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

[Richard Wilbur]

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).

>   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?  More details in next message.

1.  Are you specifying the track width, track-to-track spacing, and
board stack or is PADS determining that for you?
2.  What are you using for the ...
  a.  microstrip differential pair track widths,
  b.  intra-pair track spacing,
  c.  differential-pair-to-other track spacing?
3.  What is your copper thickness on top and bottom layers?
4.  What is the height of insulator between top layer and adjacent
ground reference layer?
5.  What is the height of insulator between bottom layer and adjacent
ground reference layer?

--
Richard