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Beyond the BooksEMC, T-Lines & PCBs
Eric Hartner Senior Engineer at National Instruments
U of M – BSE EE ’04
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Agenda
• About NI• Products and Applications• EMI/EMC• Transmission Lines• PCB Non-Idealities
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• 7,100 employees
• More than 1000 products
Dr. James Truchard CEO
National Instruments
• More than 50 international branches in over 45 countries
• Corporate headquarters in Austin, TX
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We equip engineers and scientists with systems that accelerate productivity, innovation, and discovery.
Our Mission
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What We DoWe provide graphical software and modular hardware to build measurement and control systems.
Low-Cost Modular Measurement
and Control Hardware
Productive SoftwareDevelopment Tools
Highly IntegratedSystem Platforms
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If you can turn it on, connect it, drive it, or launch it, chances are NI platform-based
technology helped make it happen
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Boeing – Reducing Aircraft Noise
• 600-1000+ -- ground based microphones• 150-250+ -- 4ch audio analyzer boards• <1ns synchronization• Lots of DSP
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CERN – Large Hadron Collider
• 27km in circumference, 150m underground• Distributed PXI system
• 200 Chassis• Provides synchronization, control, and real-time feedback• Aligns particle beam & accelerates to near the speed of
light
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NI PXIe-5451
• 2ch, differential, 16-bit, 400 MS/s/ch • Time domain, I/Q, and IF signal generation• 145 MHz analog bandwidth • 98 dB close-in SFDR at 1 MHz • ±0.34 dB flatness to 120 MHz • 2.2nV/rtHz average noise density • 25 ps channel-to-channel skew • <-146 dBc/Hz phase noise at 10 kHz
• $14k to $23k depending on memory
• Playback a recording of the entire AM/FM Spectrum
• Real world data for radio test and VnV
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NI PXIe-5644R VST
• Vector signal analyzer and generator• 65 MHz to 6 GHz frequency range• Up to 80 MHz instantaneous bandwidth• User-programmable with LabVIEW FPGA
• $45k
• Test any* wireless device• Generate any* wireless signal
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C Series
• NI 9212 ±78 mV, Isolated Thermocouple Input • 95 S/s/ch, 8 Ch Module • ±78 mV, 24-bit ADC for up to 0.01 °C sensitivity• Accuracy up to 0.29 °C• 250 Vrms, CAT II channel-to-channel isolation• 50/60 Hz rejection
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EMI/EMC
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What is Electromagnetic Compatibility (EMC)?• Preventing undesired operation in an environment with Electromagnetic Interference (EMI)
• Two Conditions• Must not suffer degradation• Must not cause degradation
• Importance• Regulation • Customer Satisfaction• Quality
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What can go wrong?
EMI in Auto Electronics - sudden acceleration issues
Effect of EMI on TV picture
USS Forrestal Fire in 1967 was caused by a missile which was accidentally fired due to an electrical power surge
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EMC Strategies: Options
• Design for EMI/EMC• Sensitive Victims• Noisy Aggressors
o If both are on your board, its called crosstalk
• Assume you will have problems and design in multiple solutions
• Easier to remove these solutions if not needed than to band-aid them in later
• Have a Plan B and Plan C, and ….
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EMC Strategies: Beyond the SchematicWhat could impact EMC that isn’t obvious from the schematic?
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?
Hidden Schematic
• Unintended Coupling
Answer: Config_Clk may couple noise onto NC223. No connectsare unterminated and thus can contribute to radiated emissions failure, Or coupling into other circuitry (crosstalk).
Question: Where is the problematic ‘hidden schematic’ above?
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EMC Strategies: Antennas
Clock Switch
What are some unintentional antennas in a design?
0.7 m
7 cm
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Fast rising edge
Near and far end crosstalk
Near end crosstalk
Far end crosstalk
Oscillation around 1.35GHz
EMC STRATEGIES: ANTENNAS1.6” of trace,
• one HiZ • one driving low to
highIs this a problem?
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Square Waves
• Where is all the energy in a square wave?• A: In the Edge
• What does the frequency spectrum of a 10MHz square wave look like?
• Is this a square wave?• Yes! It is just not 50% duty cycle.
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EMC Strategies: Return Currents
• Electrons will follow path of least impedance when returning to source
2Density
HD
1
1 I
H
D
GND
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EMC Strategies: Return Currents
Hidden Schematic => Unintended Return PathsWhere will return current flow?Can we simulate this?
For EMI purposes, the returning current path is as important as the intended
signal path!
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EMC Strategies: Minimize Loop Area
Keep in mind parasitic inductance of stitching caps and vias for return current
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Capacitors
EMC Strategies: Non-Ideal Behavior
• Know non-ideal behaviors• Evaluate the components at all frequencies you care
about!Transformers
Keep in mind for ties between grounds as well
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RF RectificationNon-linear circuits cause rectification aka “audio rectification” • Creates offset errors in DC measurements• Inputs of active devices have diode junctions
• These are non-linear circuits or RF rectifiers
Source : Analog Devices
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Transmission Lines
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Review – Electrical Length
• Signals cannot travel infinitely fast.• Limited to speed of light (in medium)
• Voltage along the length of the line at T1 & T2
T1
T2
eff
cv
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Review – Characteristic Impedance
• Impedance you would measure if the line were infinitely long
• How do I measure this?
• With the impedance of the source and receiver, used to determine amplitude of incident and reflected waves
• It’s physically the instantaneous impedance that a transition edge “sees” as it travels down the line
C
LZ 0
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Review – T-Line Propagation
• Incident into open
+1.0 V
-
+0.9 V
-
+0.5 V
-
+0.1 V
-
+0.0 V
-
C
LZ 0
Z0Z0
0
0
ZZ
ZZ
L
L
IncidentReflected VV
1.0 V
0 -
2V
Ste
p
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Review – OPEN Reflection
• When the wavefront hits an open circuit at the end of the line, the current through all that inductance has to go somewhere
+1.0 V
-
+1.0 V
-
+1.0 V
-
+1.1 V
-
+2.0 V
-
Z0Z0
0 -
2V
Ste
p
1.0 V
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Z0Z0
• Incident into short
Review – T-Line Propagation
+1.0 V
-
+0.9 V
-
+0.5 V
-
+0.1 V
-
+0.0 V
-
0 -
2V
Ste
p
1.0 V
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Review – SHORTED Reflection
• When the end is short circuited, the last capacitors discharge through that short
+1.0 V
-
+1.0 V
-
+0.9 V
-
+0.5 V
-
+0.0 V
-
Z0Z0
0 -
2V
Ste
p
1.0 V0
0
ZZ
ZZ
L
L
IncidentReflected VV
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Review – T-Line Propagation
• Incident into matched
+1.0 V
-
+0.9 V
-
+0.5 V
-
+0.1 V
-
+0.0 V
-
Z0Z0
Z0
0 -
2V
Ste
p
1.0 V
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Review – Matched Reflection
• When the impedance of the load matches the characteristic impedance of the line, the current just keeps flowing
+1.0 V
-
+1.0 V
-
+1.0 V
-
+1.0 V
-
+1.0 V
-
+1.0 V
-
Z0Z0
Z0
0 -
2V
Ste
p
1.0 V
0
0
ZZ
ZZ
L
L
IncidentReflected VV
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0V
3V
6V
-3V
1.5V
4.6VMaximum Voltage Specification
-0.5VMinimum Voltage Specification
Switching Threshold “1”
“0”
Driver
ReceiverTransmission Medium
• Reliability (long-term): exceed IC’s maximum voltage specification
• Reduced noise margin – unintended switching
Effects of a Bad Match
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Ringing Reflections - Unterminated line with low source impedance
ZS << Z0
Z0ZS
0V
2V
4V
6V
8V
10V
Receiver Input
Driver Output
50ns 100ns 200ns150ns0s
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Stair-Stepping Reflections - Unterminated line with high source impedance
ZS >> Z0
Z0ZS
0V
2V
4V
6V
50ns 100ns
Driver Output
Receiver Input
Time
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Incorrect Termination Correct Termination
Real Measurements
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Transmission Lines in PCBs
• Microstrip• Roughly half the dielectric is air,
reducing the average dielectric constant.
• Stripline• Fields contained within
dielectric.• Can be symmetric or
asymmetric.
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Transmission Lines in PCBs
• The return current must travel around the plane gap• This effectively increases the inductance, and hence the Z0
across the gap, and creates impedance discontinuities at the edges of the gap
• This can be modeled as 3 transmission lines in series• This creates a large current loop that can increase crosstalk
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Transmission Lines in PCBs
• Example - Effect of the plane gap on the signal’s edge at the receiver
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PCB – Non Idealities
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PCB Nonidealities – R, L, and C
• PCBs create non-ideal or “parasitic” components• Layered construction
• Not shown in your schematic • But important for your design
• PCB materials and layout design can have a big impact on your actual results!
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PCB Nonidealities – Resistance• All traces have some resistance
• Parasitic resistance is often noticed in power distribution causing IR drop
• Resistance limits current carrying due to heat• Skin effect increases effective resistance and
loss Vs frequency
R1
R1
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PCB Resistance – Planes and Vias• Planes are used for power delivery and ground
return
Internal Layer Case External Layer
• Above shows the effect of VIAs under a BGA reducing the plane effectiveness
• Removing VIA pads on internal layers helps the plane fill, but this is not allowed on external layers
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PCB Resistance – Leakage• PCB soldering creates surface contamination
• Flux residue, salts, environment debris
• Leakage current is proportional to voltage gradient, a problem for precision/Hi-Z circuits
• Leakage of ~10nA for 12V across two pins with flux residue is possible 1% error compared to 1uA
• Mitigated by distance, guard rings, and washing• Low impedance traces at the same potential that
can sink leakage currents.
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PCB Nonidealities – Inductance
• Inductive parasitics are most disruptive to low Z, high f sources • Power supply = high energy low impedance source• Higher frequency = higher impedance ZL=jѡL
• PCB inductance can produce damaging voltage through ringing, increase radiated emissions, crosstalk, impact frequency response and impedance match, etc
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PCB Inductance – Traces & Vias• Inductance in a trace is a function of thickness,
width, dielectric, and proximity to other conductors
• Plane behavior is similar with much lower inductance
• A single 10mil drilled via is about 1.3nH.
• PCB inductance is good for designing in PCB antennas and RF filters
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PCB Impedance – Ground Bounce• Ground Bounce is created when multiple
circuits have a common return path
• Changes in current in one circuit, effect the voltage across another circuit.
• Fix this by using a star ground technique, and good bypassing
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PCB Nonidealities – Capacitance• PCB Capacitance more of an issue with
increasing frequency and high Z nodes
• Capacitance can cause slower rise times, crosstalk, and reduced phase margin, settling errors, and dielectric absorption
• PCB plane capacitance can be beneficial by creating high frequency embedded capacitors
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PCB Capacitance – Traces and Pads• Capacitance of a trace is a function of
thickness, width, surrounding dielectric, and proximity to other conductors
• Large component pads can be compensated for by removing GND plane directly under the pad
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Scope Probes
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Probe Issue
• Same signal is measured with 3 different probes. Each probe gives a different result… what is happening?
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Probe Issue: Compensation
ALWAYS COMPENSATE PROBES
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Questions?
