Bob,
Here are some quick answers to your questions. Ask for elaborations if these responses are not sufficiently helpful.
First a comment on your measurement. I suspect you are not including enough sidebands. The sidebands are the mechanism by which aliasing is accounted for. If you are not familiar with aliasing, you should study some sampling theory. I recommend you increase the maxsidebands for both measurements. To get the true answer you would need set maxsidebands to infinity, but that would literally take forever. So instead, you want to increase the number enough to get sufficiently accurate answers without going too far, which would slow the simulations. It is difficult to know how many are too many, but it is not that sensitive. If you take a look at
http://www.designers-guide.com/Analysis/sc-filters.pdf you will see how the rate at which the compute noise level increases as the value of the maxsideband parameter is increased slows dramatically. So you just have to choose a large enough value for maxsidebands and understand that the computed results will be somewhat of an underestimation.
Basically, the noise in your circuit has some upper bandwidth, and you want to choose maxsidebands high enough so that aliasing from all frequencies up to just above that bandwidth should be included. The frequency of the highest aliased term included is fund*maxsidebands, so roughly
2*noise bandwidth < fund*maxsidebands < 10*noise bandwidth.
Now you probably do not know the noise bandwidth, but it would be the same in both of your circuits. So to get comparable results, you must set maxsidebands twice as large when simulating the second circuit to compensate for the lower fundamental frequency.
So my suggestion is to set maxsidebands to something like 20-40 in the first circuit, and 40-80 in the second. To assure that the results are comparable, set maxsidebands on the second problem to be two times as large as the value used on the first problem.
1. Are these test cases calculated correctly?
maxsidebands must be set as described above.
2. If so, why are results so different from the “41% increase” expectation?
you are only including half the aliasing in the second problem. double maxsidebands.
3. Why do we stop calculating and integrating the PSD at the beat frequency?
the spectrum of sampled data signals is periodic with the period equaling the fundamental frequency (you did use the sampled or time-domain noise feature right?). Integrating over all frequency would result in an infinite energy, which is clearly wrong. Instead you integrate from 0 to the fundamental frequency.
4. Does this ignore higher frequency noise that would worsen jitter, or is this issue somehow handled in the “noise folding” (or other) aspect that I don’t understand yet?
The effect of the higher frequency noise is accounted for with the maxsidebands parameter.
5. Could you point me to a resource that explains these issues if it is too complex to answer here?
Cannot think of any other resources. Maybe others can.
maxacfreq is a parameter of the pss analysis.
-Ken