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Altera_Forum
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15 years ago

ALTPLL constant output

Hi, I'm using a ATLPLL IP In my design. The PLL has one input clock, one areset, one Lock output and one output clock. The input clock is 50MHz . The PLL outputs should be 100MHz. I want the o...
Altera_Forum's avatar
Altera_Forum
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15 years ago

I was saying that, if the input frequency change is fast enough, the PLL can see it has a jitter and maybe able to reject it.

Obvoiusly:

1) if the wandering of input frequency is very slow -> the PLL will follow the input change.

2) it the input frequency cahnge is sudden and with a huge step -> the PLL will lose the lock

3) if the input change of frequency has a non zero average value -> in no way the PLL can guess the right multpiplication factor

The only possibility is that the change of input frequency is small in absolute terms, is fast enough to be out of the PLL bandwidth and with a zero average value (the averga is inside the PLL bandwidth). The cahnge of input frequency is then seen as a jitter and filtered by the PLL.