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transient vs. pss simulation

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Newbie level 6
Sep 4, 2020
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Hi, I want to analyze distortion in a single outpu opamp. First, I fed the circuit with a single tone signal and used transient simulation - measurement/spectrum tool to get frequency response. Then I tried PSS analysis. I got different results i.e. SFDR was different, output magnitude was different, harmonic magnitudes were different. I wonder: Should we trust transient analysis? Is PSS for switching or and oscillating circuits like switched cap, PLL, oscillator? Thank you.

Yes, PSS is for such circuits but how could you obtain Nonlinearity by Transient Analysis ?? If you did FFT/DFT to observe the spectrum, you should pay attention the parameters ( recording length, sample per period etc.) and transient result should also be collected after steady state regime not before.
But PSS analyses at steady state response therefore it's more trustable.
Normally both analysis should give the same result ( under normal conditions )

PSS validity depends on the simulated "cycle" being
- periodic in fact
- steady state in fact

You can get a valid spectrum out of any transient simulation
(but valid only for the goings-on at hand); PSS demands that
this (goings-on) be perfectly periodic and/or stable, getting
you to an ideal result for an idealized condition.

Of course modern radios seldom are periodic (frequency
hopping, xxSK modulation, data packet structure) or steady
state (pretty boring data stream). You might have to work to
create a case which both satisfied criteria, and pertains to a
useful / specified test condition.

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