I have a clocking circuit containing 2 stages, a buffer followed by a divider. I used pss with shooting method +pnoise to simultate the Rj. Assume that the clock frequency is fin at the input of the buffer and fin/2 at the output of the divider. (fin is between 10~20GHz). The beat frequency is set to be fin/2 so that pss can converge.
Rj at the 1st stage output is then calculated in two ways:
1. Use Jee in the direct plot form. The integration range is 1M~fin/2
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?
When there are circuits operating in different frequencies, should users manually compensate the difference between beat frequency and real signal frequency if Jee is used to calculate Rj?
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:
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*finNow 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)!
Thank you in advance for your kind reply, and sorry that I did not state the computation clearly.
What is concerned is the Rj of "1st stage buffer" whose output frequency is still fin. I have put that node in pnoise analysis setting and select the correct output in direct plot form for both method 1 and 2.
The computation for method 2 is like this: Rj_from_phase_noise = sqrt( integ(10**(phase_noise(f)/10), 1M, fin/2)*2 ) / (2*pi*fin)
Rj_from_phase_noise = 0.5 *Jee, while I expect the two methods should give the same results. I suspect that this is because beat frequency = fin/2 is used in PSS because a divider exists in the total clock path, and Jee result is affected by the beat frequency.
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?
First, thank you for the additional information - it is helpful to me anyway! Unfortunately for me, I am still not clear if you are comparing the random jitter number in units of absolute time or in unit intervals. Clearly if the comparison is made on random jitter and Jee on a unit intervall basis, the two will differ dependent on the frequency ratio (factor of 2 in your case).
Lawrence Guan said:
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:
that will also impact the result.
I am not convinced I've answered your question, but maybe I am closer?
I compare the random jitter in the unit of "second".
I then simulated the buffer alone with different beat frequency for PSS, but keep the input clock frequency the same. The noise plot is different with different beat frequencies fbeat=fin, fin/2, fin/3. This result is weird. Shouldn't the phase noise be independent from the beat frequency?
Below is a snapshot of the PNOISE setting: