Patent Publication Number: US-6657917-B2

Title: Selective adjustment of voltage controlled oscillator gain in a phase-locked loop

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of integrated circuits. More particularly, the invention relates to a phase locked loop circuit having reduced sensitivity to noise. The invention is used to improve the synchronization of the internal timing or clock signals within an integrated circuit such as a synchronous dynamic random access memory (SDRAM) to external timing or clock signals. The invention is also used in multi-rate high speed transmission buffers. 
     Most digital logic implemented on integrated circuits is clocked synchronous sequential logic. In electronic devices such as synchronous dynamic random access memory circuits (SDRAMs), microprocessors, digital signal processors, and so forth, the processing, storage, and retrieval of information is coordinated with a clock signal. The speed and stability of the clock signal determines to a large extent the data rate at which a circuit can function. Many high-speed integrated circuit devices, such as SDRAMs, microprocessors, etc., rely upon clock signals to control the flow of commands, data, addresses, etc., into, through and out of the devices. 
     A continual demand exists for devices with higher data rates; consequently, circuit designers have begun to focus on ways to increase the frequency of the clock signal. In SDRAMs, it is desirable to have the data output from the memory synchronized with the system clock that also serves the microprocessor. The delay between a rising edge of the system clock (external to the SDRAM) and the appearance of valid data at the output of the memory circuit is known as the clock access time of the memory. A goal of memory circuit designers is to minimize clock access time as well as to increase clock frequency. 
     One of the obstacles to reducing clock access time has been clock skew, that is, the delay time between the externally supplied system clock signal and the signal that is routed to the memory&#39;s output circuitry. An external system clock is generally received with an input buffer and then further shaped and redriven to the internal circuitry by an internal buffer. The time delay of the input buffer and the internal buffer will skew the internal clock from the external clock. This clock skew will cause signals that are to be transferred from the integrated circuit to be out of synchronization with the external system clock. This skew in the clock signal internal to the integrated circuit is furthered by the delays incurred in the signal passing through the clock input buffer and driver and through any associated resistive-capacitive circuit elements. One solution to the problem of clock skew is the use of a synchronous mirror delay. One problem with the synchronous mirror delay is that, although these type of delays require fewer clock cycles to achieve lock of an external clock signal and an internal clock signal, the synchronous mirror delay has a limited operational frequency range. Another solution to the problem of clock skew is the use of a phase locked loop (PLL) that has a greater operational frequency range. A PLL is capable of tracking phase as well as frequency of the clock signal. A typical PLL includes a voltage control oscillator (VCO). The VCO outputs a signal with frequency corresponding to a voltage. Because of the relatively large bandwidth of PLLs, oftentimes they are operated on the lower frequencies of the bandwidth. It has been found that the voltage signals controlling the VCOs are sensitive to noise, particularly at the lower frequencies. At lower voltage levels, voltage noise (or jitter) has a proportionally greater effect on the signal. Therefore, it is desirable to reduce the amount of noise or jitter on the control voltage of the VCO. At the same time, it is desired to maintain the locking function of the PLL. 
     SUMMARY OF THE INVENTION 
     The present invention solves the aforementioned problems, and decreases the amount of noise or jitter on the control voltage of a voltage controlled oscillator. 
     In one aspect of the invention, a method of decreasing voltage controlled oscillator noise susceptibility and a phase locked loop is disclosed. The method includes providing a phased lock loop (PLL) having a voltage controlled oscillator (VCO) and controlled by a control voltage VCTRL and having a VCO frequency. A desired voltage range is defined from a minimum voltage to a maximum voltage and a desired frequency range is defined from a minimum frequency to a maximum frequency. The method includes comparing the VCO frequency to the minimum frequency and the maximum frequency, and comparing VCTRL to the minimum voltage and the maximum voltage, and selectively adjusting a gain of the VCO under specific VCTRL voltage and VCO frequency conditions as determined by the comparison steps. 
     In another aspect of the invention, a method of adjusting noise sensitivity in a phase locked loop includes providing a phase locked loop having a voltage controlled oscillator (VCO) controlled by a control voltage VCTRL and having a VCO frequency. The method includes decreasing a gain of the VCO when the VCTRL is less than a defined voltage minimum and when the VCO frequency is not less than a defined frequency minimum. The method also includes increasing a gain of the VCO when the VCTRL is greater than a defined voltage maximum and when the VCO frequency is not greater than a defined frequency maximum. 
     In another aspect of the invention, a phase locked loop circuit is disclosed and includes a voltage controlled oscillator for generating a VCO frequency controlled by a control voltage VCTRL. The circuit includes a frequency comparator to compare a reference frequency with the VCO frequency and output a frequency comparison result signal. A phase and frequency detector is disclosed with a charge pump connected to the PFD and outputting the control voltage VCTRL. A filter is included to filter a target frequency range from the control voltage VCTRL, and a voltage comparator is connected to the charge pump for comparing the control voltage to a voltage maximum and a voltage minimum. The voltage comparator outputs a voltage comparison result signal. Control logic connected to the voltage comparator and the frequency comparator selectively adjusts VCO gain based on the voltage comparison result signal and the frequency comparison result signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate the best mode presently contemplated for carrying out the invention. 
     In the drawings: 
     FIG. 1 is a circuit diagram of a phased lock loop circuit in accordance with the present invention. 
     FIG. 2 is a block diagram illustrating the frequency and control voltage checks in accordance with one method of the present invention. 
     FIG. 3 is a block diagram illustrating a system in which the present invention may be used. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a preferred embodiment of the invention is exemplified and a circuit identified generally by the reference numeral  10 . Circuit  10  is referred to as a phase locked loop (PLL), which is used to synchronize an external clock signal to an internal clock signal for an integrated circuit. PLL  10  is also contemplated as being used in, for example, a multi-rate high speed transmission buffer. PLL  10  includes a voltage controlled oscillator (VCO)  12 . The VCO  12  produces an AC output having a frequency that is proportional to an input control voltage (or VCTRL) on line  14 . Frequency output by VCO  12  is output through line  16 . The signal is fed back through loop  18 , and this signal has a VCO frequency. At node  20 , loop  18  breaks into two branches, with branch  22  being used as an input to a phase and frequency detector (PFD) and a charge pump connected to the PFD (represented together by 24). Node  20  also breaks into branch  26  which is used as an input to a frequency comparator  28 . Frequency comparator  28  compares signal  26 , which has the VCO frequency, with a reference frequency  30 . Frequency comparator  28  will determine whether the VCO frequency is larger than a frequency maximum, or lower than a frequency minimum, with respect to reference frequency  30 . Frequency comparator  28  then outputs a signal  32 , or frequency comparison result signal, to control logic  34 . Reference frequency  30  is also used as an input  36  to PFD and charge pump block  24 . The PFD takes the return signal  22  from loop  18  and compares it to reference frequency input  36 . The reference frequency is windowed with a minimum frequency and a maximum frequency, to guarantee that if VCO frequency is equal to the reference frequency, then the output will not be too high or too low, respectively. In general, it is advantageous to have a smaller window, with the present invention having a preferred window of plus or minus 1% of the reference frequency. The PFD outputs a signal corresponding to the frequency deviation and phase deviation of signal  22  with respect to the input signal  36 . The output  38  of the PFD is a charge proportional to the phase difference. The output  38  is integrated and filtered by  42 ,  44  and  46  to produce VCTRL. Therefore, filter  40  is employed to eliminate, for example, a high frequency component, and therefore may be configured as a low-pass filter which may be varied by the values of capacitor components  42  and  44  as well as resistor  46  in a typical RC layout. Other filter arrangements and configurations are contemplated to obtain the desired filtering effect upon VCTRL. The resultant signal is sampled  47  by a comparator  48 , preferably a window comparator. Once the comparator  48  is able to sample the VCTRL, it can determine whether VCTRL is outside a pre-defined window defined by a voltage minimum to a voltage maximum. Comparator  48  inputs signals  50  and  52  (which may be referred to as voltage comparison result signals) into control logic  34 , where it is determined whether VCTRL is greater than a maximum voltage or less than a minimum voltage. Control logic  34  then takes the results of frequency comparator  28  and voltage comparator  48  and makes a determination as to any adjustments that must be made in VCO  12  based on the voltage level of VCTRL as well as its frequency level. Control logic  34 , through control line  36 , makes any adjustments to VCO that are necessary. The present invention contemplates that control logic  34  would adjust the gain of VCO under particular circumstances. These circumstances include specific voltage and frequency conditions of the VCTRL and the VCO frequency. The increase or decrease in VCO gain corresponds to an increase or decrease in VCTRL sensitivity to signal noise. The now-adjusted VCO is again output through VCO frequency output  16 . This process will continue until VCO output signal  16  settles, which may occur after many clock cycles, and when the signal does settle, it is output  54  to be used to synchronize the external clock with an internal clock of an integrated circuit. It is contemplated that any suitable control logic  34  may be used in order to receive the results of the frequency comparison as well as the voltage comparison, and implement if needed, any VCO adjustments that are required. It is contemplated that VCO in a preferred embodiment, is trimmed via a digital code. The net effect is that the sensitivity of the VCO block is adjusted outside the normal PLL loop to guarantee that the control voltage stays within the pre-defined range. The frequency comparison is done to make sure that the control voltage is not outside the desired frequency window. When the gain of the VCO block is reduced, the sensitivity of the VCO to noise, and consequently signal jitter, is also reduced. 
     Referring now to FIG. 2, the flowchart illustrating the method of the present invention is shown. It is to be understood that this is representative of one cycle, and consequently many more cycles may be implemented prior to settling of the circuit. The method of the present invention illustrates having both voltage range checks as well as frequency range checks. In starting  56  it is determined  58  whether VCTRL is greater than a voltage maximum. If yes  60 , it is also determined  62  whether the frequency of VCO is greater than a frequency maximum. If not  64 , the sensitivity of the VCO is increased  66 . This may be accomplished by, for example, increasing the gain of the VCO block. Following this adjustment, the cycle may be repeated  68 . If the frequency is greater than a frequency maximum  70 , no adjustments are made. The problem targeted by the present invention is for a wide bandwidth PLL where the sensitivity to noise of VCTRL occurs when the frequencies are not out of range when the control voltage VCTRL is. At this point the process again may be repeated  68 . If during determination  58  it is determined that VCTRL is not greater than a maximum voltage  72  it is also determined  74  whether VCTRL is less than a voltage minimum. In other words, is VCTRL outside the desired voltage range. If not  76 , no adjustments are made and the process may be repeated  68 . If, however, VCTRL is less than a voltage minimum  78  a further check is made to determine  80  whether the frequency output from VCO is less than a frequency minimum. If not  82 , it is an indication that even though VCTRL is outside the desired voltage range, the frequency is still above a frequency minimum and therefore it is necessary to decrease VCO sensitivity to signal noise  84 . Again, this may be accomplished by decreasing the gain of the VCO block. If, however, it is determined that the frequency is less than a frequency minimum  82 , a condition exists under which both control voltage and the frequency are less than desired minimums, and therefore the process is repeated  68  since the frequency follows the control voltage and both are outside a minimum range. The frequency and voltage comparison system recognizes that, if the control voltage is not within a given range, and the control voltage is above a voltage maximum, then the frequency should also be above the frequency maximum as well. Correspondingly, if the control voltage is not within the voltage range, and in fact is less than a control voltage minimum, the frequency should also be below a frequency minimum as well. It is when the frequency on the low end of the range, is not below its minimum at the same time that VCTRL is below its minimum voltage. Additionally, if VCTRL is greater than its voltage maximum, it is also required that the frequency also be above its frequency maximum. Adjustments will only be made when the frequency does not follow the voltage in this pattern. The selective adjusting of VCO noise sensitivity under the specific VCTRL voltage and VCO frequency conditions are determined by the comparing steps. Again, selectively adjusting or changing the VCO noise sensitivity includes decreasing a gain of the VCO when VCTRL is greater than the maximum voltage and when the VCO frequency is less than the minimum frequency, and may include increasing a gain of the VCO when the VCTRL is greater than the maximum voltage and when the VCO frequency is not greater than the maximum frequency. 
     FIG. 3 is a block diagram of a computer system  100  in which the present invention may be used. Other applications are possible and contemplated, such as in a multi-rate high speed transmission buffer. The computer system  100  utilizes a memory controller  102  in communication with SDRAMs  104  through a bus  105 . The memory controller  102  is also in communication with a processor  106  through a bus  107 . The processor  106  can perform a plurality of functions based on information and data stored in the SDRAMs  104 . one or more input devices  108 , such as a keypad or a mouse, are connected to the processor  106  to allow an operator to manually input data, instructions, etc. One or more output devices  110  are provided to display or otherwise output data generated by the processor  106 . Examples of output devices include printers and video display units. One or more data storage devices  112  may be coupled to the processor  106  to store data on, or retrieve information from, external storage media. Examples of storage devices  112  and storage media include drives that accept hard and floppy disks, tape cassettes, and CD read only memories. 
     While the present invention has been described in conjunction with preferred embodiments thereof, many modifications and variations will be apparent to those of ordinary skill in the art. For example, although the present invention is disclosed as being implemented with specific logic components, the present invention is contemplated to be used with any implementable logic devices and in other arrangements, that will perform the functionalities heretofore described. The foregoing description and the following claims are intended to cover all such modifications and variations.