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why PLL lock to different frequency with same divider ratio?

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eejli

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In the lab, my PLL locks to different frequency with different board.

For one board, PLL locks to the right frequency=Fref*divider ration N, say 1.5GHz. But for another board the PLL locks to a slightly different frequency such as 1.5GHz+100kHz(always 100kHz offset). This is a fractional N PLL.

What could be the wrong source? The VCO is tunable from 1GHz to 2GHz.

Thanks.
 

sixth

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Re: why PLL lock to different frequency with same divider ra

eejli said:
In the lab, my PLL locks to different frequency with different board.

For one board, PLL locks to the right frequency=Fref*divider ration N, say 1.5GHz. But for another board the PLL locks to a slightly different frequency such as 1.5GHz+100kHz(always 100kHz offset). This is a fractional N PLL.

What could be the wrong source? The VCO is tunable from 1GHz to 2GHz.

Thanks.

Hi, eejli
I think the problem maybe lie in the reference signal, that is, the crystal oscillator. You can measure the frequency of the crystall oscillator and check if you got a correct frequency.

sixth
 

eejli

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thanks Sixth. We did find the crystal is not that stable.
 

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