EMC Design K-E Gustafsson PCC / CMT EMC Design Group 15.1.2001

EMC DesignK-E GustafssonPCC / CMT EMC Design Group15.1.2001 Material made by: Tapio Mäntysalo, Karl-Erik Gustafsson

EMC Design General • Our products must fulfill... • Operator, regulatory and safety requirements • NMP internal requirements • Quality requirements • Potential EMC problems to solve: • Emission problems (operator, regulatory and safety issues) • ESD problems (affects FFR) • Other transient immunity problems • RF immunity problems, especially the... TAPPARA NOISE!

When You Get into an EMC Problem • Keep diary on what changes you made and what were the results. It's 99.9% sure that after two weeks, when you need the information, you won't remember it. • Remember that there might be two or more problems at the same time. If you success in removing one problem, you will not notice it! • Don't remove any changes that are 'good design' • Try to reach the absolutely best performance. You can then come back bit by bit to have manufacturable and cost effective solution • Don't waste time trying to find 'the easy one-cap solution' before you know what is happening

EMC Design Plan • Covers all interfaces • Drawing(s) that combine layout information to schematics • Clearly show EMC barrier areas • Lists sensitive PCB tracks • Those that may be affected, like audio, VCO supply and control • Those that generate noise: charger lines, SMPS, clocks, high-speed data and control, RF signals • I/O signals to external cables • Comments on shielding and how signals are filtered at shield wall • Filter properties • How ESD is suppressed at connectors and covers.

EMC Design Plan • EMC Design Plan for a product covers all relevant internal and external interfaces where EMC problems may occur. Also shielding, interface SW and mechanics should be covered in the plan. The plan should address what problems there might be, how they will be solved and what risks there are.

ImmunityElectroStatic Discharge Radiation Printed Wiring Board Transmission Lines Shielding EMC ComponentsEMC and Software

Typical Problems with Microphone Circuits • Poor immunity to own cellular transmitter noise • Poor immunity to regulatory (CE requirements) RF fields • Poor immunity to ESD • 50/60Hz noise • Charging noise • Noisy biasing voltages Principles presented in these slides are applicable to other analog interfaces as well!

Demodulation Non-linear device clips the RF signal Originally RF signal is AM-modulated, but has no audio frequency content Rs VRF Audio circuitry averages the clipped RF signal to audio band

Where Does RF Get Demodulated? • Typical microphone circuit: ** Bias Voltage Supply Connector Transmission Line Main PCB ESD Suppression and RF Filters ESD Suppression and RF Filters Biasing Microphone ESD Suppressor ESD Suppressor Semi- conductors in bias circuit Microphone Amplifier or ADC Microphone FET * **** ** **** ***** Demodulation requires a non-linear device, such as a semiconductor junction or other a voltage dependent device like varistor. * Not usually a problem ***** Nearly always a problem

Where Does RF Signal Get Coupled Into? DMRF to bias voltage requlator DMRF into additional connections of mic line, such as detection signals ** • Typical microphone circuit: DMRF directly to audio circuitry *** *** DMRF at Connecor Main PCB **** ESD Suppression and RF Filters ESD Suppression and RF Filters Biasing Microphone DMRF to PCB loop between microphone FET and RF filter DMRF to cable with poor RF isolation DMRF to cable connection at microphone PCB ** CMRF to whole microphone cable and system ***** *** CMRF = Common Mode RF Signal DMRF = Differential Mode RF Signal * Not usually a problem ***** Nearly always a problem *****

Differential Mode, Common Mode • Differential Mode = Normal Mode: disturbance here is at normal signal path - everything can be heard in audios. • Common Mode: disturbance is between signal lines and ground (ground can be also virtual return path). Disturbance cannot be heard in audio. Main PCB ESD Suppression and RF Filters ESD Suppression and RF Filters Biasing VDM Microphone VCM1 VCM = ½·(VCM1 +VCM2) VDM = VCM2 -VCM1 VCM2 Virtual Ground

Typical Antenna Sizes (Compared to a Half Wave Dipole), Lowest Tuned Frequency and Approximate (-20dB/decade) Attenuation at 1GHz 5mm  30GHz  -30dB@1GHz 5mm  30GHz  -30dB@1GHz, or 1.5m  100MHz  0dB@1GHz (if RF comes from external wires), such as loudspeakes or charger line) 5mm  30GHz  -30dB@1GHz 20mm  7.5GHz  -18dB@1GHz Main PCB ESD Suppression and RF Filters ESD Suppression and RF Filters Biasing Microphone 20mm  7.5GHz  -18dB@1GHz 10mm  15GHz  -24dB@1GHz 10mm  15GHz  -24dB@1GHz 3m  50MHz  0dB@1GHz Situation of headset, phone and charger forming a dipole antenna.

Common Mode Voltages • In the previous slides it was shown that common mode voltages can be induced at much lower frequencies than differential mode voltages. • Common mode voltage itself doesn't cause any harm - it is same voltage at both microphone lines. We shouldn't have really any RF immunity problems below 100MHz! • But... problems will occur when this common mode voltage is converted to differential mode. How can this happen? Common Mode Voltage in Cables Microphone Phone Charger Looks like a dipole antenna?

Common Mode  Differential Mode Device with microphone amplifier Microphone Charger CM Current CM Voltage Common mode current can be converted to differential mode voltage if Z1Z2 and/or Common mode voltage can be converted to differential mode if Z3Z4 Z1 Z2 Z3 Z4 Because Common mode current, not voltage, is converted by Z1 or Z2 to differential mode signal, this cannot happen in the ends of the antenna (CM current = 0). Z3 and Z4 typically can be found inside the main device. Cabling asymmetry towards ground also makes Z3Z4.

Common Mode  Differential Mode Examples Conducting objects, such as a metallic slide, close to microphone may cause CM->DM conversion Z1 Z2 Z3 Z4 At the mic end CM->DM is typically very weak Amplifier is not designed to handle large common mode RF voltages (it works probably well below 20kHz though) Cable leaks RF Connector leaks RF Electrical circuitry not balanced, for example due to capacitor tolerances Long PCB tracks, tracks have different lentgh

Differential Mode Cable Resonances - Standing Waves Zconnector Zcable1 Z1 Z2 VZ1 Reflection from Connector When Zconnector Zcable Reflection from Microphone PCB When Z1 Zcable Reflection from Cable When Zconnector Z2 Reflection from Cable When Zconnector Zcable Not matched VZ1 Low Q Perfectly mathed Frequency • When impedances are matched, there are no reflections • Removing all reflections doesn't mean you don't have any differential mode RF in the cable, but it removes the high resonance peaks and therefore you win many dB's at problematic frequencies. • Having lossy impedances, i.e. resistors, at Z1 and/or Z2 decreases the Q factor of this circuit. Therefore it will smooth the sharp resonance peaks. Resistors should match approximately to cable impedance.

Ear Pieces, Speakers and Buzzers • These are generally quite immune to RF fields • However, they are sensitive to magnetic fields of transmitter-battery current loop. Make this current loop as small as possible and also try to keep the transducers away from the center of magnetic loop • These transducers can conduct ESD pulses into sensitive circuits • Speaker or earpiece output amplifiers may be RF sensitive - the outputs may have to be filtered

ESD Problems... • Something breaks in ESD test • ASIC's • Passive components • Semiconductor devices • RF PA • Strange field failures • Applying an ESD spark causes • call drop • display blank • strange detection or behaviour of accessories • Phone charges up in end-user pocket or belt-clip: • Discharges through earpiece or headset (feels quite funny in your ear) • Discharges to desk-stand or charger • Malfunction of the phone because of the high electric field

Generation of Electro-Static Charge Nylon +++++++++++++ Nylon Separation U2 +++++++++++++ U1 - - - - - - - - - - - ABS - - - - - - - - - - - ABS Friction U1 = 100V C =10pF Q = CU = 1nAs C =0.1pF Q = CU = 1nAs U2 = 10kV • Separation decreases capacitance and increases voltage • Charged objects can transfer charge to other objects • The higher the separation speed, the higher the voltage

ESD and Plastic Covers Sensitive Circuit on a PWB Plastic Cover ESD will go through holes in plastic covers. The seams should be gas-tight to prevent this. ESD will also go through thin plastic sheets. For example breakdown voltage of PVC is about 4 times that of air. Ground trap PWB trace as aground trap Plastic Cover Air gap is long enough to prevent a spark

More on ESD Paths Sensitive Circuit on a PWB • Here the ground trap is • too far away to protect the device, and • connected to ground too far away (ESD pulse is really fast - 1cm of wire may have too much inductance for ESD) Plastic Cover Ground trap Sensitive Circuit on a PWB Floating (not grounded) metal parts may conduct the ESD spark into sensitive devices. These metal parts may include screws, display frames, keyboard parts etc. You could also consider a PCB trace as such a part! Plastic Cover

Extremely High Static Electric Field Metal Part Charged to 15kV • Even though there is no discharge to a sensitive device or signal, malfunction may occur due to high electrical field strength. Especially high impedance signals (1M and so) are problematic. • If you cannot ground the metal parts directly, you can do it with 3pF 300V varistors. These will not leak AC current and they have small enough capacitance for most purposes. 150kV/m?!?! Plastic Cover

ESD Current How to Use ESD suppressors BAD IC BAD GOOD • Place ESD suppressors at the edge of PWB, ground directly using wide trakcs and multiple vias • Route tracks through the ESD suppressor PWB

Example Schematic L3 600@100MHz R1 47 Audio Cable XEARP L4 600@100MHz R2 47 XEARN R5 14V/120pF C3 1nF R6 14V/120pF C2 1nF R3 33 XMICP R4 33 XMICN R7 14V/120pF C4 1nF C5 1nF R8 14V/120pF • At common mode ESD pulse to audio cable the ferrites will slow down the ESD current at XEAR lines so that the varistors at XMIC lines will clamp most of the ESD current. • Series resistances after the ESD supressors protect other circuitry from excess ESD current/voltage. • Varistor working voltage is 14V -> the clamping voltage may be up to 50V!

Basic Cables • Pair cable, or part of ribbon cable or flex. • Electric and magnetic fields radiate. • Radiation will increase if distance of the wires increases. • Twisted pair. • Electric and magnetic fields will cancel out in far field. • Coaxial Cable • Electric and magnetic fields will cancel out completely • Twisted pair with a shield • Properties of the twisted pair, but enhanced with a shield that creates a Faraday Cage around the cable.

E, H E, H i i Common Mode and Differential Mode Radiation Differential mode radiation Common mode radiation i

Cable Connection between two Devices Big Loop AREA BAD Device B Common Ground • Big Loop: • Common Mode Radiation • Immunity Problems Device A Device B Common Ground Device A GOOD Smaller Loop • Small Loop: • Reduced Antenna performance • at lower frequencies

Trapezoid Wave & Spectrum Envelope (Fundamental Frequency) F1=(· )-1 F0=T-1 F2=(· r)-1  = Symbol time r = Rise time f = Fall time T = Period time A = Amplitude F0 2·F0 F1 F2

Radiation from Cable Cable radiation +20db/dec Total cable radiation +20dB/dec 0dB/dec -20dB/dec 0dB/dec -20dB/dec Trapezoid wave -40dB/dec F1 F2 f

Consider Your Device as a Faraday Cage If there are wires that do not go through the hole or are grounded at the wall, the antennas still don't see each other. Shielded room with an opening. If the hole is small, the antennas will not "see" each other. Only when you put an isolated wire through the hole, the antennas will see each other. • Good cable and connector construction preserves the Faraday Cage. • Twisted pair preserves it only in differential mode, CM attenuation depends solely on electrical components • Coaxial and shieleded cables preserve also the common mode Faraday Cage. Cable Device 2 Device 1

Back to Real Life... If the shield is connected only to the other end, the shield just acts as a transmission line for the radiation. Note that this affects both emission and radiation and the process is reversible. Thumb rule example: device #2 is 8cm long. This would behave as a quarter wave antenna at about 900MHz! Cable Device 2 Device 1 'Antenna feedpoint' Antenna If the shield is connected with a piggy tail, this will result in serious degradation of the shielding performance. Thumb rule example: 2cm piggy-tail represents approximately 5cm half-wave dipole. This corresponds to 3GHz. At 1GHz the attenuation would be 9.5dB and at 100MHz about 29dB. Cable Device 2 Device 1 Antenna

Real Life Continued... If there is no shield at all, the devices will see the whole system as an antenna. Therefore we will have immunity and/or emission problems at even lower frequencies. Cable Device 2 Device 1 'Antenna feedpoint' Antenna • If the shield carries high speed data, it fill be directly fed into the piggy-tail antenna. • And vice versa, RF noise will be picked up directly to the signal path if the shield is used as signal return. Cable Device 2 Device 1 Antenna

And More... If you ground tightly one of the cables, you can reduce the common mode RF voltage to zero at that point. You will still get common mode RF current though and note that differential mode voltages exist. Cable Device 2 Device 1 No CM voltage at connectors Antenna • If you have two wires for the shield, the piggy-tail loop radiation will cancel each other in far field. • There will be a dramatic increase in shielding performance! Cable Device 2 Device 1 Antenna

An RF Tight Connector for Shielded Cable Headset Connector Housing Side View No loops in side view Phone PWB Directly and symmetrically to ground Magnetic loops and electric fields cancel out, coaxial structure is preserved as well as possible Top View Peeled cable as short as possible Any difference to coaxial RF connector structure makes shielding effectiveness of a BB connector worse!

PCB Layout Considerations - Component Placement at Connector Connector 2 Other Components GOOD BAD Connectors 1&2 Other Components EMI/ESD parts Connector 1 EMI/ESD parts • Do have well defined EMI/ESD protection areas. It will ensure that your wanted and unwanted signals do not mix. • Don't place a connector into the middle of PCB. In this case you cannot really ground the RF currents into the middle of other circuits. EMI filtering will not work properly. • If possible, put all connectors close to each other. This way you will not route large common mode RF currents through the PCB. Other Components Connector2 Connector 1 VERY BAD EMI/ESD parts Common mode RF currents

Cable Entry to PWB EMI/ESD barrier Vias Signal line Connector Ground plane EMI Filters Disturbance Currents • A signal line on a ground plane is a transmission line. • RF filters should be placed at the entry point to PWB to minimize RF currents in the ground plane. • You should have a solid EMI/ESD barrier area reserved at the PWB. Otherways the disturbance and signal currents will be mixed and the effectiveness of EMI filters will reduce considerably (tens of dB's!).

Minimizing Cross-Talk and Enhancing PCB Performance Inductive (low impedance frequencies) and/or capacitive (high impedance frequencies) coupling between traces. Coupling minimized between traces Signal goes through the pad to minimize impedance High impedance will decrease performance Separate Impedances to Ground. Common impedance at ground path and via • Especially with ESD devices it is important to reduce cross-talk • Cross-talk between "dirty" and "clean" signals is to be minimized. • Cross-talk between two "dirty" signals is not usually such an issue. • However, reducing total ground impedance with multiple vias and wide ground areas is always mark of good design.

PCB and Connector Ground currents make a loop at the connector Best practice: signals come through a ground plane to the connector. Ground currents follow the signal line as long as possible.

Fast Signal on a Ground Plane High frequency current density underneath a signal trace If the signal travels close to an opening or PWB edge, the current density is not balanced. This may cause radiation, or shielding effectiveness decrease, if a shield gasket is taking contact at this PWB edge. 20H Fast signal or supply trace, or supply plane should be kept 20 times PWB layer thikness apart from PWB edge t Cross-talk will be reduced with 2W distance. Add 1W more for vias. W >2W W

Fast Signal and a Slot in Ground Plane If the signal travels through an opening, a slot antenna at the return currents will be formed. • Clock circuits are the worst emission sources, since the fast edge density is high • Design of fast digital and RF signal traces differs in the way that the digital signal is wide bandwidth • You can use guard tracks for sensitive or fast digital signals. Guard tracks that are grounded at both ends decrease signal-to-signal cross-talk roughly by 6 dB, if there is a proper ground plane. In a two layer board the guard traces improve the situation much more. • To decrease emissions of high speed lines, you can use differential transmission.

Step Pulse in a Long transmission Line V V V Length = L Low-Z High-Z Low-Z High-Z Low-Z High-Z Transmitter Receiver V V V Low-Z High-Z Low-Z High-Z Low-Z High-Z

15 W V2 V1 5 V Zc, T Bad termination V2 5 V V1 900 W T 2T 4T 6T

Reflections Step signal when termination is correct Step signal in long transmission line, wrong termination. Frequency of oscillation depend on the length of the line. Step signal in short transmission line, wrong termination

Reflections... • …may cause jitter • …endangers signal integrity • …cause excess radiated noise (emission) • …are worst at the first receiver in the line • …will occur at every impedance transition of the transmission line • …can be reduced with proper termination and proper design of high-speed signal tracks, cables and connectors

Transmission Line • PCB tracks or lines shorter than (tr*c)/6 can be considered as lumped circuits. • tr = rise-time of the signal • c = speed of the signal, typically somewhat less than speed of light • For example, if rise-time is 1ns, then signal paths longer than 30 mm should be considered as TRANSMISSION LINES.

Terminating the Transmission Line • Transmission lines should be terminated to avoid ringing in signals. • Best termination impedance is the same AC impedance as the transmission line. RZ Z0 Z0 RZ • Two components • Capacitor is required to remove DC load from the signal • Impedance matching only at higher frequencies • Does not reduce signal rise time • One component • Reduces signal rise-time => reduces emission • Impedance matching at all frequencies

EMC Design K-E Gustafsson PCC / CMT EMC Design Group 15.1.2001

EMC Design K-E Gustafsson PCC / CMT EMC Design Group 15.1.2001

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