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nxing Community Member Offline God help the person who help themselves Posts: 46 China	phase noise after frequency dividing Aug 27^th, 2010, 5:33pm Hello everyone, I know that the phase noise of signal will be improve by 6 dB if the signal is divided by two. Can anybody explain why is that? Thanks
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pancho_hideboo

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Re: phase noise after frequency dividing
Reply #1 - Aug 27^th, 2010, 10:46pm

nxing wrote on Aug 27^th, 2010, 5:33pm:

I know that the phase noise of signal will be improve by 6 dB if the signal is divided by two.
Can anybody explain why is that?

Jitter is not changed after frequency dividers if we assume no additional jitter by frequency dividers.

Consider a definition of phase.

T1 ; Period of input signal
dt1 ; Jitter of input signal
ph1 ; phase error of input signal, ph1=(dt1/T1)*2*pi

T2 ; Period of divided signal, T2=N*T1, N ; Divide Ratio
dt2 ; Jitter of divided signal, dt2=dt1
ph2 ; phase error of divided signal, ph2=(dt2/T2)*2*pi=ph1/N

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« Last Edit: Aug 27^th, 2010, 11:54pm by pancho_hideboo »

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raja.cedt

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Re: phase noise after frequency dividing
Reply #2 - Aug 28^th, 2010, 11:36am

hi,
Phase noise means ratio of noise power in 1Hz BW to carrier power. So due to dividing there is no change in the signal, but coming to noise i guess it will be reduced by half because noise will occur mainly when ZERO crossing happening and division operation makes no of zero crossing half of the previous no.

Thanks,
Rajasekhar.

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pancho_hideboo Senior Fellow Offline Posts: 1424 Real Homeless	Re: phase noise after frequency dividing Reply #3 - Aug 28^th, 2010, 12:03pm raja.cedt wrote on Aug 28^th, 2010, 11:36am: Phase noise means ratio of noise power in 1Hz BW to carrier power. Not correct. See the followings. http://www.designers-guide.org/Forum/YaBB.pl?num=1241917016/6#6 http://www.designers-guide.org/Forum/YaBB.pl?num=1274422891
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: phase noise after frequency dividing Reply #4 - Aug 28^th, 2010, 12:16pm hi, i guess if i assume total noise dominated by PM then that Phase noise def is correct. or else could you please explain how it go down by 6db if you divide? Thanks,
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vp1953 Senior Member Offline Posts: 172	Re: phase noise after frequency dividing Reply #5 - Aug 30^th, 2010, 3:32pm Hi Raja, Assume that the total noise is dominated by phase noise and assuming that the effects of noise shows up at Zero crossing as you suggested : how does the noise power go down by 6db. When dividing by 2, the noise power is now halved and so should be down by 3dB?
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