An Introduction to Jitter Analysisbased on a training course .ppt
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1、1,An Introduction to Jitter Analysis based on a training course given by Wavecrest,2,Traditional View Of Jitter,3,What is Jitter?,Jitter - “The deviation from the ideal timing of an event. The reference event is the differential zero crossing for electrical signals and the nominal receiver threshold
2、 power level for optical systems. Jitter is composed of both deterministic and Gaussian (random) content.” T11.2 / Project 1230/ Rev 10 Fibre Channel - Methodologies for Jitter Specification page 7.,Period of clock,4,What is Jitter Composed of?,Deterministic Jitter (DJ) “Jitter with non-Gaussian pro
3、bability density function. Deterministic jitter is always bounded in amplitude and has specific causes. Four kinds of deterministic jitter are identified: duty cycle distortion, data dependent, sinusoidal, and uncorrelated (to the data) bounded. DJ is characterized by its bounded, peak-to-peak value
4、.” T11.2 / Project 1230/ Rev 10 Fibre Channel - Methodologies for Jitter Specification page 8.DJ will never grow in amplitude regardless of the number of data points such that a sufficient number of data points were taken to complete at least one complete cycle of each periodic element. Clock signal
5、s are typically susceptible to Duty Cycle Distortion (DCD) and Periodic Jitter. Deterministic Jitter is typically caused by cross talk, EMI, Simultaneous Switching Outputs (SSO), device function dependency (pattern dependant jitter) and other regularly occurring interference signals. Data signals ar
6、e also susceptible to DCD and PJ as well as Inter-Symbol Interference(ISI) and Data Dependant Jitter (DDJ),5,Where does DJ Come From?,Deterministic Jitter from Cross talk Victim line is affected by magnetic field from Driver line. Incremental inductance of victim conductor converts induced magnetic
7、field into induced current. Induced Current adds (positively or negatively) to Victim Line current increasing or decreasing potential and thus causing jitter on Victim Line,Driver Line,Current,Victim Line,Current,Mag Field,Mag Field,Mag Field,Mag Field,6,Switching Power Supply,Where does DJ Come Fro
8、m?,EMI Radiation (Amperes Law) Victim line is affected by magnetic field from EMI Source. This could be a power supply, AC Power line, RF signal source, etc. As in cross talk induced jitter, the magnetic field induces a current that is added (positively and negatively) to the victim line current the
9、reby effecting the timing of the signal on the victim line.,Victim Line,Current,Mag Field,Mag Field,Mag Field,7,Where does DJ Come From?,Noisy reference plane Noise in power planes can result in reference shifts in threshold voltages for downstream logic gates. Resultant timing shift is proportional
10、 to slew rate of input signal. The output transistor will switch when VT is exceeded at the gate. A change in ground reference at VT will result in a shift in the required voltage to switch the gate thereby delaying or advancing the switch VCO in PLL is sensitive to GND level variance.,Vout,Edge Tra
11、nsition on the input to the transistor,DT,DV,8,Where does DJ Come From?,Simultaneous Switching Outputs If all output pins switch to same state, spike currents will be induced on VCC and on GND. Spike Currents on Reference Plane can cause Threshold Voltage sense point to shift Due to the pattern sens
12、itivity and the bounded max. amplitude of edge jitter due to SSO, this is considered deterministic jitter.,9,What else is Jitter Composed of?,Random (Gaussian) Jitter (RJ) - Like all physical phenomena, some level of randomness to edge deviation occurs in all electronic signals. This component is pr
13、obabilistic in nature and is best modeled by a Gaussian function. Random Jitter is unbounded and therefore directly effects long term reliability. Where does Random Jitter Come from? Thermal vibrations of semiconductor crystal structure causes mobility to vary depending instantaneous temperature of
14、material Material boundaries have less than perfect valence electron mapping. Imperfections due to semi-regular doping density through semiconductor substrate, well and transistor elements, Imperfections due to process anomalies Thermal effects of conductor material. Thermal vibration of conductor a
15、toms effect electron mobility And many more minor contributors such as: cosmic radiation, etc.,10,What else is Jitter Composed of?,Random (Gaussian) Jitter (RJ) - Before we can talk about measuring Jitter, it is important to understand Gaussian Distributions as it relates to probability. Intro to Ga
16、ussian Distributions,Mean,Standard deviation (1s) is defined as the window which contains 68.26% of all measurements to one side of the mean. 2s contains 95.4% of all measurements. 3s contains 99.73% of all measurements. 4s contains 99.99366% of all measurements. 6s contains (100-1.973x10-7)% of all
17、 measurements. 7s contains (100-1x10-12)% of all measurements. 8s contains (100-1.244x10-13)% of all measurements. 10s contains (100-1.973x10-21)% of all measurements.,Standard Deviation (1s),Peak-Peak,Note: Peak-Peak is dependant on the sample size. Larger samples of the same distribution will most
18、 likely yield a larger peak-peak measurement. Thus, peak-peak must be discussed in context of the number of samples.,11,More Gaussian Statistics,Gaussian Statistics It is important to note that in pure Gaussian mathematics, all possible measurements are assumed to be possible. However, for all pract
19、ical purposes, the Gaussian model holds true in electronics for measurement populations not exceeding 1021 . This is equivalent to 20s (single sided). So, go ahead and use these Gaussian assumptions up to a reliability of about 20s. After that, all bets are off as to the predictability of the measur
20、ement. 20s reliability implies compliant operation for at least 321,502.06 years for a 100MHz clock,12,Why Standard Deviation?,Standard Deviation is used to predict the occurrence of outlying measurements from the mean. In electronics, it is important to know the frequency of occurrence of edges tha
21、t are too early. For example, if your system cannot tolerate clock periods that are less than 9.5ns on a 100MHz clock, you would like to know what the probability of a 9.5ns period is. Knowledge of the short period tail can tell you exactly how often a 9.5ns period occurs.The Catch This use of Stand
22、ard Deviation (1s) is only valid in pure Gaussian distributions. If any deterministic components exist in the distribution, the use of 1s based on the entire jitter histogram for the estimation of probability of occurrence is invalid.,A measurement 2s away from the mean will have a 95.4% chance of o
23、ccurring. Thus, once every 250 periods, the period is less than (mean - 2*1s). If we use the numbers from the previous slide, once every 250 periods the period is less than 9.97ns.,-1s,-2s,MEAN,13,Calculating Standard Deviation,X =,1 n,Xi,where,(10.2-10.43) 2,Std. Dev.,=,X = (10.2+10.8+10.6+10.2+10.
24、3+10.5+10.2+10.3+10.5+10.2+10.8+10.6) = 10.43ns,1 12,( Xi - X ) 2,Standard Deviation,=,+ (10.5-10.43) 2 +,+ (10.8-10.43) 2,+ (10.3-10.43) 2,+ (10.2-10.43) 2,+ (10.6-10.43) 2,(10.2-10.43) 2,+ (10.6-10.43) 2,+ (10.3-10.43) 2,+ (10.8-10.43) 2,+ (10.2-10.43) 2,+ (10.5-10.43) 2,(.23) 2,=,+ (.07) 2 +,+ (.
25、37) 2,+ (.13) 2,+ (.23) 2,+ (.17) 2,(.23) 2,+ (.17) 2,+ (.13) 2,+ (.37) 2,+ (.23) 2,+ (.07) 2,= .231ns,+(,),+,14,“Real World” Distributions,In most cases, time measurement distributions are not entirely Gaussian. Typically, some Deterministic/Systematic offset occurs to “mess-up” the distribution to
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