Patent Publication Number: US-2005116258-A1

Title: Two-modulus prescaler circuit

Description:
TECHNICAL FIELD  
      The present invention relates to a dual modulus prescaler circuit applicable to frequency synthesizers and the like. Specifically, the present invention relates to a dual modulus prescaler circuit which can operate with small signal amplitude and with reduced power consumption.  
     BACKGROUND OF THE INVENTION  
      A “divide-by 4/divide-by 5 divider” switchable between divide-by 4 and divide-by 5 modes, which is a basis for a dual modulus prescaler used for a pulse swallow type PLL synthesizer, is composed of a circuit as shown in  FIG. 1 . In  FIG. 1 , reference numerals  11  to  13  denote D flip-flop circuits (DFF circuits), and reference numerals  14  and  15  denote NOR circuits. This circuit is configured to switch between divide-by 4 and divide-by 5 modes by means of a polarity of an input signal of a terminal M.  
       FIG. 2  is a timing chart showing an operation of the “divide-by 4/divide-by 5 divider”. As shown in  FIG. 2 , in the “divide-by 4/divide-by 5 divider”, when an input of the terminal M is H 1 , an input D of the DFF 3  remains unchanged, and the input signal is frequency-divided by four with the DFF 1  and DFF 2 . On the contrary, when the input of the terminal M is Low, the DFF 3  operates, and the input signal is frequency-divided by five.  
      Further, by adding a frequency divider to the output of the circuit, a dual modulus prescaler circuit switchable between divide-by 2n and divide-by 2n+1 modes can be configured.  FIG. 3  is a block diagram of a “divide-by 32/divide-by 33 divider” using the “divide-by 4/divide-by 5 divider”. Reference numerals  11  to  13  denote D flip-flop circuits (DFF circuits);  14  and  15 . NOR circuits;  16 , an OR circuit; and  21  to  23 , T flip-flop circuits (TFF circuits).  
      In the circuit of  FIG. 3 , each of the DFF 1 , DFF 2 , and DFF 3  constituting the “divide-by 4/divide-by 5 divider” is required to operate at a frequency of an input signal. The TFF 1 , TFF 2 , and TFF 3  which receive divided signals operate at a lower frequency as they get closer to the last stage.  
      Therefore, in order to reduce the power consumption of the dual modulus prescaler, it is important to reduce the power consumption of the “divide-by 4/divide-by 5 divider”. Generally, prescalers require high-speed operation, and so bipolar processes are often used. However, CMOS prescalers have been developed along with achieving higher speed CMOS processes in recent years.  
      In the case of CMOS processes, it is possible to conceive a DFF circuit using a current mode logic circuit as shown in  FIG. 4 , which can be expected to offer faster performance than that using normal CMOS gates. The circuit of  FIG. 4  includes a master FF and a slave FF. The master FF is composed of transistors M 1  to M 6  and R 1  and R 2 , and the slave FF is composed of M 8  to M 13 , R 3 , and R 4 . I- 1  and I- 2  of  FIG. 4  denote current sources.  
      The transistors M 5  and M 6  and the transistors M 12  and M 13  are turned on and off by a clock inputted as a differential signal to switch current paths. When CN is Hi and CP is Low, the current path is through the M 5  side in the master FF. At this time, if a gate voltage IN of M 1  is higher than a gate voltage IP of M 2 , current flows through R 1 , and Low is read. If IN is lower than IP, current flows through R 2 , and H 1  is read. Simultaneously, the current path Is through the M 13  side in the slave FF, and an output is held by M 10  and M 11 .  
      When CN is Low and CP is Hi, the current path is through the M 6  side in the master FF, and data is held by M 3  and M 4 . The current path is through the M 12  side in the slave FF, and data is outputted through M 8  and M 9 . This circuit operates as a DFF circuit according to the aforementioned operations.  
      The NOR circuit used in the “divide-by 4/divide-by 5 divider” can be easily implemented only by replacing the transistor M 1  for data input in the DFF circuit of  FIG. 4  with transistors M 1 A and M 1 B connected to each other in parallel as shown in  FIG. 5 . Signals and the like in  FIG. 5  are the same as those in  FIG. 4 .  
      In the DFF circuit with a NOR gate shown in  FIG. 5 , in reading data when CP is Low, if any one of a gate voltage A of M 1 A and a gate voltage B of M 1 B or both thereof are higher than a gate voltage VR of M 2 , current flows through R 1 , and Low is read. When both of A and B are lower than VR, current flows through R 2 , and Hi is read. Therefore, this circuit operates as a DFF circuit with a NOR gate.  
      The DFF circuit with a NOR gate shown in  FIG. 5  requires a threshold voltage VR for judging whether data is Low or Hi since single-phase data is inputted. Power supply voltage necessary for operating this circuit requires being a voltage value obtained by adding signal amplitude to operating voltages of individual transistors. The DFF circuit with a NOR gate of  FIG. 5  requires a total voltage of a voltage for operating three transistors and signal amplitude.  
      Larger signal amplitude provides larger operating margins but requires larger power supply voltage. The DFF circuit with a NOR gate of  FIG. 5  requires larger signal amplitude to ensure a DC bias operating margin since single-phase data is inputted. Therefore, it is difficult to reduce the power supply voltage.  
     SUMMARY OF THE INVENTION  
      The present invention according to the application was made to solve the aforementioned problem. Specifically, an aspect of the present invention according to claim  1  is a dual modulus prescaler circuit for dividing an inputted clock signal to obtain a divide ratio which is selected by a switching signal from a combination of predetermined divide ratios. The dual modulus prescaler circuit includes: n D flip-flop circuits, n being a natural number not less than three; a first multi-input logic gate circuit including at least two input terminals; and a second multi-input logic gate circuit including at least two input terminals. An output terminal of the first multi-input logic gate circuit is connected to a data input terminal of the first D flip-flop circuit; output terminals of the first to (n-2)th D flip-flop circuits are connected to data input terminals of the second to (n-1)th D flip-flop circuits; output terminals of the (n-1)th and nth D flip-flop circuits are connected to input terminals of the first multi-input logic gate circuit; the second multi-input logic gate circuit is connected to the output terminal of the (n-1)th D flip-flop circuit and receives the switching signal, and an output terminal of the second multi-input logic gate circuit is connected to the data input terminal of the nth D flip-flop circuit. Moreover, all the aforementioned connections are connections using differential signals.  
      Another aspect of the present invention according to claim  2  is the dual modulus prescaler circuit according to claim  1 , in which a multi-input logic gate circuit is used for each of the first and second multi-input logic gate circuits, the multi-input logic gate circuit including a current source, first and second resistors each of which has an end connected to a power supply, m transistors connected in parallel, whose sources are connected to an output end of the current source and whose drains are connected to the other end of the first resistor, and further m transistors connected in series, whose sources and drains are connected between the output end of the current source and the other end of the second resistor. Herein, m is a natural number not less than two. Moreover, m differential input data are applied between gates of the transistors connected in parallel and gates of the transistors connected in series, and a differential signal is obtained as an output of the multi-input logic gate circuit at the other ends of the first and second resistors.  
      A still another aspect of the present invention according to claim  3  is the dual modulus prescaler according to claim  1 , in which a D flip-flop circuit with a logic gate circuit is used for each of combinations of the first multi-input logic gate circuit and the first D flip-flop circuit and of the second multi-input logic gate circuit and the nth D flip-flop circuit, the logic gate circuit including: first and second current sources; first and second transistors whose sources are connected to an output end of the first current source; third and fourth transistors whose sources are connected to an output end of the second current source; fifth, sixth, and seventh transistors whose sources are connected to a drain of the first transistor; an eighth transistor whose source is connected to a drain of the seventh transistor; ninth and tenth transistors whose sources are connected to a drain of the second transistor; eleventh and twelfth transistors whose sources are connected to a drain of the third transistor; thirteenth and fourteenth transistors whose sources are connected to a drain of the fourth transistor; a first resistor whose terminal is connected to a power supply and whose other terminal is connected to drains of the fifth, sixth, and ninth transistors and to gates of the tenth and eleventh transistors; a second resistor whose terminal is connected to the power supply and whose other terminal is connected to drains of the eighth and tenth transistors and to gates of the ninth and twelfth transistors; a third resistor whose terminal is connected to the power supply and whose other terminal is connected to drains of the eleventh and thirteenth transistors and to a gate of the fourteenth transistor; and a fourth resistor whose terminal is connected to the power supply and whose other terminal is connected to drains of the twelfth and fourteenth transistors and to a gate of the thirteenth transistor. Moreover, differential clock signals are applied between gates of the first and fourth transistors and between gates of the second and third transistors; a first differential data is applied between gates of the fifth and eighth transistors; and a second differential data is applied between gates of the sixth and seventh transistors.  
      Still another aspect of the present invention according to claim  4  is the dual modulus prescaler circuit according to claim  3 , in which, in the D flip-flop circuit with a logic gate circuit, the current sources are eliminated, and the sources of the first, second, third, and fourth transistors are connected to the ground.  
      In the present invention, as described above, the DFF circuit with a NOR gate is configured to receive differential inputs, so that the signal amplitude can be reduced. The use of differential signals allows the signal amplitude to be reduced to half, thus achieving reduced power supply voltage. Therefore, there are a method for constituting the DFF circuit with a NOR gate by use of a differential input/differential output NOR/OR circuit and a differential DFF circuit and a method for using a differential input DFF circuit with a NOR gate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a view showing a configuration of a conventional “divide-by 4/divide-by 5 divider”.  
       FIG. 2  is a view showing a timing chart of the conventional “divide-by 4/divide-by 5 dividers”.  
       FIG. 3  is a view showing a configuration of a conventional “divide-by 32/divide-by 33 divider”.  
       FIG. 4  is a view showing a configuration of a conventional DFF circuit.  
       FIG. 5  is a view showing a configuration of a conventional DFF circuit with a NOR gate.  
       FIG. 6  is a view showing a configuration of a circuit according to a first embodiment.  
       FIG. 7  is a view showing a configuration of a differential input/output NOR circuit according to the first embodiment.  
       FIG. 8  is a view showing a configuration of a DFF circuit with a NOR gate according to a second embodiment. 
    
    
     DETAIL DESCRIPTION OF THE INVENTION  
       FIG. 6  is a view showing a first embodiment of the present invention. This embodiment corresponds to an aspect of the present invention according to claim  1 . In the drawing, reference numerals  1  to  3  denote DFF circuits, and  4  and  5  denote NOR circuits. The circuit of  FIG. 6  is configured so that differential signals are inputted to/outputted from all the DFF circuits (D flip-flop circuits) and NOR circuits (multi-input logical gate circuits). An example of the differential input/output NOR circuit is shown in  FIG. 7 . This circuit corresponds to a NOR circuit defined by another aspect of the present invention according to claim  2 .  
      The circuit of  FIG. 7  is a NOR circuit operated in a current mode in order to ensure high speed performance. In  FIG. 7 , R 1  and R 2  denote resistors: M 1  to M 4 , transistors; I- 1 , a current source; AP, an input signal; and YP, an output signal. When the terminals AP and BP in the circuit of  FIG. 7  are Hi, AN and BN are Low. At this time, current flows through R 1  but does not flow through R 2 . Accordingly, YP is Low, and YN is Hi. When AP is Hi and BP is Low or when YP is Low and BP is Hi, current flows through R 1 . Accordingly, YP is Low, and YN is Hi. In other words, YP acts as the NOR to AP and BP. Herein, the aforementioned circuit of  FIG. 4  is used for each of the DFF circuits.  
      The circuit of  FIG. 4  includes a master FF and a slave FF. The master FF is composed of transistors M 1  to M 6  and R 1  and R 2 , and the slave FF is composed of M 8  to M 13  and R 3  and R 4 . The transistors M 5  and M 6  and the transistors M 12  and M 13  are turned on and off by a clock inputted as a differential signal to switch current paths.  
      When CN is Hi and CP is Low, the current path is through the M 5  side in the master FF. At this time, if a gate voltage IN of M 1  is higher than a gate voltage IP of M 2 , current flows through R 1 , and Low is read. If IN is lower than IP, current flows through R 2 , and Hi is read. Simultaneously, the current path in the slave FF is through the M 13  side, and the output is held by M 10  and M 11 .  
      When CN is Low and CP is Hi, the current path is through the M 6  side in the master FF, and data is held by M 3  and M 4 . In the slave FF, the current path is through the M 12  side, and data is outputted through M 8  and M 9 . This circuit operates as a DFF circuit according to the aforementioned operations. A fully differential “divide-by 4/divide-by 5 divider” can be implemented by using the circuit of  FIG. 7  for each NOR circuit and using the circuit of  FIG. 4  for each DFF circuit.  
       FIG. 8  is a view showing a second embodiment of the present invention, corresponding to still another aspect of the invention according to claim  3 . The circuit of  FIG. 8  is a circuit in which the transistor M 2  of the DFF circuit with a NOR gate shown in  FIG. 5  is replaced with transistors M 2 A and M 2 B which are connected in series. In the circuit of  FIG. 8 , differential signals are applied between AP and AN and between BP and BN.  
      In reading data when CP is Low, only in the case where AP and BP are Low, AN and BN are Hi, and current flows through R 2 . Accordingly, Hi is read. In other cases, current flows through any one or both of M 1 A and M 1 B, and any one or both of M 2 A and M 2 B are turned off. Accordingly, current flows through R 1 , and Low is read.  
      Therefore, the circuit of  FIG. 8  operates as the DFF circuit with a NOR gate. It is possible to implement the fully differential “divide-by 4/divide-by 5 divider” by applying the DFF circuit with a NOR gate of  FIG. 8  to each of combinations of NOR 1  and DFF 1  and of NOR 2  and DFF  3  in  FIG. 6  and applying the DFF circuit of  FIG. 4  to DFF 2 . Moreover, the DFF circuit with a NOR gate of  FIG. 8  and the DFF circuit of  FIG. 4  can operate even when the current sources are omitted since the clock signals thereof are differential. Accordingly, the power supply voltage can be further lowered by omitting the current sources.  
      Industrial Applicability  
      According to the present invention, all the circuits constituting the dual modulus prescaler are designed to be differential input and/or differential output circuits. Accordingly, compared to conventional circuits operating margins for signal amplitude and a signal direct current level can be increased, so that the dual modulus prescaler can operate with smaller signal amplitude. This offers advantages of reducing power supply voltage and power consumption.