why PLL lock to different frequency with same divider ratio?

[eejli]

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]

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

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]

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