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Simulators >> Circuit Simulators >> how to set initial current in the inductor of the xtal model
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Message started by hiSkill_11 on Jan 5^th, 2012, 1:03am

Title: how to set initial current in the inductor of the xtal model
Post by hiSkill_11 on Jan 5^th, 2012, 1:03am

Hi, I found it strange when I simulate the crystal oscillator with tran analysis in spectre. When I set the initial current of 1uA in the parameter of CDF of the inductor in the crystal model, I observed that the current of the inductor(sin wave) starts from 0 and begin to oscillate with 1mA amplitude, when I set it of 1mA, it starts from 0 and begin to oscillate with 1A amplitude. Then the envelope begin to decay to a stable value about 900uA. So, I am confused about relation ship between the initial current in the component parameter and the value at time 0, also the initial swing of the inductor current. And how can I set the initial current in the inductor in order to make the crystal oscillator reach the stable state quickly? Thanks.

Title: Re: how to set initial current in the inductor of the xtal model
Post by raja.cedt on Jan 5^th, 2012, 9:50am

Go to convergence aid and set initial conditions. If you use hspice just use .ic command. What is there......

thanks,
raj.

Title: Re: how to set initial current in the inductor of the xtal model
Post by rfcooltools.com on Jan 5^th, 2012, 9:54am

hi_Skill_11,

Here is what I do
Since you already where able to get to steady state once, plot the current wave form and record the value of 5 equally spaced points on a half period of the waveform starting at zero and ending at zero amps. Then get a ipwl enter the 5 points and simulate it should be at steady state within a half period.

The Q of the xtal is large enough to cause many simulators with nominal settings to converge abnormally which is why you may have seen the 1A temporarily.

http://rfcooltools.com

Title: Re: how to set initial current in the inductor of the xtal model
Post by Ken Kundert on Jan 5^th, 2012, 11:27am

This seemingly anomalous behavior is explained in section 4.3.5 of my "Designer's Guide to Spice and Spectre" book.

-Ken

Title: Re: how to set initial current in the inductor of the xtal model
Post by hiSkill_11 on Jan 6^th, 2012, 1:28am

raja.cedt wrote on Jan 5^th, 2012, 9:50am:

Go to convergence aid and set initial conditions. If you use hspice just use .ic command. What is there......

thanks,
raj.

In spectre, the convergence aid seems to have no way to set initial current but voltage. Thanks.

Title: Re: how to set initial current in the inductor of the xtal model
Post by loose-electron on Jan 6^th, 2012, 1:33pm

Everybodey should have a copy of Ken's book on their shelf.

Also, the crystal Q is so high (10E5)that it is generally
a wise course of action to drop the Q down to
something a little friendlier, like Q=100 or so.

Recognize that this is a simulator thing and the actual crystal
is narrower. But if the center band attenuation is the same (usually 0dB)
things simulate a bit easier.