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cqmyg5
Joined: 21 Oct 2003
Posts: 137
Helped: 1
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 06 Jul 2004 11:30 "Active" or "Passive" filter in PLL desi |
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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!
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ramesh
Joined: 19 Jan 2003
Posts: 1234
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| 06 Jul 2004 12:19 Re: "Active" or "Passive" filter in PLL |
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| If you need a filter with order and gain greater than 1, then go for active filter otherwise passive filter will do.
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liaowuliao
Joined: 03 Nov 2003
Posts: 7
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| 06 Jul 2004 15:15 Re: "Active" or "Passive" filter in PLL |
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| Active filter would add the Gain,but also induces the more noise.
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flatulent
Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
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| 06 Jul 2004 16:18 Re: "Active" or "Passive" filter in PLL |
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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.
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cqmyg5
Joined: 21 Oct 2003
Posts: 137
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| 07 Jul 2004 3:40 Re: "Active" or "Passive" filter in PLL |
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to flatulent:
Could you give me some information about "notch filters"? Is't same as "Band Reject Filter"?
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dinolin
Joined: 19 Jul 2002
Posts: 39
Helped: 1
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| 07 Jul 2004 3:53 Re: "Active" or "Passive" filter in PLL |
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| cqmyg5 wrote: |
to flatulent:
Could you give me some information about "notch filters"? Is't same as "Band Reject Filter"? |
Sure , Yes.
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dinolin
Joined: 19 Jul 2002
Posts: 39
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| 07 Jul 2004 3:57 Re: "Active" or "Passive" filter in PLL |
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| flatulent wrote: |
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. |
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 ....
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nyquist
Joined: 11 Jul 2004
Posts: 6
Location: Italy
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| 11 Jul 2004 16:56 Re: "Active" or "Passive" filter in PLL |
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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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