Consider you have 100ppm crystal oscillator, 80ps jitter driving a DCM that is configured to output x1.5 the input frequency. What will happen to the jitter? Will it increase or decrease? And by what factor?
All,
The 1.5 is important, but so is the origianl input frequency.
There is a jitter calculator used to calculate the jitter for any M, D and input frequency.
For M=3, D=2, and F=100 MHz, the jitter (worst case noisy system)is 630 ps peak to peak (9.5% of a UI) for V2 Pro DCM.
Thus your timing slack for this clock domain has to be comfortably better than 1/2 of 630 ps (to allow for the shortest possible period).
Aust> I agree to 100ppm is irrelevant. But according to
Symon,
Yes, it is.
In addition, its histogram is gaussian in shape (from the CLKFX) output.
Some folks have been really suprised that the jitter histogram is gaussian in shape, but does not have the same RMS to peak to peak ratio that is commonly assumed from a PLL (14 to 1), and yet it is still random....(power spectral density has no peaks).
One can think of the output of the DCM as being multiple input clock jitter histograms (which are usually very gaussian from crystal oscillators), offset by a tap or two (at random + and -). Thus the peak to peak vs RMS is closer to 4:1 or 6:1.
Aust> Hi Austin,
Peter,
Your answer is correct for the DLL outputs, CLK0, CLK90, CLK180, CLK270, CLK2X, CLK2X_b, and CLKDV.
For example, if they had said "divided by 1.5" you are correct.
But not for CLKFX, and CLKFX_b.
7 out of 9 is not all that bad......Also the 100ppm doesn't have anything at all to do with it.
I also forgot to add the square root of the sum of the squares (sqr ((630^2)+(80^2)) ~ 635 ps ) to get the total jitter out (clock + DCM).
Aust> I stand corrected, very badly so. Sigh!
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