Abstract:
The present invention relates to a ring oscillator VCO using a differential delay stage, substantially increasing the oscillation frequency. The differential delay stage utilizes a differential latch together with a pair of PMOS transistors taking the input signals through skewed delay paths. The added control NMOS transistors controls the strength of said latch and makes the ring oscillator variable in oscillation frequency.

Description:
FIELD OF THE INVENTION 
     This invention relates to oscillators, especially to a ring oscillator using a chain of delay stages, commonly used for frequency synthesis or clock generation. 
     BACKGROUND OF THE INVENTION 
     Ring oscillator has been used for communication systems, particularly for frequency synthesis or clock generation apparatus. A ring oscillator circuit constitutes several delay stages arranged in a chain forming a ring structure. Each delay stage has an electronic circuit generating an output signal that is delayed in time from an input signal. The oscillation frequency of the ring oscillator is inversely proportional to the total delay of the delay stages in the chain. 
     Henceforth, the delay of each delay stage limits the oscillation frequency of the ring oscillator. Therefore, if the delay of the delay stage can not be smaller, the oscillation frequency cannot be increased. Accordingly, it is required to develop a delay cell having a low delay to achieve high frequency oscillation. 
     However, although an invention has a simple structure, it can not produce the delay less than that of an inverter for a given fabrication process because the inverter has the simplest structure as a delay stage. 
     Thus, there is proposed a skewed delay ring oscillator having improved structure, which overcomes the fundamental limit of a delay stage for a given fabrication process, and increases the oscillation frequency of the ring oscillator. 
     The delay stage of the skewed delay ring oscillator comprises an NMOS transistor and a PMOS transistor like an inverter delay stage. However, the gate of the PMOS in each delay stage of the skewed delay ring oscillator is not connected to the output of the previous stage, but connected to the output of one or several stages before the previous stage. 
     Normally, PMOS transistors are slower than NMOS transistors in a given fabrication process. However, the skewed delay ring oscillator allows the PMOS transistor to take an earlier signal than NMOS transistor and gives the PMOS transistor more time to handle the signal. Accordingly the skewed delay ring oscillator can achieve higher frequency oscillation. 
     However, since this skewed delay ring oscillator has single-ended structure, it has a poor characteristics with respect to the power supply injected noise, and has a disadvantage in that the skewed delay ring oscillator does not vary oscillation frequency. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a delay varying capability for a delay stage that has a fully differential structure, having good rejection to the power supply injected noise, and to provide a frequency varying capability to the skewed ring oscillator. 
     The present invention is characterized in that the fully differential delay stage comprises two pair of NMOS transistors, one pair of PMOS transistors, and an optional pair of PMOS transistors. The first pair of NMOS transistors takes the differential inputs, in which one NMOS gate takes the positive polarity input signal, and the other NMOS gate takes the negative polarity input signal. The gates of the first pair of PMOS transistors are cross-connected to the differential outputs of the delay stage, which thus constitute a differential latch structure with the first pair of NMOS transistors. The gates of the second NMOS transistor pair are connected to the signal adjusting the delay of the delay stage. The gates of the optional PMOS transistor pair are connected to the differential outputs of the one stage before the previous stage through the skewed delay path and used to increase the oscillation frequency. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in form of a preferred embodiment by making reference to the accompanying drawings, in which: 
     FIG. 1 shows a circuit diagram of a delay stage according to the present invention; 
     FIG. 2 shows differential input signals and differential output signals associated with the circuit diagram of a delay stage. 
     FIG. 3A shows and FIG. 3B shows a ring oscillator construction diagram using a delay stage according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be described in more detail with reference to accompanying drawings as follows. 
     FIG. 1 shows the structure of the delay stage according to the present invention. Referring to FIG. 1, the delay stage is inputted such that the first and second input signals VIN+ and VIN− are supplied to the gates of the first and second NMOS transistors M 1  and M 2  respectively. The delay stage generates two output signals such that the first and the second output signals VOUT− and VOUT+ are generated at the drains of the first and second NMOS transistors M 1  and M 2  respectively. 
     The delay stage is constituted such that the gates of the first and second PMOS transistors M 3  and M 4  are cross-connected  11  and  12  to each of the drain of the said first and second NMOS transistors M 1  and M 2  through the fourth and third NMOS transistors M 6  and M 5  respectively, to form a differential latch structure. 
     Referring to the FIG. 2, when the first input signal  21  VIN+ is transited from 0V to VDD and the second input signal  22  VIN− is transited from VDD to 0, the gate of the first PMOS transistor M 3  is charged with VCONT-VTH 6  and the gate of the second PMOS transistor is charged with VCONT-VTH 5 . Here, VTH 5  and VTH 6  represent the threshold voltages of the first and second PMOS transistors respectively. Output signal  23  VOUT− is taken from the connection between M 1  and M 3 , and output signal  24  VOUT+ is taken from the connection between M 2  and M 4 . 
     The third and the fourth NMOS transistors M 5  and M 6  are disposed in cross-pass form to adjust the delay time of the output signal with respect to the input signal. The control voltage VCON is supplied to the gates of the third and fourth NMOS transistors M 5  and M 6  in order to control the maximum gate voltages of the fourth and third PMOS transistors M 4  and M 3  respectively. 
     The change of said control voltage VCONT further controls the strength of said latch. For a higher voltage VCONT, the third and fourth NMOS transistors M 5  and M 6  allows a higher maximum gate voltages  31  and  32  for the third and fourth PMOS transistors and forms a stronger latch. On the other hand, for a lower voltage VCONT, the third and fourth NMOS transistors M 5  and M 6  allow lower maximums gate voltages for the third and fourth PMOS transistors and form a weaker differential latch. 
     As seen in FIG. 3 b,  the third and the fourth PMOS transistors M 7  and M 8  for inputting skewed delay signals can be disposed in parallel with the first and second PMOS transistors M 3  and M 4  respectively. FIG. 3 a  shows the structure of a ring oscillator circuit using the said delay stage. Referring to FIG. 3 a,  the ring oscillator circuit has two kinds of signal paths, one kind is a normal delay path and the other kind is a skewed delay path. The normal delay path connects the input of one delay stage to the output of the previous delay stage  73 ,  74 ,  75  and  76 . The skewed delay path connects the input of one delay stage to the output of one or more stage before the previous delay stage  63 ,  64 ,  65  and  66 . 
     When the present ring oscillator comprises an odd number of delay stages, the normal delay path is used in that the positive polarity input VIN+ of the first delay stage in the chain takes the signal from the negative polarity output VOUT− from the last delay stage in the chain. The negative polarity input VIN− of the first delay stage in the chain takes the signal from the positive polarity output VOUT+ from the last delay stage in the chain. The skewed delay path is used in that the positive polarity PMOS gate VINX+takes the signal from the negative polarity output VOUT− from the one stage before the last delay stage. The negative polarity PMOS gate VINX− takes the signal from the positive polarity output VOUT+ from the one stage before the last delay stage. 
     When the present ring oscillator comprises an even number of delay stages, the positive polarity input VIN+ of the first in the chain takes the signal from the positive polarity output VOUT+from the last delay stage in the chain  78 . The negative input VIN− of the first delay stage in the chain takes the signal from the negative polarity output VOUT− from the last delay stage in the chain  72 . The positive polarity PMOS gate VINX+ takes the signal from the positive polarity output VOUT+ from the one stage before the last delay stage  68 . The negative polarity PMOS gate VINX− takes the signal from the negative polarity VOUT− from the one stage before the last delay stage  67 . 
     Adjusting the control voltage VCONT changes the oscillation frequency of the present ring oscillator in the following way. When the control voltage VCONT is high, the latch is weak and the input signal of the delay stage easily propagates to the output in short time. Therefore the delay time per each delay stage is short and the oscillation frequency is higher. When the control voltage VCONT is low, the said latch is strong and the input signal of the delay stage takes longer time to propagates to the output because the latch disturbs the signal transition. Therefore, the oscillation frequency is lower. The reason that the normal delay path is arranged in parallel with said skewed delay path is that an oscillation frequency range of the ring oscillator circuit can be widened. 
     Since the fully differential delay stage of the present invention can operate at higher speed by virtue of the simplicity and skewed signal path, it is applicable to the high frequency generation application such as clock generators and frequency synthesizers.