Loaded q_factor in time domain contradicts S-param q-factor

[yefj]

Hello , i have compared two methods of getting a loaded Q-facto.
one is using the decay of electric field shown below, the other is using S-parameters Q=w0/delta_w.
I get tottaly different results.
Where did i go wrong?
Does these methods should give us the same result?
Thanks.

 

[dick_freebird]

AC analysis uses a unit amplitude (unless told otherwise) and small signal
models, while tran uses what you give it for amplitudes and large signal.
If there is anything even a bit away from DC OP, that changes the transfer
function you would see it in tran and not in ac. I expect s-param analyses
follow ac analysis in this regard.

So I would first look to scaling factors of input sources and maybe try a
couple of input amplitudes to see if there's that kind of "different angles
make different pictures" kind of thing going on.

 

[mtwieg]

Those two methods of measuring Q are only equivalent for simple systems with a dominant second-order behavior. What is your system?

Also, the method of coupling energy in or out of the system can greatly affect the measured Q. Need a more detailed explanation of how you are performing these measurements.

 

[yefj]

i saw this youtube video which says there are two methods measuring unloaded q factor
he says there are two options as shown bellow.
I have a one port resonator.
i could not find anywhere the theory of 2.5 dB he says in the video.
is there some book that shows that?
Thanks.