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
26、 make it Non-Gaussian,Gaussian,Non-Gaussian,Tail regions still appear Gaussian,In non-Gaussian distributions, Gaussian assumptions apply to the tails (left most and right most regions) if and only if the equivalent 1s of these tail region can be calculated.,15,Non-Gaussian Distributions,Break up dis
27、tribution into tail and mid sections.,Analyze distribution by looking at tail sections separately. Allows for probabilistic estimations of out lying measurements knowledge of Gaussian component (Random Jitter) on left side of multimodal distribution enables the calculation of the probability of shor
28、t cycle measurements. NOTE: Multimodal distributions are those distributions with more than one “hump”. The non-Gaussian example shown here is referred to as bimodal. Some interesting information can be inferred from the shape of this distribution. Symmetric peaks imply equal probability of either m
29、ean point (left or right).,LEFT,RIGHT,16,Non-Gaussian Distributions,In order to determine the probability of a measurement occurring at the -3sL point, it is critical to determine the standard deviation that would correspond to a Gaussian Distribution with a identical tail region to that of our mult
30、imodal non Gaussian distribution. Note that the matched Gaussians are not necessarily the same. Either tail can exhibit a larger standard deviation,17,Determining Representative Gaussian Distributions,Tail Fit algorithm enables the user to identify a Gaussian curve with a symmetrical tail region to
31、that of the non-Gaussian distribution under evaluation. Various curves are fitted against the distribution until an optimal match is found. Then, the 1s of the matched curve is used as the standard deviation multiplier for that particular tail. This is repeated for both sides of the distribution.,Ke
32、ep adjusting 1s until tails match,18,An Example of Tail Fit Technique,The picture to the left is an example of a distribution with both Random Jitter and Deterministic Jitter. Notice how the Tail Fit curves closely match the tail regions of the distribution.,The 1sL and the 1sR can be used to predic
33、t short cycle and long cycle probability. The plot to the left is a BER plot of period jitter vs spec. This device fails a 550ps jitter spec every 20s of operation.,19,Misc. Combinatorial Logic,How does Clock Jitter Affect Systems?,Typical Synchronous Device is susceptible to short cycle errors. Thi
34、s device would latch the wrong data into the output latch if the period was to short. Therefore, the critical measurement for this device is period jitter (rising edge to next adjacent rising edge). This is a typical problem for most synchronous devices and systems. This is some times referred to as
35、 “Cycle to Cycle”,Data In,Clock In,Data Out,Input Latch,Output Latch,20,Period Stability (Period Jitter),Intel Period Stability Specification: “should be measured on the rising edges of adjacent BLCKs crossing 1.25V at the processor core pin. The jitter present must be accounted for as a component o
36、f BCLK timing skew between devices.”,tPER,CH1,CH2,Measure as tPD for combined skew and clock jitter,21,Example Debug Session,In this example, note the presence of DJ. The next step would be to determine if the DJ is due to cross talk, pattern dependency, or EMI interference. Use Accumulated Time Ana
37、lysis to determine Frequency of PJ If PJ is a multiple frequency of clock, use Function Analysis to determine where, if at all, in the pattern the jitter is a maximum.,tPD,22,What is Accumulated Time Analysis?,The key to Periodic Jitter Detection. Accumulated Time Analysis is a technique that uses a
38、ccumulated jitter to determine the presence of periodic jitter. Using ATA, the user can quickly and simply measure the cumulative amplitude and frequency of modulation for all Periodic Jitter elements riding on the clock. Using a patented normalization technique, the user can also see the worst case
39、 period deviation due to each of the PJ components. What is Cumulative Jitter? Periodic Jitter has the effect of increasing the period and decreasing the period of a clock over time. For example, suppose a clock has a frequency of 100MHz and has a 1MHz Periodic Jitter riding on it which has a peak a
40、mplitude of 2ns. The Periodic Jitter will increase the period for 50 consective periods, then, the PJ will decrease the period for 50 consecutive periods. The worst case cumulative period push out occurs after the 50 increasing periods. The worst case cumulative period contraction occurs after the 5
41、0 decreasing periods. If the user measures the time elapsed for a random sample of 50 periods, the distribution would be bimodal, and, the time distance between the two peaks would be 2ns. Further, the worst case period push out for a single period would be less than 20ps.,23,Period Decreasing,Perio
42、d Increasing,Period Decreasing,Period Increasing,The effect of a Periodic,Period Increasing In this section the period of the modulated clock is increased from the ideal. Notice how much longer the elapsed time for 5 periods. Period Decreasing In this section the period of the modulated clock decrea
43、ses from the ideal. The net effect of the shorter periods results in a canceling out of the increased periods from the prior section. Note that the same number of clocks is completed after both sections.It is important to note that the sampling of the waveform must be random so as not to filter any
44、periodic elements.,Ideal Clock,Real Clock,Period Jitter,Amplitude,24,Accumulated Time AnalysisTM,Distribution of Time Measurements,Measurement Schedule,25,Using Accumulated Time Analysis,The modulation domain plot to the left indicates a strong periodic with a cumulative jitter amplitude of 101.2ps.
45、,The frequency domain plot to the right indicates a strong periodic at about 1MHz. This tool can also be configured to normalize the peak amplitudes of each periodic element to its exact effect on a single period. This is called 1-clock normalization.,26,Pattern Dependant Jitter,In some cases, the j
46、itter on the clock could be correlated to other electronic activity near by. This could be the case in an imbedded PLL application in which some other circuit in the ASIC is causing the internal PLL to jitter tremendously. In the case of a system design, perhaps another part of the circuit board is
47、emitting an excessive amount of EMI that is interfering with the operation of the PLL or is inducing modulation on the traces distributing the clock signal. Debug this using Function Analysis The user can also debug using an oscilloscope and a pattern marker. For sampling oscilloscope, make sure the
48、 stability of the reference does not exceed the DJ being diagnosed For real time sampling oscilloscope, make sure the record length completely captures one execution of the pattern. Wavecrest Function Analysis Tool allows the user to look at each and every period after a pattern marker to evaluate p
49、attern dependant jitter.,27,Using Function Analysis,Pattern Marker Clock,28,Adjacent Cycle Jitter (Rambus Jitter),Adjacent Cycle Jitter Referred to by Rambus as “Cycle-to-Cycle” Since this is inconsistent with established definitions from Tektronix, Intel, Hewlett Packard (Agilent),Wavecrest and man
50、y others, we will simply refer to this jitter phenomena as “Adjacent Cycle Jitter” Adjacent Cycle Jitter is the worst case period deviation from one period to the next adjacent cycle. Measurement must be taken at exactly the same voltage for stop as well as start. Rambus requires that 10,000 adjacent periods be analyzed for compliance.,
