Abstract:
We describe a dual modulus prescaler that may be used in a high frequency PLL. The prescaler comprises a frequency division unit to generate a prescaled signal by dividing a frequency of an input signal by a division ratio and a frequency division ratio controller to determine the division ratio responsive to a count signal and the prescaled signal. The frequency division unit divides a frequency of an input signal by a division ratio of 2N or (2N−1) to output a prescaled signal. The frequency division ratio controller determines a division ratio responsive to a count signal and the prescaled signal.

Description:
CLAIM FOR PRIORITY  
       [0001]     This application claims priority from Korean Patent Application Number 2004-57202, filed Jul. 22, 2004 in the Korean Intellectual Property Office (KIPO). We incorporate the 2004-57202 application by reference.  
       BACKGROUND  
       [0002]     1. Field  
         [0003]     We describe a frequency divider and, more particularly, a frequency divider with a high speed dual modulus prescaler and an associated method.  
         [0004]     2. Related Art  
         [0005]     Frequency divider circuits are part of frequency synthesizers. In Radio Frequency (RF) systems, frequency synthesizers generate a local oscillator&#39;s frequency to step up or step down a frequency band.  
         [0006]     Frequency synthesizers usually include a Phase Lock Loop (PLL), and generate a frequency different from the frequency of an input signal. The PLL is a basic building block of modern electronic systems. As shown in  FIG. 1 , the PLL circuit includes a phase/frequency detector  100 , a charge pump  200 , a loop filter  300 , a Voltage Controlled Oscillator (VCO)  400 , and a frequency divider  500 .  
         [0007]     The phase/frequency detector  100  generates an up-signal SUP and/or down-signal SDN based on a phase difference between a reference signal SIN and a feedback signal SFEED. The charge pump  200  outputs a signal having a level determined by a state of the up-signal SUP and/or the down-signal SDN. The loop filter  300  removes a high frequency component of the signal provided by the charge pump  200 , and provides the input voltage VLF to the VCO  400 . The VCO  400  outputs a high frequency signal having a frequency determined by the direct current level of the input voltage VLF. The frequency divider  500  generates the feedback signal SFEED having a low frequency based on the VCO output signal SOUT. The phase/frequency detector receives the feedback signal SFEED from the divider  500 .  
         [0008]     Downstream circuitry (not shown) use the VCO  400  output signal SOUT for various applications after the PLL circuit is locked. Many embodiments of the frequency divider  500  shown in  FIG. 1  currently exist. For example, U.S. Pat. No. 6,696,857 describes the frequency divider shown in  FIG. 2 . Referring to  FIGS. 1 and 2 , dual modulus prescaler includes D flip-flops  12  and  14 , NMOS transistors MN 1  and MN 2 , a PMOS transistor MP 1 , and a NAND gate  21 .  
         [0009]     The dual modulus prescaler of  FIG. 2  receives an output signal SOUT from VCO  400  as an input signal to divide the frequency of the output signal SOUT by 4 or 3, and outputs the feedback signal SFEED.  
         [0010]     The output signal SOUT clocks the D flip-flops  12  and  14 , respectively. The NAND gate  21 , the NMOS transistors MN 1  and MN 2 , and the PMOS transistor MP 1  control the frequency division ratio of the dual modulus prescaler. When a mode signal MODE has a logic level ‘0’, an output signal of the NAND gate  21  has a logic level ‘1’. As a result, the NMOS transistor MN 1  is on and a node B has a logic level ‘0’. At this time, the NMOS transistor MN 2  is off, and a node C is not at GND. Accordingly, the D flip-flops  12  and  14  of the dual modulus prescaler divide the frequency of the input signal by 4.  
         [0011]     When a mode signal MODE has a logic level ‘1,’ on the other hand, and the output signal SFEED of the D flip-flop  14  has a logic level ‘1’, the output signal of the NAND gate  21  has a logic level ‘0’ and the NMOS transistor MN 1  is off. When an inverted output signal of the D flip-flop  14  has a logic level ‘0’, the PMOS transistor MP 1  is on, and node B has a logic level ‘1’ that turns on the NMOS transistor MN 2 . The node C, therefore, is at GND. As a result, the D flip-flops  12  and  14  of the dual modulus prescaler divide the frequency of the input signal by 3. The frequency divider shown in  FIG. 2  operates at a speed dependent on the dual modulus prescaler.  
         [0012]     The frequency divider&#39;s operating speed depends on a delay time associated with the NAND gate  21  and the NMOS transistors MN 1  and MN 2 , since these components together with the PMOS transistor MP 1  control the frequency division ratio. The delay time is related to the time until the output signal SFEED of the D flip-flop  14  and the mode signal MODE reach a node C.  
         [0013]     The dual modulus prescaler of  FIG. 2  is not suitable for a PLL system operating at a high frequency, e.g., in the range of 1 to 10 GHz due to the delay times of the NAND gate  21  and the NMOS transistors MN 1  and MN 2 .  
         [0014]     Accordingly, a need remains for a frequency divider having a dual modulus prescaler capable of operating at high frequencies.  
       SUMMARY  
       [0015]     We describe a frequency divider including a dual modulus prescaler that seeks to overcome limitations and disadvantages associated with the related art.  
         [0016]     We describe a dual modulus prescaler comprising a frequency division unit to generate a prescaled signal by dividing a frequency of an input signal by a division ratio and a frequency division ratio controller to determine the division ratio responsive to a count signal and the prescaled signal.  
         [0017]     The input signal may be generated by a voltage-controlled oscillator.  
         [0018]     The division ratio may be one of 2N and 2N−1, where N is an integer.  
         [0019]     The frequency division unit comprises N D flip-flops.  
         [0020]     The input signal is adapted to clock the D flip-flops.  
         [0021]     The frequency division ratio controller comprises at least two serially connected transistors.  
         [0022]     The frequency division ratio controller comprises a first NMOS transistor having a drain coupled to an output terminal of a first stage flip-flop in the frequency division unit and a gate to receive either the count signal or the prescaled signal. And a second NMOS transistor has a drain coupled to a source of the first NMOS transistor, a gate to receive the prescaled signal, and a source coupled to a second power supply voltage.  
         [0023]     The frequency division ratio controller comprises a first PMOS transistor having a drain coupled to an output terminal of a first stage flip-flop in the frequency division unit and a gate to receive either the count signal or the prescaled signal. And a second PMOS transistor has a drain coupled to a source of the first PMOS transistor, a gate to receive the prescaled signal, and a source coupled to a first power supply voltage.  
         [0024]     The count signal may be generated responsive to the prescaled signal.  
         [0025]     And we describe a prescaling method of a dual modulus prescaler comprising dividing a frequency of an input signal by a first division ratio responsive to a control signal and a prescaled signal, dividing the prescaled signal by a second division ratio to output the divided signal, and changing a state of the control signal after a predetermined number of clock pulses are generated responsive to the prescaled signal. 
     
    
     BRIEF DRAWINGS DESCRIPTION  
       [0026]     The above and other features and advantages of the present invention will become more apparent by describing in detail example embodiments thereof with reference to the following drawings.  
         [0027]      FIG. 1  is a block diagram illustrating a conventional phase locked loop PLL circuit.  
         [0028]      FIG. 2  is a circuit diagram illustrating a conventional dual modulus prescaler included in the frequency divider of  FIG. 1 .  
         [0029]      FIG. 3  is a block diagram illustrating a frequency divider according to an example embodiment of the present invention.  
         [0030]      FIG. 4  is a circuit diagram illustrating a dual modulus prescaler included in the frequency divider of  FIG. 3  according to an example embodiment of the present invention.  
         [0031]      FIG. 5  is a timing diagram of the dual modulus prescaler shown in  FIG. 4  when a control signal SCON has a low level.  
         [0032]      FIG. 6  is a timing diagram of the dual modulus prescaler shown in  FIG. 4  when a control signal SCON has a high level.  
         [0033]      FIG. 7  is a circuit diagram illustrating a dual modulus prescaler included in the frequency divider of  FIG. 3  according to another example embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0034]     We detail illustrative embodiments in the following description. Our intention is that specific structural and functional details are merely representative of example embodiments. The frequency divider may have many alternate forms and should not be construed as limited to the embodiments set forth here.  
         [0035]     Accordingly, while the frequency divider is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings. It should be understood, however, that there is no intent to limit the frequency divider to the particular forms disclosed, but on the contrary, to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the claims. Like numbers in the various drawings refer to like elements in the description.  
         [0036]      FIG. 3  is a block diagram of an embodiment of a frequency divider. Referring to  FIG. 3 , the frequency divider  500  includes a dual modulus prescaler  520 , a fixed division ratio scaler  540 , and a control circuit  560 . The dual modulus prescaler  520  receives a signal SOUT output from a VCO (not shown). The prescaler  520  divides the frequency of the output signal SOUT by 2N or 2N−1 under the control of a control signal SCON and a prescaled signal PDIV. The fixed division ratio scaler  540  divides the frequency of the prescaled signal PDIV by a predetermined division ratio 1/M and outputs a feedback signal SFEED. The control circuit  560  counts the number of rising edges of the prescaled signal PDIV. The control circuit  560  generates the control signal SCON having a first level, e.g. high, when the number of counted clock pulses reaches a predetermined value. The control circuit  560  provides the control signal SCON to the dual modulus prescaler  520 . The control circuit  560  may determine a division ratio of the dual modulus prescaler  520  and may include a counter. For example, the division ratio of the dual modulus prescaler  520  is 2N when the control signal SCON has a low level and the division ratio of the dual modulus prescaler  520  is 2N−1 when the control signal SCON has a high level.  
         [0037]     The frequency divider  500  shown in  FIG. 3  operates as follows.  
         [0038]     An output signal of a PLL circuit, e.g., a signal SOUT output from a VCO is divided by the frequency divider  500 . The dual modulus prescaler  520  divides the frequency of the output signal SOUT by a division ratio 2N when the control signal SCON has a low level or the dual modulus prescaler  520  divides the frequency of the output signal SOUT by a division ratio 2N−1 when the control signal SCON has a high level.  
         [0039]     The 1/(2N) or 1/(2N−1) prescaled output signal PDIV from the dual modulus prescaler  520  is scaled by a predetermined division ratio M by the fixed division ratio scaler  540 .  
         [0040]      FIG. 4  is a circuit diagram of an embodiment of a dual modulus prescaler  520  shown in the frequency divider of  FIG. 3 . Referring to  FIG. 4 , a dual modulus prescaler  520  includes a frequency division unit  521  and a frequency division ratio controller  526 . The frequency division unit  521  may include a plurality N of D flip-flops, where N is a natural number larger than or equal to 2. For example, the frequency division unit  521  may include four D flip-flops  522 ,  523 ,  524  and  525  as shown in  FIG. 4 . A VCO output signal SOUT is inputted to each of the clock terminals CK of the D flip-flops  522  to  525 . The signal SOUT, therefore, clocks the D flip-flops  522  to  525 .  
         [0041]     An output signal D 01  of the first D flip-flop  522  is applied to an input terminal D of the second D flip-flop  523 , an output signal D 02  of the second D flip-flop  523  is applied to an input terminal D of the third D flip-flop  524  and an output signal D 03  of the third D flip-flop  524 , is applied to an input terminal D of the fourth D flip-flop  525 .  
         [0042]     The output signal PDIV is output from the terminal of the fourth D flip-flop  525  of the frequency division unit  521  is outputted from an output terminal Q of the fourth D flip-flop  525 . An output signal from an inverted output terminal QB of the fourth D flip-flop  525  is fed back into the input terminal D of the first D flip-flop  522 .  
         [0043]     The frequency division ratio controller  526  includes serially connected NMOS transistors MN 3  and MN 4 . The NMOS transistor MN 3  includes a drain coupled to the output terminal Q of the first D flip-flop  522  and a gate coupled to receive the control signal SCON from the control circuit  560  ( FIG. 3 ). The NMOS transistor MN 4  includes a drain coupled to a source of the NMOS transistor MN 3 , a gate coupled to receive the PDIV signal from the output terminal Q of the fourth D flip-flop  525 , and a source coupled to a ground voltage GND.  
         [0044]     Unlike the modulus prescaler shown in  FIG. 2  that operates at a speed determined by the delay time of the NAND gate  21  and the NMOS transistors MN 1  and MN 2 , the dual modulus prescaler shown in  FIG. 3  operates at a higher speed largely determined by the transistor MN 4 .  
         [0045]     The dual modulus prescaler shown in  FIG. 3  employs two cascade connected transistors MN 3  and MN 4  rather than a logic circuit, e.g., the NAND gate  21  shown in  FIG. 2 , to speed up the operation.  
         [0046]      FIG. 5  is a timing diagram of the dual modulus prescaler shown in  FIG. 4  when a control signal SCON has a low level.  FIG. 6  is a timing diagram of the dual modulus prescaler shown in  FIG. 4  when a control signal SCON has a high level.  
         [0047]     When the control signal SCON has a logic level ‘0’, an exemplary operation of the dual modulus prescaler of  FIG. 4  is as follows. When the control signal SCON has a logic level ‘0’, the NMOS transistor MN 3  is off. The frequency of the output signal PDIV of the dual modulus prescaler  520  is equal to ⅛ of the frequency of the VCO output signal SOUT as shown in the timing diagram of  FIG. 5 . That is, when the control signal SCON has a logic level ‘0’, the frequency of the output signal of a dual modulus prescaler  520  having a frequency division unit with N D flip-flops is equal to 1/(2N) of the frequency of the VCO output signal SOUT.  
         [0048]     Referring to  FIG. 5 , the output signal D 01  of the first D flip-flop  522  changes from ‘0’ to ‘1’ at the first rising edge of the VCO output signal SOUT.  
         [0049]     The output signal D 01  is continuously maintained at ‘1’ during the first four periods of the VCO output signal SOUT, and the output signal D 01  changes from ‘1’ to ‘0’ after the first four periods of the VCO output signal SOUT. The output signal D 01  is continuously maintained at ‘0’ during the second four periods of the VCO output signal SOUT, and the output signal D 01  changes from ‘0’ from ‘1’ at the 9th rising edge of the VCO output signal SOUT. And so on.  
         [0050]     The output signal D 02  of the second D flip-flop  523  changes at the second rising edge of the VCO output signal SOUT after the output signal D 01  changes at the first rising edge of the VCO output signal SOUT.  
         [0051]     The output signal D 03  of the third D flip-flop  524  changes at the third rising edge of the VCO output signal SOUT after the output signal D 02  changes at the second rising edge of the VCO output signal SOUT.  
         [0052]     As a result, one loop period of the output signal PDIV of the fourth D-flip flop  525  is eight times a period (or a cycle) of the VCO output signal SOUT. That is, the frequency of the output signal PDIV of the dual modulus prescaler  520  is equal to ⅛ of the frequency of the VCO output signal SOUT.  
         [0053]     As shown in  FIG. 4 , each of the D flip-flops shifts latched data to next stage D flip-flop on every SOUT clock cycle. Thus, output data D 00 , D 01 , D 03 , PDIV of each of the D flip-flops changes at a sequence ‘0000’ ‘1000’, ‘1100’, ‘1110’, ‘1111’, ‘0111’, ‘0011’, ‘0001’, ‘0000’. Since one loop period corresponds to eight clock cycles of the VCO output signal SOUT, the frequency of the output signal PDIV is equal to ⅛ of the frequency of the VCO output signal SOUT.  
         [0054]     When the control signal has a logic level ‘1’, an operation of the dual modulus prescaler of  FIG. 4  is as follows. When the control signal SCON is ‘1’, the NMOS transistor MN 3  is on. The frequency of the output signal PDIV=D 04  of the dual modulus prescaler  520  shown in  FIG. 4  is equal to 1/7 of the frequency of the VCO output signal SOUT as shown in the timing diagram of  FIG. 6 . That is, the frequency of the output signal of a dual modulus prescaler  520  having a frequency division unit with N D flip-flops is equal to 1/(2N−1) of the frequency of the VCO output signal SOUT.  
         [0055]     Referring to  FIG. 6 , the output signal D 01  of the first D flip-flop  522  changes from ‘0’ to ‘1’ at the first rising edge of the VCO output signal SOUT.  
         [0056]     The output signal D 01  is continuously maintained at ‘1’ during the first three periods of the VCO output signal SOUT, and the output signal D 01  changes from ‘1’ to ‘0’ after the first three periods of the VCO output signal SOUT.  
         [0057]     When the control signal SCON has a logic level ‘1’, the output signal D 01  of the first D flip-flop  522  has a different transition point compared with the output signal D 01  when the control signal SCON is ‘0’ as shown in  FIG. 5 .  
         [0058]     The output signal D 01  is continuously maintained at ‘0’ during the second four periods of the VCO output signal SOUT, and the output signal D 01  changes from ‘0’ to ‘1’ at the 8th rising edge of the VCO output signal SOUT.  
         [0059]     The output signal D 02  of the second D flip-flop  523  changes at the second rising edge of the VCO output signal SOUT after the output signal D 01  of the first D flip-flop  522  changes at the first rising edge of the VCO output signal SOUT.  
         [0060]     The output signal D 03  of the third D flip-flop  524  changes at the third rising edge of the VCO output signal SOUT after the output signal D 02  of the second D flip-flop  523  changes at the second rising edge of the VCO output signal SOUT.  
         [0061]     As a result, one loop period of the output signal PDIV of the fourth D flip-flop  525  is seven times of the period of the VCO output signal SOUT. That is, the frequency of the output signal PDIV of the dual modulus prescaler  520  is equal to 1/7 of the frequency of the VCO output signal SOUT.  
         [0062]     The operation of prescaler  520  shown in  FIG. 6  is different from the operation of the prescaler  520  shown in  FIG. 5  in that the output terminal of the first D flip-flop  522  is pulled down to a logic level ‘0’ when the output terminal of the last D flip-flop  525  changes from ‘0’ to ‘1’ because the transistor MN 4  is on due to the high level of the output terminal of the last D flip-flop  525 . Thus, the output data D 00 , D 01 , D 03 , PDIV of the D flip-flops  522 ,  523 ,  524  and  525  changes at a sequence of ‘0000’ ‘1000’, ‘1100’, ‘1110’, ‘0111’, ‘0011’, ‘0001’, ‘0000’. Namely, the output data D 00 , D 01 , D 03 , PDIV of the D flip-flops  522 ,  523 ,  524  and  525  changes from ‘0111’ to ‘1110’ without passing through ‘1111’. Since one loop period corresponds to seven clock cycles of the VCO output signal SOUT, the frequency of the output signal PDIV is equal to 1/7 of the frequency of the VCO output signal SOUT.  
         [0063]     The gate of the NMOS transistor MN 3  included in the frequency division ratio controller  526  receives the control signal SCON. And the gate of the NMOS transistor MN 4  receives the output signal PDIV of the dual modulus prescaler  520 . Alternatively, the gate of the NMOS transistor MN 4  may receive the control signal SCON. And the gate of the NMOS transistor MN 3  may receive the output signal PDIV of the dual modulus prescaler  520 .  
         [0064]      FIG. 7  is a circuit diagram of an embodiment of a dual modulus prescaler included in the frequency divider of  FIG. 3 .  
         [0065]     A frequency division ratio controller  526  illustrated in  FIG. 7  differs from the frequency division ratio controller  526  illustrated in  FIG. 4  in that frequency division ratio controller  526  illustrated in  FIG. 7  includes PMOS transistors rather than NMOS transistors.  
         [0066]     Referring to  FIG. 7 , the frequency division ratio controller  526  includes serially connected PMOS transistors MP 3  and MP 4 .  
         [0067]     The PMOS transistor MP 4  has a drain coupled to an output terminal Q of a first D flip-flop  522  and a gate for receiving a control signal SCON.  
         [0068]     The PMOS transistor MP 3  has a drain coupled to a source of the PMOS transistor MP 4 , a gate coupled to an output terminal Q of the fourth D flip-flop  525  and a source coupled to a power supply voltage VDD.  
         [0069]     When the control signal SCON is ‘0’, the PMOS transistor MP 4  is on. When the control signal SCON is ‘1’, the PMOS transistor MP 4  is off.  
         [0070]     An operation of the dual modulus prescaler illustrated in  FIG. 7  is similar to that described earlier relative to the prescaler  520  shown in  FIG. 4 , and will not be described further.  
         [0071]     The dual modulus prescaler shown in  FIG. 7  has a time delay due to the transistor MP 3 . This time delay is an improvement to the time delay associated with other known dual modulus prescalers that include delays associated with the NAND gate  21  and the NMOS transistors MN 1  and MN 2 .  
         [0072]     The dual modulus prescaler according to an example embodiment of the present invention employs two serially connected transistors MP 3  and MP 4  rather than a logic circuit.  
         [0073]     Accordingly, the frequency divider having the dual modulus prescaler according to the embodiments described are suitable for applying to a PLL system that operates at a high frequency such as a system running between 1 to 10 GHz.  
         [0074]     While the example embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations may be made without departing from the scope and spirit of the claims.