Re: [Qucs-devel] Surprising simulation results from Qucs...

[Stefan J. <st...@gr...>]

Am Di, 6.06.2006, 18:26, schrieb Rob Frohne:

> Hi Stefan,

Hello Rob,

  I am very sorry for this late reply.  But good wheather here and the
problem being "not too easy" to deal with...

> The new circuit common_emitter4.sch works in every case, but the
> original (which appears to be the same in the GUI) doesn't work as
> expected.  I haven't compared the two netlists, because there seems to
> be a problem bringing up the text editor when I try and view the source
> in my system, but I suspect you may be able to pass on a bug report to
> the GUI developers that may save you from having to deal with this in
> the future.

In fact it was the case that your schematics (which seemed identical)
produced different netlist.  *But*: the difference was just the order
of appearance.  Thus you got different solutions because of "numerical"
(binary) noise.  Anyway that was not the actual problem.

The bigger problem was, that I alwasy assumed that the transient analysis
uses its initial DC analysis solution as initial values (i.e. starts
with loaded capacitors and inductors).  Now I found out that I intended
this with some code but actually didn't achieve it.  So now I fixed this
problem and send a patch fo applying it to CVS.  You can now choose
if an initial DC analysis is run or not.  When using it you won't have
any more problems with large time constants of some capacitors...

Cheers, Stefan.

>>> Hi Stefan,
>>>
>>
>> Dear Rob Frone,
>>
>>
>>> I can't remember the exact circuit I sent you, but if it is the one I
>>> was just playing with, (attached here as common_emitter 1.sch) its DC
>>> analysis gives me around 5.9 volts at the collector, and my hand
>>> calculations give me a saturated transistor with the DC analysis.  I do
>>> the same circuit without the capacitors (also attached as
>>> common_emitter2.sch), and I get fine results, so you are right that it
>>> has to do with the capacitors.  It seems that the DC analysis should be
>>> the same with or without the capacitors, as for DC they are open
>>> circuits.
>>>
>>> It may have to do with something I did while entering the circuit in
>>> the
>>> GUI too because I just added the capacitors to the circuit that was
>>> working (no voltage source yet) and it still works.  I'll enclose it
>>> too
>>> (common_emitter4).
>>>
>>
>> Ok.  And now you can add again your 1MHz/0.1V input voltage source to
>> common_emitter4 and set the numer of points to 10001 in the transient
>> simulation box and you will see the result which you might have expected
>> in the first place...
>>
>> Cheers, Stefan.
>>
>>
>
> --
> Rob Frohne, Ph.D., P.E.
> E. F. Cross School of Engineering
> Walla Walla College
> 100 SW 4th Street
> College Place, WA 99324
>
> (509) 527-2075
> http://www.wwc.edu/~frohro