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Measurements >> Phase Noise and Jitter Measurements >> jitter from phase noise
https://designers-guide.org/forum/YaBB.pl?num=1178678760

Message started by sivacharan on May 8^th, 2007, 7:46pm

Title: jitter from phase noise
Post by sivacharan on May 8^th, 2007, 7:46pm

Hi
I would like to know how we can derive the value of the jitter from phase noise curve.
I am attaching a paper(softlink) which describes this. But iam confused how the author found the area.
He is calculating the area in the range 12K to 10MHz. But if we see the phase noise range, it is in the order of -110dBc/Hz. The area under the curve between the freq ranges is -63dBc. How this can be possible?

http://assets.zarlink.com/CA/Phase_Noise_and_Jitter_Article.pdf

Can any one explain the calculation behind this?

Thanks.

Title: Re: jitter from phase noise
Post by imtired on Jun 5^th, 2007, 3:17pm

The paper doesn't give the function for the phase noise curve, but by eyeing the graph, this is what I calculated:

Assuming L(f) = k + a/f in the region of integration, (where a,k are constants, and f is frequency)

integral( k+a/f, from f1 to f2 ) = k*(f2 - f1) + a* ln( f2/f1 ), where k=10^-150/10, and a = 10^-140/10 * 12e3, f1=12e3, f2=20e6.

plugging in and convert to dBc:
10 * log( 1e-15*(20e6-12e3) + 1e-14*12e3*ln(20e6/12e3) ) ~ -76.8 dBc.

My answer is not the same as in the paper, but I think it's pretty close, given that I had to eyeball the function.
But this should demonstrate how to go about integrating the area under a phase noise curve, as a step to calculating the jitter.

Notice that L(f) has units dBc/Hz. Once you integrate over frequency, you get [dBc/Hz] * [Hz] = dBc.

I hope this helps you.

Regards,
Robert

Title: Re: jitter from phase noise
Post by buckaroo on Oct 23^rd, 2007, 9:41pm

as a rule of thumb, you can convert phase noise to jitter by the equation:
jitter^2=(fm^2/fo^3).L(f)
for example: 15ps, fo=21.88MHz
then @1MHz pnoise=-116.3dBc/Hz
which is 15p^2=(1M^2/21.88M^3)*10^-11.63

be careful, it won't be accurate at low frequency for its 30dBc/dec, not 20dBc/dec slope

sivacharan wrote on May 8^th, 2007, 7:46pm:

Title: Re: jitter from phase noise
Post by imtired on Nov 15^th, 2007, 11:06am

Buckaroo, your equation for computing jitter from a single phase noise offset seems highly limited to some very specific situation, whatever that may be. I would advise against making such a blanket statement on using this equation as a rule of thumb for calculating phase noise, especially for more complex phase noise curves such as found in systems with cascaded PLLs.