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It is the variation in frequency or time such as if the data packets do not follow a tight time bound or a clock frequency changes back and forth but centers around a normal value you say clock is jittery.
Depending on the situation such as in clock domains, jitter can result in malfunction or noise. However, data networks where you tend to experience delays due to jitter will hardly be ever attributed to noise.
Jittes is a (usually small and rapid) fluctuation in a
phenomenon, such as a quantity or wave, because
of noise, mechanical vibration, interfering
signals, or similar internal or external disturbances.
Noise is a random-frequency current or voltage
signal extending over a considerable frequency
spectrum and having no useful purpose, unless
it is intentionally generated for test purposes.
Jitter, when talking abour 'frequency' jitter, can be seen as follow.
Imagine you have a 100MHz clock signal. You then have a period of 10ns (10 nanosecond). Now, if you would have a device that could measure the time between each rising or falling edge of that signal, you would see that it take 10ns from a rising edge to the other raising edge, that is:
10ns, 10ns, 10ns, 10ns, 10ns, 10ns, ...
This would be a perfect (i.e. jitter-free) signal.
Now, imagine that when you measure that signal, you get the following results, for the time between each raising edge of the signal:
It is clear that on a frequency meter, you would still see that the average signal is 100MHz, but that signal also does have what's called 'jitter'. I.e. it is constantly 'wobbling'.
Jitter can be either innoffensive, usefull or undesirable. This depend on the application. In telecom, where you pass a lot of information over a single line, jitter is really not desirable as this can create bit errors if too much jitter is present. In most applications, though, jitter is innoffensive (just pass unnoticed). In modern computers, jitter is intentionally used so that the EMI get reduced (instead of having a big EMI signal, the EMI is soften, spread across a wider bandwidth).
jitter is the variation of the frquency. suppose your clock frequency wolud be 1 GHz. if your frequency increses or decrese, your clock has jitter. the resean of jitter is noise. the most important nois is the power supply noise in mixed mode designs. thermal noise of devices which used in your circuit (transistor, resistor) is the other source of jitter.
Phase noise and EYE diagram is the econventional method to meassure your jitter .
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