Abstract:
A frequency divider, comprising an input for receiving an input clock signal having a first frequency; a divider, for generating an output signal having an instantaneous frequency equal to the first frequency divided by an instantaneous division ratio; and a sequence generator, for generating a sequence of instantaneous division ratios by adding a sequence of instantaneous dither values to an integer value. The instantaneous division ratios in the sequence have a mean value that is equal to an integer desired ratio, but none of the instantaneous division ratios in the sequence is equal to the integer desired ratio.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to clock generation, and particularly, but not exclusively, relates to clock generation for power converters. 
         [0003]    2. Description of the Related Art 
         [0004]    Power converters are well-known sources of electromagnetic interference. Switching converters and DC-DC converters, when clocked at frequencies in the order of megahertz, will generate substantial tones at the fundamental frequency and its harmonics. 
         [0005]    These tones may cause problems for other components in the system. Electromagnetic (EM) pulses radiated from the chip may cause malfunction in other parts of the system. Further, in audio applications, the tones may react with non-linearities in the system and mix down in frequency, creating tones that are audible to the user. 
         [0006]      FIG. 1  shows a standard power converter system  10 . An incoming voltage V in  is input to a power converter  20  and converted to an output voltage V out , with the converter  20  being clocked at a frequency f c . V out  may be greater than V in  (as in boost converters) or less than V in  (as in buck converters). The clock frequency f c  is generated by dividing a signal of fixed frequency f REF  in a ÷N block  30 . 
         [0007]      FIG. 2  is a schematic graph showing the problem of tone generation in power converters. Sharp tones are created at the clock frequency f c  and its odd harmonics. The generation of tones at the odd harmonics arises from the Fourier transform of the square wave clock. As aforementioned, these tones are undesirable. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect of the present invention, there is provided a frequency divider, comprising an input for receiving an input clock signal having a first frequency; a divider, for generating an output signal having an instantaneous frequency equal to the first frequency divided by an instantaneous division ratio; and a sequence generator, for generating a sequence of instantaneous division ratios by adding a sequence of instantaneous dither values to an integer value. The instantaneous division ratios in said sequence have a mean value that is equal to an integer desired ratio, but none of the instantaneous division ratios in said sequence is equal to the integer desired ratio. 
         [0009]    According to a second aspect of the present invention, there is provided a frequency divider, comprising an input for receiving an input clock signal having a first frequency; a divider, for generating an output signal having an instantaneous frequency equal to the first frequency divided by an instantaneous division ratio; a word length reduction block, for receiving a fractional component of a non-integer desired ratio and outputting a sequence of instantaneous modulated outputs; and a sequence generator, for generating a sequence of instantaneous division ratios by summing a sequence of instantaneous dither values, said sequence of instantaneous modulated outputs and an integer value. The non-integer desired ratio is equal to the sum of an integer component and said fractional component, said fractional component being less than one. The instantaneous division ratios in said sequence have a mean value that is equal to the non-integer desired ratio. A partial sum of the integer value and the sequence of instantaneous dither values does not equal the integer component of the non-integer desired ratio. 
         [0010]    According to a third aspect of the present invention, there is provided a method of frequency synthesis, comprising the steps of: receiving an input signal having a first frequency; generating a sequence of instantaneous division ratios by adding a sequence of instantaneous dither values to an integer value; generating an output signal having an instantaneous frequency equal to the first frequency divided by an instantaneous division ratio. The instantaneous division ratios in said sequence have a mean value that is equal to an integer desired ratio, but none of the instantaneous division ratios in said sequence is equal to the integer desired ratio. 
         [0011]    According to a fourth aspect of the present invention, there is provided a method of frequency synthesis, comprising the steps of: receiving an input signal having a first frequency; receiving a fractional component of a non-integer desired ratio and outputting a sequence of instantaneous modulated outputs; generating a sequence of instantaneous division ratios by summing a sequence of instantaneous dither values, said sequence of instantaneous modulated outputs and an integer value; and generating an output signal having an instantaneous frequency equal to the first frequency divided by an instantaneous division ratio. The non-integer desired ratio is equal to the sum of an integer component and said fractional component, said fractional component being less than one. The instantaneous division ratios in said sequence have a mean value that is equal to the non-integer desired ratio. A partial sum of the integer value and the sequence of instantaneous dither values does not equal the integer component of the non-integer desired ratio. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the following drawings, in which: 
           [0013]      FIG. 1  is a block schematic diagram, illustrating the general form of a power converter circuit. 
           [0014]      FIG. 2  illustrates tones generated in a power converter circuit. 
           [0015]      FIG. 3  is a block schematic diagram, illustrating a clock modulation circuit, acting as a frequency divider. 
           [0016]      FIG. 4  illustrates tones generated in the circuit of  FIG. 3 . 
           [0017]      FIG. 5  is a block schematic diagram, illustrating a further clock modulation circuit, acting as a frequency divider, in accordance with an aspect of the present invention. 
           [0018]      FIG. 6  is a block schematic diagram, illustrating a further clock modulation circuit, acting as a frequency divider, in accordance with an aspect of the present invention. 
           [0019]      FIG. 7  is a more detailed block schematic diagram, illustrating the frequency divider block in the circuits of  FIGS. 5 and 6 . 
           [0020]      FIG. 8  illustrates the operation of the frequency divider block of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    One solution to the generation of the unwanted tones generated by the power converter is to modulate the clock frequency by applying some form of dither. Dither is a noise signal that is intentionally added to a signal. In some applications, dither is used to increase the accuracy of a truncated signal. In the present application, the dither is used to slightly spread the clock frequency so that not all of the energy radiated by the power converter is concentrated on the clock frequency and its harmonics. That is, the distribution of power is spread over a range of frequencies and hence the peaks are reduced. 
         [0022]      FIG. 3  shows a first clock modulation circuit  40 . 
         [0023]    The circuit  40  comprises a ÷N block  50  which receives an input signal at a reference frequency f REF . An adding element  60  adds a desired division factor N and a dither signal generated by a 1-bit dither block  70 , and outputs the sum to the ÷N block  50 . 
         [0024]    The ÷N block  50  generates an output clock signal at an instantaneous frequency which is f REF  divided by the output of the adding element  60 . This output signal is also used to clock the dither block  70 . 
         [0025]    The dither block  70  may comprise one or more of a number of random number generators that will be familiar to one skilled in the art and need not be explained in great detail here. For example, the dither block  70  may comprise a linear feedback shift register, or a loop circuit with an unstable feedback loop. 
         [0026]    Thus, in the one-bit case shown here, randomly generated 1s and 0s are added to N to shift slightly the output frequency of the system. The effect of this dither is to spread the peaks at the clock frequency and its harmonics, so that not all of the power of the system is concentrated at the discrete frequencies. 
         [0027]    The circuit  40  has a drawback, however. The average output of the dither block  70  is approximately ½, so on average the division factor will increase to N+½, and the average output clock frequency of the system will be reduced. That is, the average output frequency of the first clock generation circuit  40  is in fact f c ′=f REF /(N+½). 
         [0028]      FIG. 4  is a schematic graph showing this effect in more detail. The dashed lines show the previous positions of the tones. As can be clearly seen, the new clock frequency f c ′ and its harmonics are lower in frequency than they previously would have been. Such a reduction in frequency is also undesirable. However, the amplitude of the peaks is reduced and therefore the tones will not be as audible to an end user as they otherwise would have been. 
         [0029]    One solution to the problem of reducing the frequency of the clock is to increase the number of bits of dither to at least two. In this instance, the possible dither outputs will be −1, 0 or +1, and the average dither output is zero. However, this does not reduce the amplitude of the peaks sufficiently. 
         [0030]      FIG. 5  shows a second clock generation circuit  100 . The second clock generation circuit  100  is generally similar to the first clock generation circuit  40 . However, the dither is applied in a different way. Similar components in the two circuits  40 ,  100  have similar reference numerals and therefore will not be described in further detail. 
         [0031]    In this embodiment, the output of the dither block  70  is input to a multiplexer  80 , and the multiplexer  80  outputs the dither signal to the adding element  60 . The multiplexer  80  functions to receive the 1-bit output of the dither block  70 , and then select a dither signal of either −1 or +1. So, for example, if the output of the dither block is 0, the multiplexer may output −1, and if the dither is 1, the multiplexer may output +1. 
         [0032]    As before, the ÷N block  50  generates an output clock signal at an instantaneous frequency which is equal to the input frequency f REF  divided by the output of the adding element  60 . This output signal is also used to clock the dither block  70 . In this case, the average dither applied to the division factor is 0, and so the average frequency of the output clock signal is not shifted, and is equal to f REF /N. 
         [0033]    However, a further important point of the circuit  100  is that none of the possible outputs of the multiplexer  80  is zero. So, although the average output is zero, none of the instantaneous outputs is zero. The instantaneous frequency output of the ÷N block  50  is therefore always slightly perturbed, either positively or negatively. 
         [0034]    By never applying zero dither, the amplitudes of the tones at the central peaks of f c  and its harmonics are minimized. 
         [0035]    Variations on the circuit  100  may be thought of by one skilled in the art without departing from the scope of the invention. For example, advantageously, the applied dither may be +2 and −2, so that the peak is spread even further. If a two-bit dither signal was used, the dither outputs may be chosen as −5, −2, +2 and +5, for example. Higher numbers of bits of dither will allow the peaks to be shaped as required. However, such decisions are at the control of the system designer. The important point is that zero dither is never applied. 
         [0036]    Of course, one skilled in the art will appreciate that the overriding principle is that the sum of the inputs to the adding element  60  never be equal to the desired ratio N. That is, an alternative approach would be to input a constant value of (N−1) to the adding element  60  instead of N, and apply dither values of 0 and 2, such that the mean division ratio is still N. 
         [0037]      FIG. 6  shows a third clock generation circuit  200  wherein the clock frequency is synthesized using a fractional divide. That is, the overall division factor may not be an integer. In this case, the overall division factor is split into an integer part M and a fractional part y. 
         [0038]    The fractional input y is input to a sigma-delta modulator  210  (SDM) as will be familiar to those skilled in the art. The fractional input y may initially be described with a high number of bits. The SDM  210  reduces y to a lower number of bits, but ensures that the average output is equal to y, accurate to a high accuracy. The output of the SDM  210  may be only one bit. Thus, although the instantaneous output of the SDM  210  may be inaccurate, the average output is highly accurate. The output of the SDM  210  is added to the dither in an adding element  220 , and this combined signal is added to the integer M in a further adding element  230 . The output of the adding element  230  is then used to modulate the clock frequency f c . 
         [0039]    Thus, as before, the ÷N block  50  generates an output clock signal at an instantaneous frequency which is equal to the input frequency f REF  divided by the output of the adding element  230 . This output signal is also used to clock the dither block  70 . Again, the average dither applied to the division factor is 0, and so the average frequency of the output clock signal is not shifted, and is equal to f REF /(M+y). 
         [0040]    Sigma-delta modulation is one of several possibilities for modulating the fractional input y that will be readily apparent to those skilled in the art. In practice the SDM  210  may be any word length reduction block, such as a truncation or a noise shaper, for example. In the event that the word length reduction block is a truncation, dither may be applied to the fractional input y prior to truncation in order to improve the accuracy of the modulated output. 
         [0041]      FIG. 7  is a schematic block diagram showing one realization of the ÷N block  50  in the circuits of  FIGS. 3 ,  5  and  6 . 
         [0042]    The division is realized using a counter  240  and taking the most significant bit (MSB) of the count. An input k is fed to an adder  250 , and the signal from the adder  250  fed through a delay element  260 . The output from the delay element  260  is fed back to the adder  250 . The delay element  260  is clocked at a fixed frequency f REF  which is typically much higher than the desired clock frequency. The delay/adder cycle effectively acts as a counter in steps of k. If the input k is R bits long, the highest number the count can reach before repeating is 2 R . The most significant bit (MSB) is extracted from the output of the delay element  260  by a MSB extractor  270  and this is used as the new clock signal. Thus the output frequency f c  is the input frequency f REF  divided by the number of ‘k’s in 2 R  (f c =f REF ×k/2 R ). By adapting the input k, the output frequency of the MSB, f c , can be altered. Therefore in this embodiment the overall division factor N is 2 R /k. 
         [0043]    This is shown in more detail in  FIG. 8 . As the counter counts up, the MSB of the count is taken. The MSB output therefore has a lower frequency that may be adjusted by adjusting the size of the steps taken to reach the maximum value 2 R , i.e. k. 
         [0044]    The frequency divider described herein preferably forms part of a power converter that is preferably incorporated in an integrated circuit. For example, the integrated circuit may be part of an audio and/or video system, such as an MP3 player, a mobile phone, a camera or a satellite navigation system, and the system can be portable (such as a battery-powered handheld system) or can be mains-powered (such as a hi-fi system or a television receiver) or can be an in-car, in-train, or in-plane entertainment system. 
         [0045]    The skilled person will recognise that the above-described apparatus and methods may be embodied as processor control code, for example on a carrier medium such as a disk, CD- or DVD-ROM, programmed memory such as read only memory (firmware), or on a data carrier such as an optical or electrical signal carrier. For many applications, embodiments of the invention will be implemented on a DSP (digital signal processor), ASIC (application specific integrated circuit) or FPGA (field programmable gate array). Thus the code may comprise conventional program code or microcode or, for example code for setting up or controlling an ASIC or FPGA. The code may also comprise code for dynamically configuring re-configurable apparatus such as re-programmable logic gate arrays. Similarly the code may comprise code for a hardware description language such as Verilog TM or VHDL (very high speed integrated circuit hardware description language). As the skilled person will appreciate, the code may be distributed between a plurality of coupled components in communication with one another. Where appropriate, the embodiments may also be implemented using code running on a field-(re-)programmable analogue array or similar device in order to configure analogue/digital hardware. 
         [0046]    It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.