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Design >> Analog Design >> why do we use active filter?
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Message started by liletian on Apr 30^th, 2009, 7:34pm

Title: why do we use active filter?
Post by liletian 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,

Title: Re: why do we use active filter?
Post by pancho_hideboo on 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.

Title: Re: why do we use active filter?
Post by liletian on 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

Title: Re: why do we use active filter?
Post by sheldon on 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

Title: Re: why do we use active filter?
Post by pancho_hideboo on 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.

Title: Re: why do we use active filter?
Post by raja.cedt on 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.

Title: Re: why do we use active filter?
Post by ywguo 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.

Title: Re: why do we use active filter?
Post by liletian on 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

Title: Re: why do we use active filter?
Post by raja.cedt on 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.