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"Active" or "Passive" filter in PLL desi

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cqmyg5

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Could anyone tell me:

What's the advantage to apply a "active" loop filter in PLL? When I should use a active filter to replace a passive filter in PLL design?

Thx!
 

Re: "Active" or "Passive" filter in PLL

If you need a filter with order and gain greater than 1, then go for active filter otherwise passive filter will do.
 

Re: "Active" or "Passive" filter in PLL

Active filter would add the Gain,but also induces the more noise.
 

Re: "Active" or "Passive" filter in PLL

Active filters allow more options in the filter shape. One example is a notch which can be placed at the reference frequency to greatly reduce the spurious sidebands at that frequency on the output.

I once saw an application note by Motorola for a synthesizer for an aviation band radio local oscillator which had three notch filters to remove the first three reference spurs. The output spectrum was very low in phase noise and spurious spectral lines.
 

Re: "Active" or "Passive" filter in PLL

to flatulent:

Could you give me some information about "notch filters"? Is't same as "Band Reject Filter"?
 

Re: "Active" or "Passive" filter in PLL

cqmyg5 said:
to flatulent:

Could you give me some information about "notch filters"? Is't same as "Band Reject Filter"?
Click to expand...


Sure , Yes.
 

Re: "Active" or "Passive" filter in PLL

flatulent said:
Active filters allow more options in the filter shape. One example is a notch which can be placed at the reference frequency to greatly reduce the spurious sidebands at that frequency on the output.

I once saw an application note by Motorola for a synthesizer for an aviation band radio local oscillator which had three notch filters to remove the first three reference spurs. The output spectrum was very low in phase noise and spurious spectral lines.
Click to expand...

Passive filter can also make many shape ..... if LCR are both used.

Active fileter , with the aid of op amp or bjt fet ... , can make complex transfer function by R C only ....
 

Re: "Active" or "Passive" filter in PLL

my 2 cents:
for what i know PLL is one of those designs that prefer passive filters. That's because just after the filter you have the VCO. On the imput of the VCO you would like to have the most possible constant value (no oscillations at all neither double pole frequency response), otherwise you will see spuriouses on the spectrum. For some designs I've seen some solutions made by active filters followed by an RC backend to avoid these phenomenons. They seem to work pretty fine.
 

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