CircuitMaker Step Time puzzle

From: Terry Pinnell (terrypinDELETE_at_THESEdial.pipex.com)
Date: 11/29/04 

Date: Mon, 29 Nov 2004 20:39:37 +0000

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. 

-- 
Terry Pinnell
Hobbyist, West Sussex, UK

Relevant Pages

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