User manual ()
Contents
1. 9 standard 9 ODOM 10 ETS Lindgren Product Information Bulletin 10 2 0 Maintenances 11 Annual AUD FAO 11 DEVICE 14 3 0 Electrical Specifications 13 Model ge ee ee ree 13 a 13 AV Opera ON 15 Typical E mariana 15 PYODE SClECHOM 16 17 5 0 Typical Performance Factors 19 Magnetic Field Probes 20 20 902 Cli 21 OOS LOOP 22 Elecinic E Field Probes 23 904 Ball PrODE isa N 23 909 Stu 24 dannii 25 0 0 concent 25 6 0 Common Diagn2. that the Model Part Number 7405 907 B BN BN PSE BN110 BN220 BNL BNLN BNLN PSE BNLN110 Model Part Name Pre Amplifier series Date of Declaration 23 February 1996 to which this declaration relates meets the requirements and in conformity with the relevant EC Directives listed below using the relevant section s of the following EC harmonized standards and other normative documents Applicable Directive s lector tic Com ectve 89 3 and its directives Applicable harmonized standard s and or normative document s 801 2 1991 Electromagnetic co ility for industrial Process measurement and control Part IEC 801 4 1988 Electromagnetic for Process measurement and control t 4 Electrical fast tansientburst requirements Authorized Signatories ETS Lindgren L P ETS Lindgren L P Bryan General Manager James C Psencik Vice President of Engineering The authorizing signatures on this Declaration of Conformity document authorizes ETS Lindgren L P to affix the CE mark to the indicated product CE marks placed on these products will be distinct and visible Other marks or inscriptions liable to be mistaken with the CE mark will not be affixed to these products ETS Lindgren L P has ensured that technical documentation shall remain available on premises for inspection and validation purposes for a period ending at least 10 years after the las
3. the 16 MHz clock that are 1 13 1 104 of the 16 MHz wavelength You can then begin to zero in on undershoot and overshoot or other parasitic components You may not have to quiet the entire circuit but rather roll off the offending components What you have done is mentally transform a frequency domain failure to a time domain picture that you can work on After identifying what the signal of interest looks like on the oscilloscope it must be located within the equipment At times this will have already been accomplished during the demodulation process Example As you demodulated a 5 MHz signal maybe it became clear that the 50 MHz was pulsing on at a 40 kHz rate You may know that the only 40 kHz source in your unit is the switching rate in the power supply If nothing else in the unit operates at that frequency you have identified your source Thus the first step in identifying a signal source is to review what subassemblies in the unit may produce a signal similar to the one you are seeing radiated 31 USING SNIFFER PROBES Typically there are several possible sources for a given signal To identify the particular one in question use the sniffer probes From a set of loop probes of varying sizes start with the largest which is also the most sensitive Begin several feet from the unit and look at the signal of interest Search for the maximum and approach the unit along the line of maximum emission As you near the unit s
4. These probes are highly selective of the H field while being relatively immune to the E field Each H field probe contains a single turn shorted loop inside a balanced E field shield The loops are constructed by taking a single piece of 50 ohm semi rigid coax from the connector and turning it into a loop When the end of the coax meets the shaft of the probe both the center conductor and the shield are 360 degrees soldered to the shield at the shaft Then a notch is cut at the high point of the loop This notch creates a balanced E field shield of the coax shield The loops reject E field signals due to the balanced shield Electric E Field Probes The Model 7405 includes two E field probes the stub probe model 904 and the ball probe model 905 Due to the small sensing element the stub probe is relatively insensitive This is an advantage when the precise location of a radiating source must be determined For example while moving the stub probe over the pins of an IC chip variations can be noted at spaces as close as two or three pins By comparison the ball probe is much more sensitive The larger sensing element does not offer the highly refined definition of the source location which the stub probe allows but it is capable of tracing much weaker signals The impedance of the stub probe is essentially the same as that of a non terminated length of 50 ohm coaxial cable BALL PROBE The shaft of the model 904 bal
5. illustration is altered slightly to make the point that the impedance of the return line is distributed and that there is a distributed capacitance between the signal lines If 2 gt gt Zc amp Ze then the signal will be carried on both the signal and return lines The return current will be shunted outside the circuit From the local perspective of the unit this is acommon mode situation EMC EMI problems may be classified principally as current related or voltage related Current related problems are normally associated with differential mode situations Likewise voltage problems are normally associated with common mode circuit situations Too often solutions are attempted before the radiating parameter is understood Unfortunately solutions effective for differential mode are seldom effective against a common mode problem 35 To review the physics of the situation In a far field that is more than about one wavelength from the source the ratio of the E field and H field components to the propagating wave resolve themselves to the free space impedance of 377 ohms In the far field the E field and H field vectors will always have a ratio of 377 ohms but in the near field that ratio radically changes The ratio of E field to H field or field impedance is determined in the near field by the source impedance As you probe close to the equipment you can switch between an E field probe and an H field probe By noting the rate of change of
6. 1 000 500 Electric E Field Probes 904 BALL PROBE 1012 4 2 000 2 000 3 000 Frequency MHz 1 000 1 500 500 23 905 STUB PROBE 10 2 4 24 1 500 2 000 2 800 3 000 1 000 Frequency Preamplifier Gain 0 01 MHz 3 GHz 3000 00 2800 00 2600 00 2400 00 2200 00 2000 00 1800 00 1600 00 1400 00 1200 00 1000 00 800 00 600 00 400 00 200 00 50 00 10 00 1 00 0 01 8 8 8 8 8 8 8 8 8 8 om 8004 Preamplifier gain dB Frequency MHz 45 10 0 00 25 This page intentionally left blank 26 6 0 Common Diagnostic Techniques Before connecting any components follow the safety information in the ETS Lindgren Product Information Bulletin included with your shipment Obtaining accurate repeatable results from EMI testing requires a carefully established and calibrated test setup usually an open field test site or a shielded room Final qualification must be performed in the required test environment of a screen room or an open field site However a great deal of preliminary EMI testing can be done with a sniffer probe and signal analyzing instrument The following sections describe how sniffer probes can be used in various phases of the engineering task Locating Radiating Sources The first step is to relate the emissions failure to signals used in the Equi
7. BUILT UP ACROSS SIGNAL GROUND STUD DIRECTION OF EMANATION DIRECTION OF EMANATION The placement of ground straps changes the geometry of the radiating current loop A ground strap may reduce the signal but it will also redirect it To properly assess the modification the perimeter of the unit must be scanned 46 The more distant measurement points may lose the signal into the system noise a given solution may only redirect the beam Especially with narrow beam problems solutions frequently only shift the beam so that it radiates in a different direction After measurement points are chosen baseline the unit by measuring each point with an E field and an H field probe That way each design alternative can be implemented and measured over the same set of points 2 two procedures differ here in how they approach the measurements that have been taken e first method is based upon finding a solution with a large safety margin For example suppose a signal fails the required limit by 3 dB Once that signal is found in the lab it can be measured in the near field The goal is then to reduce in this near field the 3 dB plus a safety factor of 6 dB 10 dB This allows a large margin of error due to near field effects Additionally a solution that passes this must then be confirmed by far field measurements e second method identifies several solutions which could be effective In the pre
8. Model 7405 Near Field Probe Set User Manual METS LINDGREN An ESCO Technologies Company ETS Lindgren L P reserves the right to make changes to any product described herein in order to improve function design or for any other reason Nothing contained herein shall constitute ETS Lindgren L P assuming any liability whatsoever arising out of the application or use of any product or circuit described herein ETS Lindgren L P does not convey any license under its patent rights or the rights of others Copyright 1996 2009 by ETS Lindgren L P All Rights Reserved part of this document may be copied by any means without written permission from ETS Lindgren L P Trademarks used in this document The ETS Lindgren logo is a trademark of ETS Lindgren L P Revision Record MANUAL 7405 PROBE SET Part 399107 Rev F A E e Initial Release February 1996 January 1999 e Updates edits Updated Preamplifier October 2009 Gain chart rebrand Table of Contents Notes Cautions and 6 2 1 7 Magnetic Field 8 8 cere 9 SWD
9. afety regulatory and other product marking information This page intentionally left blank vi 1 0 Introduction The ETS Lindgren Model 7405 Near Field Probe Set includes three magnetic H field and two electric E field passive near field probes designed for use in the resolution of emissions problems The Model 7405 provides a self contained means of accurately detecting H field and E field emissions and includes a 20 cm extension handle to provide access to remote areas in larger units Made of injection molded industrial grade plastic the probes are durable light weight and compact The probes provide a fast and easy means of detecting and identifying signal sources that could prevent a product from meeting federal regulatory requirements This set is a convenient and inexpensive tool for extending the capability of a spectrum analyzer oscilloscope or signal generator A near field probe is an essential tool for quick and efficient EMC EMI engineering Using near field probes and an oscilloscope can produce the following results Gain information about the source and location of the radiation member e Reduce test expense by adding inexpensive equipment for solving EMC EMI problems e Reduce test time by pre screening various solutions and alternate implementations Magnetic H Field Probes The Model 7405 includes three H field probes of varying size and sensitivity models 901 902 and 903
10. cing their length or making them wider e Inserting ground and power grids or planes e Shielding using a ground separate from signal ground e Relocating I O cables to a lower impedance area on the ground structure Placing common mode filters on the output lines using dissipating elements Pre Screening Alternate Solutions Pre screening allows you to sort through ideas formulate test plans and take several viable solutions to the range Pre screening also provides empirical evidence that a noise reduction technique has been correctly applied and indicates when you have properly analyzed the problem to the point of designing an effective solution Testing alternate solutions can save time when troubleshooting an electromagnetic problem For example for common mode problem that involves radiation from the end of a unit with the I O connections possible solutions could include the following Improve the decoupling on the board e Improve the power and ground grading or put in a ground plane Decouple the end with the I O connections to chassis ground Place common model choke the output I O The most economical solution may be a hybrid of these options applied in conjunction Each option could be implemented a number of ways and the physical mechanization of an approach will directly impact overall effectiveness 43 Evaluating various solutions requires great skill and awareness and it 16 in t
11. e probe out of the unit when making radiated readings an attached scope probe can easily radiate and mask the real problem When done you should have a good idea of the exact location of the offending signal Diagnosing Radiation Causes A small sniffer probe can help diagnose the cause of an electromagnetic interference problem This section addresses using sniffer probes for a rough estimate of field impedance which is used to diagnose the radiation physics of a given situation Knowing the field impedance can help find solutions to EMI problems When presented with an EMC EMI problem you need to know two things 1 What is radiating inside the unit and 2 Why the component or circuit is radiating 2 dv dt dv di Ten dt dt di dv If Z is very low then gt gt dt dt di dv Z is very large then lt lt dt dt 33 Radiation is caused by an instantaneous change in current flow causing a magnetic field or by an instantaneous change of a potential difference causing an electric field Experience has shown a high degree of correlation between magnetic fields with differential mode current flow Although a change in voltage will cause a change in current and vice versa one of these vectors will predominate The impedance of the radiating source will determine whether a predominately magnetic or predominately electric field is produced Typically magnetic fields are produced by local cur
12. efore performing any maintenance follow the safety information in the ETS Lindgren Product Information Bulletin included with your shipment Maintenance of the Model 7405 is limited to external components such as cables or connectors WARRANTY If you have any questions concerning maintenance contact ETS Lindgren Customer Service Annual Calibration See the Product Information Bulletin included with your shipment for information on ETS Lindgren calibration services Service Procedures For the steps to return a system or system component to ETS Lindgren for service see the Product Information Bulletin included with your shipment 11 This page intentionally left blank 12 3 0 Electrical Specifications Model 7405 Upper Resonant Frequency 901 H Field 41 dB 790 MHz 6 cm loop 902 H Field 29 dB 1 5 GHz 3 cm loop Primary E H or H E Sensor Type Rejection 903 H Field 11 2 3 GHz 1 cm loop 904 E Field 30 dB gt 1 GHz 3 6 cm ball 905 E Field 30 dB gt 3 GHz 6 mm stub tip Preamplifier Absolute Maximum Ratings Input Voltage DC 12 VDC e Input Voltage AC 20 dBm Bandwidth 100 2 3 GHz Noise Figure 3 5 dB typical Ref 50 ohms Saturated Output Power 12 0 dBm at F 100 MHz 1 dB Gain Compression 10 0 dBm at F 100 MHz Third Order Intermodulation 23 dBm Intercept 13 This page intentionally left blank 14 4 0 Operation Be
13. fore connecting any components follow the CAUTION safety information in the ETS Lindgren Product Information Bulletin included with your shipment Typical Configuration 1 Choose the appropriate probe from the Model 7405 Near Field Probe Set See Probe Selection on page 16 Signal Analyzing Device Equipment Under Test EUT Probe 2 Connect a coaxial cable from the probe to the signal analyzing device typically an oscilloscope or spectrum analyzer If needed place the extension handle between the probe and the coaxial cable 3 Adjust the signal analyzing device as required 15 Probe Selection Choosing the correct probe is determined by the following Whether the signal is or H If the signal is primarily is E field use the ball probe or stub probe If the signal is primarily H field use one of the loop probes If unknown try one of each and select the one that best picks up the signal The strength of the signal Select a probe that adequately receives the desired signal of interest Respectively the ball probe and the 6 cm loop are the most sensitive of the E field and H field probes The stub probe and the 1 cm loop are the least sensitive The frequency of the signal If the signal is above 790 MHz the probe may go into resonance See the upper resonant frequency listed for each probe in Specifications on page 13 In this illustration a ball probe
14. ge swing e Shielding the entire radiating loop It will not respond well to partial shielding of the radiating loop Partial shielding typically occurs when the path of the return current is mapped incorrectly and not included inside the shield Filtering the radiating signal line 39 DIFFERENTIAL MODE TECHNIQUES A Filters do not work because the filter ground is floating with respect to the potential which you want to filter out FILTER Radiating Potentials B Shielding does not work because only part of the radiating loop is shielded C Twisted pairs do not work because the total loop area is only marginally changed Some traditional differential mode techniques do not work in common mode situations When differential mode solutions are applied to a common mode problem many of the techniques will prove ineffective For example e Reducing circuit loop area The radiating signal is on the signal and return path so this will be ineffective Using twisted pair wires or coax will yield little in the way of signal reduction 40 Reducing the signal voltage swing This will be ineffective when the radiating potential is developed deep in the circuitry not at the output signal driver At times the radiating potential will be built up on the power or ground system through the additive effects of a number of gates Therefore suppression of any one of these gates in isolation will not yield muc
15. h signal reduction Shielding the entire loop A problem arises when deciding where to ground the shield The radiating potential is on signal ground but if you tie the shield to signal ground you ultimately add more radiating antenna to the system Filtering the signal line A problem arises when deciding where to ground the filter Using signal ground will be ineffective because the filter will float with the radiating potential 41 COMMON MODE TECHNIQUES A Increasing the amount of decoupling between power and ground is ineffective because the radiating signal is on the signal lines B Reducing ground inductance by shortening ground leads and making them shorter does not help as this is not the problem Relocating cable shield ground points is ineffective if the cable shield itself is insufficient Some traditional common mode techniques do not work in differential mode situations Once a common mode problem is determined use techniques which good potential for success Start by analyzing the ground and power distribution system Understand what RF impedances these systems present and then reduce the excessive impedance These techniques can be tried 42 e Increasing decoupling of power to ground e Reduce lead or trace inductance by redu
16. his area that the far field near field effects can be the most misleading The E field and H field vectors are initially determined by the source impedance As you move away from the source these vectors increasingly balance until the radiating field is isolated as a plane wave with a characteristic impedance of 377 ohms In the near field the field strength can contain in addition to the radiating field a significant non radiating reactive component This reactive component does not propagate far The radiating field will fall off proportionally with the reciprocal of the first power of the distance from the source 1 R However the reactive component will fall off proportionate with the reciprocal of multiple powers of the distance from the source 1 RN Typically the reactive field will fall off at a rate approaching 1 Two points should be observed 1 Often the near field reading will be dramatically different than would be expected based on an extrapolation of the far field reading Near field readings will seem higher than expected due to the presence of the reactive field alternately it may be lower than expected because of nulls created by the interference pattern set up near the unit A reflection pattern is often established near the unit by the direct wave combining with the reflection off parts of the unit and other items in the vicinity A design which reduces field strength by attenuating the non radiating reactive fie
17. is used to examine a flat cable The distributed inductance over the length of the cables makes them particularly susceptible to common mode problems High impedance sources such as this are best examined with an E field probe The physical size of the space where the probe must fit Model 7405 includes a variety of sizes See pages 8 9 fora description of each probe 16 closely you want to define the location of the source Choose the probe that gets as close to the signal source as required Select a large probe and begin outside a unit then move closer to the source and switch to smaller probes to identify the location of the source For example the smallest probes should allow you to determine exactly which circuit on a printed circuit board is radiating This kind of refinement provides the ability to stop the radiation at the source rather than shielding an entire unit Preamplifier Use The optional preamplifier increases the sensitivity of your test system The preamplifier is connected to the input of the signal analyzing device and the coaxial cable from the probe is connected to the preamplifier A switch on the preamplifier activates power to the unit when power is activated a panel light illuminates The preamplifier is powered by a wall mounted DC power supply Both 115 VAC and 230 VAC models are available The preamplifier includes a standard DC power connector 17 This page intentionall
18. l probe is constructed of a length of 50 ohm coax The coax is terminated with a 50 ohm resistor in order to present a conjugate termination to the 50 ohm line The center conductor is extended beyond the 50 ohm termination 904 and attached to a 3 6 cm diameter 3 6 ball metal ball which serves as E field pick up The absence of a closed loop prevents current flow allowing the ball probe to reject the H field STUB PROBE The model 905 stub probe is made of a single piece of 50 ohm semi rigid coaxial cable with 6 mm of the center conductor exposed at the tip This short length of 905 center conductor serves as a monopole 6 mm stub antenna to pick up E field emanations With no loop structure to carry current the stub probe rejects the H field Standard Configuration H field probes 3 E field probes 2 e 20 cm extension handle e Carrying case Optional Items e Preamplifier including wall mounted power supply 115 VAC or 230 VAC available e Preamplifier battery charger ETS Lindgren Product Information Bulletin See the ETS Lindgren Product Information Bulletin included with your shipment for the following 10 e Warranty information e Safety regulatory and other product marking information e Steps to receive your shipment Steps to return a component for service e ETS Lindgren calibration service e ETS Lindgren contact information 2 0 Maintenance B
19. ld may show relatively little effect on the far field reading 44 OPERATOR When 26 is large Cstray 15 small be significant 2 The probe becomes part of the circuit during near field measurements There is capacitance and inductance between the circuit being measured and the probe with the associated cabling The probe will re radiate the received field altering the field being measured However technical imprecision does not necessarily eliminate a method Sometimes an attenuation of the field strength in the near field will translate into an attenuation of the far field reading As long as a linear relationship is not expected there can be real benefit from near field probing Generally a reduction of the non radiating field will also mean that the radiating field has been reduced 45 EVALUATING ALTERNATE SOLUTIONS There are two approaches that yield good results when evaluating alternate design solutions 1 first step in each procedure is to choose a set of points for example two to six points Since the object is to determine what the far field results will be most of the points should be one to four meters away Also choose one or two points close to the source If a solution results in a dramatic reduction this point may be the only one that will allow quantitative measurement of the reduction GROUND GROUND CURRENT FLOW FROM POTENTIAL
20. ostic 27 27 signal Demoadulatl ON 29 a 30 Using 32 Diagnosing Radiation 33 Common and Differential Mode Current Flow 35 Differential Mode nn rnrerenennen 40 Common Mode 16 104465 1 42 Pre Screening Alternate 43 Evaluating Alternate Solutions cccccccsssescceeeeesesesseeessseeeeeeeeeeeenaes 46 Appendix Warranty 49 Appendix EC Declaration of Conformity 51 Notes Cautions and Warnings Note Denotes helpful information intended to provide tips for better use of the product Caution Denotes a hazard Failure to follow CAUTION ne instructions could result in minor personal injury and or property damage Included text gives proper procedures Warning Denotes a hazard Failure to follow instructions could result in SEVERE personal injury and or property damage Included text gives proper procedures See the ETS Lindgren Product Information Bulletin for s
21. pment Under Test EUT being tested To do this an understanding of the nature of the time domain to frequency domain transform is necessary dBu V m Frequency MHz 27 The various specifications are given in the frequency domain so there are many dBuV at a particular bandwidth over a given frequency range However most EUT operations are characterized in the time domain 150 ns memory access time 300 V ms slew rate and so on This section presents a technique that will aid in linking emissions with the signals that create them During testing you may receive information indicating for example that it failed by 10 dB at 40 MHz and 3 dB at 120 MHz The challenge is to find the EUT function that created the emissions You may be able to connect the probe to a spectrum analyzer and locate the source locating the source of an emanating signal begins by finding the exit points Cover seams and air flow vent holes are primary suspects However many sources can emit at a given frequency Most of these emissions are non propagating reactive fields The most helpful first step in locating the sources of a propagating field is to demodulate the offending signal while it is being received in the far field Demodulation gives a time domain representation of the signal This time domain representation will appear in some way similar to an oscilloscope trace of the radiating signal 28 SIGNAL DEMODULATION Oscilloscope Video Out
22. put Equipment Under Test EUT Frequency Span 0 Hz E Stub Probe To demodulate a signal 1 Set the spectrum analyzer for a 0 Hz frequency span and tune to the signal of interest This essentially changes the spectrum analyzer into a tuned receiver and makes the display a frequency filtered oscilloscope 2 the video output off the spectrum analyzer and run it to the oscilloscope Using the oscilloscope as the display allows greater flexibility in adjusting the signal amplitude and in triggering 29 3 Obtain a clear picture of the signal produced on the oscilloscope You now have a good representation of what you are looking for when you start sniffing with the probe Produce scope photos of the demodulated trouble frequencies and then use the sniffer probes to look for similar signals in your equipment Locate close matches to the demodulated signals for clues to the source of these signals When you find the sources you will determine the subassemblies circuits or gates that need work There are several physical phenomena that cause lower frequency signals to modulate and radiate as high frequency signals A working knowledge of FM AM audio rectification and other phenomena provides greater ability to understand and interpret the data revealed by demodulated signals This understanding gives insight into the kind of radiating structure that must be present to produce the observed event and also allows grea
23. reactive field will drop off at multiple powers of the inverse of the distance 1 RN Typically the reactive field will fall off at something approaching 1 R3 Therefore we would predict these measurements relative to measurements at distance equal to one 36 Distance AtoB 1 5 2 0 3 0 Propagating Field 1 R 3 52 dB 6 02 dB 9 54 dB Reactive Field 1 83 10 57 98 18 0698 28 63 dB After the source is identified two or three angles of approach are measured A typical situation would record two points at 0 5 meters and 1 5 meters from the source along two radials from the source The signal is measured at each point with a probe which 15 highly selective of the H field and another probe which 15 highly selective of the E field The rate of fall off is noted for each probe and the relative amplitude between the probes is noted In deciding what the relative amplitude is the conversion factor of each probe must be taken into account 37 E E Field Strength H H Field Strength PF Probe Performance Factor Z Field Impedance PREAMPLIFIER OSCILLOSCOPE H FIELD E FIELD PROBE E V PF 7 10 2 20 If Z lt 377 Q then dl dt predominates and the radiator is probably differential mode If Z gt 377 Q then dV dt predominates the radiator is probably common mode Differential mode data is generally well behaved The amplitude measured with the H field probe will be significantly higher
24. rent loops within a unit These loops may be analyzed as differential mode Electric fields require high impedance sources Because the changing potential is isolated by substantial impedance on all lines into the circuit all lines will carry just the forward current gt The impedance in this context is the total impedance at the radiating frequency Often what appears as low impedance connections are actually high impedance due to the inductance in the physical circuit A common way for all lines in a circuit to become high impedance lines is for the ground servicing that circuit to contain a significant inductance At some frequency this ground inductance becomes a high impedance Because the entire circuit references ground this impedance in the ground path effectively is in series with every line in the circuit The return flow in this situation is developed by capacitive coupling to conductors external to the unit or to fortuitous conductors within the unit 34 COMMON AND DIFFERENTIAL MODE CURRENT FLOW 2 Impedence of the signal line 2 Impedence of the intended signal return Z o amp Z o Impedence between the circuit elements and true ground SIMPLIFIED DIAGRAM OF A TYPICAL CIRCUIT If Zs amp Zn lt lt 268 Z o Then le lt lt Ip THIS IS THE INTENDED DESIGN Current flow is differential and almost entirely contained in the intended conductors IfZe gt gt Z o amp Z o Then gt gt Ir The
25. t product has been manufactured 51
26. ter facility in recognizing the original signal from the altered and often distorted modulated representation Frequently the demodulated picture will contain just the transitions of a digital signal At times only the rising or falling edge will be present in a high frequency signal Understanding the radiation physics allows the appearance of the original signal to be surmised Often all that will be present in the photograph from the oscilloscope presentation is the high frequency components of a signal These waveform components are the source of the radiation EXAMPLES Getting an idea of what the waveform may look like through demodulation is not the only use for the time domain frequency domain transform Analysis can reveal the component of the waveform that is causing the problem Example If you have 16 MHz clock you have a 16 MHz problem then you know that the base signal is causing the problem More typically your probing may lead you to the 16 MHz clock when trying to find the 208 MHz problem Remember a 208 MHz signal has a wavelength of 1 13 of 16 MHz 30 If the problem is caused by a rise or fall time you may be looking for a waveform component which is between a wavelength and 1 8 of a wavelength of the radiating frequency Example In the 208 MHz example a wavelength is 1 13 of the 16 MHz clock 1 8 of a wavelength is 1 104 of a 16 MHz pulse width Look at the oscilloscope picture for waveform components on
27. than that measurement with the E field probe Also the H field will drop off at a much faster rate than the E field e mode measurements are generally less well behaved Often the best indicator is the relative amplitude The E field probe will have a much higher reading than the H field probe The drop off rate will be faster when measured with the E field probe However experience shows that the E field being a high potential field is much more susceptible to perturbation Often the reading will be sensitive to cable placement and differences in the position of the person holding the probe This susceptibility to being perturbed can be a hint that the field is coming from a high potential source 38 A qualitative knowledge of the field impedance indicates how to approach the EMC EMI design for the problem By determining the dynamics of the radiating structure it can be surmised what kinds of designs will be effective is solving the radiation problem A primarily H field problem signifies that current flow predominates The other possibility is that the problem is predominately electrical or E field In this case the field impedance is relatively high A high field impedance means there is a potential build up across some impedance and this high potential region is the radiating source A differential mode problem will respond to these types of remedies Reducing circuit loop area e Reducing signal volta
28. the field strength versus distance from the source and the relative amplitude measured by the probes the relative field impedance may be determined Low impedance sources or current generated fields initially will have predominately magnetic fields The magnetic component of the field will predominate in the near field but will display a rapid fall off as you move away from the unit This change may be observed through an H field probe Low impedance sources also will give a higher reading in the near field on an H field probe than on an E field probe Alternately high impedance sources will display a rapid fall off when observed through an E field probe There are two ways to determine the nature and source impedance the rate of fall off of the E field and H field One of these vectors will fall off more rapidly that the other e Measure both vectors at the same point and by their ratio determine the field impedance The equation E H Z is calculated and compared to the free space impedance of 377 ohms Values higher than 377 ohms will indicate a predominance of the electric field Lower values will indicate that the magnetic field component is predomination From this you can plan your approach to the problem by tailoring it to a differential model situation or a common mode situation Field theory leads us to expect a 1 R fall off for a plane wave where R is the distance from the source In the near field the non propagating
29. vious example where the signal failed by 3 dB after pre screening in the lab a variety of solutions may be selected and tested A final benefit of pre screening is that through the inevitable failures new information can be discovered For example an attempt to reduce an emission may fail the following reasons 1 diagnosis was wrong 2 technique was inappropriate to the diagnosis 47 3 4 Example 48 The technique was improperly applied An outside factor is involved such as a second source radiating at the same frequency A solution that worked in the lab and on the range before 10 00 AM failed later in the day Analysis revealed that the rise in temperature was affecting the values of decoupling capacitors making them less effective at higher temperatures Appendix A Warranty See the Product Information Bulletin included with your shipment for the complete ETS Lindgren warranty for your Model 7405 DURATION OF WARRANTIES FOR MODEL 7405 All product warranties except the warranty of title and all remedies for warranty failures are limited to two years Product Warranted Duration of Warranty Period 49 This page intentionally left blank 50 Appendix B EC Declaration of Conformity METS LINDGREN Technologies Company Declaration of Conformity We ETS Lindgren L P 1301 Arrow Point Drive Cedar Park 78613 USA declare under sole responsibility
30. witch to the next smaller probe this probe will be less sensitive but will differentiate the signal source more narrowly Often the initial probing locates where the signal is escaping from the unit indicating the point of escape from the housing Once inside the unit and inside any shielding look for the source of the signal use the smallest diameter probe available You may switch to the stub probe which is a small and insensitive E field probe that can be used to get close to the signal source Finding both the point of escape from the unit and the actual source provides choice in engineering the solution you may decide to improve the shielding or to suppress the source The more solution alternatives you identify the greater the chance of identifying one which meets all the requirements of schedule cost and performance Another procedure is to use electromagnetic probes in conjunction with regular scope probes 32 Connect a regular scope probe and switch back and forth to refine the offending components as finely as possible Using this combination can define a radiating source to a specific signal line Periodically disable portions of a circuit to make a final determination of the location of the source For example disable a line driver to see if the radiation is coming from the base unit or from a cable When disabling parts of a circuit use a sensitive probe and take readings several meters from the unit Clear the scop
31. y left blank 18 5 0 Typical Performance Factors The following graphs represent typical calibration Individual probe results may vary Probe performance factor is defined as the ratio of the field presented to the probe to the voltage developed by the probe at the BNC connector PF EN By adding the performance factor to the voltage measured from the probe the field amplitude may be obtained All probes the Model 7405 Near Field Probe Set were calibrated a transverse electromagnetic mode TEM cell which presented a 377 ohm field The H field probes only respond to the H field however the equivalent E field response is graphed This may be done if the field is assumed to be a plane wave with an impedance of 377 ohms The reason for graphing the factors this way is to allow estimation of the strength of the far field If H field amplitude is desired subtract 51 52 dB from the performance factor as indicated on the graph 19 Magnetic H Field Probes 901 6 gt ow 20 1 000 1 500 2 000 2 500 3 000 500 Frequency MHz 902 3 cm Loop Jo Oe4 120 1 000 1 500 2 000 2 500 3 000 500 Frequency MHz 21 903 1 cm Loop io 2 10 2 22 1 500 2 000 2 500 3 000 Frequency MHz
Download Pdf Manuals
Related Search