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Max The Magnificent
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Very Interesting
Max The Magnificent   12/3/2013 3:04:37 PM
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I have to admit that (coming from the days of much slower clocks and signals) I'd never really thought about this -- thanks for sharing.

tom-ii
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Re: Very Interesting
tom-ii   12/3/2013 3:12:37 PM
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From a Motorola Semiconductor Design Guide:

 

A handy rule of thumb to determine if an interconnect trace should be considered a transmission line is if the interconnect delay is greater than 1/8th of the signal transition time, it should be considered a transmission line and afforded all of the attention required by a transmission line.

MS243
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Re: Very Interesting
MS243   12/3/2013 7:02:38 PM
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This can be daunting on any type of vehicle, where many different systems signals can mix in a small space.   There are somespecial  techniques for managing this in one's reciever / baseband chain.

tom-ii
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Re: Very Interesting
tom-ii   12/3/2013 7:27:45 PM
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Often, for more complex systems, you can break it down some, and choose the "prime" frequencies - it will often be that much of the equipment uses similar frequencies, and you can throw these into your plan as well.  But you are correct - this can get onerous - and in those cases, you likely just have to use your best judgement and also lean on the experts that are working your grounding/shielding to tell you which frequencies will likely be the worst problems.

MeasurementBlues
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Re: Very Interesting
MeasurementBlues   12/4/2013 8:19:32 AM
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Openings in enclosures should also be less than ¼λ for EMI purposes. openings with lengths of that size or longer are more likely to let emissions out of the box that smaller openings.

MeasurementBlues
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Re: Very Interesting
MeasurementBlues   12/4/2013 8:25:42 AM
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"coming from the days of much slower clocks and signals"

I won't comment on Max's slower days, but another issue is power supply clock frequencies, especially when high-speed digital links are involved. power supply or other clock frequencies can couple into digital clocks, creating periodic jitter. The higher the clock rate, then less jitter it takes to ruin signal integrity.\

Signal integrity engineer Daniel Chow explains periodic jitter in Jitter: Measurement References Matter. The animated graphics really bring it to life. The article has links to numerous other articles covering jitter.

MS243
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Buses and software loops
MS243   12/4/2013 8:53:48 AM
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One often also has to factor in external buses and software loops -- software  loop repetition frequency can also add spurs and harmonics off of each fundamental, as do the SMPS switching spikes.

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.

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.

 

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.

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