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supermoment

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VCO 1/f^3 noise and LPF with 1MHz BW
May 18^th, 2010, 6:08am

the 1/f^3 noise (<100kHz) of a VCO becomes less important if it uses in a PLL with large LPF BW >1MHz. It is because VCO noise to output �of PLL (transfer function of VCO noise) is a high pass filter (HPF) relationship. In other words, the 1/f^3 will be filtered out by the LPF.

Is this statement true?

PS: the LPF is a 2nd order type.

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ywguo Community Fellow Offline Posts: 943 Shanghai, PRC	Re: VCO 1/f^3 noise and LPF with 1MHz BW Reply #1 - May 19^th, 2010, 5:39am Hi Casual, I think that statement is true. Please try to model it with verilog-A or matlab and simulate. Yawei
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supermoment Senior Member Offline Posts: 167	Re: VCO 1/f^3 noise and LPF with 1MHz BW Reply #2 - May 19^th, 2010, 6:10am why there are so many papers trying to reduce the 1/f^3 noise? Is it because the PLL LPF BW they used are <<1M Hz? especially in freq synthesizer..
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Mayank

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Re: VCO 1/f^3 noise and LPF with 1MHz BW
Reply #3 - May 20^th, 2010, 7:15pm

Quote:

Is it because the PLL LPF BW they used are <<1M Hz?
especially in freq synthesizer..

For Integer PLLs, toi keep a Loop BW of 1MHz, assumnig you want to design with Gardener's principle, your Fref or Fcomparison should be more than 10MHz ateast.
XOs generally come with haphazard frequencies like 9.6 MHz / 13 MHz, etc. in which case you need to divide down the reference freq. to an integer division of output freq. This reduces your Fref & hence you max PL BW spec.

For fractional PLLs, to control the S-D noise from dominating the output, PLL BW has to be kept low.

Freq Synthesizers can be based on both Integer as well as Fractional PLLs. SO their BW is genearlly low, but it can go above 1 MHz.

This should not be the only explanation to your original question.
Gotta think a bit more. Maybe on the lines that Flicker Noise causes Random Walk in PLLs.

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Mayank

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Mayank

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Re: VCO 1/f^3 noise and LPF with 1MHz BW
Reply #4 - Jun 4^th, 2010, 4:22am

Quote:

why there are so many papers trying to reduce the 1/f^3 noise?

Is it because the PLL LPF BW they used are <<1M Hz?
especially in freq synthesizer..

Researched.
This is indeed the only reason we keep 1/f^3 corner low for an oscillator.
PLL BWs of around 1MHz and above are diff. to design because FREFs at High Frequencies are not easily available.
Also, if you PLL Operating Frequency doesnt turn out to be an integer multiple of FREF, you gotta divide down FREF further, making it even more diff. for PLL BW to be higher than 1/f^3 corner.

That's the sole reason everyone tries to keep 1/f^3 corner LOW enough.

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Mayank.

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supermoment Senior Member Offline Posts: 167	Re: VCO 1/f^3 noise and LPF with 1MHz BW Reply #5 - Jun 4^th, 2010, 4:36am Thank you. :)
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