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Post subject: pll - active loop filter Posted: Wed Sep 28, 2005 7:30 am

Hi ,

Do any one have a design or example circuit of active loop filter ?

and what are the advantages of the active over the passive filter and how does it effect the vtune ?

Thank you ....



Post subject: active filterPosted: Wed Sep 28, 2005 12:51 pm


Active filter is useful in a case when the tunning voltage from

the IC PLL\Phase detector is not fit to the tunning voltage of the VCO.

In that case you must use Active filter to amplified the voltage.

but, it will increase your phase noise (~6dB) in the loop BW.

I dont have an exemple but you may find one at the demo`s from the

RF software like EagleWare , AWR etc.




Post subject: Posted: Wed Sep 28, 2005 2:32 pm

check out National Semiconductors for starters, in case you already haven't.

I'm sure you can find good app-notes on this website.



Post subject: active loop filtersPosted: Wed Sep 28, 2005 3:03 pm

1. Active PLL filters look very much like classical op-amp integrators, except there are one or more added resistors to set the frequency response.

2. One advantage is that you can get higher-order loops. This gives you the possibility of tracking a changing frequency better. This may also give you more reduction of the spurious frequencies that appear at a delta of the reference frequency above and below the oscillator's output frequency. These are sidebands, and are undesired garbage that you try to minimize.

3. Another advantage that you can (but don't necessarily) get is the ability to drive the phase error to zero.

4. The effect on noise depends on the noise level of the amplifier used in the active filter. There are some amazingly quiet opamps available these days!

Good Luck!



Post subject: Posted: Mon Oct 10, 2005 12:32 pm

ADISimPLL from Analog Devices is a program that will help you simulate the effects of using an active loop filter.

The Analog parts are generally better than the national parts I also find...

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