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What exactly is the clock jitter?

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Anil Rana

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highfrequencyelectronics.com jitter part 2

hi
i am new to digital design.can anybody explain me what exactly is clock jitter?
 

highfrequencyelectronics.com jitter part 2

difference between real clock and ideal.
 

    A

    Anil Rana

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clock jitter?

I think
The actual clock is not an ideal clock, it's period is not the same all the time.clock jitter represent the uncertainty of the actual clock frequency.
 

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    Anil Rana

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clock jitter?

jitter is the basically the non-ideal behavior of clock signal. Due to some noise or voltage fluctuations the clock edge may not align edge after edge. This random behavior is called jitter. This should be added to the worst case delay calculations.
Best Regards,
 

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    Anil Rana

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Re: clock jitter?

Hi
Because of certain noisy conditions, the clock time period may get increased/decreased from cycle to cycle.
and Jitter represents the variations in the clock width.
 
Re: clock jitter?

well clock jitter is basically a temporal variation in the clock pulse arrival time.. clock skew and jitter are two very important factors which determine the arrival of the global clock pulse with respect to the locally clocked domains!

u can never eliminate the skew. but u can only reduce the clock skew/jitter in a circuit!

with regards,
 

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    Anil Rana

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clock jitter?

while considering PLL jitter for setting uncertainty, consider only the cycle-to-cycle jitter only not the period jitter
 

Re: clock jitter?

I want to add something to arunragavan's answer which i hope can further clarify Skew & Jitter:
Skew: Spatial variation in clock arrival times. It is the variation, when "same" clock edge is seen by two "different" FFs.
Jitter: Temporal variation in clock arrival times. It is the variation, when two "successive" clock edges are seen by the "same" FF.

Regards,
 

Re: clock jitter?

For example, assume that the clk signal has a freq of 1 MHZ,
In ideal environment each clock edge from outside the world would arrive exactly at the time required one millionth of a secnd after its predecessor,

In the real world, however clk edges may arrive early or late a bit -- this would be the jitter.

The FPGA clock manager can be used to detect and correct this kind of jitter and to provide clean daughter clock signals from the device.

regards
raghu
 

Re: clock jitter?

Jitter is “the short-term variations of a signal with respect to its ideal position in time.”
Clock jitter is the variation in timing of a critical instant in a periodic waveform with respect to a jitter free reference.
Might be below link will be useful -
**broken link removed**
 

Re: clock jitter?

clock jitter is clock edge's random variation relate to ideal position,

and usually is cause by power supply noise and crosstalk from neighbour

signal lines.

best regards




Anil Rana said:
hi
i am new to digital design.can anybody explain me what exactly is clock jitter?
Click to expand...
 

Re: clock jitter?

And usually what causes jitter in silicon are:

1- turn on and turn off time of transistors are not perfect due to process/voltage/temprature variations. All these variation will cause the same transistor to turn on or off differently each time and that will cause jitter.

2- Noise in general is a main contributor to jitter. noise due to power line varations, transistor switching activities across the die which causes cross-talk and cross talk induces unwanted noise into transistor which in turn effects their normal operation and this translates to clock-to-data jitter.
 

Re: clock jitter?

I see some understood jitter whereas some confused jitter with skew. Haha!

Jitter - A random or temporal variation of clock phase, not arrival time. Main causes of jitter are (1) crosstalks, (2)EMI from largely from power transience and RF signals, (2) internally from common-mode and differential-mode variation. Due to the random characteristic nature of the jitter sources, jitter is random or temporal. PLL can only minimise jitter, at the expense of some drawbacks (which I am not going to tell more).

PhD (Imperial College)
 

clock jitter?

it is basically the variation in the period of clock, sow while calculating the max operating case, take the worst case.
 

Re: clock jitter?

There are three types of clock jitter: first one is :long-term jitter, the 2nd is : period jitter, the 3rd one is : cycle-to-cycle jitter. all of these three type of jitter are presentation of clcok phase noise. Just like "Distance","Speed",and"Velocity"!
 

Re: clock jitter?

what's the effect caused by clock jitter and clock skew??is there any difference?

best regards!
 

clock jitter?

for example. clock to data skew, is the arrival time difference, between the clock signal, and the data signal, from the uP to the memory. you should to control this, in layout design or floorplanning. it shouldn't be too much. there are thresholds.
maybe the memory gets the clock rising edge, and the it thinks, that it can sample the data lines. but the data lines aren't reade in that time moment, because of the skew.

normal clock skew, as i know, is the arrival difference of the same clock signal, to the different memory modules. in synchron systems (sdram) you should to hold this difference between thresholds.

pin skew: some processors doesn't like that they get a far-non-50% clk signal. maybe because of internal pipelining, or multiphase processing.

see memory/processor datasheets, memory bus specifications...
agilent has wery helpful papers about that. and the fairchild's backplane design guide is also helpful.

you should do timing analyses, to inspect them, and should control them in design stage.

these things are depending on noise, pcb line length, earlier databits(isi), unbalanced load of clock lines ...

maybe i wrote incorrect things?
 

Re: clock jitter?

Because of noisy effect, the actual clock is different from the ideal clock .
 

Re: clock jitter?

These are some good articles for understanding jitter.
 

Re: clock jitter?

Jitter—Understanding it, Measuring It, Eliminating It
Part 1: Jitter Fundamentals
https://www.highfrequencyelectronics.com/Archives/Apr04/HFE0404_Hancock.pdf

Jitter—Understanding it, Measuring It, Eliminating It
Part 2: Jitter Measurements
https://www.highfrequencyelectronics.com/Archives/May04/HFE0504_Hancock2.pdf

Jitter—Understanding it, Measuring It, Eliminating It;
Part 3: Causes of Jitter
https://www.highfrequencyelectronics.com/Archives/Jun04/HFE0604_Hancock3.pdf

Understanding Jitter
**broken link removed**
 

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