[Arm-netbook] HDMI High-Frequency Layout: Recommendations

[Richard Wilbur]

On Thu, Aug 10, 2017 at 2:01 AM, mike.valk at gmail.com
<mike.valk at gmail.com> wrote:
> GND shielding parallel to the differentials is interrupted quite
> often. Those GND tracks act as shields, for emission and reception.
> I'd try to put as much parallel GND as possible.
>
> And trace the parallel GND around the via's, see attachment.
>
> Make sure the'res as much solid GND on the layer above and below the
> traces, again shielding.
>
> Also I'd personally not use curved wriggles. HF signals travel in a
> straight direction. With curves they start diffracting and start
> bouncing cross each other and might start to radiate or echo back. But
> I see that the community is divided on that stance.

I also prefer 45 degree corners to the curves.  Looks like they only
occur in one section.

> If tight for space you can use 90% corners with a chamfered outer
> edge. I suppose the chamfer acts like a mirror.
>
> https://www.maximintegrated.com/en/app-notes/index.mvp/id/5100
> Figure 6

This is good advice for single-ended signals on a
stripline--high-speed digital and RF.  That is the situation Maxim are
addressing in the referenced document.  The signals we are dealing
with are high-speed digital but transmitted in differential mode on a
microstrip.

Single-ended signals are transmitted relative to a ground reference
and so putting ground reference next to them tends to block the
side-view of the antenna created by either microstrip or stripline,
thus reducing radiated and coupled interference.  That's a very good
thing!

microstrip (The following diagrams are in cross-section perpendicular
to the direction of signal transmission.  Think of the signal going
into the diagram away from the viewer.)

single-ended signal without ground shield traces

signal   +
dielectric       from the side we see a dipole antenna
ground   -

single-ended signal with ground shield traces
-          +       -
ground  signal  ground   -
dielectric dielectrc dielectric
ground ground ground ground   -

(ground shield traces would need some vias to connect them with ground
plane)  This blocks the view of the dipole antenna from the side and
reduces the size of the dipole antenna so that far field it is
vanishingly small being primarily the area between the ground shield
traces and the signal trace.  (Far field:  distance from microstrip at
least 10 * separation between signal and ground shield traces.)

Since we have a different geometry, the problem changes.  We are using
differential microstrips.  Differential-mode signals are transmitted
relative to each other instead of ground.  Only common-mode noise in
the signals is transmitted relative to ground.

microstrip

differential-mode signal without ground shield traces

signal+    signal-
dielectric dielectric
ground ground ground

Here the dipole antenna is limited to area between the two signal
traces, blocked on the bottom side by ground plane, and insignificant
in far field (because the traces are close together, have opposite
potential and currents, and the fields cancel each other).

I'm out of time to add detail or references, so sending now.