[Arm-netbook] Arm Netbook, Saw the update,

[Luke Kenneth Casson Leighton]

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crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68


On Tue, Jul 18, 2017 at 6:16 PM, Richard Wilbur
<richard.wilbur at gmail.com> wrote:
> On Mon, Jul 17, 2017 at 1:16 AM, Luke Kenneth Casson Leighton
> <lkcl at lkcl.net> wrote:
>> On Mon, Jul 17, 2017 at 7:26 AM, Richard Wilbur
>> <richard.wilbur at gmail.com> wrote:
>>> Wonderful!  That's music to my ears.  No major obstacles on return current path except the vias.  So when we change signal layers, we'll need adjacent ground plane-to-ground plane vias to provide a nice low-impedance path for the return current in the ground planes.
>>
>>  i've added as many of these as i can fit.  space is... very very tight.
>
> For our differential pairs we should only need one return-current-path
> via per signal via (and hopefully relatively adjacent to it) since we
> are using ground planes for our reference planes.  It is basically
> trying to provide a relatively low-impedance path for the RF
> (radio-frequency) return current in the reference planes (since when
> we switch signal layers between top and bottom, our reference plane
> changes).

 as best i can i've put the vias as close as possible.  here's some new images

 http://hands.com/~lkcl/eoma/a20_hdmi_review/

 i've also lined the bottom edge with vias, and there are a lot along
the top of the diff-pairs, separating other signals.

>>   PADS can calculate impedance based on board stack, track width and
>> track-to-track spacing... i'm... relying on that, and the fact that
>> the tracks are 50mm long.
>
> From what I'm reading this looks easiest (and best) to tackle in
> segments of no more than 15mm at a time.  Can PADS work with you on
> one section of the path at a time?

 mmm..  don't think so.  i can select a pin pair and it tells me that
the impedance is 89 ohms.

>  More details in next message.
>
> 1.  Are you specifying the track width,

 minimum 5mil.  specified in groups.

> track-to-track spacing,

 5mil minimum

> and board stack

 board thickness 47.3 (1.2mm), 1oz copper, 6.4mil on FR4.

> or is PADS determining that for you?

 how would it know?

> 2.  What are you using for the ...
>   a.  microstrip differential pair track widths,

 5 mil

>   b.  intra-pair track spacing,

 5 mil

>   c.  differential-pair-to-other track spacing?

 5 mil

> 3.  What is your copper thickness on top and bottom layers?

 1oz

> 4.  What is the height of insulator between top layer and adjacent
> ground reference layer?

 6.4 mil

> 5.  What is the height of insulator between bottom layer and adjacent
> ground reference layer?

 6.4 mil.

 standard 6 layer stack basically.

l.