Decoupling fundamental frequencies of PCB and chassis using Steinberg Octave Rule

[markpheus]

Hi guys,

Just a question regarding PCB and PCB enclosure/chassis design. Steinberg states that the fundamental frequency of the PCB and chassis be decoupled by an octave or greater but mentions noting of PCB/Chassis relative orientation or mass participation.

For example: If I mount a PCB parallel to a simply supported beam. I am comparing the Mode 1 for each, which, due to their shape and support is in Z direction for both (BTW using same coordinate system for both).

Lets say that I don't achieve decoupling, Mode 1 for beam is 600 Hz and Mode 1 for PCB is 600 Hz both modes in Z -direction.

If I now change design and have the PCB mounted perpendicular to the beam. Its critical axis is now in X-axis and at 90 degrees to the beam. Mode 1 for PCB is still 600 Hz but in X-direction and Mode 2 for PCB is in Z-direction and is 1200 Hz.

Question is: is PCB and chassis coupled because Mode 1 for PCB = Mode 1 for beam or is it decoupled because Mode 1 for beam is 600 Hz in Z-direction and the corresponding Mode (Mode 2 in Z direction) of PCB is 1200 Hz?

Thanks in advance,

Mark

 

[GregLocock]

You may get some reduction in response but you'll still have a strong 600 Hz resonance. Mode shapes don't understand our puny 3 axis coordinates, they are really energy flows, and have no trouble going round corners.

A factor of 2 on frequency separation is rather an excellent criterion, half that is good enough (a highly damped system may have enough bandwidth for 50% separation to be insufficient, but highly damped systems aren't the problem).

Cheers

Greg Locock


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[markpheus]

Thanks Greg,

I thought this much since the orthogonality of the respective modes is never mentioned!

Do you know of a source for proof of this? Book, publication or something else I can reference?

Kind regards,

Mark

 

[GregLocock]

Not really any references as such.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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