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Design >> Analog Design >> PLL Loop Filter Capacitor
https://designers-guide.org/forum/YaBB.pl?num=1184285526

Message started by imtired on Jul 12^th, 2007, 5:12pm

Title: PLL Loop Filter Capacitor
Post by imtired on Jul 12^th, 2007, 5:12pm

This is a practical question.

As an amateur PLL designer, the only caps I have ever used are ceramic, usually 0603 or 0805 size. Can other types of capacitors be used, such as tantalums, or others?

What I really would like to use is some large value (>100uF) and high voltage rating (at least 50V). With ceramic, I need to stack multiple caps in parallel to achieve this. Any help would be appreciated.

Thanks.

Title: Re: PLL Loop Filter Capacitor
Post by Berti on Jul 12^th, 2007, 11:46pm

Hi,
I can't tell you the optimal solution, but according to my experience:

- don't use electrolytic capacitors since those might produce current-spike noise
- tantalum capacitors are more expensive but don't show current-spike noise. But
ESL and ESR are usually higher than in ceramic capacitors. Therefore you probably
should include(model) ESL/ESR into your loop filter.
- Cermic capacitors are better suited for high frequencies because they are not constructed
as a coil. However, usually they are not very linear (distortion), but for a PLL loop filter
I don't think that this is a issue.

Parallel capacitors always work, but that needs space...

I hope this helps.

Title: Re: PLL Loop Filter Capacitor
Post by imtired on Jul 13^th, 2007, 11:22am

Thank you Berti for your all your comments. That definitely helps me narrow down my options. Actually I have decided to go with ceramic caps, since I can't find any large capacitors with high enough voltage ratings that can mount on a PCB in a reasonably small area.