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# the totoal current of the VCO doesn't convergence !!! why?

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#### diva

##### Newbie level 4
ebookee

When I doing the tran simulation(Using Spectre) I foud that the current of my VCO doesn't convergence !!! why?
anybody met this statues before.

For example, you spoke about a VCO. Within a loop ? Probably a PLL.
What kind of PD ? Which current ? Control input or output ?
Most probably the loop is unstable; but if you want a substantial answer you should formulate the problem in detail.
Regards

Transient convergence is most often a modeling error -
either unrealistically low capacitances (like you might
get by trying to model with behavioral or primitive /
ideal elements or by a less than complete set of active
device parasitics) or a timestep that is just too coarse
for the timescale of circuit activity, so the simulator
steps past the action, and can't "downrange past zero".
Ideal elements which have unlimited gain can without
a limiting mechanism, can force insane node voltage
or current onto other devices and push them into
unmodeled regions where singularities or further
blow-up ensues.

You could try a few of the usual approaches, such as
forcing more conservative solutions, using a method
like Euler that is more damped, etc. I'm almost
certain that the Cadence help docs have some of the

It looks like the current shown, before it quits, is in
bursts; maybe the outer loop is unstable and you're
chopping the control voltage and seeing activity / no
activity, maybe the oscillator is killed by out of range
control voltage (that would be a bad design, or a

You should be capable of probing around other parts
of the circuit and using your understanding of the
normal operation, to pick out the abnormal. Might
basic function forward-only.

### diva

Points: 2
Re: the totoal current of the VCO doesn't convergence !!! wh

dick_freebird said:
You could try a few of the usual approaches, such as
forcing more conservative solutions, using a method
like Euler that is more damped, etc. I'm almost
certain that the Cadence help docs have some of the

It looks like the current shown, before it quits, is in
bursts; maybe the outer loop is unstable and you're
chopping the control voltage and seeing activity / no
activity, maybe the oscillator is killed by out of range
control voltage (that would be a bad design, or a

.

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