Michael
I think you misunderstood what I said. What I did forget to say is that
the image plane theory uses a virtual mirror with an infinitely thin
plane. You mirror the trace on the other side of the plane, and then
remove the plane. It is "as if" there were another antenna 180 degrees out
of phase placed on the other side of the plane, with the plane removed,
since then sum of the two "antennas" is zero at the plane. It's an
approximation, but a pretty good one. Simple method of moments can be used
to determine the far field radiation pattern. At the limit, as the trace
asymptotically approaches the plane, radiated emissions approach zero.
I'm aware of all the other effects of finite conductivity metal planes.
I've had to measure field penetration through copper planes, which can be a
bear at low frequencies.
Scott
Scott McMorrow
Technical Director SI/PI
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
On Wed, Jul 6, 2016 at 1:41 PM, Michael Vrbanac <mikev@xxxxxxxxxxxx> wrote:
Scott,
Well, not exactly... if that really happened, there would be effects that
we actually do not see in practice i.e. crosstalk to signals on the
opposite side of the plane. I understand the "virtual" thing but it does
nothing to explain how it really works beyond that analogy. Forgive me for
being "picky" here but it is precisely these kinds of things that lead
people make incorrect assumptions. I see this all the time and I try to
help fix that wherever I can. Here's how it really works...
There is mutual inductance and capacitive coupling to the reference plane
by the transmission line definition that creates an out of phase
(reverse) current flow and the planar conductor skin effect/depth keeps it
from co-mingling with other signal return currents on the other side.
That out of phase current flow creates a cancellation field for the source
signal's field produced on the trace above the plane. For the single ended
case, the return current exists in the signal return plane below. For the
differential case, the return current returns in the other signal line in
the pair and the common mode residual generated due to the imbalance of the
entire signaling pair/drivers/loads appears on the plane below. If the rest
of the signaling system is balanced well, failure to couple the
differential line "slightly" to "discipline the pair routing" properly
causes increased RF common mode energy generation simply because there is
no meaningful "line balance" and that is not good for reducing RF emissions.
It's all there and the effects match the theory...
Have a great day!
Michael Vrbanac
EMC Forensics Functional/SI/PI/EMC Design
Austin, TX
On 07/06/2016 10:25 AM, Scott McMorrow wrote:
Because of the magic of the image plane.and
The image plane already causes far field cancellation for microstrip.
Essentially, on a microstrip with a 4 mil dielectric, the virtual equal
opposite image trace is 8 mils away. It's like operating two dipolemuch
antennas 180 degrees out of phase at a spacing of 8 mil. They pretty
cancel each other out.wrote:
Scott McMorrow
Technical Director SI/PI
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
On Wed, Jul 6, 2016 at 11:18 AM, Jeff Loyer <jeff.loyer@xxxxxxxxxx>
might
Something that Iââ¬â¢ve wondered about is: If microstrip (or stripline)
yetcause EMI, how come Intel never saw any significant difference when they
went to differential busses between processors? Intel replaced the
single-ended front-side bus (high EMI) with differential signaling (low
EMI) for communication between the processors and memory controller and
theIââ¬â¢ve never heard anyone write (or remark) that this made an impact
on
moreEMI signature of their servers or desktops.
*Jeff Loyer*
*From:* Sanjeev Gupta [mailto:sanjeev@xxxxxxxxxxxxxxxxxxxxxxxxxx]
*Sent:* Wednesday, July 6, 2016 6:06 AM
*To:* istvan.novak@xxxxxxxxxxx
*Cc:* doug@xxxxxxxxxx; si-list@xxxxxxxxxxxxx; Jeff Loyer; Lee Ritchey
*Subject:* Re: [SI-LIST] Re: Via fence along board edge
I have another question to EMI experts on the same lines and add few
EMpoints in the discussion..
Which is good from EMI point of view.. stripline or microstrip routing?
I conducted few EMI simulation experiments long time back and found
something interesting.
experiment was related with microstrip vs stripline radiation using 3D
striplinetool in real PCB condition and measured far-field@3m .
I found stripline radiation much higher than microstrip. This is
definitely not consistent with established myths..Normally stripline is
supposed to be better than microstrip routing for EMI. But, any
doesrouting in real PCB will come with at least 2 via transition to make
connection with components.
The conclusion was pointing towards that any via-addition in the layout
will create good amount of radiation. Therefore, stripline radiation was
worse than microstrip.
I am not sure how does this conclusion will apply to GND via stitching
fencing and ongoing discussion. But, any via addition in the layout
innot seem to be free insurance.
Another simulation experiment was related with radiation through bends
(90degree, 45 degree,45 degree mitred). I could not find any difference
degreeradiation till 10GHz. difference was marginal beyond 10Ghz. So 90
PCB.bend radiation is also a myth. Many earlier discussion on si-list also
confirms the same conclusion.
it is not possible to simulate the absolute EMI radiation from given
But relative EMI assessment for different layout condition is definitely------------------------------------------------------------------
possible.
Thanks
Sanjeev
SigIntegrity Solutions
www.sigintegrity-solutions.com
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