jobless wrote on Sep 9th, 2006, 1:22pm:Could someone please help me understand better the difference between SP and PSP analyses. I am unable to completely understand when they say in SpectreRF manual that "PSP is a small-signal analysis similar to the conventional SP analysis, except, the circuit is first linearized about a periodically time-varying operating point as opposed to a simple DC operating point."
SP is small signal s-parameter analysis calculated around a
DC operating point. That's it. The DC operating point is a linear-time-Invariant operating point, and does not allow frequency translation.
PSP is small signal s-paramter analysis calculated around a
PSS operating point. The PSS run is a period steady state, which is a periodically linear-time-Varying operating point which allows for frequency translation.
jobless wrote on Sep 9th, 2006, 1:22pm:I am unable to understand what is linearization about a periodic waveform and what is a periodically time-varying operating point. Since the waveform in the steady-state is a large signal thing, the outcome of PSP or SP such as Z11 are all time varying inside a period. Do you average it out? And therefore, depending upon the precise moment in a period, which is chosen for the linearization, the outcome of PSP and SP will vary. My question is which point in a steady-state time period is chosen for this purpose.
So SP has nothing to do with PSS. The SP analysis result will depend on the DC solution (which is done automatically, but can be forced follow a given DC operating point/nodeSet if you set the options up for this).
PSP uses the full PSS solution as its operating point. So the PSP result will depend on the PSS result.
jobless wrote on Sep 9th, 2006, 1:22pm:Say, I have an oscillator circuits (traditional LC-VCO) and I perform both SP and PSP analysis across the two oscillating nodes,that is,across the LC Tank. For this I put the psin port across the Tank with very large input resistance to avoid changing the Q of the tank. Is this method correct?
If you want to measure the differential small signal negative resistance, then a SP analysis is ok. You can use any value for the port impedance you like, it'll make no difference to the SP result (other than scaling the smith chart, as it doesn't change the DC operating point). The simulation results will show the S11 being "outside" the smith chart meaning unstability/oscillation. Plotting something like Re(ZM) will give the value of the negative resistance. Do remember the result is only a small signal result around the DC solution. One could offset the tank nodes with a real DC offset and redo the SP analysis to show the small signal negative resistance at some "snapshot" point in the swing.
If you are attempting to use PSS/PSP to do the same thing, then I recommend you simulate the circuit "driven" by the port (i.e. set to sine and given the ocsillation frequency and voltage swing you want that would normally ensue in free running transient/ ocsillator mode PSS) to get the PSS, then do the PSP sweep.
hope this helps with your understanding and best of luck with your circuit.
aw
PS. Please also note that using a balun and two ports (differential and common-mode) is a more sound way to measure differential circuits. see
http://www.designers-guide.org/Analysis/diff-sp.pdfhttp://www.designers-guide.org/Analysis/diff.pdf