PCB Level EMC Examples and Measurement Options

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PCB GUIDELINES
A look at “Common” mistakes
• David Green
• Oklahoma State University
• [email protected]
IS THIS IMPORTANT?
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Interference to other devices may cause reliability issues, safety/health
concerns, etc.
Local interference on a PCB could cause increased bit error rate (BER),
signal integrity problems, unintentional device resets/outages, voltage
spikes, increased noise in on-board peripherals, etc.
Ever hear interference generated
by your cellphone?
OVERVIEW OF BASIC PRINCIPLES
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Return current is the #1 cause for EMI
Remember, current flows in loops.
()

can generate large high frequency harmonics
What path does current take on the reference plane?
For low frequencies return current follows a path of least
resistance. For high frequencies current follows a path of least
impedance.
The impedance for an inductor increases with increasing
frequency. Must take into consideration at high frequencies.
“Ground is a place where potatoes and carrots thrive!”
– Dr. Bruce Archambeault
THE REFERENCE PLANE
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Three critical functions:
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Provides a stable voltage reference for digital signals
Distribute power to devices
Used to reduce interference between signal lines
As mentioned, low frequency return current follows path of
least resistance, whereas high frequency current follows path
of least impedance.
The overall goal is to reduce return current loop area.
Some layout considerations:
• Split plane – disrupts return current flow to excitation source. This can result in
drastically increased emissions.
• Placing holes in board for connector pins – Holes too big can essentially cause
a split. This can be corrected by allowing space in between neighboring holes
to allow return current to run through.
• When running signal traces on the reference plane keep in mind their
placement. Coupling can be reduced by placing signal lines perpendicular to
traces on top of PCB.
RETURN CURRENT PATH
Simulation frequency of 20 MHz… Sound familiar?
Top of PCB
Bottom of PCB
SPLIT REFERENCE PLANE MEASUREMENTS
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Measurements were performed in a fully anechoic chamber
Log periodic antenna
used for far field
measurements from
290 MHz – 2 GHz.
Dual Ridge Horn
antenna used for 700
MHz to 4 GHz.
REFERENCE PLANE SPLIT
Split Plane – DRG
Split Plane – Log Periodic
Solid Plane – DRG
Solid Plane – Log Periodic
SIMULATION VS. MEASURED RESULTS
Interesting!
So why the difference?
• Source power of simulation is 13dBm, measured results are 18dBm
• How do we really model connections, components, structures, etc?
• Can we trust measurements?
ADDING A STITCHING CAPACITOR TO PLANE
Offset stitching capacitor
Centered stitching capacitor
• Obvious difference between placing the capacitor at the center of the split and offset
on the split
• Not a significant difference between different values of stitching capacitors
COMPONENTS ACT THE WAY THEY SHOULD AT
ALL FREQUENCIES RIGHT?
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Short answer: Nope!
Components do not behave as they were intended to at high
frequencies
DECOUPLING CAPACITORS
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Functions
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Time domain view - Provides “charge storage” near the IC. Helps
maintain supply voltage at the device.
Frequency domain view – Switching (ie. Logic gate switching, oscillators,
etc.) can generate a wide range of frequency components. The
decoupling cap acts as a low impedance shunt.
Some layout considerations:
• Keep decoupling capacitors close to the IC of concern. Why? Because additional
inductance introduced by the trace will effect how well high frequencies are
filtered. Increased trace length = increased parasitic inductance.
• General rule of thumb is that a single .01uF capacitor is sufficient. This isn’t
necessarily true. If possible, use recommendations from IC/component
datasheet. Capacitor will have a self resonance, this can be used to determine
range of frequencies filtered.
• Use a minimum of one cap per power pin.
DISTRIBUTING POWER
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Generally, students in this course use a source multipoint
power distribution.
Multipoint systems can have common impedance coupling
Multipoint is okay and easy to implement, but be mindful of
trace locations on PCB.
Source: PCB Design Guidelines for Reduced EMI by Texas Instruments
WHAT ABOUT POWER REGULATORS/BOOSTBUCK AND OTHER SWITCHING COMPONENTS?
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Switching components, especially those which source a
significant amount of current, can cause large frequency
harmonics.
How do we suppress the generated RF noise? Use an RF choke
(ie. Ferrite bead) and/or decoupling capacitor after the
component.
OSCILLATORS
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Crystals and ceramic resonators should be placed away from
high frequency devices. This reduces capacitive coupling
between oscillator pins and PCB traces.
Oscillators should be placed close to the IC. Long trace leads
can cause unwanted coupling, compromising the signal
integrity of the clock signal.
A reference plane should be located under the oscillator
It is recommended to place a “guard ring” around the oscillator
and associated components. The guard ring should be
connected to the nearest VSS of the device.
COMMON MISTAKES IN SENIOR PROJECTS
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Leaving programmable I/O pins floating.
 Leaving output pins unconnected is okay if they are ALWAYS
output pins
 A floating input pin WILL cause chip problems.
Some considerations:
• Set unused pins to be outputs or set to be input pins and use pullup or
pulldown resistors.
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When setting up multiple μControllers/ devices for
communication, be sure they share a common reference. In the
past a team tried implementing SPI communication between
PICs. The PICs weren’t communicating to one another. A simple
wire connecting all of the device’s reference planes together
could have solved the problem. Unfortunately, this wasted a
considerable amount time.
COMMON MISTAKES IN SENIOR PROJECTS
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Check the reference plane. Traces will likely need to placed
on the reference plane side of the board. Make sure these
traces do not create a “ground island”.
If you have to place a signal line on the reference plane side
of the PCB, do not route this parallel to a PWM or similarly
switching line. An example: A couple of semesters ago a
team routed a trace going to a speaker next to a trace going
to a high-power LED that used a PWM signal to control it’s
brightness. This resulted in an audible tone in the speaker
while the LED was on.
In another project a team routed a wire near a switching
device on a PCB. Interference was picked up by the speaker.
REFERENCES
[1] Bruce Archambeault, et al., “Special Topics in EMI/EMC
Modeling,” in EMI/EMC Computational Modeling Handbook,
2nd ed. Boston: Kluwer, 2001, pp. 232-252.
[2] Texas Instruments, “PCB Design Guidelines for Reduced EMI,”
1999. [Online]. Available:
http://www.ti.com/lit/an/szza009/szza009.pdf
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