# Nature of Clock Phase Jitter in DDR/QDR™ Sync SRAM – KBA89153

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Question: What type of jitter is specified by tKC Var (clock phase jitter) in DDR/QDR™ Sync SRAM?

The QDR Consortium officially specifies input-clock phase jitter as cycle-to-cycle jitter, which measures the change in clock period measurement between any two adjacent clock cycles.

Mathematically, the cycle-to-cycle jitter can be represented as follows:

Tcycle (n) – Tcycle (n+1)
where Tcycle (n) and Tcycle (n+1) are any two adjacent cycles measured on controlled edges.

Consider the following figure, which shows the various cycles of the input clock K:

Suppose the QDR/DDR Sync SRAM is operated at 250 MHz. Then the K clock period ideally should be 4 ns.

•   For zero jitter, Tcycle (n) = Tcycle (n+1) = 4 ns.
•   If Tcycle (n) = 4 ns and Tcycle (n+1) = 3.9 ns, then the jitter would be (4 ns-3.9 ns) = 0.1 ns. This calculated jitter should be less than or equal to the tKC Var parameter specified in the datasheet for proper operation.