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Design >> Analog Design >> A question about 4th-order LPF in CP-PLL
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Message started by SemiLeon on Jul 13^th, 2007, 7:22am

Title: A question about 4th-order LPF in CP-PLL
Post by SemiLeon on Jul 13^th, 2007, 7:22am

Hi, all,
I have a question about the design of 4th-order loop filter for charge-pump PLL. Dean give a method to design a loop filter by given pole ratios. In dean's method, the poles of the filter are all on the real axis, but I think it is possible to design a filter with a pair of complex pole, and It may have better attenuation.

Please give me some advise.

Thanks and best Regards.

Title: Re: A question about 4th-order LPF in CP-PLL
Post by boe on Jul 16^th, 2007, 12:49am

Hi Leon,
If you use a complex pole pair to improve the attenuation, you are very likely to make the phase response worse.
So I expect you'll find it very hard to get a PLL with 4th-order complex-pole loop filter stable (especially over PVT & mismatch).

BOE

Title: Re: A question about 4th-order LPF in CP-PLL
Post by SemiLeon on Jul 18^th, 2007, 5:34am

Hi, BOE,
Thanks for your reply.
I think if the two highest frequency poles are made complex pair, the stability problem may be less grievous. I just want to find a way to design a passive loop filter with a pair of complex pole, and do some experiment. It may be better than we thought.

Leon

Title: Re: A question about 4th-order LPF in CP-PLL
Post by boe on Jul 23^rd, 2007, 12:51am

Hi Leon,
Let me know what you find out. I'm very interested in it. Didn't ned it yet, though...

BOE