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PSS failed to converge in SpectreRF

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vkale

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Hello,

I am trying to run a PSS analysis with SpectreRF for a Ring Oscillator and at the end of simulation, while transient simulation is completed succesfull, I get the following message from PSS: " Even after adjusting the tstab value, PSS still failed to converge".
Does anybody suggest a solution for this problem,

Thnx
 

oermens

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are you talking about a pure transient simulation or the transient that is performed before pss? are your tolerances acceptable? startup conditions?
 

vkale

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I perform a pure trnasient analysis before pss. As far as concern the tolerances I have changed iabstol=1e-9 and reltol=100e-6. I have also set the tstabmethod to 'gear2only' method for pss.




are you talking about a pure transient simulation or the transient that is performed before pss? are your tolerances acceptable? startup conditions?
 

oermens

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if you don't have this already, select save init tran in the PSS options and after simulating you can view the PSS transient using the Direct Plot menu to verify steady state oscillation exists.
 

dick_freebird

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PSS I believe wants to run over an integer number of cycles
(N=1?) of the fundamental. A ring oscillator's period is not
fixed. An arbitary run-end time is likely not to put you on a
clean wraparound.

Chaotic or subharmonic behavior also kills PSS brcause you
can't get a steady state. Stuff like charge pumping, ground
bounce, etc.
 

buckaroo

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Hi vkale,

Just remind u two things,
1. Check if the oscillating frequency you specified is the case of your osc; check if the oscillator is oscillating(save the initial result, plot the output), if not, an initial condition may be needed to do the simulation.
2. set a large # of harmonics, because ring oscillator has rich harmonics, if the # of harmonics of PSS is not large enough, it may encounter convergence problem.

Rds
Buckaroo
 

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