Re: CircuitMaker Step Time puzzle
From: John Larkin (jjlarkin_at_highSNIPlandTHIStechPLEASEnology.com)
Date: 11/29/04
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Date: Mon, 29 Nov 2004 14:35:07 -0800
On Mon, 29 Nov 2004 20:39:37 +0000, Terry Pinnell
<terrypinDELETE@THESEdial.pipex.com> wrote:
>Any CircuitMaker users have any thoughts on this puzzle please?
>I've been running the simple CMOS astable at
>http://www.terrypin.dial.pipex.com/Images/TestCursors.CKT
>
>I know the 1k resistor is far too low (330k would be good), but that's
>not the point of my post.
>
>At certain settings of Step Time (= Max Step), the waveform is not
>identical from one cycle to the next.
>
>One such critical value is 1 ms. With that chosen, successive Lows
>differ significantly in length. I've illustrated the 1 ms result here:
>http://www.terrypin.dial.pipex.com/Images/CM-StepTimePuzzle1.gif
>
>You can clearly see the variation in successive cycles. I measured
>about 4.3 ms versus 5.4 ms, while the Highs remain identical at 5.0
>ms.
>
>So, depending on which choice of cycle I happen to make, I get a
>different frequency: 97 Hz versus 104 Hz. Yet at 990 us (i.e. only a
>tiny difference), I get a *consistent* waveform and hence a consistent
>frequency measurement (97 Hz). But at Step = 100 us, I also get a
>consistent frequency, but now it's 114 Hz.
>
>Any insights appreciated please.
Consistant isn't the same as accurate.
Timesteps are usually set to a small fraction of the time any circuit
event is expected to take, like 1/100 or 1/1000 or something. You
can't expect to accurately simulate a 5 ms period when time is
quantized to 1 ms. Try 1 us maybe.
My rule is to make the time step as small as possible consistant with
simulation time. When in doubt, change the step time maybe 2:1 in
either direction; if the sim changes visibly, your steps are too
coarse by 10:1 at least.
What's a bummer is to have a circuit with a very high Q or widely
varying time constants; small dt and slow elements make for hour-long
transient runs.
John
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