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Charged capasitive layer inside a pcb - is that really so fantastic?
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AndyC_772:
The problem needs to be clearly, specifically defined before it can be answered correctly.

If the question is, "does a solid plane act to reduce interference between a noise source on one side of the board and an audio trace on the other?", then the answer is 'yes'.

If it's "does connecting a capacitor between a noise source and an audio trace reduce interference?", then the answer is 'no', quite the opposite.

FriedMule:
Thanks, my question is also therefore, to avoid the capacitor "effect" I think that i need to make some connection between the two layers.
Can I just do that by make a row of via's 1-2 mm around from the edge, Or do I have to make via's in the center of the PCB also??
AndyC_772:
No, I still think you're misunderstanding.

If one plane were connected (only) to the noise source, and the other were connected (only) to the audio signal, then the capacitor formed by the planes would be a way for noise to couple from one to the other. The effect would be small because the capacitance is small, but it would be a real effect that you could measure. You might even hear it, possibly.

However, that's not the circuit. It is NOT the case that just placing a capacitor between two signals, without actually connecting it to those signals, allows noise to couple between them.

A solid ground plane placed between two layers of the board acts as a shield. A second plane might also act as a shield, but since you already have one, there's no extra benefit especially at audio frequencies. Whether or not you join them with vias at the edges, in the corners, at a single point, or at 2mm intervals over the entire surface of the board, makes no difference.
T3sl4co1l:

--- Quote from: AndyC_772 on April 04, 2019, 12:30:36 pm ---He's right, but for completely the wrong reasons.

A capacitor doesn't block AC, it passes it -

--- End quote ---

Or, even more precisely -- a parallel capacitor to ground acts to block AC transmission (shunting it to ground), while a series capacitor acts to permit AC transmission (exclusive of DC transmission).

And, even more generally, the purpose of parallel planes within the PCB is to achieve a low impedance, nevermind the capacitance.  In general -- that is, over hundreds of MHz of bandwidth (such as matters for digital and RF circuitry, the domain where this kind of thing matters), the impedance will be up and down, not simply one way or another.  The plane helps in achieving a particularly low impedance across the band.  Compared with single capacitors, that have an impedance that falls until the SRF, then rises again.  (Mind, you still have to be careful when putting together planes and different capacitors: they can resonate against each other and make things worse.  PDN (power distribution network) analysis isn't terribly hard, and is cheap insurance!)

To echo the point others have noted: notice this is only relevant at high frequencies.  At audio frequencies (up to and including the low ~MHz, where active devices are just rolling off*), planes aren't very important.

*Rolling off, i.e., gain is near 1, and phase shift is large (>90 degrees?).  This is a sensitive region, and it doesn't take much phase shift to turn it into an oscillator.  That phase shift will arise in the form of lumped capacitance or inductance though, because again, the frequency is fairly low (say, a wavelength of ~100m).

Tim
FriedMule:
Thanks, so if we are talking about a 4 layer board, it would be fare better to use the two topmost layers for the audio (running same track on both of them), 3th layer as shield and the bottom layer to power, to lower the impedance?
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