Dear Lawrence,

Lawrence Guan said:

he beat frequency is set to be fin/2 so that pss can converge.

Great! This totally consistent with the recommendations for computing an accurate estimate of jitter components of a driven circuit or oscillator that contains dividers at the On-line support URL:

support.cadence.com/.../ArticleAttachmentPortal

2. Use Edge phase Noise in the same form to plot phase noise, and integrate the noise 1M~fin/2, convert it to Rj with respect to 2*pi*fin

Now the problem is that the Jee gives exactly twice the number of method 2. Does this mean that Jee rely on beat frequency to calculate the phase jitter instead of the real frequency?

Perhaps I am not understanding your computation of random jitter when you stated "...and integrate the noise 1M-fin/2, convert it to Rj with respect to 2*pi*fin", but if the output signal used for the edge phase noise Jee is defined as the divider output, whose frequency is fin/2, then you are sampling the noise every 1/(fin/2) seconds - not every 1/fin seconds. Therefore the frequency spectrum ion its Jee is limited to 50% of fin/2 or fin/4. Hence, should your upper limit on the integration frequency be fin/4 and not fin/2? In other words, any frequency components over fin/4 will be aliased. The spectrum of the output of the buffer at frequency fin will extend to fin/2.

Put another way, the random jitter when measured over the same frequency interval and expressed in time units, is invariant to whether the jitter measurement is made on a signal whose frequency is fin or a signal that is fin/2 (assuming the division does not add significant noise). If the measurement is made in units of per unit interval, the jitter measurement using a reference frequency of fin/2 will be 50% of the value of that using a reference frequency of fin as a result of the period difference in the two signals - not the result of a difference in jitter expressed in time units.

Once again Lawrence, I apologize in advance if I am not understanding your computation or your simulation set-up for the two cases (i.e., phase noise of buffer output and phase noise of divider output)!

Shawn

Lawrence Guan said:

If I use a smaller beat frequency (say fin/6), the Rj_from_phase_noise is still the same, but Jee changes.

Is this as expected by the tool?

In any case, the value of Jee provided by Cadence spectre is computed from the edges of the signal defined in your phase noise simulation. Hence, it will be 0.50*fin when your divider is set as the phase noise output and fin when our pnoise output is defined as the buffer output. If you include a sample ratio in the analysis, see URL:

https://support.cadence.com/apex/ArticleAttachmentPortal?id=a1O0V000009ETPrUAO&pageName=ArticleContent&oMenu=People%20who%20viewed%20this%20also%20viewed

that will also impact the result.

I am not convinced I've answered your question, but maybe I am closer?

Shawn