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The refernce frequency and the loopbandwith

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John Xu

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I have a question on the PLL. In some papers about PLL design, it is asserted that the reference frequency and the loopbandwith should have at least one decade difference to guarantee the stability. Can any one tell me why?

Thanks in advance!!!
 

John Xu said:
I have a question on the PLL. In some papers about PLL design, it is asserted that the reference frequency and the loopbandwith should have at least one decade difference to guarantee the stability. Can any one tell me why?

Thanks in advance!!!
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i think the word stabiltiy is not accurate. while the important reason for taking Loop bandwidth = Fref/10 (as an assumption) is to have better immuning for the refrunce spurs (more attenuation). so you will find that the design is matter of trade-offs as when deacresing the Loop BW will make the Loop slower while the refrunce spurs will be more attenuated.

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    J

    John Xu

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loopbandwidth =d* wn<(1/20 fref)
 

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