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Old 10-29-2005, 06:16 PM
Raymund Hofmann
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Default Spartan3 DFS & DLL Behaviour

Using the DFS to multiply a clock leaves some questions for me:

The data specifies cycle-to-cycle jitter & period jitter at the input.

A period jitter of +-1 ns seems not plausible to me.
From what ideal clock edge is the +-1ns specified ?
From the clock specified in the clocking wizard ?
Is the clock frequency specified there getting into the device
configuration ?
For a 50ns period input +1ns is +2%.
This isn't plausible to me as timing of the device changes more than 2%
with temperature and supply voltage.

The question is rather:

How would the DFS track a changing input frequency (and device
temperature variations and device supply voltage variations of course)?
Is there a limit on the speed of this change, before the DFS "unlocks" ?
+-1ns per input clock period ?

For Example a input clock

25ns period, switching to 24ns period in one cycle

A "traditional" PLL with Phase comparator, loop filter & vco has a loop
bandwidth, determining the speed of tracking.
So it's behaviour would be some kind of step response of the output

How would the DFS behave ?
Can the DFS be used to multiply a clock coming from a variable source ?
What's the tracking speed (loop bandwidth)?

Raymund Hofmann

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