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Sherlock Ohms

The Ghost Image Messed With the Display

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Rob Spiegel
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Good logic
Rob Spiegel   11/6/2012 2:13:22 PM
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This is a great example of deductive logic, chasing that one bit, and then coordinating the two clocks.

SteveG56
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Re: Good logic
SteveG56   11/6/2012 4:05:34 PM
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Rob,  I don't want to tell you how long it took to make that deduction!

Steve

JamesCAnder
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Re: Good logic
JamesCAnder   11/6/2012 4:24:52 PM
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Mr Gross,

This is why engineers are paid so well, intuition and experience can solve a lot of issues.

I am in the same boat with SteveG56.. How long did it take you to figure out the issue?

I have a similar "Ghost in the machine" problem too.. Took me a month to realize it was noise in the surrounding environment. Along the way... my intuition took me on a snipe chase..

JC

Rob Spiegel
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Re: Good logic
Rob Spiegel   11/6/2012 4:31:02 PM
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When you did get to the right solution, Steve, it must have been a big "Ah ha" moment. Looking back you probably thought, "But of course!"

SteveG56
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Iron
Re: Good logic
SteveG56   11/6/2012 4:58:36 PM
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Once we really focused on the problem it took about a day or so to figure it out, but we blamed it on bad connections, etc. for a couple of weeks.

SteveG56
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Iron
Re: Good logic
SteveG56   11/6/2012 5:00:44 PM
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You're exctly right.  The thing was, the first spread spectrum clock oscillator (I'm not a fan of those) had a narrower "spread" and didn't cause the problem.  It was when we widened the spread, to get the emitted noise spikes lower, that the ghost images started showing up.

Rob Spiegel
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Re: Good logic
Rob Spiegel   11/6/2012 5:57:54 PM
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Yes, I know how that goes, Steve, when you think the problem is in one area and it's acually in another area.

Charles Murray
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Re: Good logic
Charles Murray   11/6/2012 6:08:54 PM
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Great sleuthing work, Steve. I agree with JamesCAnder's comment below: "...intuition and experience can solve a lot of issues."

Battar
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Re: Good logic
Battar   11/7/2012 9:25:15 AM
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Steve, actually you just had to follow the 4 rules of fault-finding for technicians. It's under rule 4. The rules are - 1)Turn it off, turn it on again 2)hit it 3) read the data sheet 4) look for the problem at the last place you touched before it stopped working. That usually solves 97% of all problems.

SteveG56
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Iron
Re: Good logic
SteveG56   11/7/2012 11:08:26 AM
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You're right, Batter.  And I think the 97% statistic for step #4 is pretty close.  In this case the problem was on a board 5 slots away on the backplane and some time had elapsed since the previous design change.  Regarding the pesky spread spectrum clock oscillator, we were probably thinking if a little is good, a lot will be better.

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