1. Beyond the Books EMC, T-Lines & PCBs
Eric Hartner
Senior Engineer at National Instruments
U of M – BSE EE ’04

2. Agenda About NI Products and Applications EMI/EMC Transmission Lines
PCB Non-Idealities

3. National Instruments 7,100 employees More than 1000 products
More than 50 international branches in over 45 countries
Corporate headquarters in Austin, TX
More info attend our info session during the Fall or Spring Career Fair
7,100 employees
More than 1000 products
Dr. James Truchard CEO
© National Instruments Corporation Acquire, Analyze, and Present with LabVIEW Seminar

4. Our Mission
We equip engineers and scientists with systems that accelerate productivity, innovation, and discovery.

5. What We Do
We provide graphical software and modular hardware to build measurement and control systems.
Low-Cost Modular Measurement and Control Hardware
Productive Software Development Tools
Highly Integrated System Platforms

6. If you can turn it on, connect it, drive it, or launch it, chances are NI platform-based technology helped make it happen

7. Boeing – Reducing Aircraft Noise
ground based microphones
ch audio analyzer boards
<1ns synchronization
Lots of DSP

8. 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

9. 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 -

10. NI PXIe-5644R VST Vector signal analyzer and generator $45k
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

11. 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

12. EMI/EMC

13. 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

14. What can go wrong? Effect of EMI on TV picture
EMI in Auto Electronics - sudden acceleration issues
USS Forrestal Fire
Explosion on the deck of the USS Forrestal super-carrier in July 1967 was caused by an unguided 5-inch Mk-32 Zuni missile, which was accidentally fired due to an electrical power surge during the switch from external power to internal power → 134 dead, 161 injured, 21 aircraft destroyed, cost to US Navy - $72 million
The worst lost of live on a Navy vessel since WW II
Toyota Crisis Spotlights EMI Role in Auto Electronics, Hybrids 
03/03/10
Investigations into sudden acceleration issues have put a spotlight on everything that can go wrong with automotive electronics. “Electromagnetic interference leaves no trace. It goes away just as it came,” says Michel Mardiguian, an engineer and consultant near Paris who worked with a European automaker on a gremlin-like electronic fault. Auto engineering experts say hybrids inherently have more potential for electromagnetic interference.
Source: Interference Technology Newsletter, March 3, 2010
USS Forrestal Fire in 1967 was caused by a missile which was accidentally fired due to an electrical power surge

15. EMC Strategies: Options
Design for EMI/EMC
Sensitive Victims
Noisy Aggressors
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 ….

16. EMC Strategies: Beyond the Schematic
What could impact EMC that isn’t obvious from the schematic? 
There are a lot of potential answers to this question:
Return current for output
Output coupling (either capacitive or inductive) onto other traces in the design
Current flow from bypass capacitor and effectiveness of this across frequencies – including placement of this cap.
Effect of pin impedance (primarily inductance) for output, VCC, and GND.
Edge rate of buffer and impedance of buffer
Etc.

17. ? Hidden Schematic Unintended Coupling
In the layout example, two nets which are on adjacent balls could have crosstalk. This crosstalk could exist in the routing of the traces or in the chip bonding wires themselves. Coupling the Config_Clk clock signal to the NC223 signal has the potential to cause radiated failures because the NC line is unterminated so noise bounces back and forth many times. Granted, this is a short trace for the NC signal and the Config_Clk should probably not run all the time, but the point here is to demonstrate that there was no defined connection between the two nets in either the layout or the schematic – Hence ‘hidden schematic’.
This same concept applies to many interesting ‘problems’ that could arise, including crosstalk, ESD events & chassis ground, and conducted emissions.
Question: Where is the problematic ‘hidden schematic’ above? ?
Answer: Config_Clk may couple noise onto NC223. No connects
are unterminated and thus can contribute to radiated emissions failure,
Or coupling into other circuitry (crosstalk).

18. EMC Strategies: Antennas
What are some unintentional antennas in a design? 
Clock Switch
0.7 m
7 cm
Details on chart
The blue line is a less efficient antenna design; looks
Like they took the optimal conductor length and divided it by 12 to get this blue line for worrying about antenna effects. The dark
Green line where they claim negligible effects looks to be optimal length divided by ~200.
Example: For 266Mhz signal content, optimal antenna length is 0.34m. Still a troublesome antenna at 1.1”, negligible effects at 60 mils. Clearly, this is conservative, but I think it drives home a good point.
What are some unintentional antennas in our designs?
Cabling – try to design I/O so it can be filtered and kept away from noisy sections of the board so they don’t serve as antennas for these sources (refer to cabling picture)
Long stubs – often result from debug headers, optional circuitry, clock switches (refer to clock switch picture)
Floating planes (or planes that don’t have enough ground ties)
You can see a picture of an analog switch where the unselected net is carrying some noise due to parasitic coupling inside the switch. This unselected net is often unterminated and uncontrolled ringing results. The parasitic capacitance (green) is hidden in the schematic.
Image Reference:
compliance-club.com/pdf/DesignTechniquesPart[1-6].pdf

19. EMC Strategies: Antennas
1.6” of trace,
one HiZ
one driving low to high
Is this a problem?
Near end crosstalk
Far end crosstalk
Oscillation around 1.35GHz
Fast rising edge
Near and far end crosstalk
The Layout shows a NC7W126 Buffer capable of driving 0-> 3.3V in 500ps. This output is source series terminated and runs for 1.6 inches to a 50k resistor. There is a net with close coupling that runs parallel to which is the victim of crosstalk. The second net is not source terminated, perhaps it is an unused output routed to a debug header.
On a fast rising edge crosstalk on the HiZ victim net takes a long time to dissipate and oscillates based on the net length.
The second set of plots shows the crosstalk zoomed in on. It shows the frequency of the oscillation is around 1.35GHz. The magnitude isn’t very much, but at that frequency, a very efficient antenna is only ~3 inches long.

20. Square Waves Where is all the energy in a square wave?
A: In the Edge  𝑇𝑟𝑖𝑠𝑒 = 𝐵𝑊
What does the frequency spectrum of a 10MHz square wave look like?
Is this a square wave?
Amplitude in dB
53%
Yes! It is just not 50% duty cycle.

21. EMC Strategies: Return Currents
Electrons will follow path of least impedance when returning to source D H
Image based on formula in Howard Johnson’s Black Magic book: Distribution density = 1/(1+(D/H)^2)
GND

22. EMC Strategies: Return Currents
Hidden Schematic => Unintended Return Paths
For EMI purposes, the returning current path is as important as the intended signal path!
Where will return current flow?
Can we simulate this?

23. EMC Strategies: Minimize Loop Area
After walking through the paths, highlight the following points
If the stitching via is too far away, the loop area will grow, resulting in worse emissions
Keep stitching vias close to the via that the signal passes through
Place a stitching capacitor across a plane break to give a path for high-frequency return current
If signal switches from one reference plane to a different reference, then keep this stitching/bypass capacitor close to minimize the loop area
Give return current a good, short path, and it will be quiet. Give it a long path and it will scream and yell EMI that you didn’t build it the low impedance path that it wants to take.
Keep in mind parasitic inductance of stitching caps and vias for return current 23

24. EMC Strategies: Non-Ideal Behavior
Know non-ideal behaviors
Evaluate the components at all frequencies you care about!
Capacitors
Transformers
Keep in mind for ties between grounds as well

25. RF Rectification
Non-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
Because of the nature of the pn junction on the inputs of BJT type Opamp it will rectify the RF noise and create DC measurement error. This is what happens when you hear unwanted voices or sounds on your AM or FM radio. In that case is usually caused by the receiver as the input amplifier is suddenly affected by strong unwanted outside radio signals. RFI filter helps filtering out those interferences.
Source : Analog Devices

26. Transmission Lines

27. 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
Generally is ps/inch in a coax cable or on a PCB
Notice this is Independent of Zo
A trace is electrically long when it is 1/6 to 1/20 of the risetime based on the application requirements. (dig to RF)
6” trace on a PCB, with 1ns edge, it definitely is a T-Line T2

28. 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
Do I use a DMM in resistance Mode? (No need L/C ratio ->TDR, VNA)
Formula – Remember L/C per unit length (small electrically)

29. Review – T-Line PropagationZ0
0 - 2V
Step
Incident into open +
1.0 V -
0.9 V
0.5 V
0.1 V
0.0 V
1.0 V

30. Review – OPEN ReflectionZ0
0 - 2V
Step
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 -
1.1 V
2.0 V
1.0 V

31. Review – T-Line PropagationZ0
0 - 2V
Step
Incident into short +
1.0 V - +
0.9 V - +
0.5 V - +
0.1 V - +
0.0 V -
1.0 V

32. Review – SHORTED ReflectionZ0 Z0
0 - 2V
Step
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 -
1.0 V

33. Review – T-Line PropagationZ0
0 - 2V
Step
Incident into matched +
1.0 V - +
0.9 V - +
0.5 V - +
0.1 V - +
0.0 V -
1.0 V

34. Review – Matched ReflectionZ0
0 - 2V
Step
When the impedance of the load matches the characteristic impedance of the line, the current just keeps flowing +
1.0 V -
1.0 V

35. Effects of a Bad Match
Driver
Receiver
Transmission Medium
Reliability (long-term): exceed IC’s maximum voltage specification
Reduced noise margin – unintended switching 6V
Maximum Voltage Specification
4.6V 3V
Switching Threshold
“1”
1.5V
“0” 0V
Minimum Voltage Specification
-0.5V
-3V

36. Ringing Reflections - Unterminated line with low source impedanceZ0 ZS
ZS << Z0
10V 8V
Driver Output 6V 4V
Receiver Input 2V 0V 0s
50ns
100ns
150ns
200ns

37. Stair-Stepping Reflections - Unterminated line with high source impedanceZ0 ZS
ZS >> Z0 6V 4V
Receiver Input 2V
Driver Output 0V
50ns
100ns
Time

38. Incorrect Termination Correct Termination
Real Measurements
Incorrect Termination Correct Termination

39. 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.

40. 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

41. Transmission Lines in PCBs
Example - Effect of the plane gap on the signal’s edge at the receiver

42. PCB – Non Idealities

43. 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!

44. 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
For 1oz copper this is – Roomtemp

45. 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

46. 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.

47. 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

48. 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

49. 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

50. 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

51. 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

52. Scope Probes

53. Probe Issue
Only 1 out of the 3 probes has been compensated. An uncompensated probe will introduce problems that aren’t really there.
Same signal is measured with 3 different probes. Each probe gives a different result… what is happening?

54. Probe Issue: Compensation
To maximize the bandwidth of the attenuating probes, the probe capacitor must be adjusted such that the input capacitance of the digitizer or oscilloscope is exactly cancelled. Precise adjustment of the tunable probe capacitor to get a flat frequency response is called probe compensation. For the most accurate measurements, compensate probes for each channel (CH0 and CH1) and use them on that channel only and recompense when using the same probe on a different channel.
Any time you change probes, or plug them into another channel , you need to recompensate the probe.
ALWAYS COMPENSATE PROBES

55. Questions?