mixer  54  by combination with an oscillator frequency provided by the frequency divider  58 . The signals converted to the intermediate frequency are filtered by a subsequent selective channel filter  70  and, after demodulation in a demodulator  72 , are delivered to a receiver output  74  for further processing.  
         [0055]     The program shown in  FIG. 4 , for example, can be executed to generate a divided signal  40  in accordance with the schematic representation in  FIG. 2   e . To do so, the switch  62  is first switched to the position b in a step  76 , and the divider  66 , which in this design is implemented as a counter, is initialized. It then provides a constant level  28  until it has registered two falling edges  80 ,  82  of the undivided oscillator signal  16  in a step  78 , for example. See also  FIG. 2   a . To this end, step  78  tests whether a number p of falling edges is greater than or equal to 2. If this is the case, the output of the counter  66  is toggled to the level  26  in a step  84  and step  86  again waits for p=2 falling edges  88 ,  90  in the oscillator signal  16  (see also  FIG. 2   a ). The control unit  60  then switches the switch  62  to the position a, which corresponds to a division by n=1, in a step  92 . In steps  94  through  98 , the counter  64  counts two times to one and in the middle toggles its output level in a step  96 . Repetition of this sequence of steps  76  through  98  results in the divided oscillator signal  40  shown as the signal in  FIG. 2   e . Instead of counting only falling edges, it is also possible to count only rising edges, or even all edges.  
         [0056]     The frequency of the local oscillator  56  is set to a range between 170 MHz and 236 MHz for a universal FM receiver  46 , for example. Division by the factor 1.5 then produces a frequency of the divided signal of approximately 158 MHz for the upper band limit of 236 MHz. In a low side injection constellation, in which the divided oscillator frequency used for the mixer  54  is lower than a receive frequency (first frequency) by approximately the value of the intermediate frequency (approximately 10.7 MHz), this signal is suitable for combination with a first frequency of approximately 168 MHz. This is a frequency such as is used in the American weather band. Accordingly, the receiver can be used in a first operating state to receive signals from this FM frequency band.  
         [0057]     In a second operating state, the control unit  60  continuously switches the switch  62  to the position b, which implements a divider  66  with the value  2 . As a result, a frequency of 170/2 MHz=approx. 85 MHz to 236/2 MHz=approx. 118 MHz is delivered to the mixer  54 , which converts frequencies from approximately 75 MHz to approximately 108 MHz as selected first frequencies to the intermediate frequency in a high side injection constellation. These frequency relationships correspond approximately to the FM band used in Europe.  
         [0058]     In a third operating state, the control unit  60  continuously switches the switch  62  to the position c, which implements a divider with the value  3 . As a result, a frequency of 170/3 MHz=approx. 53 MHz to 236/3 MHz=approx. 79 MHz is delivered to the mixer  54 . This converts frequencies from approximately 63 MHz to approximately 80 MHz as selected first frequencies to the intermediate frequency in a low side injection constellation. These frequency relationships correspond approximately to the FM band used in Japan.  
         [0059]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.