Hello, my name is Taro.
This time I will write about the precision notation of clock signals.
Jitter is a function that measures the precision of a clock signal.
For more information about jitter, please refer to my previous article, " Why should I pay attention to Jitter? " for more information about jitter.
In this article, I will write about the contents that are not introduced in the previous articles.
Frequency Accuracy
A typical example of the precision notation on the datasheet of a product that outputs clock signals, such as an oscillator/oscillator, is PPM.
PPM is an acronym for Parts Per Million and represents one part per million.
For example, if the frequency accuracy of a 25 MHz oscillator is listed as ±20 PPM,
25 MHz x -20PPM = 500 Hz = - 0.5 x 10^-3 MHz
25 MHz x +20PPM = 500 Hz = + 0.5 x 10^-3 MHz
This means that the output is guaranteed to be in the frequency range of 25 - 0.5 x 10^-3 MHz to 25 + 0.5 x 10^-3 MHz.
It is important to note that the PPM value represents the accuracy of the output frequency, not what is stated about the jitter.
It is a frequency domain unit used to express the steady-state accuracy of the clock frequency, so it cannot be said to be a unit for jitter accuracy.
About Jitter Accuracy
Jitter describes the period of a clock signal, not its frequency.
As mentioned in a previous article, jitter is defined in the time domain as the error from the rising edge of the clock signal.
The common unit is ps (picoseconds). This unit is described in Clock ICs in general.
Jitter is expressed in various ways depending on the manufacturer and datasheet.
For example, "Period 16 Avg" indicates the Period jitter averaged over every 16 cycles.
It is used to see the jitter characteristics of a spread spectrum clock signal.
About UI (Unit Interval)
UI is sometimes listed as a unit to express jitter.
UI is an abbreviation for "Unit Interval" and is a unit that indicates the ratio of jitter that exists within one clock cycle of a clock signal. (1 cycle = 1UI)
UI is a unit often used in input jitter specifications for ASICs and other devices.
For example, for 25MHz 0.005UI, one 25MHz cycle is 40 ns, so the jitter component is,
40 ns x 0.005 = 0.2 ns = 200 ps
which is equivalent to a jitter component of 200 ps.
Note that in the case of UI notation, the value changes depending on the frequency.
The notation of jitter is determined by various units and measurement methods.
We will do our best to read and understand the datasheet and provide the best support to our customers!
New Engineer's Blush Blog Article List