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liletian Community Member Offline Posts: 97 MD	why do we use active filter? Apr 30^th, 2009, 7:34pm Hi Guys what is the advantages of active filter? The only advantage I can think is active filter can give you, is there any other reasons for it? Thanks,
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pancho_hideboo Senior Fellow Offline Posts: 1424 Real Homeless	Re: why do we use active filter? Reply #1 - Apr 30^th, 2009, 7:42pm liletian wrote on Apr 30^th, 2009, 7:34pm: what is the advantages of active filter? It doesn't require inductor. liletian wrote on Apr 30^th, 2009, 7:34pm: The only advantage I can think is active filter can give you, is there any other reasons for it? I can't understand what you mean.
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liletian

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Re: why do we use active filter?
Reply #2 - Apr 30^th, 2009, 7:51pm

pancho_hideboo wrote on Apr 30^th, 2009, 7:42pm:

liletian wrote on Apr 30^th, 2009, 7:34pm:

what is the advantages of active filter? The only advantage I can think is active filter can give you, is there any other reasons for it?

It doesn't require inductor.

so two advantages:
1. high gain
2. no need inductor
disadvantages:
consume power

right?

So the question is in PLL, active filter is used, the only reason is due to its high gain?
Thanks

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sheldon Community Fellow Offline Posts: 751	Re: why do we use active filter? Reply #3 - Apr 30^th, 2009, 9:47pm Liletan, You might also want to take a look at, http://en.wikipedia.org/wiki/Active_filter Not sure if high gain is helpful for a PLL loop filter, wouldn't increase the kvco and the phase noise? Best Regards, Sheldon
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pancho_hideboo

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Re: why do we use active filter?
Reply #4 - Apr 30^th, 2009, 10:53pm

liletian wrote on Apr 30^th, 2009, 7:34pm:

The only advantage I can think is active filter can give you, is there any other reasons for it?

I can't understand what you mean.

liletian wrote on Apr 30^th, 2009, 7:51pm:

So the question is in PLL, active filter is used, the only reason is due to its high gain?

Loop filter for PLL often requires very large capacitors and resistors.
If you use on-chip loop filter, such very large capacitors and resistors occupy�large area.

If you use active filter, you can reduce capacitors and resistors, although there are many problems in PLL.

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raja.cedt

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Re: why do we use active filter?
Reply #5 - Apr 30^th, 2009, 11:58pm

hi
� i think it is debatable question because-now a days most of the plls are having tough jitter specification, so in that case simple active filter wont help much, but there are another class �called tracking bandwidth plls they will utilize active components in different passion to get very good jitter specification .At the same time active filter may not reduce much area because no there is no inductor (forget about LC vco).

Thanks,
Rajasekhar.

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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: why do we use active filter? Reply #6 - May 2^nd, 2009, 7:51am liletian wrote on Apr 30^th, 2009, 7:34pm: �Hi Guys �what is the advantages of active filter? The only advantage I can think is active filter can give you, is there any other reasons for it? �Thanks, Liletian, What's your application? Perhaps we can explore the topic deeply only if we know your application.
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liletian

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Re: why do we use active filter?
Reply #7 - May 2^nd, 2009, 12:08pm

ywguo wrote on May 2^nd, 2009, 7:51am:

liletian wrote on Apr 30^th, 2009, 7:34pm:

�Hi Guys
�what is the advantages of active filter? The only advantage I can think is active filter can give you, is there any other reasons for it?
�Thanks,

Liletian,

What's your application? Perhaps we can explore the topic deeply only if we know your application.

I am thinking to use it in my PLL, but I did not see any advantages over normal RC filter yet, any suggestions?
Thanks

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raja.cedt

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Re: why do we use active filter?
Reply #8 - May 4^th, 2009, 7:01am

hi,
designers will use active filter in special cases like
1.In tracking bandwidth plls
2.To reduce filter cap size
3.To reduce impact of 1/f noise on overall phase noise

But remember all the above has to apply very carefully otherwise directly they will impact phase noise as well as jitter.

Hope this is clear,
Thanks,
Rajasekhar.

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