Understanding Radiated EMI - Silicon Labs Documents/Resources/EMI-guide.pdf · 3 What is Radiated...

Understanding Radiated EMI

Applications Engineering GroupMCU Division

2

Agenda

♦ EMI backgroundMechanisms Circuit-level causesFrequenciesMeasurementsShielding

♦ Example problem

3

What is Radiated EMI?

♦ A digital design can become an unintentional transmitter♦ Circuit elements can act as antennas

PCB tracesCables and connectionsIC's and devices

♦ This unintentional transmitter can cause problems for other intentional radio systems

108 - 136 MHz

1910 - 1990 MHz

4

Types of Radiated EMI Issues

♦ Regulatory: Fails a spec limitExamples

System clock harmonicsviolate EN55022 maximum limitsPWM signal harmonicsin an automotive display exceedmaximum level allowed by auto maker

♦ Functional: Interferes with itselfExamples

Radio scanner: System clock frequency may jam the receiverGPS blocking: 16th harmonic of system clock may block GPS reception

5

Agenda


♦ Example problem

6

Radiation Mechanism: Antennas

♦ Intentional antennas—designed to radiate

♦ Unintentional antennas—not designed to radiate (but do!)

7

Reducing EMI

♦ To eliminate EMI, the engineer mustReduce the currents or voltages exciting the antennasEliminate the transmitting antennasBlock the radiated fields

♦ In practical terms, this is done byUnderstanding and minimizing high-frequency sourcesClean PCB layoutUsing shielding

8

Agenda


♦ Example problem

9

What is the Source of EMI?

♦ CMOS digital devices are made of thousands of gates

♦ For simplicity, consider each gate as a CMOS inverter:

0

Vdd

0

Vdd

10

VDD Current

♦ In dynamic operation, transitions consume currentiCB: Crowbar current

Both gates are momentarily on at the same time, conducting current from Vdd to ground

iL: Load currentOutput of the gate is likely connected to input of another gateGate inputs are capacitive

Vddi

iCB iL

Vddi

iCB iL

11

VDD current

♦ Periodic signals through gates cause current impulses

♦ Average current depends on switching frequency

♦ Spectrum depends on switching frequencyUsually system clock Sometimes a subharmonic (sysclk/2, 3, 4, etc)

Peripherals often use sysclk/2

Vddi

12

VDD Current: DC and AC Components

♦ Think in terms of both AC and DC power suppliesWhere does the AC current come from?

♦ Ideal caseMost AC current comes from on-chip sourcesLittle or no AC current comes from off-chip sourcesSmall current loop, small antenna

13

♦ Realistic case:AC current is sourced from outside the ICOn-chip capacitors are impractical: silicon area = larger dieSome on-chip capacitance does exist, but not enough

♦ Engineer must think AC currents when designing PCB

VDD Current: DC and AC Components

14

Think Loop Area

♦ Since AC currents need to flow outside the IC, there will be currents in loops

♦ Current loops = EMI transmitting antennas

♦ Make transmitting loop antennas small !

♦ Design a short path for the currentsSource currents (from VDD)Return currents (through ground)

♦ Silicon Labs MCUs designed with adjacent power and ground pins to minimize loop area

15

Choosing Capacitors

♦ Ideal capacitor (ZC → 0 as frequency → ∞)

♦ Mag[Z] of an ideal 0.1 uF capacitor:

CjZC ω

1=

.0001 .001 .01 .1 1 10Frequency (GHz)

Capacitor Impedance

.0001

.001

.01

.1

1

10

100

|Z(1,1)|Real Capacitor

|Z(1,1)|Ideal Capacitor

CAPID=C1C=1e5 pF

PORTP=1Z=50 Ohm

16

Choosing Capacitors

♦ Unfortunately there are no ideal capacitors♦ Real capacitor: capacitor in series with parasitic inductor♦ Inductor adds impedance with increasing frequency

.0001 .001 .01 .1 1 10Frequency (GHz)

Parasitic inductance

.0001

.01

1

100

|Z(1,1)|parasitic inductor

CAPID=C1C=1e5 pF

INDID=L1L=0.61 nH

PORTP=1Z=50 Ohm

LjCj

ZC ωω

+=1

17

Choosing Capacitors

♦ Real capacitor—inductance cancels, dominates impedance

♦ A capacitor behaves differently in three frequency bands1. f < SRF: Capacitor acts like a capacitor (Z ↓ as f ↑)2. f = SRF: Reactive impedances cancel3. f > SRF: Capacitor behaves like an inductor (Z ↑ as f ↑)

CAPID=C1C=1e5 pF

INDID=L1L=0.61 nH

PORTP=1Z=50 Ohm

LjCjZC ω

ω+

−=

.0001 .001 .01 .1 1 10Frequency (GHz)

Capacitor Impedance

.0001

.001

.01

.1

1

10

100

|Z(1,1)|Real Capacitor

|Z(1,1)|Ideal Capacitor

0.1uFSRF = 20MHz

18

Choosing Capacitors

♦ Wrong capacitor may have little or no effectCapacitors are capacitors only below SRFCapacitors are inductors above SRFIncreasing inductive impedance will prevent capacitor from sourcing impulse currents

VddIC PCB

19

Choosing Capacitors

♦ Solution: select another capacitorDifferent capacitor values have different parasiticsChoose capacitor for frequency of interest

♦ Help available from capacitor manufacturersMurata tool: http://www.murata.com/designlib/mcsil/index.html

20

Capacitor Selection Examples

♦ Compare the imaginary impedance for various Murata capacitors

10pF (GRM1555C1H100JZ01)33pF(GRM1885C1H330JA01)100pF(GRM1555C1H101JZ01)1000pF(GRM1555C1H102JA011uF(GRM188F51C105ZA01)

0.03 3.1 1Frequency (GHz)

Im[Z] for various Murata caps

-40

-30

-20

-10

0

10

20

30

40Im(Z(1,1))10pF

Im(Z(2,2))33pFIm(Z(3,3))100pF

Im(Z(4,4))1000pF

Im(Z(5,5))1uF

21

Which Capacitor is Best?

♦ Use multiple capacitors in parallelExample: 10pF || 1000pF || 1uF


parallel caps

-40

-30

-20

-10

0

10

20

30

40|Z(1,1)|parallel_caps

Im(Z(1,1))parallel_caps

22

Another Reason to Keep Short Traces

♦ Connecting trace to capacitor adds series inductanceSimulate a 3mm trace with via to ground:

Trace is inductive


Connecting trace impedance

-60

-40

-20

0

20

40

60Im(Z(1,1))EM Structure 1

|Z(1,1)|EM Structure 1

23

Parallel Caps in Series with Trace

♦ Parallel capacitor combination effectiveness is reduced by additional trace inductance


parallel caps

-40

-30

-20

-10

0

10

20

30

40|Z(1,1)|parallel_caps

Im(Z(1,1))parallel_caps

24

Internal Coupling/Leakage

♦ EMI can result from AC energy coupling to digital I/O lines inside the IC

♦ Static digital I/O's may be a source of EMI♦ Possible causes:

Conduction through power supplyCapacitive couplingInductive coupling

VddIC PCB

25


♦ Consider a simplified modelThink of the EMI as a noise source with some impedance coupling it to a digital I/OCurrent at the digital I/O is from two sources

Digital driver (good)EMI (bad)

VddIC PCB

Vnoise

26


♦ How should we block the noise? Add a capacitor?

♦ May make EMI worseCapacitor provides a low-impedance path outside the ICThe low impedance path may increase current

VddIC PCB

Vnoise

27


♦ Add series resistance? May helpLess current (good and bad current) flows through high impedanceMay reduce EMI by reducing currents flowing outside IC

♦ DisadvantageAdding resistance may attenuate or distort the wanted signalFor example, it may not provide enough LED current, or may slow a signal's slew rate

VddIC PCB

Vnoise

28


♦ Troubleshooting experimentSet R = ∞ by lifting pinReduced EMI means that this pin is contributing

VddIC PCB

Vnoise

29


♦ Add an external inductor or choke?Provides high impedance for high frequencies, low impedance for low frequencies

♦ DisadvantagesInductor may actually create and radiate EMI (inductors turn electric currents into magnetic fields)Inductors cost more than resistors and capacitors

VddIC PCB

Vnoise

30

Agenda


♦ Example problem

31

Time and Frequency Domains

♦ Signals can be represented in time or frequency domainsFourier transform F: Transform between time and frequency domainsDigital designers think in time domainEMI measurements are in the frequency domain

♦ Periodic events in a circuit create distinct EMI frequenciesFrequencies often harmonics of the system clockFrequencies may be harmonics of system clock subharmonics

Example: Flash memory read every third sysclock periodDigital waveforms will create harmonics

Square wave creates odd harmonicsImpulse train creates even and odd harmonics

32

Fourier Transform Review

♦ Square waveSquare wave is composed of several odd harmonics

33

Fourier Transform Review

♦ Impulse trainA series of pulses in time is a series of tones in frequency

34

Example—Spectrum of a Square Wave

♦ F120 24.5MHz sysclock from a port pin

A

* RBW 500 kHz

VBW 2 MHz

SWT 2.5 msRef 10 dBm Att 35 dB

Start 10 MHz Stop 200 MHz19 MHz/

EXT

1 SAAVG

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:11:54

Oscilloscope (time) Spectrum Analyzer (frequency)

35

A

* RBW 500 kHz

VBW 2 MHz



EXT

1 SAAVG

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:11:54

System Clock Design Tradeoffs

♦ Difficult to change waveform

♦ Easy to change system clock

Example—C8051F120reduce sysclk using clock dividersincrease sysclk using clock multiplier24.5MHz shown here

36

Spectrum of 3.0625 MHz System Clock

A

* RBW 500 kHz

VBW 2 MHz



EXT

AVG1 SA

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:09:28

37


A

* RBW 500 kHz

VBW 2 MHz



EXT

1 SAAVG

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:10:09

38


A

* RBW 500 kHz

VBW 2 MHz



EXT

1 SAAVG

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:10:44

39


A

* RBW 500 kHz

VBW 2 MHz



EXT

1 SAAVG

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:11:54

40

Spectrum of 49 MHz System Clock

A

* RBW 500 kHz

VBW 2 MHz



EXT

1 SAAVG

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:14:42

41

Spectrum of 98 MHz System Clock

A

* RBW 500 kHz

VBW 2 MHz



EXT

1 SAAVG

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

SWP 100 of 100

Date: 23.MAY.2007 10:14:02

42

Agenda


♦ Example problem

43

Measuring EMI

♦ Measured by an accredited EMI test facility

Open-Air Test Site (OATS)

Device placed 3 or 10 meters from measurement antennaQuiet, reflection-free environmentOutdoors or anechoic chamber

44

Measuring EMI—Accredited Test Facility

♦ OATS test resultsListed in a tableIncludes all frequencies foundShows both passing/failing frequenciesData for horizontal and vertical receiving antenna polarization

45

Measuring EMI

♦ GTEM cellGTEM cell is an enclosure and antenna in one unitUsed with a spectrum analyzer and correlation softwareMany digital design companies have one for pre-compliance testingSilicon Laboratories has oneNot normally certified

46

Measuring EMI—GTEM

♦ GTEM test resultsContinuous test data vs. tabularMeasurement shows computed OATS equivalentData shows worst-case antenna orientation

OATS Equivalent Radiated Emissions

-20.00

-10.00

0.00

10.00

20.00

30.00

40.00

50.00

60.00

1.000E+01 1.000E+02 1.000E+03

f(MHz)

dBuV

/m OATS 10m (dBuV/m)CISPR-22 QPk Cl B Limits

47

Measuring EMI

♦ Loop antennaSimple and inexpensiveCan make one yourselfUsed with a spectrum analyzerGood only for relative measurements

48

Measuring EMI—Loop Antenna

♦ Loop antenna test results

Spectrum analyzer plotAmplitude units arbitrary as antenna is not calibrated

A

PASPUEM

Ref 27 dBµV

SGL1MAXH

Center 1.515 GHz Span 2.97 GHz297 MHz/

-20

-15

-10

-5

0

5

10

15

20

25

SWP 100 of 100

1

Marker 1 [T1 ]

-19.86 dBµV

43.662000000 MHz

Date: 3.APR.2007 13:47:56

49

Agenda


♦ Example problem

50

Shielding—Theory

♦ Faraday cageMade of conductive materialCharges in conductor move to cancel electric fieldFaraday cage can keep fields out or in

51

Shielding—Troubleshooting

♦ Copper Foil TapeAvailable from 3M(www.3m.com, search for '1125')

♦ Tips and TricksMake good ground connection

Leave no gapsSolder to ground in many places

Start by shielding small areasShield a device or specific traces rather than a large area: helps pinpoint EMI source

Adhesive is not conductive(Even if manufacturer says it is)Use solder for good connection

Use Kapton tape beneath copperKeeps copper tape from shorting out IC pins

52

Shielding—Production PCB

♦ Production: ShieldsCommonly used in wireless, computational productsEffective, but adds costGood source for off-the-shelf shields: Leader-Tech (www.leadertechinc.com)

53

Shielding—Troubleshooting

♦ Conductive paintUse to convert non-conductive plastic enclosures to conductive, EMI-shielded enclosuresUse in troubleshooting or productionConductive plastics commonly used in laptop PC's, mobile phones, PDA's, etcAvailable from MG chemicals:(http://www.mgchemicals.com/products/shielding.html)

54

Agenda


♦ Example problem

55

Example Problem

♦ Product is a GPS data logger using a C8051F120 MCU♦ Problem—GPS receiver sometimes loses satellite reception

♦ HypothesisEMI may be radiating from the micro in to the GPS antennaEMI may be conducted from the micro to the power supplyor control lines of the GPS

MCU

GPS

LDO

56

Frequencies

♦ KnownF120 sysclk: 98MHz (internal 24.5MHz * 4)GPS receive band: 1575.42 +/- 1MHz

♦ Are any harmonics close?15 * 98MHz = 1470MHz16 * 98MHz = 1568MHz (close, but not in GPS RX band)17 * 98MHz = 1666MHz

57

Frequencies

♦ Nominal casesysclk = 24.5 * 4

♦ From datasheetsysclk = (24.5 ± 2%) * 4

♦ Revisiting 16th harmonic16 * (24.0 * 4) = 1536MHz16 * (25.0 * 4) = 1600MHz

♦ Conclusion16th harmonic can interfere with GPS reception

1574

.42

1576

.42

1568

1536

1600

58

Frequencies—Possible Solutions

1. Use lower sysclkHigher-order harmonic at 1568MHz

1568MHz = 16 * (24.5 * 4) MHz1568MHz = 32 * (24.5 * 2) MHz1568MHz = 512 * (24.5 / 8) MHz

Higher order: lower in amplitude

2. Use more accurate sysclk1568MHz does not interfere, but 1568 ± 2% doesCrystal, typical: ± 20ppm24.5 MHz ± 20ppm = 24.500490 ~ 24.499510 MHz(24.5 *4) MHz ± 20ppm = 1567.969 ~ 1568.031 MHzHarmonics remain out of GPS RX band

1574

.42

1576

.42

1568

59

Power Supply Bypass Capacitors

♦ Insufficient power supply bypassingC8051F120 has four power supply/ground pin pairs: each should have capacitorsLack of local capacitors may cause larger current loopsSingle value of capacitor may not be effective for all frequencies

MCU

GPS

LDO

60

Power Supply Bypass Capacitors

♦ SolutionPlace bypass capacitors at each VDD pin pair (analog and digital)Use short connecting tracesConnect to ground with vias placed closeUse appropriate capacitance values

22nF: SRF = 50.6 MHz (little help at GPS frequencies)10pF: SRF = 2240 MHz

61

Data Connections

♦ EMI may conduct through data lines between MCU and GPS

MCU

GPS

LDO

MISOMOSINSS

62

Data Connections

♦ Solution—try series resistance/shunt capacitance on data lines

Try different combinationsSeries resistance onlyShunt capacitance onlyBoth resistance and capacitance

Recall that capacitance may make problem worse

MCU

GPS

LDO

MISOMOSINSS

63

Shielding

♦ Add a shield over the MCU area

MCU

GPS

LDO

64

Shielding

♦ Use ground plane as shieldMount MCU and GPS on opposite sides of PCB

65

Summary

♦ To effectively understand EMI problemsUnderstand the frequencies involved and their relation to the system clockExpect high frequency harmonicsConsider every node and trace as a potential radiating antenna

♦ To effectively troubleshoot EMI problemsThink about minimizing the power supply path for high frequenciesSelect the correct capacitorsDesign the PCB to minimize loop areasFilter signal linesUse shielding if necessary

♦ There is no single EMI fix for all problemsDon't be afraid to experiment!

66

Contact Information

♦ MCU Applications [email protected]

www.silabs.com/MCU

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