Additive and Absolute Jitter

Question What is the difference between additive and absolute jitter? Answer The jitter which exists on your final clock signal is a summed total from the various components in the clock signal generation chain. The actual calculation ends up being an RSS (root summed squares) function. When the term additive jitter is used, it excludes external components. We have provided absolute jitter numbers when the AD9525 is paired with specific mass marketed VCOs to give a more complete picture of what performance can be achieved using the AD9525.

Holdover/Switchover Functionality

Question Does the AD9525 support any holdover or switchover functionality? Answer There is no holdover function on the AD9525. The user can choose to switch references, but any phase or frequency difference between the two references will have to settle through the PLL. We did not want to add any unnecessary noise sources, so we kept only the most critical circuit elements in the device definition.

Jitter over Multiple Bandwidths

Question Why do you specify jitter over multiple bandwidths? Answer Different applications care about different phase noise ranges. When driving a data converter, phase noise from very close in (1 to 10 kHz) all the way out to the Nyquist frequency of the converter affect the SNR performance capability of the converter being clocked. In wired applications, the clock signal is typically fed to another (‘downstream’) PLL. Since PLLs have loop filters, they will filter off some portion of the jitter, meaning they only care about specific bandwidths. 12 kHz to 20 MHz is a common band to measure over for those applications.

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