[SI-LIST] Re: 2 layers PCB with no Plane(gnd or Power)
- From: "DAVID CUTHBERT" <telegrapher9@xxxxxxxxx>
- To: "Mohamad Haghtalab" <mohaghtalab@xxxxxxxxx>
- Date: Sun, 3 Aug 2008 16:08:55 -0600
I assume you mean this is a 2-layer board. The 'return' current will find
its way back although the characteristic impedance will vary and crosstalk
could be rather high.
If you don't have access to a PCB simulation package it can be simulated in
bits and pieces using free software. Parts of the board can be simulated in
the frequency domain using SONNET LITE. These models can then be turned into
LTSPICE models to see how it performs in the time domain. By exploring some
of the best and worst looking parts of the layout you will have an idea of
whether it will be successful or not.
Of course, it could be faster to order the board(s) and measure things -
view the signals at their destinations. A safe way to go is to order a
4-layer and a 2-layer version of the board. If the 2-layer does not work
well enough you have the 4-layer to fall back on. The stack-up could be if
it is a 62 mil thick board (1.57 mm):
Signal
10 mils
GND
36 mils
PWR
10 mils
Signal
The signal layers can be placed closer to the GND and PWR to reduce the
trace width and reduce crosstalk. With the stack-up shown the signal traces
will be about 20 mils for 50 ohm traces. Check this with a calculation or an
online PCB calculator. The PWR-GND spacing isn't great but that's the
tradeoff with a 4-layer board.
Dave Cuthbert
Analog: EMC, SI, RF, Power, and others
On Sun, Aug 3, 2008 at 3:50 PM, Mohamad Haghtalab <mohaghtalab@xxxxxxxxx>wrote:
> Hi all
>
> How can we analyse(simulate) PCBs (transmission line effect ,calculatig
> Z0(SI/EMI issues)...) with 2 routing layers and between them(signal
> tracks)ground Islands(cupper fills...),what's the ground associate with each
> signal track,how can we predict where the return current of each signal will
> flow?
>
> Thanks for helping
>
>
>
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