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Design >> RF Design >> why the oscillator phase noise flat out when it is far away from the center freq
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Message started by liletian on Mar 29^th, 2018, 7:04pm

Title: why the oscillator phase noise flat out when it is far away from the center freq
Post by liletian on Mar 29^th, 2018, 7:04pm

why the oscillator phase noise flat out when it is far away from the center frequency instead of 1/f^2?

Does anyone has a good explanation?

Thanks,

Title: Re: why the oscillator phase noise flat out when it is far away from the center freq
Post by Andrew Beckett on Mar 29^th, 2018, 10:46pm

Numerous discussions of this including at http://www.signalpro.biz/phasenoise.pdf - the second page shows a common shape of the phase noise. I assume you're talking about the flat bit to the right of the curve on that page? If so, this is due to the thermal noise floor (see explanations further on in that document). I'm sure there are better descriptions out there - this was from a quick google search.

Regards,

Andrew.

Title: Re: why the oscillator phase noise flat out when it is far away from the center freq
Post by R.kumar on Apr 23^rd, 2018, 5:12am

liletian wrote on Mar 29^th, 2018, 7:04pm:

why the oscillator phase noise flat out when it is far away from the center frequency instead of 1/f^2?

Does anyone has a good explanation?

Thanks,

If you were to measure the VCO output directly you would only see a -20dBc/dec clope with no thermal noise floor. However in reality the output can only be measured after going through some buffers/IO circuittry and its thermal noise dominates at large frequency offsets.