Abstract:
A digital frequency synthesizer receiving a first signal corresponding to a periodic sequence of first pulses at a first frequency and providing a second signal corresponding to a periodic sequence of second pulses at a second frequency. The synthesizer includes a first circuit clocked by a third signal corresponding to a sequence of third pulses and obtained from the first signal, the first circuit providing a fourth digital signal which, for any set of third successive pulses, increases (decreases) on each pulse and decreases (increases) at the end of said set; and a second circuit receiving the first and fourth signals and providing, for each first pulse from among some at least of the first pulses, a second pulse which is shifted with respect to the first pulse by a duration which depends on the fourth signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 07/58456, filed on Oct. 22, 2007, entitled “Digital Frequency Synthesizer,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present application relates to a frequency synthesizer capable of providing, from a first periodic signal at a first frequency, a second periodic signal at a second frequency different from the first frequency, where the ratio between the first and second frequencies can be modified. 
         [0004]    2. Discussion of the Related Art 
         [0005]    Many electronic circuits use one or several frequency synthesizers. As an example, telecommunication systems generally use frequency synthesizers which provide periodic signals in determined frequency bands to modulate the signals to be transmitted. In mobile telephony, the DCS (Digital Communication System) standard for example provides the transmission of signals having a frequency on the order of 1,800 MHz. Telecommunication systems using ISM (Industrial Scientific and Medical) frequency bands transmit, for example, signals at frequencies on the order of 2.4 GHz. 
         [0006]    An example of a conventional frequency synthesizer uses a phase-locked loop. A disadvantage of such a frequency synthesizer is that it exhibits, in operation, a generally non-negligible latency which corresponds to the duration required to stabilize the phase-locked loop. Further, such a synthesizer is essentially formed of analog circuits, which makes a modification of the synthesizer difficult. Further, the phase-locked loop generally comprises one or several filters, which may be difficult to form. 
         [0007]    There exist frequency synthesizers which provide, from a first signal corresponding to a periodic sequence of pulses at a first frequency, a second signal corresponding to a periodic sequence of pulses at a second frequency, where the ratio between the first and second frequencies can be modified. Such synthesizers are generally called digital frequency synthesizers and may essentially be formed of logic gates, especially based on MOS transistors. They however generally have the disadvantage that they have a large power consumption and that they can only operate at low frequencies. 
         [0008]    An example of a digital frequency synthesizer uses a phase accumulator. A phase accumulator is a circuit clocked by a clock signal and providing a signal corresponding to a non-perfectly periodic sequence of pulses at an average frequency proportional to the clock frequency and which corresponds to the desired frequency. However, since the signal provided by the phase accumulator is not perfectly periodic, the frequency synthesizer further comprises a correction circuit which, based on the pulse sequence provided by the phase accumulator, provides a periodic sequence of pulses corrected to the desired frequency. A disadvantage of this type of frequency synthesizer is that it cannot generally provide a signal having a frequency greater than half the clock frequency. 
         [0009]    The significant power consumption of a conventional digital frequency synthesizer results in that, when used in battery-powered systems, it is necessary to search for a compromise between the power consumption and the clock frequency that can be used to clock the synthesizer. 
       SUMMARY OF THE INVENTION 
       [0010]    There is a need for a digital frequency synthesizer using a phase accumulator and capable of providing, from a first periodic sequence of pulses at a first frequency, a second periodic sequence of pulses at a second frequency which may be lower than, equal to, or greater than the first frequency. 
         [0011]    According to an embodiment, there is provided a digital frequency synthesizer receiving a first signal corresponding to a periodic sequence of first pulses at a first frequency and providing a second signal corresponding to a periodic sequence of second pulses at a second frequency. The synthesizer comprises a first circuit clocked by a third signal corresponding to a sequence of third pulses and obtained from the first signal, the first circuit providing a fourth digital signal which, for any set of third successive pulses, increases on each pulse and decreases at the end of said set or decreases on each pulse and increases at the end of said set; and a second circuit receiving the first and fourth signals and providing, for each first pulse from among some at least of the first pulses, a second pulse which is shifted with respect to the first pulse by a duration which depends on said fourth signal. 
         [0012]    According to an embodiment, the second circuit comprises a third circuit providing a fifth analog signal which depends on the fourth signal; a source of a sixth analog signal; and a comparator receiving the fifth and sixth signals and providing the second signal. 
         [0013]    According to an embodiment, the first and third signals are identical. The source is capable of providing the sixth analog signal in the form of a first sawtooth voltage at the clock rate of the first signal. The third circuit comprises a digital-to-analog converter providing, at the clock rate of the first signal, the fifth signal in the form of a second stepped voltage which depends on the fourth signal. 
         [0014]    According to an embodiment, the source is capable of providing the sixth signal in the form of a constant voltage. The third circuit comprises a digital-to-analog converter providing a current which at least partly depends on the fourth signal; a capacitor charged by the current; and a switch assembled in parallel across the capacitor, the fifth signal corresponding to the voltage across the capacitor. 
         [0015]    According to an embodiment, the synthesizer comprises a finite state machine clocked by the first signal and capable of controlling, in a first state, the turning-on of the switch to discharge the capacitor; and causing, in a second state, the turning-off of the switch and causing the converter to charge the capacitor with said current. 
         [0016]    According to an embodiment, the first circuit comprises a first storage unit providing, at the clock rate of the third signal, a seventh digital signal; an adder receiving the seventh digital signal and an eighth signal and providing a ninth digital signal corresponding to the sum of the seventh and eighth signals; and a second storage unit receiving the ninth signal and providing, at the rate of the third signal, the eighth signal which corresponds to the last value of the stored ninth signal, the fourth signal being obtained from the eighth signal. 
         [0017]    According to an embodiment, the first circuit comprises a first storage unit providing, at the clock rate of the third signal, a seventh digital signal; a first adder receiving the seventh digital signal and an eighth signal and providing a ninth digital signal corresponding to the sum of the seventh and eighth signals; a second adder receiving the seventh signal, the eighth signal and a tenth digital signal, the tenth signal corresponding to a constant value, the second adder providing an eleventh signal corresponding to the sum of the seventh, eighth, and tenth signals; a multiplexer receiving the eighth terminal and the eleventh signal and comprising a selection signal receiving a twelfth signal provided by the finite state machine and providing a thirteenth signal equal to the eighth signal or to the eleventh signal according to the value of the twelfth signal; and a second storage unit receiving the thirteenth signal and providing, at the rate of the third signal, the eighth signal which corresponds to the last value of the stored thirteenth signal, the fourth signal being obtained from the eighth signal. 
         [0018]    According to an embodiment, the second circuit comprises N current sources, N being an integer corresponding to a power of two, each current source providing a current which depends on the fourth signal; and at least N transistors, each transistor having a first main terminal connected to one of the N current sources and a second main terminal connected to an output node, the transistor being controlled by one of N oscillating signals, the N oscillating signals being phase-shifted with respect to one another, the second signal being provided to said output node. 
         [0019]    An embodiment also provides a method for providing, from a first signal corresponding to a periodic sequence of first pulses at a first frequency, a second signal corresponding to a periodic sequence of second pulses at a second frequency. The method comprises the steps of providing, at the clock rate of a third signal corresponding to a sequence of third pulses and obtained from the first signal, and providing a fourth digital signal which, for any set of third successive pulses, increases on each pulse and decreases at the end of said set; and of providing, for each first pulse from among some at least of the first pulses, a second pulse shifted with respect to said first pulse by a duration which depends on the fourth signal. 
         [0020]    According to an embodiment, the method further comprises the steps of providing a fifth analog signal which depends on the fourth signal; and providing the second signal based on the comparison of the fifth signal and of a sixth analog signal. 
         [0021]    According to an embodiment, the method further comprises the steps of converting the fourth signal into a current; and charging a capacitor with said current, the fifth signal corresponding to the voltage across the capacitor and the sixth signal being a constant voltage. 
         [0022]    The foregoing and other objects, features, and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  schematically shows a conventional example of a phase accumulator; 
           [0024]      FIG. 2  shows an example of the variation of characteristic signals of the phase accumulator of  FIG. 1 ; 
           [0025]      FIG. 3  schematically shows a conventional example of a digital frequency synthesizer using the phase accumulator of  FIG. 1 ; 
           [0026]      FIG. 4  schematically shows an embodiment of a digital frequency synthesizer; 
           [0027]      FIG. 5  shows an example of the variation of characteristic signals of the synthesizer of  FIG. 4 ; 
           [0028]      FIG. 6  shows another embodiment of a digital frequency synthesizer; 
           [0029]      FIG. 7  shows an example of the variation of characteristic signals of the synthesizer of  FIG. 6 ; 
           [0030]      FIG. 8  shows another embodiment of a digital frequency synthesizer; 
           [0031]      FIG. 9  shows an example of the variation of characteristic signals of the synthesizer of  FIG. 8 ; 
           [0032]      FIGS. 10 and 11  show other embodiments of digital frequency synthesizers; 
           [0033]      FIG. 12  shows a more detailed embodiment of the phase accumulator of the frequency synthesizer of  FIG. 11 ; 
           [0034]      FIG. 13  illustrates the states taken by the finite state machine of the synthesizer of  FIG. 11 ; 
           [0035]      FIG. 14  shows an example of the variation of characteristic signals of the synthesizer of  FIG. 11 ; 
           [0036]      FIG. 15  shows another embodiment of a digital frequency synthesizer; 
           [0037]      FIG. 16  illustrates the operation of the synthesizer of  FIG. 15 ; and 
           [0038]      FIG. 17  shows another embodiment of a digital frequency synthesizer. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    For clarity, the same elements have been designated with the same reference numerals in the different drawings. 
         [0040]    The frequency synthesizer according to an embodiment uses a phase accumulator that can have a conventional structure or a structure slightly modified with respect to a conventional structure. The structure and the operation of a conventional phase accumulator will thus first be described. 
         [0041]      FIG. 1  shows a conventional embodiment of a phase accumulator  10  clocked by a clock signal CLK. Respectively call T CLK  and f CLK  the period and the frequency of clock signal CLK. Phase accumulator  10  comprises a first storage unit  12  (Frequency Register) in which a binary signal P is stored. Signal P comprises a number n of bits and will be called phase increment hereafter. Storage unit  12  provides phase increment P at the rate of clock signal CLK to a first input of an adder  14 . Adder  14  provides a binary signal S 1 , comprising n bits, which is stored in a second storage unit  16  (Phase Register). Adder  14  further provides a one-bit binary signal Ov, called the first overflow bit. Signal S 1  may take 2 n  different values. For each cycle of clock signal CLK, second storage unit  16  provides a signal S 2  which is equal to the last value of signal S 1  stored in second storage unit  16 . Signal S 2  is provided to a second input of adder  14 . Storage unit  16  further provides a one-bit binary signal Ov′, called the second overflow bit. Phase accumulator  10  provides a binary signal Phase which corresponds to the k most significant bits of signal S 2 , with k being possibly equal to n. 
         [0042]    The operation of phase accumulator  10  is the following: for each clock cycle, phase increment P is added to signal S 2 , that is, to the last value of signal S 1 , and the new obtained value of signal S 1  is stored in storage unit  16 . Considering that signal S 1  is initially null, for each clock cycle of index i, the new value S 1 ( i ) of signal S 1  is obtained by the following relation: 
         [0000]        S 1( i )= iP  modulo 2 n   (1) 
         [0043]    Signal S 1 , and thus signal Phase, increases “stepwise” then abruptly decreases each time signal S 1  should reach, at the next clock cycle, a value greater than or equal to 2 n . The decrease of signal S 1  or S 2  is called overflow in the following description. For each overflow of signal S 1 , first overflow bit Ov is set to “1”, for example, during a clock cycle. For each overflow of signal S 2 , second overflow bit Ov′ is set to “1”, for example, during a clock cycle. Second overflow bit Ov′ thus is substantially one clock cycle behind first overflow bit Ov. 
         [0044]    Respectively call f OUT  and T OUT  the average frequency and the average frequency of second overflow bit Ov′. Frequency f OUT  is provided by the following relation: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     OUT 
                   
                   = 
                   
                     
                       P 
                       
                         2 
                         n 
                       
                     
                      
                     
                       f 
                       CLK 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0045]      FIG. 2  illustrates the variation of signals Phase, Ov, Ov′, and CLK of phase accumulator  10  of  FIG. 1  in the case where phase increment P, n, and k are equal to 3. Signal Phase can thus take 8 different values, noted from 0 to 7. Signal Phase increases “stepwise” by decreasing on each exceeding of maximum value 7. In the example of  FIG. 2 , average frequency f OUT  Of second overflow bit Ov′ is equal to 3f CLK /8. 
         [0046]      FIG. 3  shows a conventional embodiment of a digital synthesizer  20  using phase accumulator  10  of  FIG. 1 . Such an example of a digital synthesizer is described in publication “A low-jitter phase-interpolation DDS using dual-slope integration” by Hsin-Chuan Chen and Jen-Shiun Chiang (IEICE Electronics Express, Vol. 1, N o  12, 333-338). 
         [0047]    Frequency synthesizer  20  comprises a first conversion unit  21  (Conv 1 ) receiving phase increment P and providing a terminal B 1  with a current I 1  having its intensity depending on phase increment P and which is, for example, proportional to phase increment P. Frequency synthesizer  20  comprises a second conversion unit  22  (Conv 2 ) receiving signal Phase and providing a terminal B 2  with a current I 2  having its intensity depending on signal Phase and decreasing when signal Phase increases. A switch  23  controlled by a signal SEL is capable of connecting terminal B 1  or terminal B 2  to the positive input (+) of a comparator  24 . The negative input (−) of comparator  24  is connected to ground GND. Comparator  24  provides a signal OUT corresponding to a sequence of pulses at frequency f OUT . A capacitor  25  is provided between the positive input (+) of comparator  24  and of ground GND. A switch  26  controlled by a signal SW is provided across capacitor  25 . A control unit  27  (Control Logic) receives overflow bits Ov and Ov′, signal OUT and provides signals SEL and SW. 
         [0048]    The operation of synthesizer  20  is the following: when first overflow bit Ov is at “1”, switch  23  connects terminal B 2  to the positive input (+) of comparator  24 . Capacitor  25  is then charged by current I 2  having its intensity decreasing as signal Phase increases. The voltage across capacitor  25  being positive, signal OUT is low. When second overflow bit Ov′ is at “1”, at the next clock cycle, switch  23  connects terminal B 1  to the positive input (+) of comparator  24 . Capacitor  25  is then discharged by current I 1  having a constant intensity which depends on phase increment P. Signal OUT switches from the low level to the high level when capacitor  25  is fully discharged. The switching time of signal OUT depends on the duration of the discharge of capacitor  25 , and thus on the intensity of current I 2  with which it has been charged at the preceding clock cycle. The times of occurrence of the rising edges of signal OUT are thus modulated and are exactly separated by time period T OUT . 
         [0049]    A disadvantage of synthesizer  20  is that, because of its operation, output frequency f OUT  is necessarily lower than half the frequency of clock signal f CLK . 
         [0050]      FIG. 4  schematically shows an embodiment of a frequency synthesizer  40  according to the present invention using a phase accumulator  10 , for example, the phase accumulator previously described in relation with  FIG. 1 . Synthesizer  40  comprises a phase interpolator  42 , receiving clock signal CLK and signal Phase and providing a one-bit binary signal OUT_PI, corresponding to a periodic sequence of pulses having a period T OUT     —     PI  and a frequency f OUT     —     PI . Conversely to conventional frequency synthesizers using a phase accumulator, the present frequency synthesizer uses signal Phase provided by the phase accumulator and not second overflow bit Ov′ to determine signal OUT_PI having the desired frequency f OUT     —     PI . 
         [0051]      FIG. 5  illustrates the operating principle of synthesizer  40  of  FIG. 4 . In  FIG. 5 , clock signal CLK and an example of the variation of signal OUT_PI provided by synthesizer  40  have been shown. Generally, phase accumulator  10  increases for each clock cycle signal Phase of phase increment P up to the overflow. In the interval during which signal Phase increases, phase interpolator  42  provides, for each successive pulse of clock signal CLK, a pulse of signal OUT_PI which is phase-shifted with respect to the corresponding pulse of clock signal CLK by a positive or negative phase shift, which depends on signal Phase, and which is for example proportional to signal Phase. Thereby, in an increase of signal Phase up to the overflow, the phase shift applied by phase interpolator  42  increases by a constant phase shift step. This translates as the obtaining of a signal OUT_PI which is generally shifted in frequency with respect to signal CLK. 
         [0052]    The phase shift applied by phase interpolator  42  thus follows the variation of signal Phase and in particular decreases, in absolute value, after each overflow of signal Phase. After an overflow, phase interpolator  42  cannot take into account a pulse of clock signal CLK or a value of signal Phase, or use again the last pulse of clock signal CLK to ensure the regularity in the provision of the pulses of signal OUT_PI. 
         [0053]    Period T OUT     —     PI  of signal OUT_PI is provided by the following relation: 
         [0000]        T   OUT     —     PI   =T   CLK   +dt   (3) 
         [0054]    where dt may be positive or negative according to the operation of phase interpolator  42 . The absolute value of increment dt is linked to the operating parameters of phase accumulator  10  according to the following relation: 
         [0000]    
       
         
           
             
               
                 
                   
                      
                     dt 
                      
                   
                   = 
                   
                     
                       P 
                       
                         2 
                         n 
                       
                     
                      
                     
                       T 
                       CLK 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0055]    Frequency f OUT     —     PI  is then provided by the following relation: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     OUT_PI 
                   
                   = 
                   
                     
                       f 
                       CLK 
                     
                     
                       1 
                       ± 
                       
                         P 
                         
                           2 
                           n 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0056]    The frequency synthesizer according to an embodiment is thus capable of providing, from a clock signal CLK, a periodic signal having a frequency f OUT     —     PI  that can be lower than, greater than, or equal to frequency f CLK  of clock signal CLK. The frequency synthesizer according to the present invention has the advantage that it can be almost completely formed of digital components except, possibly, for certain elements of phase interpolator  42 . 
         [0057]      FIG. 6  shows an example of a frequency synthesizer  43  having the general structure of the frequency synthesizer of  FIG. 4  and for which a more detailed embodiment of phase interpolator  42  is shown. Phase interpolator  42  comprises a digital-to-analog converter  44  (D/A) rated by clock signal CLK and receiving signal Phase. Converter  44  converts signal Phase into an analog voltage V P  which is provided to the positive input (+) of a comparator  46 . The negative input (−) of comparator  46  receives a periodic sawtooth voltage V COMP  at frequency f CLK  provided by a generator  48  (GEN). Comparator  46  provides signal OUT_PI corresponding to a pulse sequence (of variable durations) provided at frequency f OUT     —     PI . Voltage V COMP  varies between a minimum voltage V MIN  and a maximum voltage V MAX . The maximum voltage capable of being provided by converter  44  is equal to V MAX  and the minimum voltage capable provided by converter  44  is equal to V MIN . 
         [0058]      FIG. 7  illustrates the operating principle of frequency synthesizer  43  of  FIG. 6 . For each clock cycle, phase accumulator  10  provides a new value of signal Phase. In the present example, this translates as the provision by converter  44  of a new value of voltage V P  which decreases by a constant step. Simultaneously, for each clock cycle, generator  48  provides a voltage V COMP  corresponding to an increasing ramp. The time of occurrence of the rising edge of a pulse of signal OUT_PI corresponds to the time at which voltage V COMP  reaches voltage V P . As an example, five successive rising edges F 1  to F 5  of signal OUT_PI are shown. Each rising edge of signal OUT_PI is shifted by a time period Δt 1 , Δt 2 , Δt 3 , Δt 4 , and Δt 5  from the corresponding rising edge of clock signal CLK. 
         [0059]      FIG. 8  shows an embodiment of a synthesizer  50  for which the elements taking part in the management of the overflows of phase accumulator  10  have been shown. Frequency synthesizer  50  comprises a synchronization unit  52  (Synch.) receiving clock signal CLK, first overflow bit Ov, and signal Phase and providing a modified clock signal CLK_I to phase accumulator  10  and to phase interpolator  42 . Modified clock signal CLK_I enables simply ensuring the regularity of signal OUT_PI on overflows of signal Phase. 
         [0060]      FIG. 9  illustrates an example of the variation of characteristic signals of frequency synthesizer  50  of  FIG. 8  on overflow of phase accumulator  10 . In the present example, the parameters of phase accumulator  10  are the following: n and k are equal to 2 and P is equal to 1. Values 0, 1, 2, and 3 of signal Phase respectively correspond to phase shifts by 0, T CLK /4, T CLK /2, and 3T CLK /4. Seven successive cycles I to VII of clock cycle CLK are shown. For the first four cycles of clock signal CLK, signal Phase successively takes values 0, 1, 2, and 3, which causes a phase shift of the rising edge of signal OUT_PI with respect to the rising edge of the corresponding clock signal CLK successively by 0, T CLK /4, T CLK /2, and 3T CLK /4. At cycle IV, first overflow bit Ov is set to “1”. At the next cycle (cycle V), synchronization unit  52  transmits the pulse of clock signal CLK neither to phase accumulator  10  nor to phase interpolator  42 , and modified clock signal CLK_I remains at “0”. The overflow of phase accumulator  10  then occurs at cycle VI. This enables ensuring a regularity in the provision of the pulses of signal OUT_PI. At cycle VI, phase accumulator  10  provides signal Phase at value 0 and phase integrator  42  provides a pulse of signal OUT_PI which is not phase-shifted with respect to the corresponding pulse of signal CLK_I. 
         [0061]      FIG. 10  shows another embodiment of frequency synthesizer  60  simply enabling taking into account the overflows of signal Phase. For frequency synthesizer  60 , phase accumulator  10  is not rated by clock signal CLK but by signal OUT_PI. This enables ensuring for the values of signal Phase provided by phase accumulator  10  to always be provided at the right time. Interpolator  42  may further comprise a synchronization unit (not shown) receiving clock signal CLK and providing a modified clock signal from which signal OUT_PI is obtained. 
         [0062]      FIG. 11  shows another embodiment of a frequency synthesizer  70 . As will be described in further detail hereafter, phase accumulator  10 ′ has a structure slightly different from that previously described for phase accumulator  10  in relation with  FIG. 1 . Frequency synthesizer  70  comprises a finite state machine  72  clocked by a clock signal CLK 1 . Phase interpolator  42  is also rated by clock signal CLK 1 . Finite state machine  72  provides a modified clock signal CLK 2  from first clock signal CLK 1 . As will be described in further detail hereafter, signal CLK 2  partly corresponds to a periodic sequence of pulses having a smaller frequency than clock signal CLK 1 . Modified clock signal CLK 2  rates phase accumulator  10 ′. Phase accumulator  10 ′ provides first overflow bit Ov to a finite state machine  72 . Finite state machine  72  also provides a signal SEL_ADD to phase accumulator  10 ′. Signal SEL_ADD is, for example, a single-bit binary signal. Phase interpolator  42  comprises a digital-to-analog converter  74  (D/A) receiving signal Phase and a signal S C  provided by finite state machine  72 . Signal S C  is, for example, a one-bit binary signal. Converter  74  provides a current I having its amplitude depending on the value of signal Phase and of signal S C . Current I provided by converter  74  is likely to take 2 n  values, noted L 0  to L 2   n −1. Current I charges a capacitor C having one terminal connected to the output of converter  74  and having its other terminal connected to a source of a reference voltage, for example, ground GND. The voltage across capacitor C is designated with reference V CAP . A switch M, for example, a MOS transistor, is assembled in parallel across capacitor C and is controlled by a signal S M  provided by finite state machine  72 . Voltage V CAP  is applied to a positive terminal (+) of a comparator  76 . The negative terminal (−) of comparator  76  receives a constant voltage V COMP  provided by a voltage generator  78  (GEN). The output of comparator  76  corresponds to signal OUT_PI. 
         [0063]      FIG. 12  shows an embodiment of phase accumulator  10 ′. Phase accumulator  10 ′ comprises the same elements as phase accumulator  10  shown in  FIG. 1 . Phase accumulator  10 ′ further comprises a multiplexer  80  receiving, on a first input (A), signal S 1  provided by adder  14  and providing a signal S MUX  to storage unit  16 . Further, phase accumulator  10 ′ comprises an adder  84  receiving, on a first input, signal S 1  provided by adder  14  and, on a second input, a signal ADD equal to value “1”. Adder  84  provides a signal S 3  to a second input (B) of multiplexer  80 . Multiplexer  80  comprises a selection terminal receiving signal SEL_ADD provided by finite state machine  72 . As an example, when signal SEL_ADD is at “0”, signal S MUX  is equal to S 1  and when signal SEL_ADD is at “1”, signal S MUX  is equal to S 3 . 
         [0064]    The operation of frequency synthesizer  70  will now be described for a specific example in which P is equal to 1 and n and k are equal to 2. Current I provided by converter  72  is likely to take 4 values, noted L 0  to L 3 . Further, current L 0  corresponds to the null current, current L 2  is equal to two thirds of current L 3 , and current L 1  is equal to one third of current L 3 . 
         [0065]      FIG. 13  illustrates an example of an operating method of finite state machine  72 . In this embodiment, finite state machine  72  is likely to occupy one state out of five, respectively called “R 1 ”, “V”, “C 1 ”, “C 2 ”, and “R 2 ” hereafter. Finite state machine  72  switches from one state to another state at the frequency of clock signal CLK 1 . In the present example, modified clock signal CLK 2  provided by finite state machine  72  has a frequency which is approximately four times smaller than the frequency of clock signal CLK 1 . 
         [0066]    At step  90 , finite state machine  72  is at state “R 1 ” (Reset 1 ). It then causes the turning-on of switch M, causing the discharge of capacitor C. Further, converter  74  is controlled by signal S C  so that current I is equal to L 0 . Further, if an overflow will occur at the next cycle of modified clock signal CLK 2 , that is, if first overflow bit Ov is at “1”, finite state machine  72  sets signal SEL_ADD to “1”. In the opposite case, it sets signal SEL_ADD to “0”. The method carries on at step  92 . 
         [0067]    At step  92 , finite state machine  72  switches to state “V” (Variable). It causes the turning-off of switch M, causing the charge of capacitor C with the current. Finite state machine  72  controls, with signal S C , converter  74  so that current I is at a value L 1 , L 2 , or L 3  according to the value of signal Phase. As an example, current I is at L 3  when signal Phase is at 1, current I is at L 2  when signal Phase is at 2, and current I is at L 1  when signal Phase is at 3. The method carries on at step  94 . 
         [0068]    At step  94 , finite state machine  72  switches to state “C 1 ” (Constant 1 ). Finite state machine  72  causes the turning-off of switch M, causing the charge of capacitor C with current I, and it further controls converter  74  so that current I is at value L 3 . The method carries on at step  96 . 
         [0069]    At step  96 , finite state machine  72  switches to state “C 2 ” (Constant 2 ). Finite state machine  72  causes the turning-off of switch M, causing the charge of capacitor C with current I, and it further controls converter  74  so that current I is at value L 3 . The process carries on at step  98 . 
         [0070]    At step  98 , the finite state machine determines from the value of first overflow bit Ov whether an overflow will occur at the next cycle of modified clock signal CLK 2 . If not, the method carries on at step  90 . If so, the process carries on at step  100 . 
         [0071]    At step  100 , finite state machine  72  switches to state “R 2 ” (Reset 2 ). It then causes the turning-on of switch M, causing the discharge of capacitor C. Further, converter  74  is controlled so that current I is equal to L 0 . Further, finite state machine  72  delays modified clock signal CLK 2  by a cycle of clock CLK 1 . The process then returns to step  90 . 
         [0072]      FIG. 14  shows an example of variations of characteristic signals of synthesizer  70  of  FIG. 11 . A periodic signal CLK superposed to signal OUT_PI and having its frequency f CLK  equal to the one quarter of the frequency of signal CLK 1  has been shown. As an example, the frequency of clock signal CLK 1  is 2 GHz. According to the value of signal Phase, voltage V CAP  across capacitor C increases more or less rapidly and reaches comparison voltage V COMP  at times regularly spaced apart at frequency f OUT     —     PI . The expression of frequency f OUT     —     PI  for synthesizer  70  can be deduced from relation (5), considering that 4(2 N −1) values of signal Phase are used to describe 360°: 
         [0000]    
       
         
           
             
               
                 
                   
                     f 
                     OUT_PI 
                   
                   = 
                   
                     
                       f 
                       
                         CLK 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     
                       1 
                       + 
                       
                         P 
                         
                           4 
                           × 
                           
                             ( 
                             
                               
                                 2 
                                 n 
                               
                               - 
                               1 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0073]    where f CLK2  is the frequency of clock signal CLK 2 . 
         [0074]    According to a variation of the previously-described embodiment, a buffer memory may be provided on the transmission line of signal Phase between phase accumulator  10  and phase interpolator  42 , a buffer memory may be provided on the transmission line of signal S C  between finite state machine  72  and phase interpolator  42  and on the transmission line of signal S M  between finite state machine  72  and phase interpolator  42 . Each buffer delays the transmission of the signal that it stores, for example, by one cycle of clock CLK 1 . In this case, finite state machine  72  may receive second overflow bit Ov′ instead of first overflow bit Ov and may determine whether an overflow of signal Phase occurs based on second overflow bit Ov′ which has the advantage of being generally better stabilized than first overflow bit Ov. 
         [0075]      FIG. 15  shows another embodiment of a digital frequency synthesizer  110 . In this embodiment, phase interpolator  42  comprises an interpolation circuit  112  such as that described in publication “A 10-Gb/s CMOS Clock and Data Recovery Circuit With an Analog Phase Interpolator” by R. Kreienkamp, U. Langmann, Ch. Zimmermann, T. Aoyama, and H. Siedhoff (IEEE Journal of solid-state circuits, vol. 40, N o  3, March 2005, pp. 736-743). 
         [0076]    Circuit  112  comprises four differential pairs  114 A to  114 D, each comprising a first MOS transistor  115 A to  115 D and a second MOS transistor  116 A to  116 D. For each differential pair, the drain of transistor  115 A to  115 D is connected to a node A 1  and the drain of transistor  116 A to  116 D is connected to a node A 2 . Further, for each differential pair, the sources of transistors  115 A to  15 D and  116 A to  116 D are connected to a terminal of a current source I A  to I D  having its other terminal connected to a source of a first reference voltage, for example, ground GND. Node A 1  is connected to a source of a second reference voltage VDD via a resistor R 1 . Node A 2  is connected to source VDD via a resistor R 2 . The voltage between nodes A 1  and A 2  corresponds to signal OUT_PI. 
         [0077]    A clock signal V CLK,I  is applied between the gate of transistor  115 A and the gate of transistor  116 A, and the clock signal complementary to V CLK,I  (that is, phase-shifted by 180° with respect to V CLK,I ) between the gate of transistor  115 B and the gate of transistor  116 B. A clock signal V CLK,Q  phase-shifted by 90° with respect to signal V CLK,I  is applied between the gate of transistor  115 C and the gate of transistor  116 C, and the clock signal complementary to V CLK,Q  (that is, phase-shifted by 180° with respect to V CLK,Q ) is applied between the gate of transistor  115 D and the gate of transistor  116 D. Signals V CLK,I  and V CLK,Q  may correspond to pulse sequences, to sinusoidal signals, or to triangular signals. 
         [0078]    Phase interpolator  42  also comprises a control unit  118  (Logic Module), rated by clock signal CLK, receiving signal Phase and providing control signals S IA  to S ID  to current sources I A  to I D . The frequency of signal V CLK,I  is equal to an integral multiple, possibly equal to 1, of the frequency of clock signal CLK. 
         [0079]    Differential pairs  114 A to  114 D are driven by clock signals in quadrature. The sum of currents I A  to I D  is constant so that the peak-to-peak amplitude of signal OUT_PI remains constant. Circuit  112  provides a voltage OUT_PI corresponding to a clock signal phase-shifted with respect to signal V CLK,I , the value of the phase shift being imposed by the values of current I A , I B , I C , and I D . Currents I A  and I D  are in fact integrated by the stray capacitances of the MOS transistors or by capacitors, not shown, provided in parallel with resistors R 1  and R 2 . 
         [0080]      FIG. 16  illustrates the curves of variation of currents I A  to I D  according to the phase shift of the signal OUT_PI desired for interpolation circuit  112 . 
         [0081]    In operation, the values of currents I A  to I D  are controlled by the corresponding control signals S IA  to S ID  which are themselves determined by control unit  118  based on signal Phase. For each new value of signal Phase, control unit  118  determines new values of control signals S IA  to S ID  so that the phase shift of signal OUT_PI with respect to signal V CLK,I  depends on signal Phase and is, for example, proportional to signal Phase. 
         [0082]      FIG. 17  shows another embodiment of a digital frequency synthesizer  120 . Phase interpolator  42  comprises four CMOS inverters  122 A to  122 D, each comprising a P-channel MOS transistor  124 A to  124 D and an N-channel MOS transistor  126 A to  126 D. For each CMOS inverter, the drain of transistor  124 A to  124 D and the drain of transistor  126 A to  126 D are connected to a node B. For each CMOS inverter, the source of transistor  124 A to  124 D is connected to a terminal of a current source I A  to I D  having its other terminal connected to a source of a first reference voltage VDD. For each CMOS inverter, the drain of transistor  126 A to  126 D is connected to a source of a second reference voltage, for example, ground GND. The voltage at node B corresponds to signal OUT_PI. Further, for each CMOS inverter, the gate of transistor  124 A to  124 D is connected to the gate of the associated transistor  126 A to  126 D. 
         [0083]    A clock signal V CLK,I  is applied between the gates of transistors  124 A and  126 A and the gates of transistors  124 B and  126 B, and a clock signal V CLK,Q , phase-shifted by 90° with respect to signal V CLK,I , is applied between the gates of transistors  124 C and  126 C and the gates of transistors  124 D and  126 D. Signals V CLK,I  and V CLK,Q  may correspond to pulse sequences, to sinusoidal signals, to triangular signals. 
         [0084]    Phase interpolator  42  also comprises a control unit  128  (Logic Module), rated by clock signal CLK, receiving signal Phase and providing control signals S IA  to S ID  to current sources I A  to I D . The frequency of signal V CLK,I  is equal to an integral multiple, possibly equal to 1, of the frequency of clock signal CLK. 
         [0085]    CMOS inverters  122 A to  122 D are driven by clock signals in quadrature. The sum of currents I A  to I D  is constant so that the peak-to-peak amplitude of signal OUT_PI remains constant. Signal OUT_PI corresponds to a clock signal phase shifted with respect to signal V CLK,I , the value of the phase shift being imposed by the values of currents I A , I B , I C , and I D . Currents I A  and I D  are in fact integrated by the stray capacitances of the MOS transistors. 
         [0086]    In operation, the values of currents I A  to I D  are controlled by the corresponding control signals S IA  to S ID  which are themselves determined by control unit  128  based on signal Phase. For each new value of signal Phase, control unit  128  determines new values of control signals S IA  to S ID  so that the phase shift of signal OUT_PI with respect to signal V CLK,I  depends on signal Phase and is, for example, proportional to signal Phase. 
         [0087]    In the embodiments previously described in relation with  FIGS. 15 and 17 , the number of differential pairs or of CMOS inverters may be greater than 4 while corresponding to a power of 2. 
         [0088]    Specific embodiments of the present invention have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, phase interpolator  42  may be formed with MOS transistors and finite state machine  72  will then be adapted to this structure. 
         [0089]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.