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現在2183件がヒットしています。check
Hello, this is Taro.
This time, I will write about the precision notation of the clock signal.
Jitter is a function that measures the accuracy of the clock signal.
Please also refer to the article I wrote about Jitter earlier, "Why should I care about Jitter?".
This time, I will write about the contents that I did not introduce in the previous article.
About frequency accuracy
PPM is a typical example of the accuracy notation described in the datasheets of products that output clock signals, such as resonators/oscillators.
PPM is an acronym for Parts Per Million and stands for 1 part per million.
For example, if the frequency accuracy of a 25 MHz oscillator is listed as ±20 PPM,
25MHz x -20PPM = 500Hz = - 0.5 x 10^-3MHz
25MHz x +20PPM = 500Hz = +0.5 x 10^-3MHz
From the calculation result, it means that it guarantees output in the frequency range from 25 - 0.5 x 10^-3 MHz to 25 + 0.5 x 10^-3 MHz.
Please note that the PPM value represents the accuracy of the output frequency, not what is described for Jitter.
Since it is a frequency domain unit used to express the steady-state accuracy of the clock frequency, it cannot be said to be a unit that expresses the jitter accuracy.
About Jitter Accuracy
Jitter is described in terms of the period of the clock signal, not its frequency.
As I wrote in my previous article, Jitter defines the error from the rising edge of the clock signal in the time domain.
A common unit is ps (picoseconds). This unit is described in general clock ICs.
Jitter is represented differently by different manufacturers and datasheets.
For example, "Period 16 Avg" indicates the average value for every 16 cycles of Period Jitter.
It is used when looking at the Jitter characteristics of a spread spectrum clock signal.
About UI (Unit Interval)
UI is sometimes described as a unit that represents Jitter.
UI is an abbreviation for Unit Interval, a unit that indicates the ratio of jitter that exists within one period of the clock signal of the clock period. (1 cycle = 1UI)
UI is a unit commonly used in input Jitter specifications such as ASICs.
For example, in the case of 25MHz 0.005UI, one period of 25MHz is 40 ns, so the Jitter component is
40ns x 0.005 = 0.2ns = 200ps
, which agrees that the Jitter component is 200 ps.
Please note that in the case of UI notation, the value changes depending on the frequency.
Jitter's notation is derived from various units and methods of measurement.
I will do my best to read the data sheet and provide the best support to our customers!