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MeasurementBlues
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Re: Buses and software loops
MeasurementBlues   12/4/2013 10:16:57 AM
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Also with regards to FPGAs, another test is to drive as many gates as possible all at once for testing power integrity on a board. That causes the FPGA to draw maximum current so you can see what it does to the power rails due to inductance in the power delivery network. Too deep a dive in power can cause havoc and you probably need better or more bypass capacitors on the board.

MeasurementBlues
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Re: Buses and software loops
MeasurementBlues   12/4/2013 10:07:33 AM
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When performing an emissions test, you usually want worst case and thus many systems have diagnostic software for the purpose of emissions testing. It would be like exercising more FPGA gates than are used in normal operation. Or you might drive a a whole lot of gates at once for a worst-case condition.

Max The Magnificent
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Re: Buses and software loops
Max The Magnificent   12/4/2013 9:57:41 AM
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@MeasurementBlues: Being more of a hardware type, I hadn't thought of software as a possible interference problem.

I agree -- this had never struck me -- very interesting

MS243
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Re: Buses and software loops
MS243   12/4/2013 9:53:54 AM
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Display's also radiate  -- CRT's were kind of bad with the magnetic deflection, and the new LCD, etc displays scan the array to update it -- this means that there are effectively long rows of horizontal and vertical ITO traces being switched on and off to make each pixel light or dark -- and ITO must be placed over the whole array to reduce these emmisions -- DO-160 is often hard to pass, for a display designed only to pass  FCC/CE

MS243
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Re: Buses and software loops
MS243   12/4/2013 9:47:15 AM
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SW Loops --And you change to a different memory type, with different wait states then everything changes also.

zeeglen
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Re: Very Interesting
zeeglen   12/4/2013 9:42:20 AM
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>Of course, there are cable grounding and shielding issues. The most significant is to make a 360° connection with a shield around a connector.

And make the shield connection to the chassis, not digital ground.  I once had to clean up a horrendous mess where the designer connected coaxial cable shields to his pcb digital ground plane and could not understand why the radiated emissions were so strong.


zeeglen
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Re: Buses and software loops
zeeglen   12/4/2013 9:35:17 AM
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>software  loop repetition frequency can also add spurs and harmonics

At a radiated emissions test site a manager type once told me to "Keep moving the cables around until it passes".  I can see the new version now - "Just keep tweaking the code until it passes".


MeasurementBlues
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Re: Buses and software loops
MeasurementBlues   12/4/2013 9:07:55 AM
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MS243, thanks for the tip on software loops. Being more of a hardware type, I hadn't thought of software as a possible interference problem.

MeasurementBlues
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Re: Very Interesting
MeasurementBlues   12/4/2013 9:06:20 AM
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Of course, there are cable grounding and shielding issues. The most significant is to make a 360° connection with a shield around a connector. No pigtails or you get emissions.

Understanding grounding, shielding, and guarding in high-impedance applications

Shields are your friend, except when... (Part 3), has links to Part 1 and Part 2

Grounding and shielding: No size fits all a piece I wrote in 2001, still relevalnt.

 

tom-ii
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Re: Very Interesting
tom-ii   12/4/2013 8:54:04 AM
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Excellent point.  It might also be noted that those openeings can create really strange effects and "nodes," too.

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