It is extremely difficult to create appreciable radiation from any 50 ohm
trace over a continuous plane. Simply using image theory you'll see that
the antenna return image antenna is so close as to have incredibly like
radiation efficiency. In the case of microstrip (the only trace
construction that can potentially radiate) trace height over the plane is
usually 5 mil or less. The image antenna is therefore 10 mil away. The
trace and it's out of phase image are effectively so close that the fields
cancel in space, except at extremely high frequencies. How high? Well, 10
mil at an effective propagation delay of 120 ps/in is a 1.2 ps of delay.
That's equivalent to a lambda of 833 GHz.
Now, full disclaimer, I'm not an antenna guy. But on axis orthogonal to
the PCB plane the net far field strength is essentially zero at frequencies
that are significantly below 833 GHz. Maximum field strength with be at
about 45 degrees. Do the trig and field cancellation at say Two decades
down at 8.33 GHz would be extremely low.
Now you can argue that emi can occur with very low radiated power, which is
true. But the board is surrounded by a shielded enclosure which further
attenuates fields.
I've not read the paper, but it is much more likely that there is a metal
structure in the phone that is in proximity to the serpentined traces and
is coupling to them.
Scott's rule of thumb: "if you've designed the board with 50 ohm traces and
have a continuous ground plane, then the EMI problem is coupling to
unshielded metal, such as an outer shield that is too close, or a cable
that is laying on top of the trace."
Prove me wrong.
How do you explain this to a customer? Simply do the first principles
analysis and step them through it.
Regards,
Scott
On Sat, Jun 5, 2021, 11:36 AM Nicholas Franconi <
nicholas.g.franconi@xxxxxxxxx> wrote:
Also, that paper you reference has actually been retracted.
https://ieeexplore.ieee.org/document/8114328
Nick Franconi
On Sat, Jun 5, 2021 at 11:12 AM Chuck Corley <corley@xxxxxxxxx> wrote:
Just to add something to that, in my post I was referring to EMIhttps://www.researchgate.net/publication/321143234_Differential_Lines_Paired_with_Serpentine_as_Potential_EMI_Aggressors_in_Mobile_Electronic_Devices
(emissions).
There is also another side issue of crosstalk between traces that your
customer might possibly be thinking of. There is the possibility of
long serpentine traces increasing crosstalk between traces and causing
signals to fail. Especially if making space for the serpentine traces
causes the layout to pack traces closely together, making the crosstalk
worse.
Chuck
---
Chuck Corley
On 2021-06-04 21:01, Zachariah Peterson, NWES wrote:
Hi Chuck,
That's exactly how it was done: only routing over a solid ground plane
with no gaps, so I'm unsure what my client could be referring to. I'm
now wondering if this is a case where the client got this idea because
someone did the routing incorrectly in the past (routing over a gap or
whatever), and then this guy is blaming in on the structure of
serpentine routing.
--
Zachariah Peterson
On 2021-06-04 20:49, Chuck Corley wrote:
Short answer: If you have a good plane ('reference plane') as you
should have under those traces to set the proper trace impedance
then they will couple to that plane and not significantly radiate.
If you didn't have a solid reference plane close under your traces
then you've built an antenna and they will radiate. Some people do
this accidentally, by having big gaps or other bad features in their
reference plane.
Chuck
---
Chuck Corley
On 2021-06-04 20:16, Zachariah Peterson, NWES wrote:
Hi everyone,
While I was conducting a design review today, my client expressed
some
concern over serpentine routing that was used to enforce length
matching
between an SDRAM chip running at 166 MHz (1 ns rise time) and an
MCU.
The concern was that the serpentine routing structure would create
an
EMI problem. The traces are routed according to specs in the
datasheet
with a reference plane on the adjacent layer.
I have never heard of serpentine routing creating a new EMI
problem or
making some existing EMI problem worse. I started looking into it
and I
find mixed recommendations. This is one instance where I won't
trust
application notes, they all give inconsistent guidance, and they
show
poor images of the exact way serpentine routing should be
implemented.
I looked into the literature and there are some studies on EMI,
effects
on eye diagrams, and reflections from serpentine routing
structures.
Here's one example:
------------------------------------------------------------------
If anyone can provide some perspective, or if someone can confirm
this
is just another signal integrity myth, I'd appreciate it. Thanks
in
advance.
--
Zachariah Peterson
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject
field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List forum is accessible at:
http://tech.groups.yahoo.com/group/si-list
List archives are viewable at:
//www.freelists.org/archives/si-list
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
