[Arm-netbook] EOMA68-A20 2.7.4 pre-production prototypes received and working

[mike.valk at gmail.com]

2017-06-21 20:04 GMT+02:00 Luke Kenneth Casson Leighton <lkcl at lkcl.net>:
> On Wed, Jun 21, 2017 at 6:26 PM, Wolfram Kahl <kahl at cas.mcmaster.ca> wrote:
>> On Wed, Jun 21, 2017 at 12:00:07PM +0100, Luke Kenneth Casson Leighton wrote:
>>> .On Wed, Jun 21, 2017 at 9:35 AM, mike.valk at gmail.com
>>> >>  in a stack you tell the factory what thicknesses you want, as well as
>>> >> what material in between, and what thickness of that, too.  so you get
>>>
>>> > So it doesn't have to be a problem. As long as you control the layers
>>> > that have traces have the same thicknesses.
>>>
>>>  technically correct but far too much risk and hassle.  you end up
>>> tying the PCB layout to a specific PCB manufacturing factory.
>>
>> Are there high-frequency risks/problems with switching back and forth
>> between the layer pair.
>
>  there are.

Every set of parallel wires act as both inductors and capacitors.
https://www.engineersgarage.com/sites/default/files/imagecache/Original/wysiwyg_imageupload/4214/CIRCULAR%20CONNECTORS%203.JPG.

With DC inductance is less of a problem. But with HF signals you don't
want return signals canceling out the main signal.

That's why the lines need to be parallel and of equal length.

And why I hate the length matching only on the end of the line.

> the more VIAs you have the more EMI there is.  R.F. (and
> HDMI is R.F.)

HDMI is HF, High frequency, which causes RF, Radio Frequency's, emissions.

EMI, Elektro Magnetic Interference, is the result of RF hitting your
signal line and creating noise and distortion.

Every electrical current causes an EM field. But with DC it is static.
With HF is dynamic making to harder to read the signal properly and
without errors.

>
>  the best track layouts use curves not 45 degree transitions.

That might not be true. Curves might actually be more problematic.
When the signal hits a wall it deflects. Like light on a mirror. With
curves the signal starts bouncing in zigzag pattern. Making the
distance traveled more unpredictable. And in worst case the signal
starts traveling backwards creating echo's.

HF Signals also tend to move on the outside of a conductor/track.

I guess that's why via's are so bad. The are round and change route at
90 degrees, downward or upward. So the signal starts bouncing and
echoing. Creating RF noise.

But with BGA IC's you have no other option than to use VIA's and
sometimes signals need to cross so you have to as well.

> the best layouts have no track changes at all, are as symmetrical as
> possible, are completely surrounded symmetrically by the exact same
> amount of space on either side, and the exact same number of vias on
> both sides of the track.  and also are impedance matched in terms of
> distance between the pairs, width of the tracks, *and* the distance
> between layers *and* the dielectric constant of the insulation between
> the layers.
>
> it's a pretty heavy-duty amount of requirements.
>
> l.
>
> _______________________________________________
> arm-netbook mailing list arm-netbook at lists.phcomp.co.uk
> http://lists.phcomp.co.uk/mailman/listinfo/arm-netbook
> Send large attachments to arm-netbook at files.phcomp.co.uk