Abstract:
Provided is a circuit to convert input CMOS level signals having a predetermined duty cycle to CML level signals having a higher duty cycle. The circuit includes two differential transistor pairs connected together. The two differential pairs are constructed and arranged to use gates of the associated transistors as inputs to receive and combine a number of phase shifted CMOS input signals. The combined CMOS input signal are converted to CML level signals which are provided as circuit outputs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. Non-Provisional application Ser. No. 09/953,279, filed Sep. 17, 2001 is now U.S. Pat. No. 6,794,907, which claims the benefit of U.S. Provisional Application No. 60/233,181, filed Sep. 15, 2000, all of which are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to clock converters. More particularly, the present invention relates to a circuit configured to convert a clock signal having a complimentary metal oxide semiconductor (CMOS) duty cycle level to a clock signal having a current mode logic (CML) duty cycle level. 
   2. Background Art 
   High speed communication systems utilize a variety of different approaches to optimizing the performance of their associated system clocks. As clock rates increase to accommodate the demands of these high speed communications systems, the room for clock tolerances decreases. For example, small variations in a clock&#39;s output signal, known as jitter, may have a crippling effect on the operation and synchronization of interrelated clock dependent circuits. Furthermore, clock stability may be critical to the operation of logic circuits that are dependent upon the rising and falling edges of the clock&#39;s output signal. Thus, even a small amount of jitter in a clock&#39;s output signal may significantly alter the clock signal&#39;s duty cycle, consequently degrading the communication system&#39;s overall performance. 
   Particular integrated circuit technology types, such as CMOS and CML, are typically associated with specific duty cycle values. For example, CMOS systems normally produce signals having a duty cycle around 25% and CML systems normally produce signals having a duty cycle around 50%. The higher duty cycle characteristics of CML make it better suited for higher speed applications. Also, as known in the art, CMOS circuits operate at logical high voltage levels from about 0 to 2.5 volts, thus creating about a 2.5 volt peak-to-peak swing. On the other hand, CML level circuits operate around 1.5 volts to 2.5 volts, thus producing a 1 volt peak-to-peak swing. Some applications, however, may require attributes of both CMOS and CML technology. One approach to satisfying this requirement is the ability to convert CMOS signals into CML signals. 
   For example, a variety of conventional CMOS based frequency divider circuits receive a master clock signal as an input and produce a number of multi-phase signals as an output. These multi-phase divider circuits may be used to reduce the overall number of oscillators required on a given semiconductor chip, for example, thereby making available additional room on the chip to place more circuitry. Although beneficial in this capacity, these CMOS multi-phase divider circuits are inherently slow and their low duty cycle signals are susceptible to supply coupling, which causes jitter. As a result, there is a need for a device to convert a CMOS multi-phase output clock signal having a duty cycle of about 25% into a CML level clock signal having a duty cycle of at least 50%. 
   BRIEF SUMMARY OF THE INVENTION 
   Consistent with the principles of the present invention as embodied and broadly described herein, an exemplary circuit includes a first pair of transistors having gates thereof respectively forming first and second circuit inputs, sources thereof being connected together, and drains thereof being connected together and forming at least a first circuit output the exemplary embodiment also includes a second pair of transistors having gates thereof respectively forming third and fourth circuit inputs, sources thereof being connected together, and drains thereof being connected together and forming at least a second circuit output. Sources of the first pair of transistors are connected to the sources of the second pair of transistors. 
   Features and advantages of the present invention include the ability to convert a lower duty cycle clock signal in CMOS to a higher duty cycle clock signal in CML. Such a circuit may be ideal for use where both CMOS and CML technologies are used together, such as the low jitter and high speed environments of variable control oscillators used in phase locked loop (PLL) circuits. Additional features include the ability to insure excellent rejection of common mode voltages associated with circuit power supplies. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and, together with the description, explain the purpose, advantages, and principles of the invention. In the drawings: 
       FIG. 1  is a block diagram of an exemplary converter constructed and arranged in accordance with the present invention; 
       FIG. 2  is a schematic diagram of the exemplary embodiment of  FIG. 1 ; and 
       FIG. 3  is an illustration of input and output signals associated with the schematic diagram of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description of the present invention refers to the accompanying drawings that illustrate exemplary embodiments consistent with this invention. Other inventions are possible and modifications may be made to the embodiments without departing from the spirit and scope of the invention. Therefore, the following detailed description is not meant to limit the invention. Rather, the scope of the invention is defined by the appended claims. 
   It would be apparent to one of ordinary skill in the art that the present invention, as described below, may be implemented in many different embodiments. Thus the operation and behavior of the present invention will be described with the understanding that modifications are variations of the embodiments are possible, given the level of detail present herein. 
     FIG. 1  illustrates an exemplary embodiment of the present invention. In  FIG. 1 , an exemplary signal converter  100  is shown. The converter  100  includes input ports IP 1 , IP 2 , IP 3 , and IP 4  which are configured to receive CMOS level multi-phase signals as inputs. The converter  100  also includes an inverting output port OUTN and a non-inverting output port OUTP, both configured to produce CML level signals as outputs. A more detailed view of embodiment of  FIG. 1  is shown in  FIG. 2 . 
     FIG. 2  is a schematic diagram of the exemplary signal converter  2  shown in  FIG. 1 . The converter  100  includes a first pair of transistors  200  electrically coupled to a second pair of transistors  202 . The first and second transistor pairs  200  and  202  are known in the art as differential pair transistors. Also included in the converter  100  is a transistor  204  to supply constant current to the differential transistor pairs  200  and  202 . 
   The first differential pair of transistors  200  includes NMOS field effect transistors (FETs)  206  and  208 . The transistor  206  includes a gate  210 , a source  212 , and a drain  214 . Similarly, the FET  208  includes a gate  216 , a source  218 , and a drain  220 . The gates  210  and  216  are operatively configured as the input ports IP 1  and IP 2  respectively, as shown in  FIG. 1  and discussed above. Also as shown, the sources  212  and  218  are connected together. The second differential pair of transistors includes FETs  226  and  228 . The FETs  226  and  228  respectively include gates  230  and  232 , sources  234  and  236 , and drains  238  and  240 . The drains  214  and  220  of respective transistors  206  and  208 , are connected together. Further, the gates  238  and  240  are operatively configured as the input ports IP 3  and IP 4  respectively, shown in  FIG. 1 . 
   The inverting output port OUTN is formed of a connection node between the drains  214  and  220 . A resistor R 1 , having one end connected to the inverting output port OUTN, is connected between the inverting output port OUTN and a source drain voltage supply source VDD. The non-inverting output port OUTP is formed of a connection node between the drains  238  and  240 . Another resistor R 2  is provided, having one end connected to the non-inverting output port OUTP and the other end connected to the source drain voltage supply VDD. Differential output CML signals are produced across the output ports OUTN and OUTP in response to multi-phase input CMOS signals. The type and impedance of resistors R 1  and R 2  are not critical to the present invention. However, these features may vary based upon various circuit design goals, such as a bandwidth, amplitude, and output swing of the associated output signals. 
   The transistor  204  provides constant current to the transistor pairs  200  and  202 . The transistor  204  includes a source  242  connected to a ground node  244  and a drain  246  connected to the sources  212 ,  218 ,  234  and  236 . As discussed above, the converter circuit  100  is configured to receive multi-phase CMOS level signals at a duty cycle of about 25% and convert the input CMOS level signals to CML level signals having a duty cycle of about 50%. 
     FIG. 3  is a timing diagram of exemplary multi-phase CMOS level input signals and exemplary CML level output signals. In  FIG. 3 , CMOS level input signals IS 1 –IS 4  are representative of four phases of an input clock signal. For purposes of illustration, IS 1 –IS 4  are respectively shown to be at 0°, 90°, 180°, and 270° phase and are respectively received at input ports IP 1 , IP 2 , IP 3  and IP 4 . The input CMOS signals are combined and converted to provide the CML level differential output signals OS 1  and OS 2 . OS 1  and OS 2  are provided at the output ports OUTP and OUTN respectively. The timing diagram depicting t 1 –t 5 , shows the input timing of the signals IS 1 –IS 4 . Next, the operation of the converter  2  will be described. 
   The CMOS input signals IS 1 –IS 4  may be generated by techniques known in the art, such as by use of a multi-phase divider circuit (not shown). As shown in  FIGS. 2 and 3 , at time (t 1 ), the input signals IS 1 –IS 4  are respectively provided to the input ports IP 1 , IP 2 , IP 23  and IP 4 . Each of the signals IS 1 –IS 4  is shown to be shifted in phase from all of the other signals by 90°. Next, at time (t 2 ), input signal IS 1  goes high and transistor  206  turns on. As a result, the output signal OS 1 , produced at output port OUTP, goes high and output signal OS 2  produced at output port OUTN, goes low. 
   At time (t 3 ), IS 2  goes high and transistor  208  also turns on. Transistors  206  and  208  remain on until time (t 4 ). At time (t 4 ), output signal IS 2  goes low and transistors  206  and  208  turn off. Additionally, at time (t 4 ), the input signal IS 3  goes high, transistor  226  turns on, the output signal OS 1  goes low, and the output signal OS 2  goes high. At time (t 5 ), input signal IS 4  goes high, transistor  228  turns on, OS 1  remains low and OS 2  remains high. Finally, at time (t 6 ), input signal IS 4  goes low, the transistor  228  turns off and OS 1  and OS 2  repeat the cycle that began at time (t 2 ). Thus,  FIG. 3  illustrates and actual conversion of the input CMOS level signals IS 1 –IS 4  to output CML level signals OS 1  and OS 2  using the exemplary technique described above. 
   The output signals OS 1  and OS 2  are differential in nature. That is, a signal produced at the output port OUTP necessarily includes the presentation of an inverted version of the produced signal at the output port OUTN. The inverted signal is equal in amplitude but opposite in phase in relation to the signal produced at OUTP. Also, as shown in  FIG. 3 , the output CML level signals OS 1  and OS 2  have duty cycles of about 50%, whereas input signals IS 1 –IS 4  operated at duty cycles of about 25%. 
   Low jitter is achieved because the converter  100  has excellent circuit symmetry. That is, the gate capacitance of transistors  206 ,  208 ,  226  and  228  are substantially equal. Thus, this circuit provides even loading from one stage of the converter to the other, that is, from one transistor to the other. Also, the differential circuit symmetry ensures excellent power supply and common mode voltage rejection. 
   The foregoing description of the preferred embodiments provide an illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible consistent with the above teachings or may be acquired from practice of the invention. Thus, it is noted that the scope of the invention is defined by the claims and their equivalents.