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1、翻译（中）一种新型通用全数字时钟匀滑技术秦晓懿,王瀚晟,曾烈光(清华大学电子工程系微波与数字通信技术国家重点实验室.北京100084)摘要: 针对锁相环在匀滑含有低频大幅度的抖动和漂移的时钟时有诸如同步和捕捉范围很窄容易失锁等缺点,提出了一种新的全数字时钟匀滑技术统计预测法.其主要思想是用一个周期的统计结果来预测下一个周期的处理值从该方法的原理和性能分析可知,其可有效的匀滑各种抖动和漂移.抖动积累较小,同步和捕捉范围很宽,可以匀滑不同频率的时钟.另外,改进的变周期的统计预测法也可使捕捉时间降到合适的长度,以适用于不同场合.关键词: 时钟匀滑技术;抖动;漂移;统计预测法中图分类号: TN913.
2、24, TN914.3 文献标识码: A 文章编号: 0372-2112(2001)09-1276-041.引言 传统的时钟匀滑技术一般采用锁相环,但要滤出频率较低幅度较大的抖动甚至漂移,锁相环的带宽需要很窄(如要求小于十分之几Hz甚至更小).此时模拟锁相环的元件尺寸将变得过大而难以实现.同时,窄带宽的锁相环其同步范围和捕捉范围常常很小.在初始情况或非正常情况下容易由于较大的瞬时频差或相差而失锁锁相环能匀滑的时钟频率仅在中心频率附近的范周内.同一设计难于匀滑不同速率.另外,锁相环是用输入输出时钟的相位差来对VCO频率进行调整,当输入时钟抖动较大,由于输出时钟受到输入时钟的影响,两者的相位差并不
3、能准确的反映输人时钟的情况,输入时钟的抖动容易传递到输出时钟上,从而增加抖动的积累.本文则提出了一种新的通用时钟匀滑技术统计预测法.该方法可有效匀滑各种抖动或漂移,抖动积累较小,同步范围和捕捉范围很宽,可以对不同频率进行匀滑,并可全数字实现,便于集成.2.基本原理围1为统计预测法的原理框图.图中写时钟是待匀滑的时钟,读时钟为匀滑后的时钟.统汁周期控制可与写时钟或高速时钟相关.周期统计计数通过对一个周期内写时钟或其信息与高速时钟的关系的统计,预测出下一个周期应扣除的高速时钟脉冲个数(对采用扣除值的电路而言,一般为电路实现方便,高速时钟选得比读时钟的r倍高,以避免有增加高速时钟脉冲的操作)或应生成
4、的时钟脉冲个数.周期统计计数在具体实现时可有多种方案,例如:(1)由高速时钟产生与写时钟标称频率相同的固定参考时钟,周期统计计数模块统计出一个周期内固定参考时钟与写时钟的个数差异来预测时钟扣除值或生成值;(2)直接对一个周期内的写时钟个数进行统计,则下一个周期的预测生成值为写时钟个数统计值的r倍.当统计周期足够大时,可以认为相邻周期内写时钟的情况近似相等,因此可将前一周期的统计值作为后一周期的预测值.时钟综合模块则随周期统计计数输出的扣除值或生成值进行对应的扣除操作或生成操作(如图2所示).图中时钟扣除的功能为扣除脉冲上每有一个脉冲,则扣除高速时钟的一个脉冲.为得到高质量(低抖动和漂移)的时钟
5、.通常有两种策略:(1)使扣除脉冲或生成脉冲的位置尽量均匀分布,则其最小抖动可为仅由一个高速时钟周期的空缺带来的抖动变化若高速时钟约为生成脉冲的m倍,则生成脉冲的抖动约为1/m UI,当m足够大时,生成脉冲可直接作为匀滑后的读时钟输出(即r=1).(2)在扣除操作和生成操作后增加r分频模块,则可将读时钟的抖动减小为分频前的l/r.在缓冲存储器中,通过比较读时钟与写时钟的相位差来给出相应的溢出或取空指示,以提前或延后读时钟的相位例如对扣除值而言,溢出指示将控制时钟综合模块,瞬时地减少扣除一个高速时钟脉冲,使读时钟的相位前移1/r UI;反之,取空指示则瞬时地增加扣除一个高速时钟脉冲.该过程是为了
6、调整缓冲存储器中写数据与读数据之间的初始相对位置,因此只有在初始状态或非正常状态下,才可能送出溢出或取空指示,其它状态下并不发生这个过程.故其对抖动和漂移的分析不会产生影响.若不需得到与匀滑后时钟同步的数据时.图l中虚线框内的部分可以略去.当周期统计计数模块采用第一种方案时,统计预测法与锁相环结构很相似,但其本质差别在于:锁相环是根据读写时钟的相位差异来改变读时钟,用读写时钟的相位差作为控制只能使读时钟能跟踪写时钟的变化,若写时钟有短时的较大幅度的跃变,读时钟的相位抖动也容易随之增大,因此并小能很好的抑制抖动和漂移.而统计预测法则根据写时钟与固定参考时钟的频率差异来产生读时钟,其更能如实的反映
7、写时钟的变化,因此抖动积累更小一些.另外,当要匀滑的时钟频率较高时,高速时钟的频率可能会过高,以致在集成电路中难以实现.此时,可将对高速时钟的处理改为对几个频率相同相位不同的较低频时钟的处理例如，设高速时钟频率为,则改为对频率为/r,相位依次落后l/r UI的r个时钟进行处理.通过对r个时钟的切换，实现对相位的滞后与超前控制,相当于对高速时钟的扣除与增加操作.当将切换为(0ir-1)时,相当于相位滞后l/r UI;当将切换为时,相当于相位超前l/r UI;读时钟的抖动仍约为l/r UI.3.结语统计预测法作为一种通用的时钟匀滑技术，可以有效的滤出低频、大幅度的抖动或漂移、且其具有锁相环电路没有
8、的一些优点：抖动积累更小；在获得好的性能的同时，同步和捕捉范围仍然很宽，可对不同频率的时钟进行匀滑.另外，变周期的统计预测法也可使捕捉时间降到合适的长度，以应用于不同场合.附录4 翻译（英）A New Universal All-Digital Clock Smoothness TechniqueQIN Xiao-yi, WANG Han-sheng, ZENG Lie-guang(State Key Laboratory on Microwave &Digital Communication, Department of Electronic Engineering, Tsinghua Un
9、iversity, Beijing 100084, China)Abstract: As pulls have some disadvantages such as the narrow synchronizing and pull-in ranges to smooth clocks with low-frequency large-amplitude jitter and wander, this paper presents a novel universal digital clock smoothness techniquethe counting prognostication m
10、ethod. The key element of this technique is to use counting results in a cycle to prognosticate numbers in the next cycle. Principles and performance analyses are given to show that it can efficiently smooth jitter and wander, jitter accumulation is small, and the synchronizing and the pull-in range
11、 is very wide to smooth different frequencies. In addition, for applications in some special circumstances, the cycle varied counting prognostication method is also presented to diminish the capture time.Key words: clock smoothness techniques; jitter; wander; counting prognosticationDocument code: A
12、 Article ID: 0372-2112 (2001) 09-1276-04 1. Introduction Smoothing by traditional clock PLL is typically used, but filter out low frequency, large amplitude jitter or drift, take a very narrow bandwidth phase-locked loop (if requested less than a few tenths of Hz or less). At this time analog PLL co
13、mponents size will become too large and difficult to achieve. Meanwhile, the narrow bandwidth of the PLL capture range and scope of its synchronization is often very small. In normal circumstances, the initial conditions or easily as the larger instantaneous frequency difference or the difference be
14、tween the loss of lock PLL clock frequency can be smoothed only in the center frequency in the vicinity of Van weeks. The same design with different rate smoothing difficult. In addition, the PLL is input, the output clock phase to adjust the frequency of the VCO, when the input clock jitter is larg
15、e and the output by the input clock of the clock, the phase difference between the two does not accurately reflect the situation of the input clock, input clock jitter can easily be transmitted to the output clock to increase the accumulation of jitter. This is a novel universal clock smoothing tech
16、niques - statistical prediction method. This method can be effectively smoothed, all kinds of jitter or drift, jitter accumulation of small, simultaneous range and capture range is very wide, can smoothing of different frequencies, and can all-digital implementation, easy integration. 2. Basic princ
17、iples Circuit 1 is a block diagram of statistical prediction. The figure was to be smoothing the clock is the clock, read clock to the clock after smoothing. Statistically cycle control may be related write clock or high-speed clock cycle by cycle counting statistics clock or to write a letter.The r
18、elationship between income and high-speed clock statistics, predict the next cycle should be deducted from the number of high-speed clock pulse (on the use of net value of the circuit, the circuit is generally convenient, high-speed clock selection read clock r times than high in order to avoid an i
19、ncrease in operating high-speed clock pulse) or the number of clock pulses to be generated. cycle counting statistics can be realized in a variety of specific programs, such as: (1) written by a high-speed clock generation and clock frequency of the same nominal fixed reference clock cycle count sta
20、tistics Statistic module within a period fixed reference clock and write clock of the clock net to predict the number of differences in value or generate value; (2) directly to a write clock cycle the number of statistical, the next generate value for the forecast period to write the clock value of
21、r times the number of statistics. When the survey cycle is large enough, you can write that next to the clock cycle is approximately equal to the situation, it could be the statistical value of the previous cycle as a cycle after predictive value. Clock module is integrated with the cycle count stat
22、istics generated output of less value or less value for the corresponding operation or build operation (Figure 2). The figure for the net function of the clock pulse deducted every one pulse, then less a high-speed clock pulse to be high quality (low jitter and drift) of the clock. There are usually
23、 two strategies: (1) to deduct the location of pulse or pulses generated as uniformly distributed, then the minimum jitter for only by a high-speed clock cycle jitter caused by changes in vacancies, if high-speed clock pulses generated approximately m times, the generated pulse jitter is about 1 / m
24、 UI, when m is large enough, the generated pulse can be directly read as the clock after smoothing output (r = 1). (2), net of operating and generating operation frequency after the increase in r module can be read clock frequency jitter is reduced to pre-l / r. In the buffer memory, by comparing th
25、e read clock and write clock phase to give the corresponding overflow or take air directions to advance or delay the phase of clock time the value of such deduction, the overflow direct control of the clock synthesis module reduce the deduction of a transient high-speed clock pulse, the phase of the
26、 clock forward to reading 1 / r UI; the other hand, means to take an empty net of currency is a transient increase in high-speed clock pulse. Adjustment of the process is to write data buffer memory and read data in position between the initial orange, only in the initial state or normal state, it m
27、ay send overflow or to take an empty instruction does not occur under other conditions this process. Therefore, its analysis of the jitter and drift does not produce impact. If received and without smoothing the data after the clock synchronization, figure l in part within the dashed box can be omit
28、ted. When the cycle count statistics with the first program module, the statistical prediction method and the phase-locked loop structure is similar, but the essential difference is: read and write clock PLL is based on the phase difference to change the time clock, the clock used to read and write
29、phase as the control can only read the clock to track the write clock changes, if the clock for short time to write more substantial jump, read the clock phase jitter is also easy to increase with and, therefore, inhibition of small to very good jitter and drift and statistical prediction rule to wr
30、ite the clock and according to the frequency difference between the fixed reference clock to generate the read clock, the clock more accurately reflect the changes in writing, so a smaller number of accumulated jitter. In addition, when the smoothing higher clock frequency, high-speed clock frequenc
31、y may be too high, resulting in the integrated circuit is difficult to achieve. At this point, the treatment can change the clock speed will be on the same number of frequency, phase difference The low-frequency clock processing, for example, setting high-speed clock frequency, the frequency was cha
32、nged to r, followed by backward phase l/r UI r-clock processing. r clock by switching to realize the phase lag and advanced control, which is equivalent to the net and to increase high-speed clock operation. When the switch to (0ir-1), the equivalent phase lag l/r UI; When the switch too, is equival
33、ent to phase advance l/r UI time clock jitter is still about l/r UI. 3. Conclusion Statistical prediction method as a universal clock smoothing technique can effectively filter out low frequency, significant jitter or drift, and it has a phase-locked loop circuit without some of the advantages: a smaller jitter accumulation; to get good performance at the same time, synchronization, and capture still very wide range, can be of different frequency clock smoothing. In addition, variable cycle, statistical prediction can also catch down the right length of time, to apply to different occasions.