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Design >> RF Design >> phase noise after frequency dividing
https://designers-guide.org/forum/YaBB.pl?num=1282955615

Message started by nxing on Aug 27^th, 2010, 5:33pm

Title: phase noise after frequency dividing
Post by nxing on 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

Title: Re: phase noise after frequency dividing
Post by pancho_hideboo on 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

Title: Re: phase noise after frequency dividing
Post by raja.cedt on 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.

Title: Re: phase noise after frequency dividing
Post by pancho_hideboo on 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

Title: Re: phase noise after frequency dividing
Post by raja.cedt on 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,

Title: Re: phase noise after frequency dividing
Post by vp1953 on 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?