Troubleshooting Radiated Emissions
Contents
1. DM currents Source le Load Signal R Signal Return lc v Venp1 Venp2 i d 3m Phasor from far wire Phasor from near wire Resultant phasor Figure 5 Conunor mode currents in a circuit loop The source is a digital signal with harmonics and we ll assume a resistive load Because the phasor current in the far wire is in the same direction as the phasor current in the near wire the resultant phasor is relatively large compared to that produced by differentia mode current phasors In this case lowering the harmonic content by slowing the digital riseAalk times or diverting blocking the CM current is very important in limiting radiated emissions The equation for calculating the emission level in volts meter for a CM signal is shown below in Equation 2 A JSL Ec max 1 25710 a Equation 2 Ficld level V n due to CM current where f frequency Hz L length of the wires m and d the measurement distance typically 3m or 10m Note that the exponent is now 6 rather than 14 a much larger number thus the emissions from CM currents are typically much higher than that from DM currents So how do we minimize the emissions froma CM circuit loop First we must realize that frequency fand the length of the cabling Lis likely fixed by the design However Ic which is an undesirable signal may be reduced by either using a lower impe2. above that we measure RE If your product uses a high frequency crystal oscillator with fast edge speeds the harmonic content can be estimated with the formula in Equation 3 1 Equation 3 Miximal RE frequency estimate where f EMI frequency Hz and t risctime s For example with 1ns logic the harmonic content may be centered around 300 MHz Another rule of thumb is that for frequencies below about 300 MHz the problem is most likely due to common mode emissions from cables and above that the problem is most likely radiation from slots or seams in the metal chassis or circuit board radiation DIMENSIONS The dimensions of physical structures are also an important factor in troubles hooting an emissions problem Recall that the wavelength m of a resonant wire at frequency f in free space is 3x10 f Equation 4 Wavelength ofa wire where c speed of light in m4 and f frequency in Hz The dimensions in physical structures like circuit boards must be reduced by the velocity factor of the board material example 4 7 for FR4 circuit boards However typical cables such as USB or video are approximately 1m long and can be considered as being in free space Wires or slots may resonate strongly at multiples of a quarter wavelength For example a 1m long cable has a full wave resonance of 300MHz but may also radiate strongly at 150MHz and 75MHz Slots or seams of 8 to 5 15cm may resonate in the area
3. of 500 MHz to 800 MHz As a general rule of thumb radiating cables or chassis slots of 1 20 wavelength or greater start to become significant coupling paths for RE PROBES There are a variety of useful probes that may be used to troubleshoot RE problems E field H field and current probes The E field and H field probes are easily made in the lab All are available from several manufacturers An E field probe may be made by extending the center conductor about 0 5 cm froma section of semi rigid coax or high quality flexible coax then attaching a coax connector to the other end Shorting of the probe to circuit traces may be avoided by wrapping insulating tape around the end A useful H field probe may be fashioned by looping the center conductor around and soldering it to the shield to forma small loop of 0 5cm to 5 cmin diameter the larger the loop the more sensitivity A better H field probe design uses semi rigid coax to form the loop see examples in Figure 7 These probes are then connected to the input of an EMI receiver or spectrum analyzer to display the harmonics as the probe is brought into close contact with the circuit traces or chassis slots Depending on the diameter of your H field probe you may need to use a broadband preamplifier between the probe and analyzer a Figure 7 Examples of commercial E field and H field probes from Beehive EA En Electronics Figure 8 Examples of home made H field
4. probes 1 Probe manufacturers include Fischer Custom Communications www fischercc com Beehive Electronics www beehive electronics com or Teseq www teseq com 2 I made my own broadband preamp using a MiniCircuits model ZX60 3018G S which covers 20 MHz to 3000 MHz at 18 23 dB gain and 2 7 dB noise figure It sells for USD 50 SAFETY amp EMC 2008 29 Figure 9a Use of simple H field probes to lie ate emission sources Meerkat Main PC Board 100MHz 100MHz XTAL Harmonics feeds PLL on cable Creating 133MHz clock and maybe EB 933MHz ham Heatsink is Ok but hot Hot 100MHz 100 Mhz harm harmonics undemeath 50MHz ham 24MHz XTAL i mmayralen be lt Cabins Ok creating the 933MHz harmonic at harmomic 933MHz 933MHz harmonic coupling to I O pins due to proudmity to 24 MHz xtal oscillator Figure 9b I map out all the potential sources documenting them on a photograph of the circuit board Duracell 28A x 2 BV ea in AA holder Mini Circuits ZX60 3018G S 50 Figure 10 I made my own broadband preamp using a Mini Circuits model ZX6 h3018GS It is powered it with two 6V Duracell 28A batteries which fit in a standard AA battery holder The amplifier covers 20 to 3000 MHz at 18 23 dB gain and is used to boost the probe signals TROUBLESHOOTING STEPS Generally once you are finished mapping out your sources you should start with the lower harmonics and work upwards Often lower fr
5. that Ip is likely fixed by the design Likewise frequency f is probably fixed However length of the loop L and distance s may be reduced In other words the area of the loop may be decreased to reduce emissions This is an important point to keep in mind during circuit layout Placing a crystal oscillator one common source of harmonics and resulting emissions close to the circuitry that requires the clock signal is a good design practice Likewise the use of multi layer boards with full ground planes serves to reduce the loop area substantially Now let s consider common mode CM currents and how they are generated because it is not intuitive as to how current may travel the same direction through both the signal and signal return wires in a system Referring to Figure 5 note that due to finite impedance in any SAFETY amp EMC 2008 G3 5 grounding system including circuit board grounds there will be a voltage difference between any two points within that ground This is denoted by Vaan and Vexm in the figure This difference in potential will drive CM currents through common cabling or circuit traces between circuits or sub systems These CM currents may be generated on circuit boards or within sub systems inside product enclosures Because the current phasors are additive the resulting radiated phasor may be quite large compared to those generated by DM currents Therefore CM currents tend to be more of an issue than
6. 5 Troubleshooting Radiated Emissions A Practical Approach Kenneth Wyatt Abst r act Because time to market and budget factors often drive many of today s high tech designs electromagnetic compatibility EMC issues often surface at the last moment in the design cycle potentially delaying product introductions Very often simple pre compliance measurements and techniques can identify issues early when the cost of implementation is substantially lower and design improvements may be made with less impact on schedules This paper describes a number of simple techniques and tools useful in characterizing the radiated emissions RE of a design at various stages of development and will better prepare products for a successful radiated emission qualification Keywords radiated emission field probe troubleshooting kit Introduction There are usually five key threats that comprise most electromagnetic compatibility EMC problems radiated emissions ESD susceptibility to RF fields power disturbances and internal crosstalk Of these radiated emissions RE can be the most difficult EMC test for a product to pass Because emissions limits are established worldwide products that don t meet the limits may not be placed on the market The best way to achieve this is through proper product design but often these design techniques are not taught in universities nor are these techniques fully understood by many experienced engin
7. amp EMC 2008 and records procurement and incoming goods examination manufacture process control and process examination routine examination and confirmation examination examination on instruments and equipments control on unqualified products internal quality examination coincidence of products and packaging removal and deposition of products totally in ten aspects Those requirements would ensure that batch manufactured certificated products can comply with requirement of CESI certification standard and consist with certificated type of sample I believe CESI electronic component certification will be an effective assistant means for your products to enter China market Please browse relative information at www cesi ac cn Contact 86 10 6400 858084029206 EMC design of products and use of harmonic comb generators for predicting shielding effectiveness He has been published in magazines such as RF Design EMC Design amp Test Electronic Design Microwave Journal HP Journal and several others Kenneth is a senior member of the IEEE and a long time member of the EMC Society where he serves as their official photographer He is also a member of the dB Society and is a licensed amateur radio operator His comprehensive yet practical EMC design measurement and troubleshooting seminars have been presented across the U S Europe and Asia He currently resides in Colorado and may be contacted at ken emce seminars con His We
8. b site is www emic senunars com which can be accepted by 3C certification The components having acquired CESI certification don t need to be tested again during 3C certification process of equipments That also means the manufacturers having obtained CESI marks on their products have passed the certification of national level certificate authority As a result value added of products will undoubtedly increase As a matter of fact certificated components can be used by electronic equipment manufacturers more preferably consequently it will lead to the increase of product sales volume and bring more benefits to the manufacturers
9. dance ground or by blocking with a ferrite choke or with CM filters or by diverting through proper mechanical design This is an 34 SAFETY amp EMC 2008 important point to keep in mind during system design The solution to most EMC problems is to Control The Path of Current Troubleshooting Philosophy Radiated Emissions In troubleshooting any radiated emission problem it s useful to think of the problem in the form of a source path receptor model See Figure 6 below Receptor receiver Transfer coupling path Source emitter Seam Slot Radiated Emission Oscillator Cable TV Radio Figure 6 Source Path Receptor model Typically the source of radiated emissions is a high frequency crystal oscillator or other high frequency fast edged high current signal ASICs FPGAs and A D or D A converters may also generate these high frequency harmonics The path is the coupling mechanism or the means by which the high frequency energy is being radiated The receptor in most cases is the EMI receiver at the test site with specified emission limits By using various probes it should be possible to identify the source or sources Once the sources are identified the path or coupling mechanism must be identified and fixed What s difficult is that there may be multiple sources and coupling mechanisms to identify and fix before passing results are achieved In addition if a fix is imp
10. e currents are really phasors When the phasor produced by the far wire is added to the phasor produced by the near 5 wire the result is the difference in phasor magnitudes which produce a small resultant phasor and relatively low emission as shown in Figure 4 The area of the loop the current levels and harmonic frequencies dictates how much radiation will be produced Source Ip Load Signal RL Signal Return f Ip i d 3m Phasor from far wire Phasor from near wire Resultant phasor Figure 4 Differentia mode currents in a circuit loop The source is a digital signal with harmonics and we ll assume a resistive load Because the phasor current in the far wire is opposite the phasor current in the near wire the resultant phasor is relatively small compared to that produced by commorr mode current phasors However reducing the loop area is very important in limiting radiated emissions The equation for calculating the emission level in volts Aneter for a DM signal flowing in a loop is shown below in Equation 1 2 s1310 PEE D max Equation 1 Field level V n due to DM current where f frequency Hz L length of the wires m s spacing between wires m and d the measurement distance typically 3m or 10m So how do we minimize the emissions from a DM circuit loop Note that the area of the loop is Le s First we must realize
11. eers In many cases EMC is considered as black magic and many products must be tested repeatedly through a system of trial and error in order to finally pass as NUTA oe Figure 1 Photo of a typical 3m radiated emission test chamber 32 SAFETY amp EMC 2008 This is unfortunate because the emissions a product may produce is easily understood if the designer considers that it s the high frequency currents in circuit loops that tend to broadcast these emissions These circuit loops may be in the form of printed circuit traces differential mode currents or cables connecting two subsystems common mode currents There may also be combinations of these phenomenon The circuit and system design level of a product usually falls within the domain of the electronic engineer The other consideration is the shielding properties of the product which typically falls within the domain of the mechanical engineer Ideally these to must work together as a team to address the whole product in order to be successful in addressing EMC Background Theory In order to better understand RE and how to troubles hoot your product we must review how harmonics are created and then understand differential mode DM and common mode CM currents and how they get generated General design techniques are mentioned but specific design practices are a subject for another paper A periodic square wave Figure 2 may actually be represented by a se
12. equency sources will cause significant high frequency harmonics depending upon the rise time By fixing the low frequency source you Il often resolve high frequency harmonics as well Next check cables and then the enclosure CABLES Check your cables first as they are often the worst offenders Moving a hot cable will alter the RE levels I usually unplug all cables then try plugging each one in individually to find all that are radiating Remember that there may be more than one bad cable Snapping a ferrite choke around the base of the cable will probably help as an interim fix I ve found that most cable emissions are very likely due to poor grounding at the I O connector Cable currents may also be measured directly versus frequency with a current probe Examples are shown in Figure 1 Examples of commercial current probes ails gt Note a lack of good connection between A chassis enclosure and connector ground z Figure 11 pan 60 i R orreen sata TREDUEDY oma Figure 12 Poor I O connector grounding to the chassis allows the cable to radiate and usually fail the RE test A lack of solid ground can allow CM currents generated inside the product to flow out the I O cable and radiate sually causing RE failures The included graph shows poor margins to the CISPR 11 Class A 3m RE limit for ISM products in this case ITE products such as PG and printers have a limit 10 dB lower Look
13. etermine the appropriate fixes with your mechanical engineer It s also possible to use a differential probe and high bandwidth oscilloscope to measure any voltage 5 differences between pieces of sheet metal on the chassis enclosure If any voltage is measured it indicates a poor connection and potential leakage Figure 13 shows howto place one tip on each side of the joint Ideally the voltage should measure zero Figure 15 Use ofa differential probe and higlr frequency oscilloscope or spectrum analyzer to measure the potential difference across suspected gaps in the seams of a product enclosure Ideally this voltage should measure zero TROUBLESHOOTING KIT For speedy troubles hooting and analysis I ve assembled an EMC troubleshooting kit into a portable case which can be wheeled right to an engineer s workbench Major contents include a small spectrum analyzer a broadband preamplifier small antennas various probes and other accessories Other useful items to include into your troubleshooting kit include ferrite chokes aluminum foil copper tape power line filters signal filters and various values of resistors and capacitors Figure 15 shows an overall view of the contents ed OP Figure16 Contents of the special EMC troubles hooting kit I ve assembled I can probe for various RE problems as well as test for ESD and radiated immunity Performing these tests early in the design cycle results in a greater c
14. hance of passing the required EMC product qualification tests Summary In order to pass required EMC tests for radiated emissions it is necessary to understand the basic concepts of current flow through loops as well as differential and common mode currents and how they re generated Troubleshooting an existing design is simply the process of identifying the likely sources determining the coupling mechanisms through probing and applying temporary fixes Once these fixes have been applied and the product passes then the electronic and mechanical engineers may determine the most cost effective solutions Obviously troubleshooting or characterizing products early in the design cycle Continued on Page50 SAFETY amp EMC 2008 By 5 If there is only one type in an application unit the type of sample should be delivered Representative samples should be chosen and delivered if certification application ofa series of products is within the same application unit d Relativestandards Refer to Table frompage 44topage 47 e Factory inspection On site examination on factories is an important precondition for enterprises to obtain CESI certification for the first time as well as mantaining effectiveness of certificate Usually the examination includes initial examination and at least one time reinspection per year as to the latter the enterprises would be informed or not informed beforehand Content of factory examination includes exa
15. impedance Z graph of a typical current probe courtesy of Fischer Custom Communications The x axis is frequency while the y axis is dBQ Ue this to calculate the value of IC given the measured voltage at the probe terminals Vany and Zr Pela P r Zra dBuA dBu V Equation 5 Calculation of Ic given the measured V and Z from Figure 14 Next plug Ic into Equation 2 to calculate the predicted E field emission level in V n Converting this to dBuV n will indicate a pass or fail due to the cable being measured SLOIS amp SEAMS Once the cables and associated I O connectors are addressed it s time to probe for radiation leakage through slots or seams in the chassis Remember that the length of the slot or seam is important Any seam with leakage whose effective length is longer than 1 20 of a wavelength at the harmonic of concern has the potential to be an effective radiator For example a slot of 2 5cm can just start radiating harmonics at 1000 MHz I use a permanent marking pen to record the areas of leakage and frequencies of concern from every seam lot on the chassis Once these are marked I ll carefully cover over all the openings with copper tape and re measure the RE levels Keeping an eye on the RE levels I ll start removing the tape piece by piece to determine which slots or seams are actually causing problems Often just a fewslots or seams are causing the most problems Once the leakages are identified you can d
16. ing from ocio 1000MHz TEST SETUP Current probe on USB cable Connection between connector ground shell and chassis enclosure made with screwdriver blade Before After Some harmonics dropped by 10 15 dB Figure 13 Cables should be tested individually Here I have a current probe clamped around the cable under test and am monitoring the harmonics with a simple hand held spectrum analyzer As I ground the connector shell to the chassis with the screwdriver blade the harmonics are reduced 10 to 15 dB 3 Commercial current probes are available from Fischer Custom Communications www fischercc com Teseq www teseq com or Solar Electronics www s olar emc com 4 The handheld spectrum analyzer being used for the cable test is made by Thurlby Thander Instruments wwwv tti test com It sells for approximately USD 1500 and covers 150 kHz to 1 GHz 36 SAFETY amp EMC 2008 It is possible to actually predict whether a particular cable will pass or fail by measuring the CM current at the offending frequency solving for IC Figure 14 and Equation 5 below and plugging this into Equation 2 to solve for the field level in V m The length of the cable is L and the offending harmonic frequency is f Use a test distance of either 3 or 10m to predict the outcome at those test distances FFERI 4 30 ap Z WHO SAOSY 9P SONVGSdWI HJASNYHL 0 1MHz 1MHz 1OMHz 100MHz 1000MHz FREQUENCY Figure 14 Transfer
17. mination on certified products and requirements on quality system of the factory which involve factory s responsibility and resource documents Continued from page 37 are preferred in order to reduce overall implementation costs Kenneth Wyatt Sr EMC Engineer Wyatt Technical Services LLC holds degrees in biology and electronic engineering and has worked as a senior EMC engineer for Hewlett Packard and Agilent Technologies for 21 years He also worked as a product development engineer for 10 years at various aerospace firms on projects ranging from DC DC power converters to RF and microwave systems for shipboard and space systems A prolific author and presenter he has written or presented topics including RF amplifier design RF network analysis software Continued from page 47 Safety and EMC Testing Center of Electronics Industry SEC is legally owned by China Electronics Standardization Institute shortened as CESI SEC is a government approved Testing Lab approved by the former Ministry of Electronic Industry of the P R C specialized insafetyand EMC testing of electronic products aiming at supporting the government on quality supervision SEC is an independent and non profit testing body SEC was authorized by Certification and Accreditation Administration of the People s Republic of China CNCA to evaluate safety and EMC performance of products CESI mark is one of several marks designated by CNCA 50 SAFETY
18. ries of more basic signals called basis functions Assuming the rise and fall times of the square wave are straight up and down an infinite number of harmonically related basis functions or sine waves are required Digital circuitry today uses rise and fall times of sub nanoseconds which can generate harmonics of several hundreds to thousands of MHz Figure 2 A periodic square wave digital signal The building of a square wave Gibbs effect 1 oak N o8 N o7 oat 03 02 0 1 a 1 4 1 n 1 4 4 0 20 40 60 80 100 120 140 160 Figure 3 A representation of the square wave is comprised of a linear comb ination of basis functions or sine waves Image courtesy of Math Works Differential mode DM currents are caused by digital signals and their harmonics traveling through circuit loops The larger the loop the stronger the fundamental and harmonic emissions We want to minimize the area of any circuit loops through use of ground planes typically by use of multi layer circuit boards For low cost products multi layer boards may not be feasible so other design techniques must be used to minimize these loops Let s consider a simple circuit loop with a square wave source and resistive load The fundamental signal current plus all the related harmonic currents will circulate around the loop as shown Let s assume we have a receiving antenna 3m away from this circuit loop Thes
19. roperly installed the emission can actually get worse That s probably why the field of EMC is considered so mysterious By using a structured approach the troubleshooting phase should go smoothly Generally you ll want to diagnose the issues first then try various fixes Leave these fixes installed as you continue the troubleshooting process If you set up an antenna and EMI receiver or spectrum analyzer a fixed distance away 1 to 3m from where you re troubleshooting you can monitor your results real time IDENTIFY THE SOURCES tThe first step should always be toidentify the likely sources Ifyou re failing at 300 MHz or 500 MHz for example are these the third or fifth harmonics of a 100 MHz clock oscillator How about the memory clocking Generally memory address and data busses are fairly random The exception would be the AO or DO line which is clocking at a relatively non random rate What about clock lines to ASICs or FPGAs If you have multiple crystal oscillators which could be the cause of a particular harmonic spraying freeze spray on one then the other can often identify the offending oscillator FREQUENCY The frequency is key to any radiated emission problem As a quick rule of thumb the higher the frequency the more likely the coupling path is radiated The lower the frequency the more likely the path is conducted In fact the common break frequency is 30 MHz Below that we measure conducted emissions CE
Download Pdf Manuals
Related Search