Near-Field Methods of Locating EMI Sources

Transcription

1 Nea-Field Methods of Locating EMI Souces Contact Infomation: Vladimi Kaz Vladimi Kaz Cedence Technologies, Inc A Caldwell D., Soquel CA USA 1995 Cedence Technologies, Inc. Published in Compliance Engineeing Magazine, May/June 1995 The nea-field test is a valuable tool in EMC pecompliance. This aticle is a bief oveview of the fundamentals of the nea-field measuements and a pactical note of whee and how the test should be applied. Vitues of the Nea-Field Testing Most enginees and manages in the manufactuing business would athe not know anything about electo-magnetic compatibility (EMC). But this is whee the Fedeal Govenment steps in to help. In most counties, no electonics poduct may be sold unless it complies with the equiements of the local govenmental EMC agency. Many peviously exempt industies, such as medical, etc., now have to ente the wold of compliance. The adiated EMI compliance test is clealy defined in FCC p.15 and many othe intenational and militay standads and egulations. The specifics vay slightly, but the pinciple is the same: the poduct unde test is placed at some distance fom a well-defined antenna and the amount of adiation is measued acoss the equied fequency band. In othe wods, the measuements ae done in the fa field. Testing in the fa field means that the eceiving antenna is placed at a sufficient distance fom the poduct unde test whee the adiation iegulaities esulting fom the poduct s complex geomety ae added togethe and ae measued as a whole. In essence, the fa field test can tell whethe the poduct passes o fails as a whole but it cannot point the souce of a poblem. Using only the fa-field test, one cannot tell whethe an opening in the metal enclosue leaks too much adiation o a cable connected to the poduct caies too much RF enegy that gets adiated. It is simila to tying to find out what specifically makes a noise unde the hood of a ca fom a distance of 10 feet. One can guess that it might be a belt by its signatue squeak, but which one out of thee o fou is impossible to tell. Of couse, a diligent mechanic can eplace all the belts just in case, taking too much time and unning epai costs too high, but what if at the end it may not be a belt at all? In ode to locate the souce of the poblem, why not come close to the ca and take a look? As with the ca, electonic assemblies also need close up inspection. Unfotunately, fa-field antennae won t be able to help -- they ae just too big fo the job. The ight tool is called a nea-field pobe. It can identify the exact souces of emissions down to a component o a tace. With its help the whole poduct can be scanned in a matte of minutes and the leaks and poblematic aeas can be located on the spot. The disadvantage of the nea-field pobe is that thee is vey little coelation between nea-field and fafield measuements. This is due to the complex geomety and uneven adiation pattens of the tested poduct. Howeve, one thing is cetain: the highe the adiation level in the nea field, the highe it is in the fa field. Theefoe, tests pefomed in the nea field give designes a feel of how the poduct will behave in the fa field test. The nea-field test can map the electomagnetic field aound the poduct, easily identifying spots with high adiation levels. Designes can then make changes to eithe eliminate o contain the emissions. Nea-Field Pime The measuements done fo agency appoval ae pefomed at a distance of 3m o 10m. At these distances, the natue of the electomagnetic field is detemined by the souce, and the distance fom the souce. The popeties of the mateial between the souce and the point of measuement (in ou case it is only ai) also influence the eading. Page 1 of 7

2 As the test pobe gets close to the souce, the natue of the electo-magnetic field changes. Nea the souce of adiation the field poduced is mostly a function of the popeties of the souce. If the field is geneated by an object with high cuent and low voltage, the field poduced will be magnetic in natue, o H-field. If the souce has high voltage but little o no cuent, the esulting field is electic, o E-field. The electomagnetic field can be chaacteized by its wave impedance. Just like Ohms law stating that R= V, the wave I impedance Z W is a atio of the values of the electic and magnetic components of a paticula field. At any given point, the wave impedance is: Z W = E H (1) At some distance fom the souce, called the tansitional egion, both fields will end up with the same wave impedance that theeafte stays the same. This wave impedance, called the chaacteistic impedance, is the popety of the media in which the field is popagated. Fo ai, this impedance is 377W. As seen fom the equation above, a constant wave impedance means that the atio of E and H fields beyond the tansitional egion becomes the same fo all types of fields egadless of how they began. The field befoe the tansitional egional is called the nea field (o Fesnel egion), and the field beyond it is called the fa field (o Faunhofe egion). It is impotant to emembe that thee is no such thing as a puely electic field o a puely magnetic field. A conducto with a substantial cuent causing an H field still caies some small voltage that ceates an E field, albeit small in magnitude. Similaly, an antenna o an unteminated wie still caies some cuent via paasitic capacitance to gound. As seen fom the equation above, pedominately magnetic (H) fields ae chaacteized by low wave impedance, while mostly electic (E) fields have vey high wave impedance. The distance fom the souce also factos into the wave impedance equation. Electic and magnetic fields attenuate as this distance inceases. Each field begins to poduce its complementay field. As a esult of this, the wave impedance changes with the distance fom the souce. Fo an H-field, the wave impedance inceases with the distance fom the souce, fo an E-field, the wave impedance deceases. The fomula fo a magnetic souce is: Z W = 120p and fo an electic souce is: Z W = 120p (2) 2 +1 (3) whee is the distance fom the souce. Page 2 of 7 Figue 1. Wave Impedance as a Function of a Distance Figue 1 shows the tansition fom the nea field to the fa field. As seen, in the nea field each component has to be consideed sepaately. The tansition zone lies at a point of: = l 2p (4) whee is the distance fom the souce; l is the wavelength of the signal. Figue 2 shows how fa the tansition zone is fom the souce as a function of signal fequency. As seen, fo the fequencies of inteest (30MHz to 1GHz) all adiated compliance tests (classes A and B) ae conducted completely in the fa-field.

3 Distance, m Nea-Field Tansitional Region The Validity of the Nea-Field Measuements Ou inteest is in compliance measuements. As discussed above, fa-field measuements pefomed at the final compliance test do not help in locating the souce of the poblem. Nea-field measuements ae the only way to pinpoint souces of offensive adiation. But how good and eliable ae these tests and how to intepet thei esults? Since nea-field eadings ae geatly dependent on the geomety of the souce and its popeties, any attempt to povide coelation between measuements pefomed in the nea field to those done in the fa field will not delive usable esults. The only coelation is that, in geneal, the stonge the field nea the souce, the stonge it will egiste in the fa field. In addition to the basic coelation poblem, the following issues haunt the pecision and epeatability of nea-field measuements: Small movements of the nea-field pobe may poduce much geate elative change of distance fom the souce than the same absolute change in the fa-field. Readings of the field stength will be affected accodingly. A nea-field pobe has finite dimensions. Slight epositioning of the pobe vs. the device unde test (DUT) may put the nea-field antenna in a place whee the geomety of the DUT altes field 450 Fequency, MHz Figue 2. Tansitional Region as a Function of Fequency stength eadings, thus endeing epeatability of the measuements next to impossible. Thee ae just too many vaiables to make the numeical esults of nea-field tests tustwothy. Of couse, one could calibate the nea-field pobe and poduce an elaboate specification of its pefomance. If the eadings taken with the nea-field pobe cannot be tusted, what ae the benefits of using it? This question may especially intigue those enginees used to pecision in measuements. The answe is simple: the nea-field test povides only elative esults. The nea-field test can show whee the emission is stonge and whee it is weake and what type of field is dominant (i.e. what specifically is causing it). With the help of the nea-field test, expeience and intuition one can easily isolate the souce of the poblem and take actions to solve it. Types of Nea-Field Pobes H-Field Pobes The magnetic, o H, field is caused by cuent. Examples of H-field souces ae PCB taces with elatively high cuent, teminated wies and cables, etc. Sometimes, an H-field is geneated by cuents in the metal enclosue whee it was induced by highlevel EM adiation fom the cicuit boad inside. Appopiately, the sensing element of an H-field is a simple coil which is inductively coupled to the emitting tace o wie. An example of such a pobe is shown in Figue 3. The H- field pobe is connected to a spectum analyze via a cable o is incopoated into a self-contained device. Cente Wie Shield Coax Cable Figue 3. Simplest H-Field Pobe The H-field pobe povides maximum output when its loop is aligned with the cuent-caying wie and is in immediate poximity to it. Both these featues seve the pupose of locating exact souces of adiation. Using the diectional popeties of the H- pobe, the use can actually follow the wie o tace with the excessive adiation. By otating the pobe, Page 3 of 7

4 the taces with little o no adiation can be exempt fom suspicion. Cuent, ua Fequency, MHz 750 An impotant thing to know is that not evey spectum analyze o voltmete has a full-wave ectifie. A pactical implication of this is that a typical H-pobe may poduce diffeent esults at otation. Not ecognizing this fact can lead to ignoing impotant souces of stong adiation. Some H-field pobes have two coils positioned in opposite diections and a balun tansfome. Such a pobe has much bette bi-diectional popeties. The diamete of the loop detemines seveal impotant paametes: sensitivity of the pobe, its fequency esponse, and the ability to identify exact souces of emission. The lage the diamete of the loop, the moe magnetic field flux lines the loop will coss and the bette the inductive coupling will be with the offending wie. This esults in highe sensitivity of the lage loop. At the same time, a lage loop may not be able to diffeentiate between seveal taces, no to tell which one is the culpit. The lage loop has highe inductance that the smalle one. It leads to the loop s lowe esonance fequency beyond which the loop is unusable. Some manufactues poduce a kit of seveal H-pobes suited fo diffeent fequency bands. The ideal case, Wie Length, m Page 4 of 7 howeve, would be a small loop with sufficient sensitivity as it would satisfy the majo equiement: the ability to locate souces of adiation within the equied fequency band. Even a small cuent in a conducto can poduce a noticeable electo-magnetic field if the conducto is sufficiently long. The field stength can be calculated as: E = 1.25 Ifl whee E is field stength, mv/m I - cuent though the conducto, ma f - signal fequency, MHz l - length of conducto, m - distance fom the conducto, m (5) Figue 4 shows what cuent stength will poduce a field of 100mV/m at a distance of 3m as a function of signal fequency and conducto length. As seen, a nea-field H-pobe that can eliably detect 5mA is well suited fo the job. E-Field Pobes The electic, o E field is caused by voltage. Examples of E-fields ae unteminated cables and wies, PCB taces leading to high-impedance logic, whethe it be high-impedance inputs o ti-state outputs of logic ICs. A most common home-gown E-field pobe fo the nea-field is vey simple - it is just a piece of shot wie extending fom the shielded coaxial cable, as shown in Fig. 5. The smalle the antenna (the extended piece of the cental wie), the easie it would be to pinpoint the exact souce of adiation. Sensitivity, howeve, will Cente Wie Coax Cable Figue 5. Simplest E-Field Pobe suffe. The sensitivity of the nea-field pobe/spectum analyze combination is impotant not in absolute numbes, but athe in the stength of the field it is able to detect. A typical nea-field pobe is usually attached to the spectum analyze. A pobe combination that can detect electic field with a stength of ~5 mv/m is adequate fo use with equipment that is to be cetified fo FCC class B. A less sensitive pobe combination will miss impotant

5 emission souces, while one too sensitive can pompt a panic situation whee even a quite lame and insignificant emission spot will be teated as an emegency. Points of Inteest A nea-field test can do the following: pinpoint the exact location of a component, cicuit aea o tace with high emission levels pedict adiated EMC test esults pe-test you poduct pio to the official compliance test in ode to identify potential poblems ealy quickly toubleshoot a poduct which failed the compliance test find EM leakage on enclosues, cables and connectos find souces of intefeence peventing a poduct fom functioning popely. It is vey impotant to use nea-field pobes popely and to coectly intepet the esults. Nothing can substitute fo hands-on expeience with actual neafield pobes and with coelating the esults of the fa-field test with nea-field measuements. Howeve, thee ae some notes of inteest that may help in nea-field tests. Thee is seldom an exclusively electic o an exclusively magnetic field. Always use both pobes in ode to undestand the natue of emission and what you ae fighting against. As an example, if a PCB tace geneates esponse only in an E-pobe, but not in an H-pobe, it is pobably an electic field. Howeve, it is unlikely to find many occasions whee only an H-pobe is affected. Usually, if the tace emits a magnetic field, an E-field is also pesent. This happens because taces almost neve cay cuent with vey little voltage. High-level adiation egisteed with the nea-field pobe is not necessaily an indication of a poblem in the fa field. Just the opposite could be tue -- the nea-field pobe may not show the field stength of the aea which is the cause of the compliance failue. In ode to undestand why, one needs only to ealize that in the nea field the pobe picks the signal only at a paticula spot, while in the fa-field the field stength geatly depends on the antenna popeties of the adiating object. As an example, a shot tace on the PCB with a stong emission level may be much less dangeous than a longe one with a lowe emission level. A single small-size stong emission leak fom an othewise shielded enclosue may be much less dangeous than lukewam adiation fom a lage aea o aound a long seam in the shield. If the offending fequency is well-known and if the field caying it is not the stongest one in the poduct, a spectum analyze will be needed. If the pobe is used in the ealy stages of development whee it is not yet clea what fequency will be of concen, a simple boad-band level indicato will do an adequate job. In many cases it may actually do a bette job than a spectum analyze in seaching fo the highest emission level, since the spectum analyze may not povide a total signal level in the band eadings. It is difficult to subjectively judge it by looking at the sceen. Case Study: Detemining Shielding Needs Place the PCB assembly into an enclosue. The pototype will do since this is the time to decide on the shielding needs. Powe it up. Pass the nea-field pobe ove all extenal sufaces of the enclosue. Notice whee the emission is the stongest. If thee ae only one o two stong emission spots and the est of the poduct is elatively quiet, then placing localized small shields ove these spots may suffice. If thee is a mild o high adiation level ove a boad aea, moe compehensive shielding may be needed. Afte the shielding is pototyped, epeat the scan. The nea-field test will tell you whethe the shielding did its job. Good shielding will educe the nea-field emission down to a backgound level. Sometimes moe than one iteation of a shield s configuation may be needed. Afte the decision on one o two types of shielding is made, the poduct needs to go to a cetified lab fo a fa-field test. This test will show whethe the shielding did its job adequately and the shield design can be finalized at the pototype stage. Without the nea-field test applying shielding is somewhat like flying blind. As a esult, moe than one tip to the lab could be needed, with its Page 5 of 7

6 inceased cost and loss of pecious time. The neafield test helps design pope shielding at a much ealie stage in poduct development, saving both time and money. Timing is Eveything Thee comes a time in the design of many an electonic poduct when it is taken to a cetified laboatoy fo EMC testing. The esult of this test detemines whethe the poduct can be shipped o needs to be edesigned in ode to comply with the egulations. Such a time comes at the wost possible moment in the schedule -- the poduct is fully designed and debugged, the pototype is assembled, mateials fo inventoy have been odeed, poduction is planned, Maketing and Sales ae loiteing in the engineeing quates tying to glean how soon they can begin selling the poduct, and time is unning out. Evey day of delay can cost a company thousands (o even millions) of dollas in evenues. It is inconceivable fo uppe management that such an insignificant engineeing pocedue as an EMC compliance test can actually bing the whole opeation to a halt, should the poduct fail the test. It is even moe supising to many that the last minute quick fix fo the compliance poblems may aise the cost of the poduct seveal dollas, uining evenue plans, pofit objectives, etc. Since EMC compliance is almost neve consideed to be among the pime objectives of poduct development, it is seldom given enough attention. Typically, an EMC test is not a pat of the plan, but athe a scheduled event somewhee at the end of the poject. Yet, if one is to calculate the damage non-compliance could bing to a business, suely the bight minds in a easonable company would assign it a vey high pioity fom the vey beginning. The wasted time due to unfoeseen EMC poblems could be weeks, if not moe. The thousands of dollas spent on unnecessay tests and evisions of the pototype at the last moment fades in compaison to the lost evenues due to a schedule slip and the cost of added unplanned EMC potection pats to evey shipped unit. As Wall Steet Jounal epots in its Aug. 5/94 issue, Compaq had to delay shipment of thei Elite line of notebook computes fom Mach to June due to the FCC compliance poblems. How many companies can absob such impact on evenues? Smooth and timely passing of the EMC compliance test without emegency expenses is almost neve an accident; it is a esult of a good planning. The ealie electomagnetic compliance entes the scene, the fewe supises will be waiting at the end. Most of the factos detemining the EMC fate of the poduct ae decided upon at the ealiest stages of design: the poduct s physical achitectue, the numbe of the PC boads in the poduct, the equied cable inteconnects, the numbe of layes in each boad, the numbe of diffeent clocks and the clock fequencies, the type of enclosue and its mateial, etc. Most of these decisions will be made fa ahead of having a fully woking system that is testable fo compliance. Some lage pecent of guesswok is unavoidable. But having the ight tools fo the job can damatically educe the pue guesswok down to educated estimates, and futhe, to some level of confidence. The standad fa-field compliance test fo adiated emission is not well suited fo the ealy stages of poduct development. The fist pototype boad is seldom the same as its final vesion; cable assemblies ae spaghetti-like; and the enclosue is nowhee in sight. Futhemoe, the fa field test cannot tell whee to look fo the poblems. The nea field test can become the pefect tool fo ealy pe-compliance wok. Pefoming a nea-field scan on the vey fist evision of the PC boad can eveal futue hot spots at the time they can be made hamless at the lowest cost. Taces can be eouted, gound planes added, filte components, such as feite beads, etc. can be accommodated. Ealy knowledge of the adiation pattens can tell the mechanical enginee exactly whee to pepae to accommodate shielding, should it become necessay. This alone can save hundeds of thousands of dollas in last-minute tooling changes. Ealy detection can not be achieved with only the fa-field test. No one would be able o willing to take ealy pototypes to the EMC laboatoy fo outine diagnostics. As fo the financial aspects, a full-blown fa-field setup on site may cost a pohibitive amount of money. On the othe hand, nea-field pobes can be puchased fo as little as $600, and can and should be as common fo an Page 6 of 7

7 electonics enginee as a multimete o an oscilloscope. ScanEM Pobes -- The Right Tool Cedence Technologies manufactues fully selfcontained nea-field EMI pobes: ScanEM -EC fo detecting electic fields and ScanEM Ò -HC fo detecting magnetic fields. In self-contained mode, ScanEM pobes do not need a spectum analyze o any othe equipment to opeate and equie no cods. Each pobe weighs ~2 ounces (57g), and is poweed by two AAA batteies that ae included with the poduct. ScanEM-EC uses a small monopole antenna, and ScanEM-HC employs 1/4 (~6mm) diamete coil. The pobes can locate souces of emission of vey small size -- often down to a single pin of the connecto o a single tace on the PC boad, o a wie in a cable assembly. Since in the beginning of poduct development it is not entiely clea which fequency may cause poblem at the compliance test, ScanEM measues field stength in a boad band. ScanEM-EC has a fequency esponse fom 2MHz to 2GHz, though it was epoted by customes to pefom well up to 3GHz. ScanEM-H woks between 1MHz and 1GHz. ScanEM pobes do not povide absolute values of the field stength. Absolute numbes bea little significance in the nea-field measuements. Rathe, ScanEM pobes povide elative field stength data. In ode to have eal-time esponse, both ScanEM pobes have a 5-LED level ba with esolution of ~1dB/LED. In addition to visual indication, both pobes ae equipped with a speake that poduces a tone with the pitch dependent on field stength. Since the human ea is extemely sensitive to diffeences in the pitch of sound, the slightest change in field stength can be obseved by listening, without looking at the pobe s display. The sound can be disabled with an easily accessible switch. In ode to accommodate fo vaious field stengths, ScanEM pobes have a Level dial that can set the backgound level ove a boad ange of amplitudes. The use would bing ScanEM to an aea whee emissions ae consideed to be at a backgound level. The Level dial would be then adjusted so that only the fist (geen) LED is on. Afte that, the suspected aeas of the poduct would be scanned. The LED level ba and the pitch of the sound show deviations in field stength vs. the set backgound level. If the field stength at any point is lowe than the backgound level, ScanEM will be quiet and the lights will be off (squelch mode). This squelch featue allows ScanEM to eact only to fields stonge than the peset backgound level. This acceleates seach fo the hot emission spots. The momentay powe button povides powe to ScanEM only when depessed. This pevents accidental battey dischage. It should be clea that ScanEM pobes do not attempt to be a eplacement fo a spectum analyze. Howeve, ScanEM pobes povide essential functionality fo nea-field testing at a minimum cost and with geat convenience. ScanEM pobes ae vey easy to use and equie vitually no taining. Each pobe is about the size of a candy ba and fits easily inside a shit pocket. ScanEM ae available as a kit. The ScanEM pobes kit model CTK015 includes both ScanEM pobes, RF cable fo connecting to a spectum analyze o an oscilloscope along with a handy, duable plastic case. ScanEM pobes ae made in the U.S.A. Page 7 of 7