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phase of series LC impedance

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willdoit

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Hi

I understand that the impedance looking across series LC must have the phase component varying as an ideal step function.

I have been trying to replicate this in spice but I always see a gradual transition from 90deg to -90deg... I modified the gm for my simulation as well..

Any suggestions on which spice parameters need to be tweaked or if there is some problem in my understanding.

Thanks
 

Assuming you have ideal L and C in your simulation, SPICE apparently has a better understanding of the problem.
 

    willdoit

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FvM,

SPICE has poor understanding of high-Q circuits.. I was looking for tweaks to get it reasonably fixed..

I hope you also agree that the phase must be either 90 or -90 in non-lossy circuit or else power dissipation is implied..
 

willdoit said:
FvM,

SPICE has poor understanding of high-Q circuits.. I was looking for tweaks to get it reasonably fixed..

Hi willdoit,

I have simulated a LC-circuit with SPICE: L with a series resistance of 1E-9 ohm and C with a parallel resistor of 1E12. The phase jump from 0 deg to -180 deg was like an ideal step . Do you want anything more ?
 

    willdoit

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Hi LvW,

I see something like the attached response.. i don't know why it is not as desired and as in your simulation.. which simulator are you using?

I used 1e-9H inductor and 1e-5F cap.. simulated from 150KHz to 15MHz
 

Willdoit, at the moment I have no explanation for your simulation result.
Check the characteristics of the parts used by the program from Linear Tech.
Perhaps they are lossy.
Obviously, the LC-circuit is a lowpass and the phase must start at 0 deg - contrary to your graph.
As mentioned before, my simulaltor (ORCAD/PSpice) works perfectly and presents correct results: Lowpass with heavy peaking around 160 kHz and a phase response between 0 deg and -180 deg.

Correction: peaking at 1.6 MHz

Added after 31 minutes:

Willdoit, do you know that the inductance in LTspice has a 1mohm series resistance as a default ?
 

    willdoit

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LvW said:
Obviously, the LC-circuit is a lowpass and the phase must start at 0 deg - contrary to your graph.

Thanks LvW for confirming that there is a problem in simulation only.

Just a point to note - The phase of current must be -90deg at low frequency (-j/wC dominates) and +90deg at high frequency (jwL dominates) and never 0 deg w.r.t. voltage. Its in lowpass RC filter that phase is 0 deg at low freqs..
 

Thanks LvW for confirming that there is a problem in simulation only.

I don´t see any problem. The simulation engine works properly, does it not?
(General comment: In most - if not in all cases - the simulator works correct and the user makes interpretation errors. Example: Bias point calculation of an opamp with POSITIVE feedback. The simulator calculates a stable bias point. Question: Who is wrong? The simulator or the user who expects instability?).

Just a point to note - The phase of current must be -90deg at low frequency (-j/wC dominates) and +90deg at high frequency (jwL dominates) and never 0 deg w.r.t. voltage. Its in lowpass RC filter that phase is 0 deg at low freqs.

Yes, if you speak about the phase of the current you are right. I was referring to the phase of the voltage of the LC divider circuit which acts as a lowpass.
 

    willdoit

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