Abstract:
A method and a circuit are described for generating a frequency signal having fine frequency control, and which are suitable for implementation on an-integrated circuit. The output frequency is generated having a controllable relationship with an oscillator frequency by using multiple phases of the oscillator signal. The output is provided by selecting a signal from the plurality of phases, and the frequency control is achieved by varying the selection cyclically, so that the output signal may be composed of segments of different phases. The cyclic selection is performed at a controllable rate to achieve stable generation of an original frequency signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to a digital frequency generation method and apparatus. The invention is particularly suitable for use to generate a pulse signal, such as a clock signal, but it is not limited exclusively to this. The invention is also particularly suitable for implementing on an integrated circuit, but again, the invention is not limited exclusively to this. 
     BACKGROUND OF THE INVENTION 
     There are many applications for which a controllable or variable frequency generator is needed, for example, in mobile communications (mobile telephones), or when it is desired to synchronise two circuits. Other applications include data and clock recovery (when it is desired to generate a clock signal synchronised to a data stream which may include dither). 
     Particular problems can arise when it is desired to generate an original pulse signal having a frequency which is controllable very finely, for example, in very fine frequency steps. Generally, digital oscillators having a fixed frequency can be implemented relatively easily, for example, based on a crystal or other high Q circuit, but it is difficult to produce a controllable output frequency other than the crystal frequency (or integer division multiples thereof). 
     Fine frequency control has been achieved digitally using oversampling techniques such as direct digital synthesis, and using phase locked loops. However, direct digital synthesis techniques suffer from the disadvantage that they require an original clock frequency which is very much higher than the output frequency. Moreover, direct digital synthesis requires fairly complex circuitry to implement. 
     As an example, FIG. 1 illustrates a general circuit for direct digital synthesis. The output waveform is stored in a read only memory (ROM)  10 , which is addressed by a phase address register  12 . Each time a clock pulse is received from the fixed frequency oscillator  14 , the value in the address register  12  is updated by the adder  16  incrementing the current address value by a phase increment value stored in increment register  18 . The digital output from the ROM  10  is converted to an analogue signal by an output digital to analogue converter (DAC)  20 . 
     The output frequency depends on the number of address bits in the adder  16 , the increment value stored in register  18 , and the fixed clock frequency. Frequency control is achieved by varying the increment value stored in register  18 , the smallest frequency step achievable being dependent generally on N/F where N is the number of bits in the adder, and F is the fixed frequency. Fine frequency control is obtainable only by using an overly high fixed frequency F, and a relatively high number of arithmetic adder bits N. This requires the use of high speed arithmetic circuitry. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised bearing the above problems in mind. 
     In contrast to the prior art, one aspect of the present invention is to generate an output frequency which has a controllable relationship with an oscillator frequency by using multiple phases of the oscillator signal. The output is provided by selecting a signal from the plurality of phases, and the frequency control is achieved by varying this selection cyclically, so that the output signal may be composed of segments of different phases. 
     Each time the selection is changed, the period between a pulse of the preceding segment and a pulse of the new segment will be different from the basic period of each phase (i.e. either longer or shorter than the basic period). By controlling the rate at which the different phases are repetitively selected, an output signal can be generated having an effective frequency different from the basic oscillator frequency. 
     An important advantage of this invention is that it does not require a oscillator frequency which is much higher than the frequency desired to be generated. The generator can output a controlled frequency which is close to the oscillator frequency. Also, multiphase oscillators can be implemented very easily, for example, on an integrated circuit. 
     Multi-phase oscillators have been used hitherto in data/clock recovery applications, although not for original frequency generation. An example of such an application is illustrated in FIG.  2 . The different phase outputs  22  from a inulti-phase oscillator  24  are fed to a selector switch  26  which is operable to select one of the multiple phase signals  22  as the final output  28 . The switch  26  is controlled by an up/down counter  30 , the value held in the counter controlling which of the multi-phase signals  22  is selected by the switch  26 . 
     The output  28  is synchronised relative to an incoming pulse signal  32  by means of a phase detector  34  and a loop filter circuit  36 , the output of which controls the up/down counter  30 . The circuit attempts to correct phase differences between the output signal  28  and the incoming signal  32 , by selection of the multi-phase output signals  22 . If the generated output  28  leads the incoming signal  32 , then the counter  30  is controlled to decrement the count value, to thereby control the selector switch  26  to select a respective one of the multi-phased signals  22  having a phase lag. Similarly, if the output signal  28  is lagging behind the incoming signal  32 , then the counter  30  is controlled to increment its count value, to thereby control the selector switch  26  to select a respective one of the multi-phased signals  22  with a more advanced phase. 
     The above circuit is only effective when the frequency of the incoming signal  32  is the same as, or very close to, the fixed frequency of the oscillator  24 . If the frequencies are slightly different, then the circuit will tend to cycle through the multi-phase outputs to minimise the phase difference. 
     The circuit of FIG. 2 may be regarded as simply as an “aligner” for aligning the output from the oscillator  24  with an incoming pulse signal  32 . The output  28  simply mimics a clock signal to match the incoming pulse signal  32 . Therefore, such a circuit is not suitable as a stand-alone, or original, frequency generator because it relies on receiving an input pulse train having the same characteristic frequency as that to be generated. 
     To the best of the inventor&#39;s knowledge, the technique of cyclically selecting the outputs from a multi-phase oscillator has not hitherto been used for original frequency generation. 
     In one specific aspect, the invention provides a digital frequency generator, comprising: 
     an oscillator circuit with means for producing multiphase output pulse signals; 
     switch means for outputting a selected signal from the plurality of multiphase pulse signals from the oscillator circuit; and 
     cyclic selection means driven by a signal from the oscillator circuit or from the switch means, and operable to control the switch means to vary cyclically the selection of the signal used as the output signal, to thereby generate an output signal having a frequency different from the oscillator frequency. 
     In a closely related aspect, the invention provides a method of digital frequency generation, comprising: 
     generating a plurality of multi-phase pulse signals all having a common predetermined frequency; and 
     cyclically selecting at a controllable or predetermined rate, different ones of the multi-phase pulse signals for output, to thereby generate an output signal having a frequency different from the oscillator frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are now described by way of example only, with reference to the accompanying further drawings, in which: 
     FIGS. 1 and 2 are schematic block circuit diagrams showing two prior art arrangements; 
     FIG. 3 is a schematic block circuit diagram of an embodiment of the present invention; 
     FIG. 4 is a timing diagram showing the principle of signal selection; 
     FIG. 5 is a schematic block circuit diagram of a basic first embodiment of a multi-phase ring oscillator; 
     FIG. 6 is a block schematic circuit diagram of a second embodiment of multi-phase oscillator; 
     FIG. 7 is a schematic circuit diagram of a delay element used in FIG. 6; and 
     FIG. 8 is a timing diagram showing operation of the delay element of FIG.  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, a frequency generator  50  implemented on an integrated circuit comprises an oscillator  52  producing multi-phase outputs  54 . Generally, the oscillator will operate at only a predetermined frequency, which may be governed by one or more external elements, such as a crystal (not shown). The oscillator has N outputs, each of which is separated by a phase difference of 360°/N from adjacent phase outputs. In the present embodiment, the number of outputs N is 16, so that the outputs are separated by a phase angle of 22.5°. The signals are illustrated in FIG. 4, and are labelled CK(n), where n is 0, 1, 2, . . . 15. 
     The N outputs  54  are fed as inputs to a selector switch  56  having a single output  58 . The switch  56  is operative to couple a selected one of the multi-phase signals  54  to the output  58 , the signal being selected in accordance with a control signal  60  to the switch  56 . The input  60  represents a number (between 0 and 15) corresponding to the respective multi-phase input to be selected. 
     In use, the selector switch  56  is controlled by a circuit (denoted by broken line  62 ) to vary cyclically the selection of the multi-phase signals  54  for the output  58 . In this embodiment, the circuit  62  is clocked by the output  58 , and includes a first divide-by-N counter  64  and a second increment/decrement-by-R produce the control signal  60 . The control signal cycles the selection of the multi-phase signals  54  at a rate dependent on the ratio R/M. 
     In more detail, the first counter  64  produces a single output pulse to the second counter  66  for every M pulses generated on line  58 . For each pulse generated by the first counter  64 , the second counter  66  is incremented (or decremented) by the value R to output a new count value. Each time that that the second counter  66  cycles through 0 or 15, the count value is reset to perform modulo counting. 
     In general, the frequency Fou- on the output line  58  will be:          F   OUT     =       [     N     N   ±     (     R   M     )         ]     ×     F   IN                              
     where F in  is the frequency of the oscillator  52 . 
     In the above formula, the±operation depends on whether the second counter  66  is arranged to increment, or decrement, by the value R. In the illustrated embodiment (see also FIG. 4 ), the signals  54  include a progressive phase lag with increasing n. For example, CK 1  lags behind CKO by 22.5° C., and CK 2  lags behind CK 1  by a further 22.5° C. Therefore, if the second counter  66  increments the count value (i.e. the “+” operation in the above formula), successively selected phases will lag each other, leading to a reduction in the output frequency. Conversely, if the second counter  66  decrements the count value (the “−” operation in the formula), successively selected phases will lead each other, resulting in the output frequency being higher than the input frequency. 
     FIG. 4 illustrates a simple timing diagram for the frequency generator when M=1 and R=+2 (the + sign indicating that the second counter  66  will increment the count value). The second counter  66  will initially hold an arbitrary value, say 6, such that the initial phase signal  54  selected for output will be CK 6 . 
     With the value M set to 1, the first counter/divider  64  is transparent and pulses are passed from the output  58  directly to the input of the second counter  66 . 
     The falling edge transition of CK 6  (at time T 1 ) triggers the second counter  66  to increment the current counter value by R, to 8. This value is passed as the control input to the selector switch  56  at time T 1  (or very shortly thereafter). Thus, after the point T 1 , the output signal  58  will consist of a signal segment from CK 8 . Similarly, the falling edge transition of CK 8  (at time T 2 ) triggers the second counter  66  again to increment the current counter value by R, to 10. Thus, after the point T2, the output signal will consist of a signal segment from CK 10 . The output signal will have an effective frequency of (16/18)×Fin. 
     It will be appreciated that the provision of two counters  64  and  66  in this embodiment, the first as a divider and the second as a multiplier, enables the output frequency to be controlled according to the ratio R/M, which provides excellent versatility of frequency selection. 
     In this embodiment, the counters  64  and  66  are each programmable, so that appropriate values of M and R, respectively, can be set. However, in other embodiments one or both of these counters may be have a fixed value of M or R. In particular, the second counter  66  may be an increment only, or a decrement only, counter. Also, if the desired value of M is 1, then the first counter  64  may be omitted altogether. 
     In this embodiment, the multi-phase signals  54  are coupled in a uniform sequence CK 0 , CK 1 , CK 2 , . . . CK 15  to the inputs of the selector switch  56 , so that they can be selected in order. However, in other embodiments, the multi-phase signals  54  may be arranged in a different order, for example, to generate higher frequencies. Such different ordering is merely equivalent to a different switching order by the selector switch  56 , but may be used to simplify the counters  64  and  66  to be used. 
     In this embodiment, the multi-phase signals  54  are separated by a generally uniform phase increment. This is advantageous in enabling fine frequency control. However, in other embodiments, a non-uniform phase increment might be used. 
     In FIG. 3, the control circuit  62  is driven from the output  58 . Alternatively, the circuit  62  may be driven by a different signal derived from the oscillator  52 . For example the circuit  62  may be driven directly by one of the multi-phase signals  54 . An important feature of this embodiment is that the selector switch  56  be driven by an “internal” signal of the frequency generator, rather than by an external reference signal having the same frequency characteristic as that desired. It is this property which enables the circuit to generate an original frequency, and distinguishes the circuit from an aligner as described earlier. 
     Any suitable oscillator  52  having multi-phase outputs  54  may be used as desired. FIG. 5 illustrates a simple form of ring oscillator which may be used. In FIG. 5, the oscillator  52  consists of a number N of gates  70  coupled in a continuous cascade arrangement, with the input of each gate being driven by the output of the preceding gate in the ring. Provided that the requirements are met for oscillation, such a ring circuit would oscillate at a basic frequency of Fosc=1/(N×Td) where Td is the propagation delay of each gate. Each gate produces an output which is delayed in to phase relative to the preceding gate. In the circuit illustrated, the gates  70  are inverter gates, and so adjacent gates would also include an additional 180° phase difference. (If necessary an additional gate (not shown) would be included to produce the net phase feedback phase change need for oscillation). For example, for 16 gates labelled GO to G 15 , the multi-phase outputs illustrated in FIG. 4 would be produced as follows: 
     G 0 -CK 0   
     G 1 -CK 9   
     G 2 -CK 2   
     G 3 -CK 11   
     G 4 -CK 4  . . . 
     G 7 -CK 15  . . . 
     G 15 -CK 7   
     Although the circuit illustrated in FIG. 5 is simple, it is not generally suitable for implementing in an integrated circuit because the oscillation frequency is not very stable. The propagation delay Td depends on many factors, including temperature, and may be subject to power supply noise. 
     To make such an oscillator controllable and of sufficient quality for most no applications, the “ring” may be placed in a feedback loop such as a phase-locked loop. However, this is not the only means of achieving stability. A technique which is particularly suitable for implementing a multi-phase oscillator on silicon, can provide many phases, and has other advantages such as good supply rejection, is to use differential controllable delay elements in a phase locked loop. The delay of the elements may be controlled by voltage, current or charge. 
     Such a circuit is illustrated in FIG.  6 . For the sake of brevity, a six-phase oscillator is illustrated. In FIG. 6, the oscillator “ring” is formed by three voltage controllable delay elements  72 ,  74  and  76  each having differential inputs and outputs. The outputs of the first delay element  72  are coupled to the inputs of the second delay element  74 , whose outputs are, in turn, coupled to the inputs of the third delay element  76 . The outputs of the third delay element  76  are coupled in antiphase to the inputs of the first element  72  to complete the ring. Each element  72 ,  74  and  76  produces two opposite phase output signals, and it is assumed for fine frequency control applications that the delay Td, and therefore the phase increment, is the same in each delay element. For example, the first delay element  72  can produce the opposite phase signals CK 0  and CK 3  of a six phase output, the second delay element  74  can produce CK 1  and CK 4 , and the third delay element  76  can produce CK 2  and CK 5 . 
     The phase locked loop includes a reference oscillator  78 , a phase detector  80  for comparing the ring output to the reference oscillator output, and a loop filter  82 . The output of the loop filter  82  is a voltage control signal for controlling the delay of the elements  74 ,  76  and  78 . 
     FIG. 7 illustrates an example of a CMOS differential delay element for use in the circuit of FIG.  6 . The delay element consists of a pair of CMOS transistors  84  in a differential long tailed pair configuration with respective bias transistors  86 . The current through the transistors  84  is regulated by a fifth transistor  86 , and is controlled by the voltage applied to the gate of the fifth transistor. The controllable delay results from the gate capacitance of the differential transistors  84 . If the gate capacitance is C, then, when an input transition occurs, the rate of change of will be:                 V   OUT            t       =     I   C                            
     Thus, the switching speed of the transistors  84  is proportional to the current I, and reducing the current can increase the effective switching delay introduced by the transistors. 
     Although the circuit of FIG. 7 is a CMOS circuit, the same basic circuit can be implemented in almost any technology. 
     The present invention, particularly as described in the preferred embodiments, can provide a controllable and stable digital frequency generator, which can be suitable for implementing in a variety of integrated circuit technologies, requires only relatively straightforward circuitry, and can avoid the need for a very high speed clock. In particular, fine frequency control can be achieved even at frequencies close to the clock frequency. There are many applications for the invention include data and clock recovery, and frequency synthesis or generation, for example, for telecommunications (such as mobile telephones). 
     It will be appreciated that the above description is merely illustrative of a preferred form of the invention, and that many modifications may be made without departing from the invention. 
     Although features believed to be of particular importance have been set out in the appended claims, the Applicant claims protection for any novel feature or combination of features described herein and/or illustrated in the drawings, irrespective of whether emphasis has been placed thereon.